Development of an efficiency ranking system for beef cows and effects on feed intake, ruminal fermentation, NDF turnover, and apparent total tract digestibility

Abstract Beef cows (n = 100) were ranked for efficiency based on cow rump fat thickness at calving, calving date, and calf weaning weight (% dam BW) over 2 years. The nine most (ME) and least efficient (LE) cows were used to compare feed intake and ruminal fermentation using four 26-day periods with decreasing dietary nutrient density. There were no phenotype × diet interactions for variables of primary interest. Rump fat and calf weaning weight were greater, and the calving date was earlier for ME cows than LE cows (P ≤ 0.032). The ME cows were lighter (P < 0.001) but had similar DMI (P = 0.93) to the LE cows, resulting in greater dry matter intake (DMI) %BW (P < 0.001). Ruminal contraction amplitude height and area (P ≤ 0.015) and ruminal digesta weight were greater for LE than ME cows (P = 0.043). Ruminal ash-free neutral detergent fiber (aNDFom) passage was greater for ME cows than LE cows (P = 0.047), but the rate of aNDFom degradation did not differ (P = 0.69). Total tract digestibility did not differ. Efficient cows had greater rump fat, weaned heavier calves, ate more relative to their BW, had a smaller ruminal digesta mass, and had greater ruminal passage of aNDFom without reducing digestibility.


Introduction
Beef cattle are often managed in extensive grazing systems during the fall and winter, where they typically consume low-quality forages (Western Canadian Cow-Calf Survey 2017).The ability of cows to maintain body condition with minimal supplementation has helped cow-calf producers impose low-cost production systems (Canfax Research Services 2020).Thus, cows that efficiently utilize low-quality forage as nutrients to maintain their body reserves, support reproductive success, and wean a heavy calf are more profitable (Cushman et al. 2013).There are several approaches that have been used to quantify feed efficiency for the cow-calf sector; however, the reliance on gain as a key metric in many efficiency systems limits its applicability to mature, non-growing cows where revenue is generated from the sale of their calves.Residual feed intake (RFI) has been promoted as a potential efficiency measure for cows, although adoption of RFI is limited by the high cost of identifying low-RFI animals, the difficulty in measuring individual feed intake, and practical limitations with the measurement of RFI using high-forage diets (Arthur and Herd 2008).Furthermore, cattle may change their RFI classification when evaluated in consecutive feeding tests, even when fed the same diet (Durunna et al. 2011(Durunna et al. , 2012)), and while having multiple cows consume feed from the same bunk allows for measurement of feed intake, feed sorting behaviour may alter the composition of the diet for others eating from the same bunk, challenging whether nutrient consumption is directly proportional to DMI.As such, efficiency factors that result in a consistent outcome and allow for evaluation when fed high-forage diets are needed for beef cattle.
The ability of a cow to produce a calf on an annual basis influences the profitability of cow-calf operations.In addition, cows that calve early within the calving season produce calves that are heavier at the time of weaning due to more days available for growth (Cushman et al. 2013).These cows have a longer duration for uterine involution and resumption of estrus prior to the start of the breeding season, and as a result, they are less likely to be culled due to reproductive failure (Lesmeister et al. 1973;Cushman et al. 2013).Maintenance of, and the trajectory for, body condition score (BCS) as parturition approaches and in early lactation impact the duration of the postpartum interval.Cows that approach parturition with an adequate BCS have a shorter postpartum interval than those that lose body condition as parturition approaches (Osoro and Wright 1992;Hess et al. 2005).Therefore, measures of efficiency for the cow-calf sector should include the aforementioned factors as they affect lifetime productivity and profitability (Rahnefeld et al. 2011;Scholljegerdes and Summers 2016;Snelling et al. 2019;Broleze et al. 2020).
The objectives of this study were to develop a ranking system to classify the long-term productive efficiency of beef cows and to evaluate whether efficient and inefficient cows differ in their ability to utilize forage.The hypothesis was that efficient cows will have a greater DMI than inefficient cows due to greater ruminal turnover rates and motility and a smaller ruminal digesta mass.Furthermore, these responses will be consistent across diets of varying quality.

Materials and methods
Use of cows in this research was approved by the University of Saskatchewan Research Ethics Committee (protocol 20200079) and followed the guidelines of the Canadian Council on Animal Care (Ottawa, ON, Canada).

Management of cows under extensive feeding conditions
Two years of data collection were conducted to complete the phenotype assessment.One hundred yearling Black Angus heifers were selected and managed over 2 years (2018)(2019)(2020) under an extensive production system utilizing tame pasture mixes for grazing during the summer, stockpiled forage during the fall, and whole-crop barley in a swath grazing system over the winter.Cows were housed in large drylot pens during calving.Cows were checked for pregnancy during the fall of each year via ultrasonography, and estimated fetus ages were recorded.Calving date, calf sex, calving difficulty (0 for unassisted, 1 for assisted), and calf body weight were measured within 24 h of parturition.In addition, cow body weight, cow BCS on a scale of 1 to 5, and cow rump and rib fat thickness using ultrasonography (Aloka SSD-500; 17 cm 3.5 MHz linear transducer: Aloka UST-5044-3.5;Hitachi Aloka Medical Ltd., Tokyo, Japan) with rib fat measured between the 12th and 13th ribs and rump fat measured at the apex of the biceps femoris muscle between the hook and pin bones (Realini et al. 2001) were recorded at calving and weaning.Due to cow culling in year 1 and missing data, the final selection was based on 86 cows.

Phenotypic selection
Cows were ranked using a percentile scoring systems in each of the 2 years based on cow rump fat depth at calving, calving date, and calf weaning weight as a percentage of cow body weight.Cow rump fat at calving was used as an indicator of the ability of cows to maintain BCS under extensive winter grazing management.Rump fat was measured using ultrasonography at calving and was weighted the greatest at a maximum of 10 points (Table 1) as it was a direct measure of the cow's response.The calving date was used as an indicator of reproductive fitness.Cows were ranked on a percentile basis for the day of calving within the calving season, with cows calving earliest receiving a maximum of eight points.The year 2 calving date was based on the gestational age of the calf determined at pregnancy checking to place emphasis on rebreeding and to allow for completion of the subsequent phase of the study.As back fat thickness may also influence the ability of cows to re-breed and conceive, a discount was applied to the weighting factor with a maximum of eight points.Calf weaning weight is important for prof- itability, but is impacted by many factors besides the dam, so it was weighted the least of the selection criteria with a maximum of six points.Calf weaning weight as a percentage of cow body weight was conducted independently for heifer and steer calves and was unadjusted for age (Table 2).Cows that were open at pregnancy check or had a calf die before weaning were deducted two points.The total score of the three attribute percentiles was then summed for each cow within each year.The cumulative percentile scores were plotted to evaluate cows that were above (most efficient; ME) or below (least efficient; LE) the 50th percentile in both years.Using this phenotypic scoring system, the 10 cows with the greatest (ME) and least (LE) scores (n = 20) were selected.

Preparation and management of the selected cows
The 20 ME and LE cows were subsequently moved to the University of Saskatchewan Livestock Research Building (Saskatoon, SK, Canada) and were group housed in an outdoor pen on a free-choice grass hay diet.Prostaglandin (Lutalyse, Zoetis, Parsippany-Troy Hills, NJ, USA) and dexamethasone were used to abort the fetus (Johnson et al. 1981) to avoid the confounding effects of pregnancy on ruminal fermentation, digesta passage, and apparent total tract digestibility.Cows were evaluated for pregnancy 7 days after prostaglandin and dexamethasone treatment, and cows that did not abort were 1 Cows that did not calve or did not have a live calf at weaning were deducted two points from weaning weight.
2 Score was calculated based on percentile ranking in the herd with a maximum score of 6 for cows with either bull or heifer calves.
treated again and rechecked.Cows were then surgically fit with a 7.6 cm ruminal cannula (Model 4C; Bar Diamond Inc., Parma, ID, USA).Twenty-one days after surgery, the cannulas were replaced with a 9 cm cannula (Model 9C; Bar Diamond Inc.), and the cows were provided with an additional 29 days for recovery and training.Two cows were removed from the study due to aggressive behaviour resulting in nine ME and nine LE cows.Cows were housed in individual pens (9 m 2 ) with a rubber mat on the floor.Pens were scraped daily and washed every 2 days apart from during collections when washing was not permitted.Environmental enrichment was provided via nose-to-nose contact with neighbouring cows and a suspended ball in each pen.If weather and sampling permitted, cows were provided with 3 h of outdoor access in a group pen.Cows were fed daily at 10:00h and the refusals were removed prior to the feeding time.Minerals and urea were combined and supplied on a daily basis using a separate feeder due to difficulty homogenously mixing them into the dry diets.Cows had ad libitum access to water in their pens.

Experimental design
Cows were blocked into one of two wings of the barn and randomly assigned to an individual pen, with five ME and four LE cows in block 1 and four ME and five LE cows in block 2. The experiment consisted of four consective 26-day periods, with all cows in both blocks offered the same diet within each period.Diets were formulated using the nutritional dynamic system (RUM&N, Reggio Emilia, Italy) and progressively decreased in quality as indicated by reduced silage inclusion, increased straw inclusion, and chemically with increasing neutral detergent fiber (NDF), undigestible neutral detergent fiber (uNDF), and decreasing starch (Table 3).Crude protein (CP) was formulated to be adequate for non-pregnant and non-lactating cows in all diets.The four diets were formulated to represent the varying diet quality that cows may have access to over the year, especially the low-quality forages that are often fed over the winter.The cows were offered the same diet within each period to allow for comparisons among efficiency rankings while avoiding the confounding effects of diet sequence and time.For the diets, hay and straw were chopped through a 15.2-cm primary and 5.1-cm secondary screen, respectively, using a Haybuster H1100E (DuraTech Industries, Jamestown, ND, USA) and blended using a Keenan Mechfiber320 mixer wagon (Alltech Farming Solutions Ltd., Borris, Co. Carlow, Ireland).Five minutes of mixing time from the last ingredient addition was used to ensure a homogenous mix was produced.Silage and blended hay-straw samples were collected every 4 and 8 days, respectively, and dried for 72 h in a forced air oven at 55 • C to determine the DM concentration to ensure diets were mixed accurately on an as-is basis.

Data and sample collection
Cow body weight, rib fat, and rump fat thickness were recorded on days −1 and 1 and day 26 of each period.Cow rib and rump fat were measured using ultrasonography, with rib fat measured between the 12th and 13th ribs and rump fat measured at the apex of the biceps femoris muscle between the hook and pin bones (Realini et al. 2001).Subcutaneous fat was determined by the same person throughout the study to ensure consistent measurements.
Feed intake was monitored throughout the study to ensure ad libitum intake of feed by targeting 15% of offered feed being refused.The amounts of feed offered and refused were weighed and used for data analysis from days 14 to 21.A subsample of refusals (10% of the total weight) from each day were collected and composited to yield a single sample for each cow in each period.Individual feed ingredient samples (200 g/day) were collected from days 14 to 21 and a period composite was prepared.Feed ingredients and feed refusals were stored at −20 • C until they were removed for particle size separation using the Penn State Particle Separator with sieve openings of 19, 8, and 4 mm, and a pan (Nasco, Newmarket, ON, Canada).Sorting indices were calculated for each cow as the actual intake of particles retained on each screen as a percentage of the predicted intake (Leonardi and Armentano 2003).The predicted intake of each screen was based on the particle size of the feed offered and the amount of feed offered.The feed ingredients and refusals were also dried in a forced air oven at 55 • C for 72 h to determine DM before being ground using a hammer mill (Christie-Norris Laboratory Mill, Christie-Norris Ltd., Chelmsford, UK) to pass through a 2.5-mm screen.Samples were then re-ground to Table 3. Ingredient and chemical composition of the high-quality (HQ), medium high-quality (MHQ), medium-quality (MQ), and low-quality (LQ) diets fed to 18 mature beef cows in over four consecutive 26-day periods.pass through a 1-mm screen and analyzed for organic matter (OM), CP, starch, water-soluble carbohydrates, ash-free neutral detergent fiber (aNDFom), acid detergent fiber (ADF), and uNDF at Cumberland Valley Analytical Services (Waynesboro, PA, USA).Ash was analyzed according to method 942.05 of AOAC (2000) with the modifications of using a 1.5-g sample weight, a 4-h washing time, and a hot weight.The OM was calculated as ash subtracted from 100%.CP was determined using method 990.03 of AOAC (2000) with a Leco FP-528 Nitrogen Combustion Analyzer (St.Joseph, MI, USA).Starch was analyzed according to Hall (2009), with a correction for free glucose and water-soluble carbohydrates according to Dubois et al. (1956).Ether extract was determined according to AOAC (2000) method 2003.05 using the Tecator Soxtec System HT 1043 Extraction Unit (Tectator, Foss, Eden Prairie, MN, USA).The aNDFom was determined according to van Soest et al. (1991), except that Whatman 934-AH glass microfiber filters with 1.5 μm particle retention were used, and samples were ashed at 535 • C in a furnace for 2 h.Method 973.18 of AOAC (2000) was used to determine ADF, but Whatman 934-AH glass microfiber filters with 1.5 μm particle retention replaced the fritted glass crucible.The uNDF content was determined as the aNDFom left after 240 h of in vitro digestion, measured according to Goering and Van Soest (1970).
To determine the net energy of maintenance (NE m ) requirements, energy balance equations from NASEM (2016) were used.To determine the NE m concentration of the diet, apparent total tract nutrient digestibility (described below) was used to calculate total digestible nutrients (TDNs) using the equation TDN (kg) = CP digested + aNDFom digested + starch digested + (ether extract digested × 2.25).The TDN was then converted to DE, assuming that 1 kg TDN = 4.4 Mcal of DE and DE was converted to ME assuming a constant efficiency of 0.82.The ME was then converted to NEm according to NASEM (2016), and the difference between NE m consumed and NE m required was calculated.For RFI, calculations followed those described by Durunna et al. (2011).However, as the period length is shorter than suggested, caution should be applied when evaluating these data.

Ruminal fermentation
Ruminal digesta collection was initiated on day 17 at 07:00 h and continued every 12 h with a 4-h offset among days.Thus, samples were collected at 07:00 and 19:00 h on day 17, 11:00 and 23:00 h on day 18, 15:00 h on day 19, and 03:00 h on day 20.Ruminal digesta samples (250 mL/region) were collected at the ruminal-fluid-ruminal-mat interface from the cranial central and caudal central regions of the rumen.The ruminal digesta from each region were combined and strained through two layers of cheesecloth.Ten milliliters of ruminal fluid were then added to 2 mL of metaphosphoric acid (25% w/v) or 1% sulfuric acid for analysis of short-chain fatty acids (SFCAs) or ammonia, respectively.The ruminal fluid samples were stored at −20 • C before being thawed.A composite sample was prepared by transferring 1.5 mL from each time point for each cow within each period.The SCFA concentration was determined using gas chromatography with a flame ionization detector (Agilent 6890; Agilent Technologies Canada Inc., Mississauga, ON, Canada; Khorasani et al. 1996).
Ruminal pH was measured using the strained ruminal fluid at each sampling timepoint described above.The hand-held pH meter (Accumet AP110; Fisher Scientific, Pittsburgh, PA, USA) was standardized in buffers 4 and 7 prior to each sampling.The mean, maximum, and minimum pH values were calculated.

Ruminal motility
Ruminal motility was measured in five cows/block on day 22 and four cows/block on day 23 from 11:00 to 15:00 h.Motility was measured by placing a weighted balloon (Party City, Saskatoon, SK, Canada) filled with 1 L of water into the ventral sac of the rumen.The balloon was connected to a disposable blood pressure transducer (MLT0699; ADInstruments Inc., Colorado Springs, CO, USA) as described by Egert et al. (2014).The pressure transducer was connected to a recording device (PowerLab 9/35, ADInstruments Inc.) through bridge amplifiers (FE224; ADInstruments Inc.), which took a mV reading every 200 ms and was calibrated to 20 and 100 mmHg prior to recording (MLA6595; ADInstruments Inc., Colorado Springs, CO, USA).Rumen pressure data were analyzed using LabChart Pro software (ADInstruments Inc.) for contraction frequency, time between contractions, contraction duration, contraction amplitude, and total contraction area (Pereira et al. 2022).Contraction frequency, time between contractions, contraction duration, and contraction amplitude were measured by calculating the integral of the smoothed pressure signal, filtering to remove noise, using cyclic measurement detection with filtering using a high pass cutoff of 0.01 Hz, and a minimum peak height of 3.5432 to 11.1200 mmHg.The minimum peak height used was adjusted for each cow in each period to ensure contractions were detected.The total contraction area was calculated by summing the areas from each contraction event.

Ruminal NDF turnover
Ruminal NDF turnover was determined by completely evacuating the rumen of all digesta at 07:00 h (prior to feeding) on two consecutive days (days 24 and 25; Linton and Allen 2008).Ruminal digesta was weighed, thoroughly mixed, and two 5 L samples were collected before the digesta was returned to the rumen.Each 5 L sample was separated into the liquid and solid portions using a wine press (Harvest Bounty Wine Press; Pleasant Hill Grain LLC, Hampton, NE; Karnati et al. 2007).The liquid and solid portions were weighed before being dried for 120 h at 55 • C in a forced-air oven.The duplicate samples of the dried solid portion from each evacuation were composited and sent to Cumberland Valley Analytical Services (Waynesboro, PA, USA) for determination of uNDF and aNDFom, as previously described.The ruminal rate of aNDFom passage (k p ) was assumed to be equal to the ruminal rate of uNDF passage, and the ruminal rate of aNDFom degradation (k d ; %/h) was calculated as (intake of potentially degradable aNDFom/ruminal pool size of potentially degradable aNDFom) − k p (Dado and Allen 1995).Intake of uNDF and aNDFom was calculated from d 24 DMI and the composited feed and refusal uNDF and aNDFom concentrations.The ruminal pool size was the product of ruminal solid fraction DM weight and the ruminal solid fraction uNDF and aNDFom concentrations.

Apparent total tract digestibility
Foley bladder catheters (24 Fr 75 mL Bardex Lubricath Catheter; C.R. Bard Inc., Covington, GA, USA) were inserted and infused with 80 mL of saline solution on day 16 of each period.Cows were tethered, and cows were provided with 1 day to acclimatize before collections began.Total fecal collections began on day 17 of each period at 07:00 h and continued for 96 h.Pens were scraped every 4 h, and the weight of the feces was weighed at each timepoint.A representative sample of feces was collected (2% of the collected weight) and used to prepare a composite for each cow.Fecal samples were stored in a plastic container at −20 • C.After compositing, the fecal samples were thawed and dried in a forced-air oven at 55 • C for 120 h to determine the DM concentration.Dried samples were ground to pass through a 2.5-mm screen using a hammer mill (Christie-Norris Laboratory Mill, Christie-Norris Ltd., Chelmsford, UK).The ground fecal samples were sent to Cumberland Valley Analytical Services (Waynesboro, PA, USA) for determination of OM, CP, starch, water-soluble carbohydrates, aNDFom, ADF, and uNDF, as previously described.
Total urine output was collected during the same period as fecal output.Urine was collected in 20-L carboys filled with 300 mL of HCl.Urine was mixed and weighed daily at 07:00 h and a 30-mL subsample was collected and stored at −20 • C. Daily urine subsamples were then composited based on daily urine output.Urine pH was tested in the carboys daily to ensure that the 300-mL of HCl was sufficient to maintain pH below 4. The composited urine samples were analyzed for urine N using a LECO FP-528 analyzer (LECO Instruments ULC, Mississauga, ON, Canada).

Statistical analysis
All statistical analysis was conducted using the Statistical Analysis Systems software (SAS version 9.4; SAS Institute, Inc., Cary, NC, USA).The mixed procedure of SAS was used, and the model included the fixed effects of phenotype, diet, the two-way interaction, and the random effect of block.The cow rump fat and calf data from the 2 years of extensive management were also analyzed using the mixed procedure of SAS with the fixed effects of phenotype, year, and the two-way interaction.Means were determined to be different when P < 0.05.Sorting behaviour was also analyzed using a two-tailed t-test to determine if individual treatment means differed from 100, with significance being declared when P < 0.05.Correlation and regression of cow rump fat at calving, calving date, calf weaning weight, and yearly selection point totals were analyzed using the CORR procedure and REG procedure of SAS, respectively.

Results
Rump fat thickness of the 86 cows ranged from 2.0 to 6.0 mm with a mean of 3.7 ± 0.8 mm in year 1 and 2.0 to 8.0 mm with a mean of 4.3 ± 1.2 mm in year 2 (Table 1).The minimum rump fat thickness score was 0:00 for both years 1 and 2. The rump fat score had a maximum of 10.00 with a mean of 3.25 ± 2.81 in year 1 and a maximum of 9.88 with a mean of 3.69 ± 2.89 in year 2. Calving date was measured as 1 Greatest SEM for the two-way interaction is presented.
2 Cow rump fat was measured using ultrasonography during calving. 3Calving date is the difference in days between the calving date and the first cow to calve for year 1 and the difference between a cow's estimated gestation length at pregnancy checking and the longest cow's estimated gestation length in year 2. 4 Calf weaning weight was corrected for sex according to Minyard and Dinkel (1965) and then calculated as a percentage of the dam body weight.the difference between a cow's calving date and the date of the first cow to calve in year 1, and the difference in a cow's gestational age and the oldest gestational age in year 2. Calving dates ranged from 0 to 76 days with a mean of 27 ± 14 days in year 1 and from 0 to 44 days with a mean of 20 ± 14 days in year 2. The minimum calving date score was 0.00 in both years 1 and 2, and the maximum score was 8.00 and 7.96 in years 1 and 2, respectively.The mean score in year 1 was 5.17 ± 1.55, and 5.55 ± 1.93 in year 2. The weaning weight of bull calves as a percentage of cow BW ranged from 33.0 to 59.0% in year 1 and 24.0% to 51.0% in year 2 (Table 2).The mean bull calf weaning weight was 44.1 ± 6.2% of the dam's BW in year 1 and 37.7 ± 5.5% in year 2. The weaning weight of heifer calves in year 1 ranged from 0.0% to 53.0% and in year 2 ranged from 20.0% to 46.0%.The mean weaning weight of heifer calves was 39.9 ± 9.2% of dam's BW and 34.8% ± 5.5% of dam's BW in years 1 and 2, respectively.The score for bull calf weaning weight ranged from 0.00 to 6.00 in years 1 and 2, with a mean of 2.87 ± 1.82 and 2.85 ± 1.77 in years 1 and 2, respectively.The minimum score for heifer calf weaning weight was −2.00 in year 1 and 0.00 in year 2, with a maximum of 6.00 in both years.The mean score for heifer calf weaning weight was 2.98 ± 1.92 and 2.80 ± 1.82 in years 1 and 2, respectively.When comparing LE and ME (Table 4), ME cows had greater rump fat, calved earlier in the calving season, had a heavier calf weight at weaning whether represented in kg or as a function of cow BW.That said, calf birth weight was not affected by the phe-notype (P = 0.33).Year effects were also present with lesser rump fat, lesser calf birth weight, and lesser calf weaning weight when represented as a function of cow BW in year 2 than year 1.
The calving date of the 86 cows over 2 years of extensive grazing management was not correlated with rump fat thickness at calving or calf weaning weight (Table 5; P ≥ 0.45).Calf weaning weight as a percentage of cow BW was negatively correlated to rump fat thickness at calving (P = 0.001).The year 1 weaning weight score was positively correlated with the year 2 weaning weight and year 1 total score (Table 6; P ≤ 0.003) and tended to be negatively correlated with the year 2 rump fat (P = 0.062).The year 1 weaning weight was not correlated with the year 1 rump fat and calving date or year 2 calving date and total points (P ≥ 0.67).The year 1 rump fat was not correlated with the year 1 calving date or the year 2 weaning weight and calving date (P ≥ 0.13) but was positively correlated with the year 2 rump fat and years 1 and 2 total points (P ≤ 0.017).The year 2 weaning weight was not correlated with the year 2 calving date (P = 0.44) but was positively correlated with the year 1 and 2 total points.The year 2 weaning weight also tended to be negatively correlated with the year 2 rump fat (P = 0.085).The year 2 rump fat and calving date were positively correlated with the year 2 total points (P < 0.001), and the years 1 and 2 total points were also positively correlated (P = 0.010).
Rump fat measured at calving, calving date, calf birth weight, and calf weaning weight as a percentage of cow BW and on a kilogram basis of the nine LE and nine ME cows during 2 consecutive years of extensive management were not affected by the phenotype × year interaction (Table 4; P ≥ 0.22).Rump fat thickness was greater in ME cows than LE cows (P < 0.001) and was greater in year 2 than year 1 (P = 0.024).The calving date did not differ between years (P = 0.49) but was earlier for the ME cows (P < 0.001).Calf birth weight did not differ between phenotypes (P = 0.33) but was greater in year 2 than year 1 (P > 0.001).Calf weaning weight as a percentage of cow BW and on a kilogram basis were both greater for ME than LE cows (P ≤ 0.032), but only calf weaning weight as a percentage of cow BW was greater in year 1 than year 2 (P = 0.018).Low-efficiency cows had 9 bull calves and ME cows had 10, with 12 of the bull calves being born in year 1 and 7 in year 2. Nine heifer calves were born to the LE cows and eight to the ME cows.In year 1, 6 heifer calves were born and in year 2, 11 were born.
Starting BW (643 and 595 kg; P < 0.001; Table 7) and ending BW (633 and 618 kg; P < 0.001) were both greater for LE than ME cows, respectively.Starting BW was lightest when cows were fed the HQ diet relative to all other diets (P < 0.001).Diet had no effect on ending BW (P = 0.18) and there was no phenotype × diet interaction for starting or ending BW (P > 0.98).Likewise, there were no phenotype × diet interactions for measures of rib or rump fat thickness (P > 0.17).
There was no phenotype effect on the change in rib or rump fat thickness (P > 0.51).Starting rib fat did not differ between phenotypes (P = 0.14), but starting rump fat was thicker for ME than LE cows (P = 0.019).Ending rump fat was also thicker for ME than LE cows (P = 0.019), while ending rib fat followed the same tendency (P = 0.070).Starting rib and rump fat were thinnest in the HQ diet, intermediate in the MHQ diet, and thickest in the MQ, and LQ diets (P < 0.001).Ending rib and rump fat were thinnest when fed the HQ diet and thickest in the MHQ, MQ, and LQ diets (P < 0.025).Rib and rump fat changes were greatest while cows were fed the HQ diet, intermediate for the MHQ diet, and least for the MQ and LQ diets (P < 0.001).As BW was lighter, ME cows had lesser NE m and greater energy balance than LE cows.While NE m and energy balance were affected by diet, NE m increased and energy balance decreased as diet quality decreased.While periods were short in duration, RFI tended (P = 0.095) to be lesser for LE (0.16 kg/day) than HE (0.18 kg/day) cows with no effect of diet or interaction of phenotype and diet.
There were no interactions between phenotype and diet on measures of DMI (P ≥ 0.36; Table 8).While DMI (kg/day) was not affected by phenotype (P = 0.93), when reported as a percentage of BW, ME cows had greater DMI than LE cows (2.16% and 2.00%, respectively; P < 0.001).In addition, DMI (kg/day and % BW) were greatest for cows fed the HQ and MHQ diets, intermediate when fed the MQ diet, and least for the LQ diet (P < 0.001).Phenotype had no effect on NDF intake (kg/day, P = 0.99); however, similar to DMI, when reported as a percentage of BW, ME cows had a greater intake than LE cows (1.14% and 1.06%, respectively; P < 0.001).NDF (kg/day) and uNDF (% BW) intake were greatest when fed the MHQ diet, and decreased as diet quality decreased, although the least intake occurred when fed the HQ diet (P < 0.001).NDF intake (% BW) was greatest when fed the Table 6.
Pearson correlation coefficients for years 1 and 2 weaning weight, rump fat thickness, calving date, and total scores in a herd of 86 cows measured over 2 consecutive years.Calf weaning weight was corrected for sex according to Minyard and Dinkel (1965) and then calculated as a percentage of the dam body weight. 2 Cow rump fat was measured using ultrasonography at calving. 3 Calving date was measured as the difference in days between the calving date and the first cow to calve for year 1 and the difference between a cow's estimated gestation length at pregnancy checking and the longest cow's estimated gestation length in year 2.  1 Highest SEM for the two-way interaction is presented.
2 Sorting index was calculated as actual intake of retained particles on each screen/predicted intake (Leonardi and Armentano 2003).a,b,c Means with different superscripts in a row are significantly different (P < 0.05).z Indicates difference from 100 (P < 0.05) using two-tailed t-test.
MHQ diet, intermediate for the MQ diet, and least when fed the LQ and HQ diets (P < 0.001).UNDF fiber intake, represented as a percentage of BW, was greater for ME than LE cows (P < 0.001).
The only sorting index value that did not differ from 100 was the fraction on the pan when cows were fed the HQ diet (P > 0.050; Table 8).The phenotype did not affect the sorting of any of the fractions (P ≥ 0.24).Particles retained on the 19 mm screen were preferentially consumed when fed the HQ diet and were increasingly selected against as diet quality decreased (P < 0.001).Particles retained on the 8-mm screen were selectively consumed to the greatest extent when cows were fed the MQ diet, intermediate and selected for when fed the MHQ and LQ diets, and selected against when fed the HQ diet (P < 0.001).Cows fed the MQ and LQ diets selected for particles retained on the 4 mm screen, while those fed the HQ and MHQ diets selected against these particles (P < 0.001).Fine particles were most greatly selected for when fed the LQ diet, with less preferential selection when fed the MQ diet, not selected or refused when fed the HQ diet, and selected Table 9. Ruminal pH, short-chain fatty acid (SCFA) concentrations, and ammonia concentrations from beef cows phenotypically classified as least efficient (LE; n = 9) or most efficient (ME; n = 9) and fed a high-quality (HQ), medium high-quality (MHQ), medium-quality (MQ), or low-quality (LQ) diet during four consecutive 26-day periods.against in the MHQ diet (P < 0.001).There were no phenotype × diet interactions in any of the fractions (P ≥ 0.28).
The phenotype and the phenotype × diet interaction did not affect mean, minimum, or maximum ruminal pH or total SCFA concentration (P ≥ 0.11; Table 9).Mean and minimum ruminal pH were greatest when fed the LQ and MQ diets, which did not differ, intermediate for the MHQ diet, and least for the HQ diet (P < 0.001).Maximum ruminal pH was greatest when fed the LQ diet; the LQ and MQ diets did not differ, and the MQ and MHQ diets did not differ, although the LQ and MHQ diets were different, and the HQ diet was the least (P < 0.001).Total ruminal SCFA concentration was not affected by phenotype (P = 0.12) or the phenotype × diet interaction (P = 0.78); however, concentrations were greatest when fed the HQ diet, with no difference between the HQ and MHQ diet, the MHQ and MQ diet, or the MQ and LQ diet (P < 0.001).There were no phenotype or phenotype × diet interaction effects (P ≥ 0.33) on the molar proportions of SCFA except for isovalerate, which was affected by the phenotype × diet interaction (data not shown; P < 0.001), where the LE cows (2.1%) had a greater molar proportion than the ME cows (1.4%) fed the HQ diet.The molar proportion of acetate was greatest for cows fed the LQ diet, and concentrations decreased as diet quality increased (P < 0.001).For propionate, butyrate, and isobutyrate, molar proportions were greatest when fed the HQ diet, intermediate for the MHQ diet, and least when fed the MQ and LQ diets, although the MHQ and MQ diets did not differ for isobutyrate (P > 0.001).Valerate was greatest for cows fed the HQ diet and decreased as diet quality decreased from HQ to LQ (P < 0.001).Caproate was greatest when fed the HQ diet and least when fed the MQ and LQ diets (P < 0.001).Ruminal ammonia concentrations were not affected by phenotype (P = 0.52) or the phenotype × diet interaction (P = 0.57) but were greatest when fed the HQ and LQ diets, intermediate for the MQ diet, and least for the MHQ diet, with no difference between the HQ and MQ diets (P < 0.001).
Ruminal contraction frequency, duration, and time were not affected by phenotype, diet, or the phenotype × diet interaction (P ≥ 0.11, Table 9).Ruminal contraction height and peak area were not affected by the phenotype × diet interaction (P ≥ 0.27), but ruminal contraction height was greater in LE than ME cows (P = 0.015) and was also greater when fed the HQ and MHQ diets than the MQ and LQ diets (P < 0.001).The ruminal contraction peak area was also different among phenotypes (P = 0.009) and diets (P = 0.036), where the LE cows had a greater peak area than ME cows and the HQ diet was greater than the MQ and LQ diets, with the MHQ diet not different from the HQ, MQ, and LQ diets.
Total, solid, and liquid ruminal digesta weights, whether reported on an as-is or DM-basis, were not affected by the phenotype × diet interaction (P ≥ 0.84; Table 10).However, total digesta and the solid digesta fraction reported on an asis basis (P = 0.043 and P < 0.001, respectively) and a DM basis (P = 0.013 and P = 0.018, respectively) were greater for LE cows than ME cows.The liquid fraction did not differ on an asis basis (P = 0.48) but tended to have a greater quantity of DM for LE cows than ME cows (P = 0.097).Total ruminal digesta weight (as-is basis) was greatest when fed the LQ diet and decreased as diet quality increased (P < 0.001).The weight of the solid ruminal digesta fraction (as-is basis) was greatest when fed the HQ and MQ diets, least for the MHQ diet, and the LQ diet did not differ from the other diets (P = 0.032).The liquid fraction digesta weight when reported on an as-is (P < 0.001) and a DM (P < 0.001) basis was greatest when fed the LQ diet, intermediate for the MQ and MHQ diets, and least in the HQ diet.For both the total and solid rumen digesta weight (DM basis), the LQ and MQ diets were the greatest, the HQ diet Table 10.Ruminal motility, digesta pool sizes, and NDF turnover for beef cows phenotypically classified as least efficient (LE; n = 9) or most efficient (ME; n = 9) and fed a high-quality (HQ), medium high-quality (MHQ), medium-quality (MQ), or low-quality (LQ) diet during four consecutive 26-day periods.was the least, and the MHQ diet did not differ (P < 0.001 and P = 0.024, respectively).The aNDFom pool size was greater for LE than ME cows (P = 0.028) and did not differ between the MQ and LQ diets, the LQ and MHQ diets, and the MHQ and HQ diets, with HQ differing from the MQ and LQ diets, MHQ differing from MQ, and LQ differing from HQ (P < 0.001).In the aNDFom fraction, the potentially digestible aNDFom did not differ between phenotypes (P = 0.42), but the uNDF pool size was greater for LE cows (P < 0.001) than ME cows.The potentially digestible aNDFom pool size was greatest for cows fed the HQ diet, intermediate for the MHQ diet, and least when fed the LQ diet, with MQ only differing from LQ (P < 0.001).The ruminal uNDF fraction was greatest when fed the LQ and MQ diets, intermediate for the MHQ diet, and least when fed the HQ diet (P < 0.001).The total ruminal digesta DM and weight on an as-is basis, when presented as a % BW, did not differ between phenotypes or the phenotype × diet interaction (P ≥ 0.30), and only differed among diets on an as-is basis where ruminal digesta weight (% DM) was greatest when fed the LQ diet and decreased as diet quality increased (P < 0.001).The rate of aNDFom passage and degradation were not affected by the phenotype × diet interaction (P ≥ 0.58); however, ME cows had a greater rate of aNDFom passage from the rumen than LE cows (P = 0.047).In addition, the aNDFom passage rate was greatest when fed the MHQ diet, intermediate for the HQ and MQ diets, and least for the LQ diet (P < 0.001).The rate of aNDFom degradation was not different between phenotypes (P = 0.69) but was greatest when fed the LQ diet, intermediate for the MHQ diet, and least when fed the HQ diet, with MHQ only differing from the HQ diet (P < 0.001).
The daily fecal DM output was not affected by phenotype or the phenotype × diet interaction (P ≥ 0.62) but was greatest when fed the MHQ and MQ diets and least for the LQ diet, with HQ not differing from the other diets (P < 0.001; Table 11).Apparent total tract digestibility of DM, OM, CP, aNDFom, ADF, starch, and ether extract did not differ among phenotypes or the phenotype × diet interaction (P ≥ 0.23) but did differ between diets (P < 0.001).The apparent total tract digestibility of DM and OM were greatest when fed the HQ and MHQ diets, intermediate for the MQ diet, and least when fed the LQ diet.The apparent total tract digestibility of CP was greatest when fed the MHQ and MQ diets and least for the HQ and LQ diets.The apparent total tract digestibility of aNDFom was greatest when fed the MHQ diet, intermediate when fed the LQ diet, and least when fed the HQ diet, with the MHQ diet only differing from HQ.The apparent total tract digestibility of ADF was greatest when fed the MHQ diet, in-Table 11.Fecal output, apparent total tract digestibility, urine output, and nitrogen balance for beef cows phenotypically classified as least efficient (LE; n = 9) or most efficient (ME; n = 9) and fed a high-quality (HQ), medium high-quality (MHQ), medium-quality (MQ), or low-quality (LQ) diet during four consecutive 26-day periods.termediate for the MQ and LQ diets, and least for the HQ diet.Starch total tract digestibility was greatest when fed the MHQ and LQ diets, intermediate for the MQ diet, and least in the HQ diet.Ether extract total tract digestibility only differed when fed the HQ diet, which was greater than the MHQ, MQ, and LQ diets.The daily urine output, urinary and fecal N output, urinary and fecal N concentrations, total N output, and N balance were not affected by the phenotype × diet interaction (P ≥ 0.37; Table 11).The daily urine output tended to be greater for LE than ME cows (P = 0.097) and was greatest when fed the HQ diet, intermediate for the MHQ and MQ diets, and least when fed the LQ diet (P < 0.001).Daily urinary N output tended to be greater in the LE cows than the ME cows (P = 0.054) and was greater when cows were fed the HQ diets than the MHQ and LQ diet, with the MQ diet not differing (P = 0.036).The urinary concentration of N did not differ between phenotypes (P = 0.54) but was greatest when cows were fed the LQ diet, did not differ between the MQ and MHQ diets, and was least in the HQ diet (P < 0.001).Daily fecal N output did not differ between phenotypes (P = 0.81), but N concentration tended to be greater for the ME cows (P = 0.095).Fecal N output was greatest when fed the HQ diet, not different between the MHQ and MQ diets, and least for the LQ diet (P < 0.001), while the N concentration was greatest for the HQ diet and decreased as diet quality decreased (P < 0.001).Total N output as a percentage of N intake did not differ between phenotypes (P = 0.15), but was greatest for the LQ diet, moderate for the HQ and MQ diets, and least for the MHQ diet (P < 0.001).Nitrogen balance also did not differ between phenotypes (P = 0.30) but was greatest when cows were fed the MHQ diets, moderate for the HQ and MQ diet, and least for the LQ diet (P < 0.001).

Discussion
Development of a novel efficiency classification system for beef cattle Efficiency of mature breeding beef cows is more difficult to define than that of growing cattle due to a lack of easily identified and measured variables that can be used to denote cow performance.Measures of cow efficiency usually relate to feed intake, sometimes measured during a growing phase and assumed to extend to later life stages of life, or have been based on the performance of their calves prior to weaning (Arthur and Herd 2008;Callum et al. 2019).While evaluating feed consumption is logical considering that feed accounts for up to 41% to 60% production costs in Canada (Canfax Research Services 2020), such approaches rarely consider the assessment of output variables that drive reproductive success and profitability for cow-calf operations.In the present study, a novel ranking system was used to characterize the efficiency of cows based on relatively long-term assessments, including body condition at calving, timing of calving within the calving season, and weaning weight of heifers and steers as a proportion of cow BW.Specifically, rump fat measured at calving was used as an indicator of the ability to maintain body condition over the winter under an extensive grazing system.Rump fat depth was used instead of BCS given the quantitative nature of rump fat and the fact that rump fat and BCS are positively correlated and both indicative of body fat reserves (Looper et al. 2010).Past research has indicated that a greater BCS at calving has been reported to hasten the postpartum interval (Osoro and Wright 1992;Hess et al. 2005).Rump fat also received the greatest weighting in the current scoring system, as it was assumed to be a response directly impacted by nutrient intake and utilization by the cow under extensive winter grazing conditions.
The second efficiency criterion was the calving date within the calving season.Cows that calve earlier in the current calving season tend to also calve earlier in the following year, are older when they are first diagnosed as open, stay in the herd longer, and wean calves that are older and heavier (MacGregor and Casey 1999; Cushman et al. 2013).Others have suggested using the calving interval as an indicator (Hess et al. 2005); however, the use of the calving interval prevents the selection of cows within a single season.The third component of the efficiency selection was calf weaning weight when represented as a percentage of the dam's body weight.Given the differing growth potential for heifer and steer calves (Minyard and Dinkel 1965), values were established independently for dams with heifer and steer calves.While distinct performance indicators were used, it was recognized that these factors may be, at least, partially autocorrelated.As such, the ranking criteria were weighted to place greater emphasis on direct measures of the cow and lesser weighting on factors where management might confound outcomes.Despite the assumption that traits might be highly autocorrelated, the post-hoc correlation analysis did not yield such findings.We observed that calf weaning weight was negatively correlated with rump fat thickness; however, neither calf weaning weight and calving date nor rump fat thickness and calving date were correlated.The correlation between rump fat thickness and calf weaning weight was −0.248, and most of the correlation appears to come from year 2.There was also a correlation, although only 0.276, between year 1 and 2 total points, which suggests that the phenotypic selection is at least partially consistent from year to year.Despite the correlation, at least two years of data are recommended to ensure a more consistent and accurate selection.
The results of our combined efficiency ranking yielded cows that had greater rump fat, calved earlier in the calving season, and weaned heavier calves as a percentage of cow BW.For example, ME cows had 4.7 mm of rump fat at calving compared with LE cows, which had 3.5 mm.In addition to greater rump fat at calving, cows selected as ME had greater starting and ending rump fat than LE cows during the intensive portion of this study, regardless of the diet fed, highlighting consistency in this measurement outcome.The ability to deposit more rump fat may be explained by the lighter BW and, consequently, the lesser NE m requirements and greater energy balance.The thicker rump fat combined with lesser NE m requirements for the ME cows may have contributed to their greater reproductive fitness (Whitman 1974;D'Occhio et al. 2019).Correspondingly, ME cows calved earlier, with a mean calving date of 14 days relative to the start of calving compared with a mean of 30 days for LE cows.Sex-corrected calf weaning weight was also greater for ME cows than LE cows whether considered on a calf BW basis (510 and 464 kg, respectively) or calf BW represented as a percentage of cow body weight (43.8 and 35.5%, respectively).Despite the greater weaning weights, calf birth weight did not differ between ME and LE cows, further supporting the idea that calves born earlier are heavier at the time of marketing (Cushman et al. 2013).

Lack of interactions between the efficiency phenotype and dietary treatment
In the present study, we characterized DMI, ruminal fermentation characteristics, ruminal pool size and motility, and apparent digestibility for LE and ME cows using four consecutive 26-day periods where the quality of the diet progressively decreased.The use of multiple diets was to determine whether phenotypic responses would differ by diet quality, as previous studies have reported that diet may influence feed efficiency rankings, at least for RFI (Durunna et al. 2011(Durunna et al. , 2012)).We did not observe phenotype × dietary interactions for any of the primary response variables observed.The lack of interactions infers that long-term selection based on performance over two annual cycles can be used to identify cows that express a consistent phenotypic response, regardless of dietary characteristics.

Differences in digestive physiology among LE and ME cows
Measures of efficiency for growing cattle generally classify efficient cattle as those that gain more while consuming less feed (Koch et al. 1963;Arthur and Herd 2008), and it has been extrapolated to suggest that cows may have lesser feed requirements for maintenance functions (Herd and Arthur 2009).In the present study, ME cows did not have a lesser DMI than LE cows, as they consumed more when DMI was expressed as a percentage of cow BW.Reis et al. (2021) calculated an energy efficiency index as the ratio of metabolizable energy required by the cow for maintenance, gestation, and lactation to calf weaning weight, where high-efficiency cows had a lower energy efficiency index.Reis et al. (2021) also found that high-and low-efficiency cows did not differ for DMI, despite high-efficiency cows having lighter BW.Highefficiency cows in this study were lighter at the start and end, which is consistent with previous studies that found heavier cows had longer gestation lengths, younger calves at weaning, and lower cumulative lifetime production (Snelling et al. 2019).Using NASEM (2016) equations, the lighter BW for ME cows resulted in lower NE m requirements and greater energy balance.However, the greater DMI (% BW) of the ME cows contradicts RFI-based measures of efficiency, even on highforage diets (Arthur and Herd 2008;Fitzsimons et al. 2014).In fact, while our periods of measurement were short, ME cows tended to have a greater RFI value than LE cows, indicating that the current ranking system and use of RFI are not in agreement.
As could be expected, aNDFom intake followed patterns observed for DMI, with no difference between ME and LE cows when reported as kg/day, but ME cows ate more aNDFom and uNDF as a percentage of BW.The greater aNDFom, uNDF, and DM intake as a percentage of cow BW for ME relative to LE cows indicates that metabolizable energy intake and potentially metabolizable protein supply may have been proportionally increased, especially when paired with the lack of apparent total tract digestibility differences.The previous suggestion is further supported by greater energy balance for ME cows than LE cows, regardless of dietary composition.
In the present study, differences in DMI (% BW) between the ME and LE cows can partially be explained by differences in ruminal digesta mass and rate of passage.High-efficiency cows had smaller total and solid ruminal digesta weights on both as-is and DM basis.This smaller ruminal digesta mass, coupled with greater rates of aNDFom passage, may have allowed the smaller ME cows to consume the same DMI as the LE cows when reported on a kg/day basis but a greater DMI as a percentage of BW.It is possible that the lower rate of aNDFom passage without differences in degradation rate and digestibility may have limited DMI for the LE cows, as a larger ruminal digesta volume and mass generally decrease DMI (Schettini et al. 1999;Whetsell et al. 2004).Supporting the previous theory, ME cows had smaller ruminal pools of aNDFom and uNDF, despite a consuming the same amount of aNDFom (kg/day).Interestingly, the potentially degradable aNDFom pool size was not different between the ME and LE cows, and sorting indices were not different, suggesting that ME cows may have a greater ability to selectively retain potentially degradable aNDFom in the rumen.Lund et al. (2007) reported that dairy cattle selectively retain digestible NDF in the rumen, most likely through differences in the distribution of digestible and indigestible NDF in the plant and differences in buoyancy and size of the plant components.It may be possible that the selective retention of potentially digestible aNDFom further contributed to enhanced energy intake by supporting rump fat accretion.However, there were no differences in the total tract digestibility of aNDFom or the ruminal aNDFom k d despite a greater ruminal aNDFom k p for the ME cows.Okine and Mathison (1991) reported that the ruminal rate of passage also increased when nonlactating dairy cows were fed above maintenance requirements.However, in the study by Okine et al. (1989), the fractional rate of NDF degradation decreased with increasing levels of maintenance requirements.Okine and Mathison (1991) further reported that greater NDF degradation occurred post-ruminally as cows were fed above maintenance requirements, which may explain why there was no difference in total tract digestibility of aNDFom despite the greater ruminal aNDFom k p for ME cows in this study.The greater ruminal aNDFom passage rate may have increased microbial protein flow out of the rumen, contributing to a greater microbial efficiency for ME cows than LE cows (Sniffen and Robinson 1987).
In addition to greater DM and aNDFom intake as a proportion of BW, it is likely that differences in performance between the ME and LE cows may be related to differences in their maintenance energy requirements.Using the equations in NASEM (2016), the NE m balance of the ME and LE cows could be calculated for each diet using the mean BW and DMI and the NE m of the diet.The overall calculated NE m balance was 3.83 Mcal/day for the ME cows and 3.17 Mcal/day for the LE cows.These data suggest that the greater energy balance for the ME cows was likely due to a lesser BW but similar DMI and total tract digestibility when compared with the LE cows.
Ruminal motility is an important factor regulating the mixing of digesta within the rumen and the passage of digesta out of the rumen (Okine et al. 2011).High-efficiency cows had lower amplitude and had ruminal contractions with a shorter duration than LE cows.Okine et al. (2011) reported that reticular contraction duration was more important for digesta passage out of the rumen than contraction frequency.However, we observed that ME cows had greater passage of NDF out of the rumen despite ruminal contractions of shorter duration.The ME cows in this study had a smaller ruminal digesta mass, and hence less ruminal distention, which plays an important role in the regulation of ruminal motility (Dado and Allen 1995).Deswysen et al. (1987) suggested that an increase in strong ruminal contractions may be indicative of weaker or less efficient reticular contractions.Strong primary contractions of the rumen are involved in the movement of feed to the cranial region of the reticulo-rumen, which may limit the consumption of new feed.Thiago et al. (1992) reported that despite greater intensity of ruminal contractions on a hay vs. silage diet, the fractional rate of passage from the rumen was not greater, further suggesting factors beyond ruminal contraction frequency and amplitude may play a role in particle passage out of the rumen.While contractions are indicative of mixing in the rumen, we did not assess rumination activity, the size of the omasal orifice, or fibrous particles in the feces, and as such, we are unable to conclude whether reticular motility or other factors regulating ruminal NDF residence time in the rumen were causative for the change in aNFDom turnover.
Despite ME cows having a greater ruminal passage rate for aNDFom and DMI (% BW), they did not differ in the rate of aNDFom degradation or for total-tract digestibility of DM, OM, CP, aNDFom, ADF, starch, or ether extract relative to LE cows.This further suggests that greater DMI as a function of BW rather than greater digestibility may be the primary factor resulting in greater fat thickness in ME cows.However, ME cows had a greater ruminal rate of passage but no difference in the ruminal rate of NDF degradation or total tract digestibility.That said, the aNDFom turnover model evaluated (Linton and Allen 2008) only considers potentially degradable aNDFom and undegradable aNDFom using 240 h of in vitro incubation to assess these values.It may be possible that LE and ME differed in the passage of potentially digestible aND-Fom.For example, the CNCPS model (Raffrenato and Van Amburgh 2011) characterizes aNDFom into rapidly digestible, potentially digestible, and undigested aNDFom pools.A shift in the passage rate of individual pools could facilitate the lack of difference in aNDFom digestibility between ME and LE while still allowing for greater rates of aNDFom passage out of the rumen.Alternatively, greater passage of potentially degradable aNDFom could suggest greater post-ruminal digestion for ME than LE cows.Okine and Mathison (1991) reported that with increasing DMI, ruminal retention time decreased but lower digestive tract retention time increased resulting in an increase in total tract NDF digestibility.Okine and Mathison (1991) further suggested that as DMI increases, the digestion of feed post-ruminally becomes more important.In the present study, we were unable to assess the site of digestion and, other than measuring apparent total tract digestibility, did not evaluate fecal characteristics to determine if large intestinal fermentation may have differed.Other studies that have compared digestibility responses among efficiency groups have found conflicting results.De La Torre et al. ( 2019) compared high-and low-RFI beef cows and found that low-RFI cows had greater DM and OM digestibility but were not different for NDF digestibility.Johnson et al. (2019) reported greater DM, OM, and NDF digestibility in low-RFI growing beef heifers, while McDonnell et al. (2016) only observed differences in OM digestibility.Other studies have found no differences in diet digestibility between RFI classifications (Cruz et al. 2010;Lawrence et al. 2011) or have attributed differences in digestibility to the lesser DMI rather than efficiency classification (Cantalapiedra-Hijar et al. 2018).Although ME cows had a greater DMI (% BW) and ruminal aNDFom k p , total tract digestibility did not differ between ME and LE cows, suggesting that diet digestibility is not likely to be affected by the efficiency classification used in this study.
Ruminal fermentation did not differ between efficiency classifications, with only the proportion of isovalerate being affected by the phenotype × diet interaction.High-efficiency cows had greater proportions of isovalerate when fed the HQ diet but did not differ from LE cows when fed the MHQ, MQ, or LQ diets.McDonnell et al. (2016) also found greater proportions of isovalerate in low-RFI-growing heifers fed either grass silage, ryegrass pasture, or a 70:30 total mix ratio of corn silage:concentrate.The differences in isovalerate proportions were relatively small, and it is not clear how the proportion of isovalerate may help explain outcomes given that rates of SCFA production and clearance and the ruminal volume were not known.Studies comparing ruminal fermentation among RFI groups have reported inconsistent results.Despite the inconsistent results from RFI in relation to ruminal fermentation patterns, the results from our study were supported, as there were no differences in the ruminal rate of degradation of aNDFom or apparent total tract digestibility of any of the nutrients evaluated.

Effects of dietary composition
In the present study, cattle were fed common diets within a period, and a clear dietary sequence from those expected to have high digestibility to those with low digestibility was imposed.As such, diet and the period of the study are inherently confounded.That said, as could be expected, DMI was affected by diet.Given that aNDFom and uNDF intake were least for HQ, greatest for MHQ, and decreased from HQ to MQ, and that the ruminal uNDF pool size increased as the expected fermentability of the diet decreased, it is likely that metabolic regulators of intake may have limited DMI when fed HQ and that physical fill may have regulated intakes for all other diets (Allen et al. 2009).
Ruminal fermentation patterns followed expected outcomes for a forage-based diet with the lowest ruminal pH when cows were fed the HQ diet, with pH generally increasing as diets were formulated to be less digestible.Corresponding to ruminal pH changes, the molar proportions of SCFA decreased as diet quality decreased, likely indicative of reduced rates and extents of ruminal fermentation, and these diets increased the ruminal liquid pool size, which may have diluted the SCFA concentration.The molar proportions of acetate and propionate increased and decreased, respectively, as diet quality decreased, yielding results consistent with previous studies (Sutton et al. 2003;Penner et al. 2009;Walsh et al. 2009).As diet quality decreased, molar proportions of butyrate, isobutyrate, valerate, isovalerate, and caproate decreased, in agreement with Walsh et al. (2009), who reported similar results when increasing the ratio of barley straw to grain in silage.
Despite the greatest DMI and smallest ruminal digesta mass, the ruminal rate of NDF passage was less for HQ than the MHQ diet and similar to the MQ diet.This may be due to the assumption that the rate of uNDF passage is equivalent to the rate of NDF passage out of the rumen.The uNDF may have been associated with more large particles in the HQ diet, which would cause them to have a greater retention time in the rumen and lower the k d .The ruminal rate of degradation of NDF was also the lowest in the HQ diet, likely a result of the greater DMI and smaller ruminal digesta mass.Dry matter intake was likely regulated by rumen fill in both the MQ and LQ diets because they had the smallest intakes but the greatest total and solid DM ruminal digesta masses.This suggests that about 9 kg of DM is the maximum weight that the cows in this study could hold in their rumen.Because of this slow ruminal turnover, cows fed the LQ diet had the slowest ruminal rate of passage and the greatest ruminal rate of NDF degradation due to the long ruminal retention time.The greater k d of cows fed the MQ diet than the HQ diet is likely a result of lesser DMI and greater ruminal digesta mass.
The only measures of ruminal motility that differed among diets were ruminal contraction amplitude and contraction area, but it should be noted that ruminal contraction amplitude and area are inherently autocorrelated.The ruminal contraction amplitude and area were greatest for the HQ and MHQ diets, suggesting that contraction height and peak area are driven by DMI rather than ruminal digesta weight.We speculate that contraction characteristics such as amplitude and contraction duration may be affected by ruminal digesta mass and the insulating effect it may have to alter the contraction signal to the pressure transducer.Future research is needed to quantify the impact of contraction force and frequency on the mixing of ruminal digesta differing in weight and dry matter concentration.
Dry matter and OM apparent total tract digestibility both decreased as diet quality decreased, as was expected (Jung and Allen 1995).However, aNDFom and ADF digestibility only followed this trend when fed the MHQ, MQ, and LQ diets.When cows were fed the HQ diet, aNDFom and ADF digestibility were less than the MHQ, MQ, and LQ diets.Dry matter intake and ruminal aNDFom k p were greatest when cows were fed the HQ diet, suggesting that the high turnover rate when fed the HQ diet may have limited aNDFom and ADF digestibility.

Conclusions
A phenotypic selection approach utilizing cow rump fat at calving, calving date, and calf weaning weight allowed for the ranking of efficient and inefficient cows in a herd over 2 years.High-efficiency cows weighed less, had thicker rump fat, calved earlier, and had heavier calf weaning weights than LE cows.In addition to greater performance under extensive management, the ME cows had consistent responses across diets of varying forage quality, as there were no phe-notype × diet interactions.The more efficient cows may have had greater energy retention, considering the ME cows were lighter over the course of the study and had similar DMI and total-tract digestibility, suggesting more energy was partitioned to body condition, reproduction, and lactation.The ability of the ME cows to consume more DMI as a percentage of BW was driven by a greater ruminal aNDFom passage rate, a smaller ruminal digesta mass, and potentially by differences in ruminal motility.

Table 1 .
Rump fat and calving date for 86 cows over 2 consecutive years and the ranking score used.
1 Rump fat score was calculated based on percentile ranking in the herd with a maximum score of 10 points. 2 Calving was recorded as days relative to the start of the calving season.Year 2 calving date was predicted by gestation length via ultrasound pregnancy checking.3Calvingdate score was calculated based on percentile ranking in the herd, with a maximum score of eight points.

Table 2 .
Calf weaning weight for the 86 cows evaluated over 2 consecutive years and ranking score for calf weaning weight.

Table 4 .
Rump fat measured at calving, calving interval, and calf weaning weight for cows phenotypically classified as least efficient (LE; n = 9) and most efficient (ME; n = 9) when measured over 2 years.

Table 5 .
Pearson correlation coefficients and the respective P-value for calf weaning weight, rump fat thickness, and calving date from 86 cows measured over 2 years.
1 Cow rump fat was measured using ultrasonography during calving. 2 Calving date is the difference in days between a cow's calving date and the first cow to calve for year 1 and the difference between a cow's estimated gestation length at pregnancy checking and the longest cow's estimated gestation length in year 2.

Table 7 .
Start and ending BW and rib and rump fat measurements of beef cows phenotypically classified as least efficient To determine NE m consumed, total digestible nutrients was calculated as TDN = CP digested + aNDFom digested + starch digested + (ether extract digested × 2.25).The TDN was then converted to NE m using equations in NASEM (2016) with a static efficiency of conversion from DE to ME of 0.82.The energy balance was then determined by calculating the difference between energy consumed and energy required for maintenance.
Durunna et al. (2011)ficient (ME; n = 9) and fed a high-quality (HQ), medium high-quality (MHQ), medium-quality (MQ), or low-quality (LQ) diet during four consecutive 26-day periods.1 Greatest SEM for the two-way interaction is presented.2Thenetenergyfor maintenance (NE m ) was calculated following equations in NASEM (2016).3 4esidual feed intake was calculated as described byDurunna et al. (2011); however, as periods were only 26 days, readers should interpret these data with caution.a,b,c Means with different superscripts in a row are significantly different (P < 0.05).

Table 8 .
Sorting behaviour and DM and nutrient intake for beef cows phenotypically classified as least efficient (LE; n = 9) or most efficient (ME; n = 9) and fed a high-quality (HQ), medium high-quality (MHQ), medium-quality (MQ), or low-quality (LQ) diet during four consecutive 26-day periods.