Given that sow's parity influences litter size and piglets’ birthweight, this study was designed to test the hypothesis that differences in muscle characteristics of piglets are affected by parity and also that secondary muscle fiber number could affect piglets’ birthweight.
MATERIAL AND METHODS
Animals were used and cared for in accordance with the Committee of Ethics in Animal Use (CEUA) no. 26368.2012.23.
Eighty-one Topigs×Danbred sows from the 1st to the 7th parity and their litters were used to identify the differences in the semitendinosus muscle characteristics of piglets according to parity of sows. The sows were identified by ear tags and were given similar diets and husbandry conditions. Estrus detection was done by passing a young boar (8 to 12 mo of age) twice daily (between 0800 and 0900 and from 1600 to 1700). When estrus was detected, sows were inseminated at 24, 36 and 48 h after estrus and gilts were inseminated at 12, 24 and 36 h after estrus. For practical reasons, this timing was not strictly followed, but was quite close. Insemination was done within a week with 85 mL of semen (3×109 live sperm cells) from different boars distributed homogeneously over all sows in order to avoid any boar effect.
The sows were allocated into individual cages during gestation and into maternity pens during lactation. All of them had similar backfat thickness (determined using P2 ultrasonic measurement) and feed intake during gestation was restricted at 2.5 kg
−1 animal
−1 d
−1 until 84 d of gestation and 3.5 kg animal
−1 d
−1 between 85 and 114 d of gestation. At 111 d of pregnancy, the sows were transferred to the maternity pen. They received no feed on the day of delivery. The diet is described on
Table 1.
Piglets’ birthweight (within 5 min after birth), gender, total number of piglets born, and the numbers of piglets born alive were recorded. The litter's average weight was calculated, and the piglets with weights equal to or near to the litter average weight were sacrificed (three piglets per sow, totaling 233 piglets) for the collection of the semitendinosus muscles. The muscles were weighed and stored for 24 h in Bouin's solution for fixation and later preserved in 70% alcohol.
The muscle samples were submitted to dehydration in solutions of increasing concentrations of alcohol, diaphanized in xylol using an automatic tissue processor Leica TP1020, embedded in paraffin, and then mounted on histological slides. Sections 5 µm thick were cut from the middle portion of the muscle using a rotary microtome LAB-MR500 and stained with hematoxylin and eosin for histological evaluation. Motic Images Plus 2.0 software was used to obtain the images and to measure the area of the semitendinosus muscle, and the number of secondary muscle fibers was estimated by counting eight randomly selected fields (36.7 mm2 per field) from each slide using an Olympus BX 50 microscope. Knowing the total area of the muscle, the area of the field, and the average number of fibers per fields, it was possible to estimate the total number of secondary muscle fibers in the whole muscle by simple multiplying these variables.
Data were analyzed using SAS software (Littell et al.
1996) as a completely randomized design, with unequal frequencies. Homogeneity of the data was tested using Bartlett's Test for homogeneity of variance. To evaluate the effect of parity on each characteristic, regression trials were done considering the sow as an experimental unit (the average of the three piglets euthanized per sow) and the correlation between characteristics was tested by Pearson Correlation. For the evaluation of gender and weight class on muscle fiber number, a 3×2 factorial arrangement (weight class and gender) was used. The model was:
where Y ij is the dependent variable, m is a constant; T i is the weight classs effect, S j is the gender effect, T sij is the weight class and gender interaction, and e ij is the residual error. When treatment effects were significant a Tukey test at 0.05% was performed, considering each piglet as an experimental unit and gender and weight class as treatments.
RESULTS AND DISCUSSION
Several reports have studied the impact of nutrition during gestation on fetal and post-natal development (Wigmore and Stickland
1983; Dwyer et al.
1994; Musser et al.
2004; Nissen et al.
2003; Gatford et al.
2003; Bee 2004; Ferguson et al.
2006; Karunaratne et al.
2007; Cerisuelo et al.
2009; Rehfeldt et al.
2012) as well as line, gender, litter size and additive genetic variation (Miller et al.
1975) on porcine muscle development. The present results present a new perspective in this regard – the effect of parity.
No significant effect of gender was found for body weight of piglets, number of muscle fibers, or semitendinosus muscle weight and area at birth. The effect of gender on muscle area is controversial. Some authors have shown larger muscle areas in gilts than in barrows (Solomon et al.
1990; Larzul et al.
1997), whereas others found no differences (Sosnicki
1987; Ender
1994). According to Bee et al. (
2004), these results illustrate the difficulty in sampling procedures and analyses techniques for myofiber counting due to the high subjectivity.
A quadratic effect of parity was found on the semitendinosus muscle weight of the piglets at birth (
P<0.05;
R 2=0.44), the semitendinosus muscle area (
P<0.05;
R 2=0.64), and the number of secondary muscle fibers (
P<0.05;
R 2=0.54) (
Fig. 1).
Due to the larger number of muscle fibers to be formed, the secondary myogenesis can vary substantially (Zhu et al.
2008) depending on the sow's age and parity, the genotype, the placental and uterine size and functional capacity, the uteroplacental transfer of nutrients and oxygen, and the metabolic and hormonal status of the embryo (Ashworth et al.
2001; Reynolds et al.
2005). Young sows (parities 1 and 2) are still growing during their gestation and, therefore, they have proportionally higher energy and protein requirements than adult sows (Eissen et al.
2000). In this regard, parity can affect the partition of nutrients between the maternal and the embryonic tissue, and prenatal undernutrition is related to lower muscle fiber numbers, myonuclear numbers, or muscle DNA content (Wigmore and Stickland
1983; Handel and Stickland
1987; Dwyer and Stickland
1991; Rehfeldt et al.
2001).
Also, considering that the skeletal muscle is not a priority during the early development of embryos (Zhu et al.
2008), it is reasonable to suggest that embryo requirements for muscle development are not met during late gestation, as suggested by Nissen et al. (
2004), in young sows, resulting in an impaired development of the muscular tissue when compared with 3rd and 4th parity sows.
Intrauterine crowding could be the main cause of this impairment in high parity sows (parities 5, 6, 7). Town et al. (
2004) have suggested that intrauterine crowding on the 30th day of gestation can affect muscle fiber differentiation through a down expression of some myogenesis-regulating factors, and Foxcroft and Town (
2004) hypothesized that intrauterine crowding could affect the development of fetal organs and muscle fiber and type, as intrauterine growth retardation does.
There were positive correlations for the weight of the semitendinosus muscle and the number of muscle fibers (P<0.05; 0.39), for the area of the muscle and the number of muscle fibers (P<0.001; 0.63) and for muscle area and semitendinosus muscle weight (P<0.001; 0.64).
Together, these results suggest that 3rd and 4th parity sows are not affected, or are less affected, by factors impairing fetal growth, resulting in better fetal muscular characteristics, in particular muscle weight and muscle area, which improves fetal development by a higher muscle growth rate due to better protein turnover, as shown by Nissen et al. (
2004).
Parity had a quadratic effect (
y=11.2174+2.3573
x – 0.20489
x 2,
R 2=0.66) on the total number of piglets born, on average birthweight (
P<0.05;
R 2=0.54), and a tendency to a linear effect (
y=13.02+0.268931
x,
R=0.07) on the number of piglets born alive (
Table 2). It is known that parity influences productivity, mainly because of the number of piglets born alive and the average litter weight (Holanda et al.
2005; Cavalcante-Neto et al.
2011). Our results partially agree with those of Mahan and Peters (
2004), who found that the total pigs born tended to increase as parity increased, but there was parity effect on the number of live pigs, and litter and 0-d-old pig weights increased as parity increased.
The highest litter birth weight was achieved by 4th parity sows, with 1.16% and 5.24% differences compared with 5th and 3rd parity sows, the second and third best results, respectively, being 4.03% the difference between them. The higher litter birth weight for 5th parity sows when compared with 3rd parity sows was due to the total number of piglets born, which was 9.15% higher. In addition, the highest average birth weight was found in 3rd parity sows (5.56% higher than 5th parity sows), followed by 4th and 2nd parity sows.
There were correlations for piglets’ birth weights and muscle weight (
P<0.001; 0.73), muscle area (
P<0.001; 0.54), and number of muscle fibers in the semitendinosus muscle (
P<0.05; 0.39). These results show that the greater weight of piglets at birth was mainly due to semitendinosus muscle weight and secondarily to muscle area, and not to semitendinosus muscle fiber number. However, an influence of piglets’ weight class on the number of semitendinosus secondary muscle fibers (
Table 3) was also found, where light-weight piglets had lower numbers of semitendinosus secondary muscle fibers when compared with heavier piglets.
This result is in agreement with many authors (Powell and Aberle
1980,
1981; Wigmore and Stickland
1983; Handel and Stickland
1987; Foxcroft et al.
2006; Tristán et al.
2009) who observed that light-weight piglets had a lower total number of muscle fibers due to a lower number of secondary muscle fibers, compared with heavy piglets. Nissen et al. (
2004) found greater amounts of DNA and RNA per fiber in high-weight pigs than in low-weight pigs and suggested that this supports an increased protein turnover.
Moreover, Rehfeldt and Kuhn (
2006) observed lower muscular protein concentration and activity of creatine kinase (a marker of muscular differentiation) in low-weight than in high-weight piglets.
These results suggest that the higher number of fetuses in high parity sows, which can mimic intrauterine growth retardation, affects fetal development by reducing the average birth weight, due at least in part by impairing muscle accretion, as suggested by Alvarenga et al. (
2013). Even with a low correlation between birth weight and the number of secondary fibers in the muscle, these fibers are important during fetal development, since impairment in prenatal cell proliferation, differentiation, and protein accretion in secondary muscle fibers leads to low-weight piglets (Rehfeldt and Kuhn
2006).