Effect of age, weight, and sire on embryo and fetal survival in sheep

The goal was to estimate the heritabilities and genetic variances for embryo and fetal survival (ES) in sheep along with the effect of premating ewe weight, age, and bilateral or unilateral ovulation on ES. The data consisted of 11,369 records on ovulation rate and litter size. Statistical models for ES included year and ovulation rate as fixed effects, premating ewe weight, and age as covariates, and sire of embryo, maternal grandsire (MGS), and permanent maternal environmental effects of the ewe as random effects. The variance components were estimated using REML. In ewes that survived to yr 6, the mean litter size was 1.87, 2.05, 2.01, 2.07, and 1.91 ± 0.04 in ewes of age 2, 3, 4, 5, and 6 yr, respectively. Litter size was less in ewes of age 2 and 6 yr compared to ewes of age 3, 4, and 5 yr (P < 0.01). Ovulation rate was lower at age 2 yr and increased from age 2 to 6 yr (P < 0.05). Two-year-old ewes had lower ES than 3-yr-old ewes (P < 0.01) and the probability of ES decreased after age 3 yr (P < 0.01). Thus, ES contributes significantly to lower fertility in 2-yr-old ewes. In ewes with high ovulation rates (i.e., 5 corpora lutea, CL), more balanced ovulations (i.e., 2 or 3 CL on each ovary) tended (P = 0.06) to be associated with increased ES. A quadratic relationship was observed between ewe weight and litter size (P < 0.01) and a positive linear relationship between premating ewe weight and ovulation rate (P < 0.01). A quadratic effect of ewe weight on ES was observed, with decreased ES for low and high ewe weights (P < 0.01). The optimal ewe weight for ES increased with ovulation rate, which is consistent with the requirement of greater body reserves for maintaining a larger number of fetuses during gestation. A quadratic relationship between ewe weight and the probability that a ewe is able to maintain a pregnancy was also observed (P < 0.05). Pregnancy loss is due to failure of the embryo or fetus or failure of the dam to maintain the pregnancy. The sire of the embryo only influences the embryo, whereas the MGS influences both the ewe and the embryo. The heritability for the direct additive effect on ES in ewes that lambed was 0.0081 ± 0.0139, and the heritability for the maternal additive effect was 0.0447 ± 0.0242. The permanent maternal environmental variance component was significant and explained 8.5% of the phenotypic variance. Thus, genetically, the dam's ability to maintain a pregnancy has 5.5 times the effect on pregnancy loss than the embryo's ability to survive, and this, in turn, was only half the size of the permanent environmental effect. Therefore, selection among dams based on the mean embryonic survival of their embryos will provide an effective way to improve embryonic survival.