Fetal development in the pig in relation to genetic merit for piglet survival.

The objective of this study was to investigate if litters with different genetic merit for piglet survival differ in late fetal development. In total, 507 fetuses from 46 litters were delivered by Caesarean section at, on average, d 111 of gestation. All litters had known estimated breeding values for piglet survival (EBVps). The obtained range of EBVps of the litters was continuous, and the difference between litters with the lowest and highest EBVps was 16.4%. Analysis of relationships between fetal characteristics and EBVps was performed with litter averages, using linear regression analysis with inclusion of EBVps as a covariate. An increase in EBVps of the litter was associated with decreases in average placental weight (P = 0.01) and within-litter variation in placental weight (P = 0.02), and an increase in average placental efficiency (P = 0.08). Average fetal length decreased with increasing EBVps (P = 0.04), but weights of liver (P = 0.02), adrenals (P = 0.0001), and small intestine (P = 0.01) showed relative increases with increasing EBVps. Average serum cortisol concentrations increased with increasing EBVps (P = 0.0001), but the other blood characteristics (hematocrit, glucose, fructose, albumin, estradiol-17beta) were not related to EBVps. Glycogen concentrations in liver (P = 0.07) and longissimus dorsi muscle (P = 0.04) and total liver glycogen content (P = 0.05) increased with increasing EBVps, whereas heart glycogen concentration decreased with increasing EBVps (P = 0.005). The percentage of carcass fat increased with increasing EBVps (P = 0.05). Relationships of relative liver weight, relative small intestinal weight, and liver and muscle glycogen levels with EBVps were absent after adjustment for differences in cortisol levels between litters. The observed differences in fetal development in relation to EBVps suggest a higher degree of physiological maturity in litters with high EBVps. Differences in fetal cortisol most likely accounted for most of these maturational differences. The results imply that selection for improved piglet survival will lead to slightly smaller piglets that nevertheless have an improved ability to cope with hazards during birth or within the first days of life.