Influence of parity and litter size on estrous cycles and progesterone patterns in sows

Post-weaning patterns of serum progesterone were studied in 32 primiparous and 67 multiparous sows (approximately 35 sows per week for 3 weeks). Blood samples were obtained on Days 0, 7, 10, 14, 17, 21, 24 and 28 after weaning. There was a significant parity effect for days to estrus (17.83 ± 2.44 for primiparous sows vs 5.86 ± 1.60 for multiparous sows; P<0.05). Litter size was different for Week 3 vs Weeks 1 and 2 (5.47 ± 0.49 vs 8.43 ± 0.49 and 7.36 ± 0.38, respectively; P<0.05). Within parity groups there was a low correlation between litter size and days to first estrus (r = 0.21, P = 0.24 for primiparous sows and r = -0.11, P = 0.36 for multiparous sows). Sows were categorized by serum progesterone patterns for further analysis. The xis categories were 1) sows that had low progesterone on the day of weaning exhibited estrus within 7 d of weaning and became pregnant (n = 59); 2) sows with high progesterone levels (14.6 ng/ml) on the day of weaning (n = 9); 3) sows with regular returns to estrus after mating (n = 14); 4) sows with delayed returns to estrus after mating (n = 7); 5) behaviorally anestrous sows (n = 4); and 6) anovulatory anestrous sows (n = 6). The distribution of sows into the various progesterone categories was different for the two parity groups using the Chi-square test of independence (P = 0.005), with a smaller proportion of primiparous sows in Group 3 (3.2% vs 19.4%; NS) and a larger proportion in Group 6 (18.75% vs 0; P = 0.02) when compared with multiparous sows. Because an outbreak of transmissible gastroenteritis virus (TGE) occurred during the last week of the study, the distribution of sows into progesterone groups by week was examined. The proportion of normal and anestrous sows did not appear to be influenced by week of weaning. The outbreak of TGE that occurred in Week 3 may have influenced the proportion of sows that were in Group 2 that week. The major problem seen with primiparous sows in this study was that of anovulatory anestrus, while the primary problem seen with multiparous sows was the occurrence of regular returns to estrus after weaning.