Reproductive performance of Pelibuey and Blackbelly sheep under tropical management systems in Mexico

An analysis of reproductive performance records from 1988 to 1994 of seven hair sheep herds in the dry tropics of Mexico was conducted on 785 Black Belly (BE) and 1810 Pelibuey ewes (P). Reproductive management on four farms included the continuous presence of the male (1471 lambings), and the three other herds had biannual breeding exposure to rams (1057 lambings). In 7 years, the combined herds produced 2528 parturitions from 2585 ewes bred (0.97 births per year per ewe). Number of parturitions were 1.10 year(-1) for BE (863 births in 785 ewes) and 0.92 for P (1665 births in 1810 ewes). Prolificacy was 1.46 lambs per birth with a total of 3690 lambs in 7 years; per breed it was 1.37 for BE and 1.55 for P lambs per parturition (P > 0.05); except for first lambing in both breeds, averaging 1.05 lambs per ewe (P < 0.01). There was an average of 530 (+/-299) lambs per year with a mortality of 15%; still births, malnutrition, diarrhea and pneumonia accounted for most of the losses, Mortality increased to 23% when more than two lambs were nursed by one ewe and reduced to 8% when only one lamb was born. Average age at first lambing was 465 days (15.25 months) for the combined herds. Yearly average (1988-1994) for lambing interval (LI) was 265, 227, 238, 225, 252, 230 and 258 days, with an average of 242 days (+/-16). Quarterly seasonal birth distribution was 50% from January to March (n = 1264); 25% from April to June (n = 632); 15% from July to September (n = 379) and 10% from October to December (n = 252). No statistical differences were observed among farms in reproductive performance. Seasonality of reproduction was demonstrated with minimum (7%) of lambs born in the fall and 50% in the winter, Rainfall had a significant correlation with fertility and estrous production (P < 0.05), but temperature and photoperiod did not. Grass growth seemed to be the key factor influencing sexual activity in ewes. Interval per lambing was statistically different (P < 0.01) with extended intervals for those ewes lambing during the autumn, winter and spring, compared with the summer lambing ewes, Rearing management had a statistical effect on interval per lambing(P < 0.05) increasing when lactation length augmented from 60 to 120 days. Fertility was higher (P < 0.05) for BE as compared with P, but the latter had a better prolificacy (P < 0.05) during the study. Parturition showed seasonal distribution but 1.5 lambings per year could be achieved throughout with good reproductive management.