Effects of twinning on postpartum reproductive performance in cattle selected for twin births

The effects of twinning, dystocia, retained placenta, and body weight on po stpartum reproduction were evaluated for 3,370 single and 1,014 twin births . Females were bred by AI for 40 d followed by 20 or 30 d of natural servic e with equal numbers bred and calved in spring and fall. Percentage of dams cyclic by the end of the AI period was lower (P < .05) for dams birthing a nd nursing a single calf (92.4%) than for dams birthing twins and nursing z ero (98.7%) or two (94.7%) calves. Whereas the interval from parturition to first estrus was shorter (P < .01) for dams birthing and nursing a single (56.9 d) than for dams birthing twins and nursing one (68.5 d) or two (69.6 d) calves, length of the interval was further reduced by dystocia in nonla ctating dams of either twins or singles (type of birth x dystocia, P < .05) . Ensuing pregnancy rates were also affected by type of birth and dystocia. Without dystocia, dams birthing and nursing a single calf had a higher pre gnancy rate (79.2%) than dams birthing twins and nursing one (61.7%) or two (66.3%) calves, whereas the lower ensuing pregnancy rates associated with dystocia in dams of singles (71.9%) resulted in similar rates among dams of singles and twins with dystocia (type of birth x dystocia; P < .01). Havin g a retained placenta resulted in a lower incidence of (93.5 vs 96.4%, with vs without; P < .05) and a longer interval to (64.7 vs 59.2 d; P < .01)est rus while reducing subsequent pregnancy rates ((X) over bar = 9.6%) in 3 of the 7 yr evaluated (retained placenta x year, P < .01). Because all parous females were bred during the same calendrical period, the shorter gestatio n length for twin calves (275.6 vs 281.3 d) resulted in a longer interval f rom parturition to conception for twin births, whereas means for conception date differed by only 2 d between dams of twins and singles. Furthermore, a reduction (P < .01) in the interval to conception occurred with dystocia in dams of singles (89.3 vs 85.0 d, without vs with dystocia) and of twins nursed by zero (116.9 vs 83.5 d), one (100.2 vs 92.8 d), or two (96.1 vs 97 .2 d) calves. Another detriment to fertility was the higher incidence of fe tal mortality or abortions associated with twin vs single pregnancies (12.4 vs 3.5%; P < .01). However, despite the lower conception rates for dams of twins, the increased prolificacy provides an opportunity to increase total beef production with a twinning technology.