Determinants of pregnancy success following fixed-time artificial insemination in Nelore cows under field conditions: a large-scale study.

This retrospective, observational study examined the associations of female category, body condition score (BCS), timing of progesterone device removal, and repeated device use with pregnancy per fixed-time artificial insemination (FTAI) in Nelore females managed under field conditions. Records from 109 farms in Minas Gerais, Brazil, comprising 31,358 cyclic females (10,079 heifers and 21,279 multiparous cows) at their first FTAI of the breeding season, were analysed. Associations were estimated using multivariable logistic regression, which modeled all factors jointly and included farm as a random effect to account for clustering of females within farms; pairwise contrasts were adjusted for multiple comparisons, and exact p-values are reported. The overall pregnancy rate was 51.5%, with no difference between heifers (50.9%) and multiparous cows (51.8%; p > 0.05). In multiparous cows, BCS was positively associated with pregnancy: high-BCS cows had greater odds of pregnancy than moderate- (OR 1.90; p = 0.003) or low-BCS cows (OR 2.40; p < 0.001); no such association was observed in heifers. After multivariable adjustment and correction for clustering and multiple comparisons, the day of device removal (D8-D10) was not associated with pregnancy in any category. Pregnancy did not differ across the first three device uses; however, in multiparous cows a pre-specified contrast indicated lower odds of pregnancy at the fourth use than at earlier uses (OR 0.80; p = 0.028). Under these field conditions, BCS in multiparous cows was the main factor associated with pregnancy, whereas the day of device removal and device reuse up to the third time were not.