Climatic conditions, twining and frequency of milking as factors affecting the risk of fetal losses in high-yielding Holstein cows in a hot environment.

An epidemiological study of risk factors for fetal losses was carried out on 62,403 high-yielding Holstein cows in 29 large highly technified dairy herds in northern Mexico (25° N; 23.5 °C mean annual temperature). Multivariate multiple-group response model indicated that fetal losses between 43 and 260 days of pregnancy were 23 %. Heat-stressed cows at conception (temperature-humidity index, THI >82) were 14 times more likely (P < 0.01) to present fetal losses than not heat-stressed cows (27 vs. 18 %). Heat-stressed cows at 60 days of pregnancy (THI >82) were 4.5 times more likely (P < 0.01) to present fetal losses than cows suffering heat stress in early gestation (29.1 vs. 17.7 %). The proportion of cows experiencing fetal loss was lower for multiparous than primiparous cows (odds ratio; OR = 0.7). Cows with twin pregnancies had significantly increased chances of losing their fetuses than cows with a single fetus (33.6 vs. 20.7 %; P < 0.01). Cows with three milkings per day were 30 % more likely (P < 0.01) to lose their fetuses than cows milked twice daily. Cows calving in winter and spring had significantly increased chances of losing their fetuses than cows calving in summer and fall (30-35 vs. 4-5 %; P < 0.01). It was concluded that, in this particular environment, heat stress exert a great influence on fetal losses in high producing Holstein cows.