Effect of reproductive indices and bovine somatotropin on milk yield in Holstein cows experiencing climatic sub-fertility

Abstract

This study aimed at determining the association between reproductive indexes and the total milk yield in non-planed lactations ≥600 days in sub-fertile Holstein cows treated or not with recombinant bovine somatotropin (rbST) and experienced prolonged hyperthermia. A total of 1141 cows with a single lactation were enrolled in the study: 622 were not treated with rbST (control) and 519 were treated with rbST (500 mg every 14 days throughout lactation). Across parities, control cows with >480 days open, produced 5642 kg more milk (p<0.01) than cows with <380 days open. rbST-treated cows produced 5994 more (p<0.01) kg of milk during the entire lactation than cows with <380 days open. In control cows receiving ≥10 services, the average total milk yield was 1811 kg higher (p<0.01) than in cows receiving ≤10 services. In rbST-treated cows, the average total milk yield was 1680 kg higher (p<0.01) in cows receiving ≥10 services, compared to cows receiving ≤10 services. The overall conception rate for all services was 51% and 73% (p<0.01) for control and rbST-treated cows, respectively. In conclusion, the greater the delay to get pregnant and the greater the number of services per lactation the higher the total milk yield per lactation were observed. Thus, in this unusual situation where sub-fertile cows are retained with >380 days open and they continue to be inseminated after 10 unsuccessful services, the impact of delayed conception is not entirely negative as this disadvantage is more than outweighed by the extra milk that is produced when lactation is extended.