Influence of Bovine Somatotropin on Characteristics of Estrus and Reproduction of Holstein Cows During Summer Months

Abstract

A field trial was conducted to evaluate the effects of bovine somatotropin (bST) on estrous characteristics of multiparous, lactating Holstein cows (n = 44) during the summer months. Cows were randomly assigned to receive bST every 14 d or no bST (control) and were fitted with a HeatWatch® transmitter (HW; DDx Inc., Denver, CO). A temperature-humidity index (THI) was calculated for the day of estrous onset. Ranges of THI were 68 to 80 (THI < 72 = THI 1 [no heat stress], THI 72 to 76 = THI 2 [mild heat stress], THI > 76 = THI 3 [moderate heat stress]). Duration of estrus, number of standing events, and quiescent periods between standing events were similar (P>0.10) between bST and control cows. The number of standing events was influenced (P<0.05) by THI and averaged 20.3 ± 3.0, 8.6 ± 1.3, and 8.9 ± 1.2 for THI 1, THI 2, and THI 3, respectively. Quiescent periods observed between standing events tended (P=0.11) to be influenced by THI and were 1.2 ± 0.8, 3.0 ± 0.4, and 3.0 ± 0.4h for THI 1, THI 2, and THI 3, respectively. Days to first service were less (P=0.07) for bST-treated cows (90.9 ± 5.1 d) than for control cows (105.9 ± 6.1 d). First service conception rates and pregnancy rates were similar (P>0.10) between bST-treated and control cows. We conclude that estrous characteristics and pregnancy rates of lactating dairy cows were not influenced by bST treatment. However, the number of standing events was reduced, and quiescence between mounts tended to increase, in lactating dairy cows as ambient temperature and humidity increased.