Effects of timed insemination and supplemental beta-carotene on reproduction and milk yield of dairy cows under heat stress.

In three experiments, we tested the efficacy of timed artificial insemination (AI) and beta-carotene supplementation for improvement of reproduction and milk yield. Experiments 1 and 2 were conducted during hot months, and Experiment 3 was conducted during cooler months. Cows were fed rations supplemented with beta-carotene at 0 or 400 mg/d per cow for > or = 15 d before the first AI. Cows were inseminated at each observed estrus after 70 d (Experiment 1) or at 50 d postpartum (Experiments 2 and 3) or were included in a timed AI program [d 0 (i.e., approximately 40 or 60 d postpartum), 8 micrograms of GnRH agonist; d 7, 25 mg of PGF2 alpha; d 9, 8 micrograms of GnRH agonist; d 10, AI] for first breeding. Pregnancy rate at first AI was similar among groups, but the percentage of cows that were pregnant by 90 d postpartum was greater for cows in the timed AI group in Experiments 1 (16.6% vs. 9.8%) and 2 (34.3% vs. 14.3%) but not in Experiment 3 (24.1% vs. 28.7%). Overall, beta-carotene had no effect on reproductive function. For cows fed supplemental beta-carotene for > or = 90 d, however, pregnancy rate at 120 d postpartum was increased in Experiment 1 (35.4% vs. 21.1%). In all experiments, beta-carotene increased cumulative milk yield on the last test day by 6 to 11%. In conclusion, timed AI can improve pregnancy rates during periods of heat stress. Supplemental beta-carotene may increase pregnancy rates for cows in the summer and can increase milk yield.