The effect of dietary and climatic factors on fertility, and on plasma progesterone and oestradiol-17 beta levels in dairy cows.

The effects of high dietary protein concentrations, high ambient temperatures and low dietary beta-carotene levels on fertility and on plasma progesterone and oestradiol-17 beta levels in high-yielding dairy cows, are described. High-yielding dairy cows were fed diets containing 15 or 20% crude protein. Cows fed the higher level of protein were less fertile and had a lower plasma progesterone concentration during the oestrous cycle preceding the first insemination. This phenomenon was especially pronounced in cows fed a diet containing 85% concentrates and 15% hay, where plasma progesterone levels during days 9-17 of the oestrous cycle were 2.5-3.7 ng/ml in cows fed the high level of protein and 4.0-5.5 ng/ml in cows fed the low level of protein (P less than or equal to 0.05). In cows fed 50% more energy before parturition, plasma progesterone levels did not differ between animals fed the low and high protein diets. Heat stress, which caused a rise in body temperature, decreased fertility and plasma progesterone levels during the luteal phase of the oestrous cycle but increased plasma oestradiol-17 beta levels during the 36 h preceding the onset of oestrus from 5 to 7 pg/ml (P less than or equal to 0.005). The effect on fertility of supplementing the diet of dairy heifers and cows with beta-carotene is reviewed. In some of the experiments where the basal plasma beta-carotene levels were lower than 50 micrograms/100 ml, beta-carotene supplementation significantly increased fertility. However, in no case was fertility affected when plasma beta-carotene levels exceeded 150 micrograms/100 ml. The possible relationships among body weight changes, energy balance, plasma progesterone levels and fertility of dairy cows are discussed.