Triennial Lactation Symposium: Effects of stress on postpartum reproduction in dairy cows.

The objective was to review the effects of production stressors on reproductive performance of dairy cows. It has been well documented that genetic selection for milk yield over the last 50 yr has been associated with reduced fertility. In addition to negative associations between yield and conception rate, there is also an association between milk production and expression of behavioral estrus. Stress caused by production diseases in high-yielding dairy cows also contributes to the problems of poor fertility. Lameness results in reduced intensity of estrus and can contribute to ovulation failure, which is largely due to reduced pre-ovulatory estradiol secretion and failure of the LH surge. Mastitis has been associated with prolonged intervals to dominant follicle selection, and in animals with uterine infection the dominant follicle grows slower and produces less estradiol. In a recent study, we identified that milk yield was associated with an increased incidence of uterine infection, which is known to contribute to reduced fertility and prolonged calving-to-conception intervals. The incidence of uterine disease was 73% in high-yielding, compared with 45% in low-yielding cows. As well as effects at the ovary, various models of stress have also been shown to perturb endocrine secretion in the hypothalamus and anterior pituitary. In conclusion, the adverse effects on fertility associated with genetic selection for yield in dairy cows is, in part, associated with increased incidences of production disease-induced stress but is also associated with high milk yield.