Models for prediction of days to first ovulation based on changes in endocrine and nonendocrine traits during the first two weeks postpartum in Holstein cows.

Regression models were developed for prediction of days to first postpartum ovulation in Holstein cows milked twice daily. Variables subjected to least squares multiple regression analysis included experimental data collected during weeks 1 and 2 postpartum from 20 cows that ovulated first between 15 and 34 days postpartum. Changes in serum estradiol (AE), number of ovarian follicles ;~ 10 mm diameter (ANOF), coefficient of variation of serum LH and glucocorticoid concentrations (/XLHCV, /XGLCV), fat-corrected-milk (Z~FCM), energy intake (AMCAL) and percentage change in body weight (% z~BW) between weeks 1 and 2 postpartum were independent variables and days to first ovulation (DFO) was the dependent variable. A prediction equation with nine variables (/xE, ANOF, ALHCV, AGLCV, % Z~BW, /XE x /xNOF, /XLHCV x AFCM, &MCAL x %•BW and AFCM x % Z~BW) resulted in an R 2 value of .86 (adjusted R 2 = .74). The most important criteria for predicting DFO were AE, /XNOF and AE • ANOF. A conceptual model was proposed to describe endocrine events leading to initiation of cyclic ovarian activity in dairy cows. The model is based on evidence that early postpartum follicular development and estradiol secretion are stimulated by FSH secretion. Increased levels of estradiol then induce LH synthesis and release which in turn causes final follicular maturation and ovulation. (