Relationships of hormones, temperature, photoperiod, and other factors to voluntary intake of dry matter in pregnant dairy cows prior to parturition.

Voluntary intake of dry matter per 100 kg body weight (defined as intake) was quantified in 86 pluriparous Holstein cows which calved during 19 consecutive mo. Least squares techniques were used to examine the association of intake during the last 8 days before calving with numerous independent variables representing seasonal and physiological factors. The data were summarized as prepartum days 7, 4, and 1 (means of days 8 to 6, 5 to 3, and 2 to 0, respectively). Intake was lowest on prepartum day 1. Main effects of diet-group, health status, and month were not related significantly to intake when covariates were included in the statistical model. Intake decreased with increasing age of cows, but it was not related independently to body condition score. There were seasonal variations in intake which could be accounted for statistically by either month as a main effect or by quantitative indices of seasonality (temperature, photoperiod, plasma prolactin). Intake was lowest in February, June, and July and highest in October. Calf birthweight was related to intake in an unexpected manner. Intake was lowest in dams carrying either small or large calves. Prolactin of plasma was associated independently with intake, but this hormone may be just another measure of seasonal variation. All variables accounted for 58% of the total variance in intake. When seasonal variations in intake were accounted for statistically by other variables, none of the steroid hormones (progesterone, estrone, estradiol-17 beta, and estradiol-17 alpha) were associated independently with variation in voluntary intake. Associations of cow age and of climatic indices with intake were greater during days 7, 4, and 1 prepartum as compared to day 250 of gestation, days .5 to 2.5 postpartum, and days 3 to 30 of lactation.