Variables associated with peripartum traits in dairy cows. IV. Seasonal relationships among temperature, photoperiod, and blood plasma prolactin.

Concentrations of prolactin in plasma were measured in 176 dairy cows and heifers from 13 days before calving to 2.5 days after calving over 21 mo. Prolactin averaged 35.1, 115.0, and 34.4 ng/ml prepartum (days -13 to -2), peripartum (days -1.0 to +.5), and postpartum (days +1.5 and +2.5). Season of the year affected prolactin in all periods. The linear covariate of daily photoperiod (hours of daylight per 24 h) accounted for as much variation in prolactin prepartum and postpartum as did linear covariates of both photoperiod and average daily temperature. However, it was possible to account for additional seasonal variation in prolactin peripartum by addition of the temperature covariate to the photoperiod covariate. Although photoperiod was related either directly or indirectly more than temperature to factors affecting prolactin seasonally, these statistical inferences cannot prove that prolactin is more dependent on photoperiod than on temperature because the two metereorological measures were correlated (r = .84). When the data were grouped for correlation analysis by months, correlations between temperature and prolactin among prepartum samples collected in the spring and in the fall were positive, small but significant.