Variables associated with peripartum traits in dairy cows. IX. Relationship of season and other factors to blood plasma concentrations of progesterone and the estrogens.

Least-squares associations of month, average daily temperature, photoperiod (h daylight/24 h) and other factors with plasma concentrations of hormones during the prepartum period were examined in 172 Holstein cows and heifers. The data were summarized and expressed as values for prepartum d 7, 4 and 1, which represented the means for d 8 to 6, 5 to 3, and 2 to 0. respectively. All animals calved during a period of 19 consecutive months. Across days, least-squares means for plasma hormones were 4.0 ng/ml for progesterone (P4), 232 pg/ml for estradiol-17 beta (E beta), 478 pg/ml for estradiol-17 alpha (E alpha), 1,970 pg/ml for estrone (E1) and 2,680 pg/ml for total estrogen (TE; sum of E beta, E alpha and E1). Variations in plasma concentrations of certain steroids were associated (P less than .05 to P less than .01) with diet group (E beta), calf sex (TE), prepartum day (all hormones), month (E beta, E alpha) and linear covariates of cow weight (E beta, E alpha) calf birth weight (E beta, E alpha, E 1, TE), temperature (P4, E beta) and photoperiod (P4, E beta). The independent linear regressions for temperature and for photoperiod in prediction equations for each plasma hormone were always inversely related. The combined use of these regressions only partially accounted for fluctuations in the least-squares monthly means for hormones. Changes in least-squares monthly means for P4, E beta and E alpha tended to be only partially paralleled during the year. Nonparallel trends among hormones were most apparent during May to July (decreasing P4 but increasing E beta and E alpha), September to November (decreasing P4 and E beta but increasing E alpha) and January and February (only E alpha elevated). We hypothesize that seasonal differences in parturient traits reported for cows may be explained in part by the seasonally associated nonparallel changes in plasma concentrations of P4, E beta and E alpha.