Influence of Season on Sexual Development in Heifers: Age at Puberty as Related to Growth and Serum Concentrations of Gonadotropins, Prolactin, Thyroxine and Progesterone

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Abstract
An experiment was done to test the hypothesis that seasonal changes in environment during the 1st and 2nd 6 mo. of life influence age at puberty in heifers. Twenty-eight Angus .times. Holstein heifers, born in March (M) or Sept. (S), were reared under natural conditions until 6 mo. of age. From 6 to 12 mo. of age, heifers were reared in environmental chambers programmed to simulate seasonal changes in temperature and photoperiod characteristic of spring, summer and early autumn (Sp-F chamber) or autumn, winter and early spring (F-Sp chamber). S were younger (P < 0.06) at puberty than M, and Sp-F were younger (P < 0.08) than F-Sp for both M and S. Mean ages at puberty were 295 for S, Sp-F; 319 for S, F-Sp; 321 for M, Sp-F and 346 days for M, F-Sp. Average daily gain (ADG) between 6 and 9 mo. of age [1.03 kg/day (S) vs. 0.91 kg/day (M)] and mean concentrations of serum luteinizing hormone (LH) between 6 and 7 mo. of age [3.45 mg/ml (S) vs. 0.47 ng/ml (M)] were greater (P < 0.01) for S than M, suggesting an association between these traits and date of birth effects on age at puberty. Differences in these traits did not seem to be involved in the chamber effect on age at puberty, since ADG from 6-9 mo. of age was greater (P < 0.05) for F-Sp heifers and chamber did not generally affect LH concentrations. Serum concentrations of FSH were not significantly influenced by month of birth or chamber, but concentrations tended to decrease with age. Serum concentrations of thyroxine (T4) were higher in M than S at 6 mo. of age (7.8 .mu.g/dl vs. 6.3 .mu.g/dl) but not at other times, and chamber did not have a significant affect. Prolactin (Prl) concentrations paralleled patterns of temperature and day length and did not appear to be related to age. Although cattle are not seasonal breeders, season of birth and season of attainment of puberty influence age at puberty in heifers. Season may have influenced age at puberty by affecting serum concentrations of LH or Prl or growth rate.