Genotype × Environmental Interactions on Reproductive Traits of Bovine Females. I. Age at Puberty as Influenced by Breed, Breed of Sire, Dietary Regimen and Season3

Abstract

The influence of breed or breed of sire and dietary regimen on age at puberty in heifers was determined in two experiments. Exp. 1 was a 2 × 2 factorial involving animals of two breeds (Hereford and Holstein), born January through June, 1969 and fed one of two dietary regimens, High (H) and Low (L). Exp. 2 was a 4 × 2 factorial involving crossbred, autumn born heifers out of Holstein dams, sired by Angus (AF), Hereford (HF), Simmental (SF) or Chianina (CF) bulls and fed one of two dietary regimens. In Exp. 1, Herefords were older at puberty than Holsteins. In Exp. 2 CF and SF were older than AF and HF, and CF were older than SF at puberty. In both experiments, H heifers reached puberty sooner than L heifers. There were no breed or breed - of - sire × diet interactions. In Exp. 1, there was a season of birth × diet interaction (P<.01). Heifers on the H regimen that were born in the spring reached puberty earlier than winter-born H heifers while the opposite occurred among L heifers. Regression coefficients of age at puberty on birth date (January 1 = d 1) were −.69 (P<.05), −.19, −1.00 (P<.005) and 1.86 (P<.01) for the Holstein H and L and Hereford H and L groups, respectively. Most of the animals in the first three groups reached puberty before June of the year following their birth, while Hereford L-fed heifers reached puberty between June and August, suggesting that date of attainment of puberty may have affected the relationship between age at puberty and birth date. In Exp. 2, the winter environment appeared to delay the onset of puberty. Most of the L heifers reached puberty in the summer, autumn or subsequent spring and few became pubertal during the intervening winter, causing a bimodal distribution of the dates of puberty attainment. Most of the H-fed heifers reached puberty before November, but puberty appeared to be delayed in those that did not. In Exp. 1, H heifers were heavier and had greater weight/height at puberty and consumed more TDN to puberty than L heifers. The opposite occurred in Exp. 2, presumably because seasons of birth differed between experiments and more L than H heifers did not reach puberty before winter in Exp. 2. Because H and L heifers reached puberty at different weights and levels of fatness, as measured by weight/height, dietary regimen did not appear to affect age at puberty by altering the time needed to attain a certain body weight or composition. We concluded that heifers fed diets low in energy reached puberty later than those fed diets high in energy and that breed or breed - of - sire differences in age at puberty were not affected by dietary regimen. Season affected age at puberty and may have altered the effect of nutrition and other factors on pubertal traits.