Asociación Latinoamericana de Producción Animal (ALPA)
Abstract
Two hundreds and forty seven Holstein heifers born during four consecutive years were for this research to describe effects of season of birth and bST injections on growth rates of heifers fed two diets and during two growth periods. For both experimental groups, treatments consisted of control (.68 kg/d) and fast growth rates (.900 kg/d) and injected with bST either 12.36 mg/d or 500 mg/14 d. All heifers were fed corn silage beginning at about 80 d of age and grain concentrates were top-dressed on the silage to provide energy, CP, and minerals to support desired ADGs. All heifers were weighed and height at withers recorded biweekly, adjustments to feed offered were made within several days after weighing. Heifers were group fed once daily. Water was available ad libitum. Data for body weight and height were analyzed by least squares analysis of variance using the mixed model procedures of SAS. Significant differences were detected for effects of season of birth, diet, bST and the two-way interaction, year × season, for both the overall and peripubertal growth periods. Significant effects were detected for year during the peripubertal growth period and for the two-way interaction, season × bST, during the overall growth period. The covariate initial body weight was significant during both growth periods. Growth rates of heifers were increased by bST irrespective of whether heifers were fed for normal or rapid growth rates, or the season in which heifers were born. This suggested that bST improved growth rates by increasing feed efficiency and nutrient distribution to priority organs. Energy storage as protein accretion (skeletal and muscle growth), instead of fat deposition, may result in larger heifers at age of breeding. Thus, bST injected heifers could be bred earlier to calve at a younger age
