023 Pre-Weaning Injections of Bovine Somatotropin Enhanced Puberty Attainment of Bos Indicus-Influenced Beef Heifers

Abstract

A 3-yr study evaluated the effects of three 14-d apart injections of bovine somatotropin (bST) to suckling beef heifers on pre-weaning (yr 1, 2 and 3) and post-weaning (yr 1 and 2) growth and puberty achievement. On d 0 of each yr, Angus × Brangus heifers (n = 15 heifers/treatment/yr; BW = 147 ± 20 kg; Age = 134 ± 11 d) were stratified by BW and age, and randomly assigned to receive s.c. injections of saline (SAL; 5 mL; 0.9% saline) or half-dose of bST (250 mg of sometribove zinc; Posilac, Elanco, Greenfield, IN) on d 0, 14 and 28. Cow-calf pairs were allocated to 4 bahiagrass (Paspalum notatum) pastures (7 to 8 pairs/pasture/yr) from d 0 until weaning (d 127). Unshrunk BW and blood samples were collected on d 0, 14, 28 and 127. From d 127 to 346, heifers were pooled by treatment and allocated to bahiagrass pastures (1 pasture/treatment/yr) and fed blackstrap molasses-based concentrate at 1.1% BW (DM basis). Post-weaning blood samples were collected every 9-10 d to determine plasma progesterone (P4) concentrations. Heifers were considered pubertal when 2 consecutive plasma samples had P4 ≥ 1.5 ng/mL. Effects of treatment × yr and treatment × yr × time were not detected for any variable measured in the study (P ≥ 0.69). During pre-weaning phase, bST heifers had greater pre-weaning mean plasma IGF-1 concentrations (115 vs. 102 ± 3.8 ng/mL; P = 0.02; yr 1 and 2 only) and ADG from d 0 to 42 (1.15 vs. 1.07 ± 0.026 kg/d; P = 0.03), but had less ADG from d 42 to 127 than SAL heifers (0.74 vs. 0.80 ± 0.022 kg/d; P = 0.04). Hence, BW at weaning did not differ between bST and SAL heifers (261 vs. 259 ± 1.3 kg, respectively). During post-weaning phase, bST heifers had similar ADG from d 127 to 347 (0.17 vs. 0.12 ± 0.07 kg/d; P = 0.11), and BW and age at puberty (290 vs. 291 ± 6.9 kg and 395 vs. 419 ± 14 d, respectively; P ≥ 0.15), but greater puberty achievement at start of breeding season (63 vs. 44 ± 7.9%) compared to SAL heifers. Hence, three half-dose injections of bST administered to suckling beef heifers at 14-d intervals, between 135 and 163 d of age, may be a feasible management practice to enhance puberty attainment at the start of the breeding season.