Pre-Weaning Injections of Bovine Somatotropin Altered Liver Gene Expression, and Enhanced Puberty Attainment and Calving Rates of Bos Indicus-Influenced Beef Heifers

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 growth and reproductive performance (yr 1 and 2 only). 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 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 (7-8 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 soybean hulls-based concentrate at 1.1% BW (DM basis). Liver samples were collected via needle biopsy on d 0, 42, and at start of breeding season (d 262) of yr 3 only (6 heifers/treatment). Blood samples were collected every 9-10 d from d 127 to 346. Heifers were considered pubertal when 2 consecutive plasma samples had P4 ≥ 1.5 ng/mL. Resulting interactions among treatment, yr, and day were not detected for any variable measured in the study (P ≥ 0.12). During pre-weaning phase, bST heifers had greater ADG from d 0 to 42 (1.15 vs. 1.07 ± 0.026 kg/d; P = 0.03), but similar BW at weaning compared to SAL heifers (261 vs. 259 ± 1.3 kg, respectively; P = 0.25). Liver mRNA expression of GHR-1B and IGF-1 did not differ on d 0 and 42 (P ≥ 0.15), but was greater for bST vs. SAL heifers on d 262 (P ≤ 0.02). During post-weaning phase (d 127 to 347), bST heifers had similar ADG (0.17 vs. 0.12 ± 0.07 kg/d; P = 0.11), a greater (P ≤ 0.02) percentage of heifers attaining puberty at initiation of the breeding season (53 vs. 27 ± 7.9%), pregnancy (93 vs. 70 ± 6.6%) and calving rates (90 vs. 53 ± 7.3%) compared to SAL heifers. Calving distribution did not differ between treatments (P ≥ 0.15). Therefore, three 250 mg injections of bST administered at 14-d intervals, between 135 and 163 d of age, enhanced reproductive performance of bos indicus-influenced beef heifers.