Feeding strategies for rearing replacement beef heifers in a grazing system under tropical conditions

Our objective was to evaluate the effects of pre- and post-weaning protein supplement on the productive response and nutritional and metabolic status of replacement beef heifers grazed on tropical pasture. Fifty-six Nellore heifers (7.7 ± 0.1 months age) with an average initial body weight (BW) of 237 ± 3.5 kg were distributed in a completely randomized design in 2 × 2 factorial arrangement. The four strategies were: US-ME, unsupplemented heifers at pre-weaning and supplemented with 5 g/kg BW at post-weaning; US-HI, unsupplemented heifers at pre-weaning and supplemented with 7.5 g/kg BW at post-weaning; ME-ME, heifers supplemented with 5 g/kg BW at pre-weaning and post-weaning; and ME-HI, heifers supplemented with 5 g/kg BW at pre-weaning and 7.5 g/kg BW at post-weaning. On average, crude protein (CP) intake and CP: digested organic matter (DOM) ratio were influenced (P < 0.05) by protein supplement in the post-weaning period, with a higher intake of CP and CP:DOM ratio in HI heifers than in ME heifers (average CP of 0.812 vs 0.665 kg/day and average DOM of 204 vs 174 g CP/kg). Nonetheless, the post-weaning protein supplement did not affect the intake of total dry matter (DM), pasture DM, organic matter (OM), neutral detergent fiber corrected for ash and protein residue (apNDF), non-fibrous carbohydrates, and DOM. The CP digestibility was highest (P = 0.005) for HI heifers than for ME heifers (average of 0.732 vs 0.645 g/g DM). The pre-weaning and post-weaning protein supplement did not affect the concentration of insulin-like growth factor–1 (IGF-1), progesterone, albumin, globulins, glucose, and triglycerides, and liver mRNA expression of carbamoyl phosphate synthetase-1 (CPS-1) and phosphoenolpyruvate carboxykinase (PEPCK). Finally, the pre-weaning and post-weaning protein supplement did not affect growth performance, carcass traits, and body growth in the post-weaning. Supplementation during pre-weaning does not bring any carry-over effect on the productive performance and metabolic status of heifers supplemented during post-weaning. However, supplementation in post-weaning with 5 or 7.5 g/kg BW allows a compensatory growth of heifers that are unsupplemented at pre-weaning. Therefore, the supply of 7.5 g/kg BW of protein supplement for beef heifers only during the post-weaning period is recommended.