Performance of grazing suckling beef calves receiving different protein regimens.

Background: to ensure continuous calves' growth, the supply of supplements is necessary to impact the animals' tissue response. Objective: the aim of this study was to evaluate the effects of supplementation with different protein regimens on performance in suckling beef calves. Methods: fifty suckling Nellore calves with average initial body weight (BW) of 114 ± 2.4 kg and 105 ± 3.0 days age were used and distributed in a completely randomized design with two supplementation treatments: Low protein (LOW) and High protein (HIGH). The supplements were composed of soybean meal, cornmeal, wheat meal, mineral mixture, and molasses, containing 150 g CP/kg dry matter (DM) and 300 g CP/kg DM for LOW and HIGH respectively, and provided in amounts of 6 g/kg BW. The animals underwent 78 experimental days, and were kept in eight 5 hectares paddocks each, formed by Brachiaria decumbens and equipped with drinkers and feeders. For productive performance evaluation, all calves were weighed at the beginning and end of the experiment after 14 h for solids fasting. Also, on the 78th-day experimental carcass images were obtained by ultrasound between the 12th and 13th ribs over the longissimus muscle to measure the ribeye area and backfatthickness on the Longissimus muscle (FAT-L), and between the ischium and pubis to the backfat-thickness on the rump (FAT-R). Biopsies of Longissimus dorsi muscle were performed at 75 days to measure the number and diameter of muscle fibers. Results: no difference (p > 0.05) was observed for average daily gain (LOW = 0.98 vs HIGH = 1.00 kg/d) or final BW (LOW = 191 vs HIGH = 193 kg) throughout the experiment. Likewise, no effect (p > 0.05) of the amounts of protein supplementation was verified on ribeye area (LOW = 36.7 vs HIGH = 38.8 cm2), FAT-L (LOW = 1.38 vs HIGH = 1.54 mm), FAT-R (LOW = 2.06 vs HIGH = 2.30 mm), or number (LOW = 141 vs HIGH = 142) and diameter (LOW = 29.2 vs HIGH = 30.9) of muscle fibers. Conclusions: supplementation with 150 g CP/kg DM in the amount of 6 g/kg BW is recommended for suckling beef calves in grazing. [ABSTRACT FROM AUTHOR]