Effects of feeding lubabegron on apparent absorption and retention of nitrogen, fractional degradation rate of skeletal muscle, and glucose tolerance in finishing feedlot steers

Lubabegron (LUB; Experior®; Elanco Animal Health, Greenfield, IN), a β-adrenergic modulator, is the first U.S. Food and Drug Administration approved product that reduces ammonia gas emissions per kilogram of live weight and hot carcass weight in beef steers and heifers fed in confinement for slaughter. However, there is limited information about the effect of LUB on nitrogen retention, skeletal muscle turnover, and glucose metabolism in beef cattle. Therefore, 16 Angus steers (initial BW = 553 ± 25 kg) were used in a randomized complete-block-design to examine the impact of LUB feeding on apparent nitrogen retention and excretion, fractional degradation rate of skeletal muscle, and glucose tolerance. Steers were individually housed in outdoor dirt-surfaced pens and fed a steam-flaked corn-based diet once daily in amounts to allow ad libitum access to feed over a 24 h period. Treatments were negative control (CON; no β-ligand) and LUB (fed at a rate of 3.2 g Experior/ ton of DM for the last 91 d on feed). A glucose tolerance test (GTT) was conducted twice during the study, and total fecal and urinary output were collected for 5 d on all steers. On day 96, following a 4-day LUB withdrawal period, all animals were slaughtered at a USDA-inspected abattoir, and carcass data were collected. Data were analyzed using the mixed models procedure in R. Initial and final body weights, dry matter intake, average daily gain, and feed efficiency were similar (P > 0.05) between treatments. Skeletal muscle degradation and fractional protein breakdown rate were reduced (P < 0.01) when cattle fed LUB were compared to cattle fed CON. Neither treatment nor a treatment × time interaction influenced glucose, insulin, or non-esterified fatty acids concentrations throughout the GTT. Apparent nitrogen absorption and retention were not different across treatments (P > 0.05), but total urinary output was reduced (P = 0.02) when cattle were fed LUB. Steers receiving LUB had a 9.21% reduction (P = 0.004) in calculated cumulative ammonia gas emissions compared with control steers. Longissimus muscle (LM) area was greater (P < 0.01) in steers fed LUB compared to steers fed CON, whereas yield grade tended (P = 0.09) to be greater in steers fed CON than in steers fed LUB. These results indicate that LUB supplementation decreased muscle protein turnover, improved LM area, and reduced total urinary output in feedlot cattle, without affecting nitrogen balance.