Effects of dietary level of ruminally protected choline on performance andcarcass characteristics of finishing beef steers and on growth and serum metabolites in lambs

Ruminally protected choline (RPC) was evaluated in two experiments. In Exp. 1, beef steers (n = 160; average initial BW = 350.9 kg) were fed a 90% con centrate diet with either 0, .25, .5, or 1.0% RPC (DM basis) for 112 to 140 d. Feeding .25% RPC increased ADG 11.6% compared with 0% RPC, but response s diminished with increasing RPC level (cubic response, P < .10). Daily DMI was not affected by RPC level, but feed:gain was improved 6.8% with .25% R PC compared with 0% RPC, and responses diminished with increasing RPC level (cubic response, P < .10). Carcass yield grade increased Linearly (P < .10 ) as RPC level increased, but marbling score was lower for all three RPC-co ntaining diets than for the 0% RPC diet (quadratic response, P < .05). In E xp. 2, 20 Suffolk lambs (initial BW = 29.8 kg) were fed an 80% concentrate diet for 56 d with the same RPC levels as in Exp. 1. Serum triglycerides (T G) and cholesterol (CLSTRL) were measured in weekly blood samples, and inte nsive blood samples were collected on d 28 and 56 to evaluate serum insulin (INS), GH, and NEFA. For the 56-d feeding period, ADG responded quadratica lly (P < .10) to RPC level, but DMI and feed:gain were not affected. Serum INS and NEFA concentrations increased linearly (P < .05) and serum GH respo nded cubically (P < .05) to RPC level on d 28, but no differences were note d on d 56. Serum TG concentrations in weekly samples increased Linearly (P < .10) with RPC level, but, averaged over all weeks, serum CLSTRL concentra tions did not differ (P > .10) among treatments. Quantities of carcass mese nteric (P < .05) and kidney fat (P < .10) increased linearly, but longissim us muscle and Liver fat contents did not differ (P > .10) among RPC levels. Supplementing RPC in high-concentrate diets improved performance, but resu lts were not consistent among RPC levels or between cattle and sheep. Poten tial effects of RPC might be mediated through alterations in fat metabolism and(or) metabolic hormones related to fat metabolism.