Feedlot performance and carcass characteristics of Holstein steers as affected by source of dietary protein and level of ruminally protected lysine and methionine.

The objective of this study was to determine the effects of source of dietary CP and level of ruminally protected lysine and methionine (RPLM) on feedlot performance and carcass characteristics of Holstein steers during a growing-finishing trial (266 d). A total of 168 Holstein steers (182.7 +/- 27.5 kg) were used in a completely randomized design experiment (eight treatments; three pens of seven steers/treatment). Steers were given ad libitum access to high-concentrate diets (13% CP) containing 71% whole shelled corn, 10% corn silage, 4% condensed distillers solubles, and 15% protein supplements (DM basis). Treatments were arranged as a 2 x 4 factorial. The main factors were two sources of dietary CP and four levels of RPLM. The sources of dietary CP were soybean meal (SBM) or SBM and urea (SBM-U). Urea-N replaced 50% of SBM-N in the SBM-U diet. The levels of RPLM were 0, 5, 10, and 15 g per steer daily. No interactions (P > .10) between source of dietary CP and level of RPLM were observed for feedlot performance or carcass characteristics. Feedlot performance showed an advantage (P < .10) to feeding SMB during the first 84 d of the trial and an advantage to feeding SBM-U during the last 98 d of the trial. However, feedlot performance for the whole trial and carcass characteristics (except for fat thickness) were not affected (P > .10) by the source of dietary CP. Steers fed diets containing SBM-U had 12% less (P < .10) fat thickness than those fed diets containing SBM. Supplementation of diets with increasing levels of RPLM did not affect (P > .10) ADG or carcass characteristics. However, DMI and gain:feed showed cubic (P < .10) responses to increasing dietary level of RPLM. Supplementation of RPLM at the 10 g/d level improved gain:feed by 12% during the last 98 d of the trial, and this was a direct response to the cubic effects of RPLM on DMI. Results suggest a cost advantage for replacing 50% of SBM-N with that from urea in high-corn diets without negative effects on feedlot performance or carcass characteristics of growing-finishing Holstein steers with extended feeding periods (266 d). These types of diets seem to meet the amino acid requirements and are not limiting in lysine and methionine.