Evaluation of slowly degraded proteins: dehydrated alfalfa and corn gluten meal.

A system was developed for evaluation of protein sources in ruminant diets. Two steer growth trials were conducted to determine if the system would differentiate between protein sources. Corn cob, corn-based diets were supplemented with urea or a test protein: soybean meal (SBM), a SBM-bentonite complex (SBMB), direct-cut dehydrated (DCD) or wilted dehydrated alfalfa (WD) or corn gluten meal (CGM). A combination of DCD and CGM was also evaluated (DCGM). Diets were 61% TDN and 11.596 crude protein equivalent (CPE). Experimental diets'contained 20, 30, 40, 50 or 60% supplemental N from natural sources. Ad libitum intake of the urea-fed steers was used to establish the intake of all other steers, with corrections made for maintenance requirement. In trial 1, diets that were supplemented with SBM, DCD, WD or DCD-corn gluten meal (DCGM) produced faster (P<.05) gains than diets in which urea supplied all the supplemental N (means averaged across all levels of natural protein). In trial 2, diets supplemented with DCD, SBMB, CGM or DCGM produced greater gains than diets in which urea was used as the supplemental N source. Animals supplemented with natural protein required less (P<.05) feed per unit of gain than did the urea-supplemented animals. Protein efficiency, calculated as added gain per unit increase in natural protein above that produced by cattle fed the urea control were greater (P<.05) for the DCD, CGM and DCGM (.9, .7, .8) than SBM (.4). (