The objective of this Purdue University trial was to compare the effects of a natural vs. conventional feedlot protocol on performance and carcass characteristics of spring-born Angus heifers (595 lb). The conventional protocol consisted of an 85% corn-based diet plus feed additives (Rumensin, Tylan, and MGA) and an implant (Revalor I-H) at both day 30 and day 120. The natural protocol consisted of the same 85% corn-based diet but with no additive or implants.
Heifers were fed to a common 12th rib fat depth endpoint of 0.5 in. Conventional heifers were significantly greater in avg. daily gain (2.95 vs. 2.62 lb), finished weight (1109 vs. 1061 lb), dressing percent (64.4 vs. 63.2%), and hot carcass weight (714 vs. 670 lb).
Although ribeye area was larger in conventional heifers (11.8 vs. 11.4 sq. in.) there were no significant differences for 12th rib fat thickness, marbling score, yield grade, or feed efficiency.
Natural heifers tended to have a greater percentage of heifers grading USDA Choice or Prime (100 vs. 90.5%). However, the combination of heavier carcass weights along with acceptable quality grade resulted in a greater numerical carcass value for conventional heifers ($944.96 vs. $910.24).
Due to cost of additives and implants, the cost to finish weight was $13.20/head greater for conventional heifers.
Overall, there was a net increase of $21.52 in favor of the conventionally fed heifers (Gunn et al. 2007. Midwest Section ASAS. Abstract 312).
Dr. Rick Rasby, Professor of Animal Science
Animal Science, University of Nebraska–Lincoln, Lincoln, NE