Impact of Diet Type on Repeatability of Feed Efficiency in Beef Cattle

As the beef industry has pushed for improvements in feed efficiency (FE) over the last several years our knowledge regarding factors that influence FE has evolved. Work from our multidisciplinary FE project has focused on the repeatability of FE across feeding phases. Briefly, this study utilized 985 steers (464 kg BW) fed across 6 replicated trials. Steers received roughage or whole shell corn-based growing diets for at least 69 d at University of Missouri and were ranked for residual feed intake (RFI) within diet type. Steers were shipped to Iowa State University and blocked to pens based on growing diet and growing period RFI (classified as low, medium, or high) to receive either a dry rolled corn or by-product-based (40% distillers grains, 20% soyhulls) finishing diet. All steers received ractopamine hydrochloride at 200 mg/steer daily for the last 28 d on feed. Results from this project suggest that FE is repeatable across feeding phases (growing and finishing), regardless of diet type. However, growing phase diet type influenced the means by which steers achieved that efficiency ranking in the finishing period. Steers identified as highly efficient when grown on corn remained highly efficient in the finishing phase because they consumed approximately 6% less DM, but gained similarly to steers ranked as lowly efficient during the growing phase. Interestingly, steers identified as highly efficient when grown on roughage diets remained highly efficient in the finishing phase not because they ate less DM, but because they improved daily gains by more than 8.5% while maintaining DM intake similar to those ranked as less efficient during the growing phase. This suggests that steers identified as highly efficient on a roughage-based growing diet may have the ability to extract more nutrients from the diet. Indeed, total tract fiber digestibility, estimated using titanium dioxide as an inert marker, was much greater for highly efficient vs. lowly efficient steers fed roughage-based diets. In contrast, nutrient digestibility wasn’t different between FE classifications for corn-fed steers, suggesting the ability to extract more nutrients from the diet may be more important in separation of FE classifications in high roughage than high grain diets. Indeed, findings from others suggest that rumen microbe populations are less variable in cattle with superior FE, perhaps suggesting that differences among rumen microbe species may explain some of the differences among FE phenotypes of cattle.