Using Soy Protein in Growing and Finishing Cattle Diets

Abstract Over the last two decades, distillers grains (DGS) have become the default protein supplement for growing and finishing cattle diets in the Midwest. Rapid expansion of ethanol production resulted in abundant supplies of low-priced DGS which were readily adopted by cattle feeders. However, it is worth re-evaluating conventional wisdom, particularly as feed price dynamics change and as we strive to capitalize on cattle growth genetics. Our research group has been examining how soy protein can be used to either completely or partially replace DGS in growing and finishing cattle diets. Our initial work showed no practical differences in finishing cattle diets containing solvent-extracted soybean meal compared to DGS despite differences in RUP concentration; however, including additional soybean meal with DGS resulted in 4 kg numerically greater HCW. Growth responses in diets containing heat-treated soybeans have been more compelling. Replacing 60% of DGS with extruded-expelled soybeans resulted in 10 kg greater HCW (P = 0.02). In a follow-up study, we evaluated three different isonitrogenous dietary treatments: modified DGS control, complete replacement with full-fat extruded soybeans, or modified DGS with sufficient added corn oil so that dietary fat content was equivalent to the full-fat extruded soybean treatment. Feeding full-fat extruded soybeans increased HCW (489 kg) compared to modified DGS, with (481 kg) or without added fat (476 kg; P = 0.01) with corresponding responses in ADG and G:F (P = 0.01). Although the increased HCW resulted in greater value per steer, the increased revenue was insufficient to pay for the greater diet costs when full-fat extruded soybeans completely replaced DGS. For our most recent finishing study we partially replaced DGS with full-fat soybeans (6% diet DM inclusion) as a strategy to economically increase cattle performance. In that experiment, partially replacing DGS with full-fat extruded soybeans increased HCW by 12 kg (426 and 438, respectively; P = 0.01). We included a beta-ligand (lubabegron; Experior, Elanco Animal Health; Indianapolis, IN) in the two experiments evaluating full-fat extruded soybeans. Heat-treated soybeans increased ADG relative to diets where DGS was the sole supplemental CP source while the beta-ligand was fed, and in all three studies cattle growth was increased in the interim period immediately following administration of a growth promotant implant. It appears that based on our data blending DGS with heat-treated soybeans may act synergistically to enhance cattle growth and that this response is magnified while growth promoting technologies are used. Further research is warranted to better understand mechanisms of action, optimal dietary inclusions, and applications for roughage-based growing diets.