ABSTRACT

Beef cattle are able to utilize high fiber feedstuffs to produce a high-quality end product for consumers; however, beef cattle are at a disadvantage relative to swine and poultry in their ability to readily convert feed resources to lean tissue. Therefore, enhancing feed efficiency (FE) in beef cattle is at the forefront of the industry. Biological mechanisms explaining individual animal variation in FE are poorly understood, but differences among proteolytic systems are hypothesized to play a role in this variation. Because these systems are critical to postmortem muscle fiber degradation and increases in tenderness as beef ages it is important to understand the implications for selection for more FE cattle on ultimate tenderness of beef. Additionally, little is known about how cattle diet composition impacts steak tenderness and diet components often vary widely across geographical location. Cattle producers are able to utilize waste byproducts from production of fuels such as ethanol, but they require an understanding of the feeding value of the product to safely utilize these affordable feedstuffs. Thus, the subsequent trials were designed to: 1) determine the influence of FE on meat tenderness attributes of beef steers, and 2) evaluate the use of post-ethanol extraction sorghum silage as an alternative forage in growing and finishing diets on steer performance, carcass characteristics and nutrient digestibility. Within the initial research objective, growing phase FE classification, was determined from a growing phase (GP) residual feed intake (RFI) test where cattle were fed whole shell corn (MU-Corn) or roughage based (MU-Rough) diets. Within each growing phase diet the 12 greatest (HFE; average RFI – 3.33 à ± 0.77, SD) and 12 least (LFE; average RFI 2.90 à ± 0.94, SD) efficient steers were selected (48 steers total), and transitioned to a corn (ISU-Corn) or by-product (ISU-Byp) based diet for finishing. Growing phase diet and FE classification elicited no difference for d 2 troponin- T degradation or calpain 1 autolysis; however, d 14 troponin-T was affected by finishing diet with greater amounts of protein degradation in steaks from steers fed ISU-Corn vs. ISU-Byp. Calpastatin activity measured on d 2 tended to be greater in HFE steers than LFE steers. An interaction between ISU diet and FE classification was noted for steak lipid content and marbling score with greater lipid in steaks from LFE vs. HFE within ISU-Byp finished steers. Measurements of beef tenderness, using the Warner-Bratzler shear force (WBSF) method, indicated that steers grown on a roughage-based diet (MU-Rough) had greater WBSF than steers grown on corn (MU-Corn). It is improbable that differences in WBSF were obtained from differences in ADG during the GP because ADG across FE classifications were similar for both MU-Corn and MU-Rough (1.94 kg/d) fed steers. The influence of finishing period feed efficiency (G:F) on meat tenderness attributes was determined fitting a slope regression model. Slope estimation for d 2 calpastatin and d 2 troponin-T differed from zero only for LFE steers receiving MU-Corn:ISU-Byp, indicating as G:F increases for steers receiving this respective diet there is a noted decrease in calpastatin activity with a subsequent increase in troponin-T degradation. Interestingly, the slope for d 14 troponin-T of HFE steers receiving MU-Rough:ISU-Corn differed from zero, suggesting as finishing phase G:F increases there is a decrease in troponin-T degradation. Within the second research objective, total tract apparent digestibility of organic matter (OM), ether extract (EE), crude protein (CP), and neutral detergent fiber (NDF) were greater, and acid detergent fiber (ADF) digestibility was decreased for steers fed diets containing the post-ethanol extraction sorghum silage relative to a medium quality grass hay; however, no differences were noted for the digestibility of DM and starch. Although growing phase digestibility was different, overall steer performance and carcass characteristics were not affected by the inclusion of post-extraction sorghum silage in feedlot diets, thus suggesting post-ethanol extraction sorghum silage may be utilized effectively as an alternative forage source in feedlot diets. These findings provide opportunity for a new forage by-product feed to be incorporated into feedlot diets and suggest that protein metabolism is influenced by FE phenotype, though the importance in explaining biologic variation in FE remains to be elucidated. Additionally, this research has revealed interactions among FE phenotype and cattle diet composition and further research is warranted to better understand the implications of nutritional management across different FE classifications, and the alterations in protein expression that may lead to differences in beef tenderness.