Effects of protein and fat concentration in coproduct-based growing calf diets on performance and carcass composition

Angus×Simmental crossbred heifers (n=150) and steers (n=100) were used to evaluate 1 of 5 growing diets: 1) a corn-based growing diet (CRN); 2) a high-fat, high-protein coproduct blend; 3) a high-fat, low-protein coproduct blend; 4) a low-fat, high-protein coproduct blend; and 5) a low-fat, low-protein coproduct blend in a 2×2+1 factorial arrangement. Low-protein and low-fat diets were formulated to be isonitrogenous and isofat to CRN (16.0% CP and 3.0% fat), and high-protein and high-fat diets were formulated to have 20.0% CP and 5.0% fat, respectively. Calves were weaned at 85±1.2 d, blocked by weight, and allotted to pens (10 calves/pen) within sex (10 pens of steers and 15 pens of heifers). The objective of this experiment was to determine if the concentration of protein or fat or their interaction in coproducts used in growing diets fed to early-weaned calves affects feedlot performance or carcass composition. Starting on d 0, calves (141±1.2 d of age) were fed experimental diets for 112 d and then fed a common feedlot diet for an additional 112 d. Body weight, hip height, and ultrasound data were collected at the end of each 112-d feeding phase. Carcass data included HCW, LM area (LMA), 12th-rib back fat (BF), marbling score (MS), KPH, and USDA quality grade. There was no fat×protein interaction (P≥0.27); therefore, only main effects are discussed. No effects (P≥0.47) of CRN, protein, or fat were detected for BW at d 112 or 224. Increased dietary protein resulted in greater (P=0.04) ADG at d 112 compared to calves fed low protein. Feeding cattle CRN decreased (P=0.04) DMI and increased (P<0.01) G:F during the growing phase compared to coproducts. High-fat diets increased (P=0.05) BF in calves at d 112 compared to low-fat diets. High-protein diets decreased (P=0.02) ultrasound MS at d 112 compared to low-protein diets. Carcasses from cattle fed high-fat diets had greater (P=0.03) MS compared to those from cattle fed low-fat diets. No differences (P≥0.14) were observed for HCW, LMA, BF, KPH, or yield grade. These data indicate that final BW was unaffected by protein and fat content of growing calf diets but that increased dietary fat and low dietary protein increased MS.