Changes in Feed Intake, Growth, Feed Efficiency, and Body Composition of Beef Cattle Fed Forage then Concentrate Diets

The objective of this experiment was to determine changes in production traits and body composition of beef steers and heifers when fed a forage-based ration followed by a concentrate-based ration. Cattle were progeny of composite breed cows bred to Charolais, Simmental, and Red Angus bulls. Approximately 4 wks after weaning, steers (n=71) and heifers (n=79) were placed on a ration containing (DM basis) 69.8% corn silage, 30% alfalfa hay, and 0.2% salt. Feed intake was measured for 84 d and BW were measured at 7 time points to calculate ADG using a quadratic regression of BW on time. Cattle were then transitioned to a concentrate-based finishing ration containing (DM basis) 67.8% dry-rolled corn, 20% wet distillers grains with solubles, 8% alfalfa hay, and 4.2% vitamin/mineral supplement. Data were analyzed using a mixed model (SAS) with sex, sire breed, dam breed, period (forage vs. concentrate), and sex × period as fixed factors, sire as a random variable, and period treated as repeated variable. This allowed the calculation of correlations among residuals from within periods. Cattle consumed approximately 30% more DM on the concentrate-ration compared to the forage-ration (P<0.01) and steers consumed about 10% more DM than heifers (P<0.01). The residual correlation between forage and concentrate DMI was r = 0.51, indicating a positive association of DMI between the 2 rations. Regardless of sex, ADG was greater on the concentrate ration (0.9 vs. 2.1 kg/d; P<0.01); however, the residual correlation was r = -0.09. The residual correlation coefficient for G:F was r = -0.17 and RFI was r = 0.38. Increase in LM area was greater for heifers (P<0.01) and was greater during the concentrate-ration feeding period (P<0.01) and the residual correlation was r = -0.35. There was a sex × feeding period interaction for back fat change (P<0.01), indicating that steers had a greater increase in fat thickness on the forage ration while heifers had a greater fat thickness increase on the concentrate ration. The residual correlation for back fat gain was r = -0.27. There was a sex × feeding period interaction (P<0.01) for change in intramuscular fat (IMF), with steers gaining more IMF on the forage-ration (0.50 vs. 0.33%), but heifers gaining more IMF on the concentrate ration (1.64 vs. 0.65%). Data indicate there is correlation across different diets for feed intake and composition of growth traits in beef cattle. USDA is an equal opportunity provider and employer.