079 Dietary Energy Utilization in Limit-Fed Beef Cattle Purchased

Diet digestibility typically increases with intake restriction, but an interaction between intake and diet composition affecting this response could cause substantial overprediction of feeding requirements in limit-fed systems. Feed costs are typically the greatest variable cost associated with cow-calf production, especially in intensive management systems; overprediction of requirements unnecessarily increases costs. Twenty crossbred (3/4 Angus × 1/4 Nellore) cows were used in a 20 × 6 incomplete Latin square to describe effects of dietary energy concentration and intake level on nutrient digestion and apparent energy availability. Treatments were arranged as 5 × 4 factorial. Diets differing in energy concentration were constructed by substituting dry rolled corn for wheat straw in a total mixed ration; such that corn concentration in the diets was 0% (1.090 Mcal NEm/kg), 16% (1.335 Mcal NEm/kg), 32% (1.580 Mcal NEm/kg), or 48% (1.825 Mcal NEm/kg). Each diet was fed at 5 levels of intake. Minimum intake levels for each diet were designed to meet 75% of NRC estimates of energy requirements for a 454-kg mature, dry, open cow. Maximum levels of intake were estimated to meet energy requirements for a 390-kg primiparous cow at peak lactation (6.09 kg/d) gaining 0.14 kg/d. Intermediate intake levels were evenly spaced between the minimum and maximum for each respective diet. All levels of intake for all diets met or exceeded projected protein requirements. Experimental periods were 14 d, (10 d for adaptation to treatments, 4 d for measurement of digestion). A diet × intake interaction (P < 0.01) was observed for apparent DE concentration. Apparent DE concentration decreased (P < 0.01) with increasing intake of 32 and 48%, tended to decrease (P = 0.09) as intake of 16% increased, and was not affected (P = 0.29) by intake of 0%. Although a diet × intake² interaction was not observed (P = 0.45) for apparent total DE intake, there was a tendency (P = 0.11) for the rate of increase to decline with intake of 48%. Apparent diet DE concentration increased with intake restriction, with the extent and rate of improvement increasing with dietary energy density. Data suggest that energy delivery is under predicted in limit-fed, high energy diets, especially at low intakes.