107 Effects of Dietary Energy Density and Intake on Energy Requirements in Beef Cows

Abstract Fifty-six gestating cows (393 ± 34.2 kg) were used to examine effects of dietary energy concentration and intake on energy utilization. Treatments were arranged as 4 × 2 factorial with four levels of dietary energy concentration and two levels of intake. Diets were constructed by substituting dry rolled corn for wheat straw in a total mixed ration; corn concentrations in the diets were: 16% (1.335 Mcal NEm/kg; 15.5% CP), 32% (1.580 Mcal NEm/kg; 16.5% CP), 48% (1.825 Mcal NEm/kg; 17.5% CP), and 64% (2.070 Mcal NEm/kg; 18.5% CP). Diets were not formulated to be isonitrogenous. Cows were individually fed using Calan gates for 112 d at: maintenance (M), designed to meet predicted energy requirements for a dry, 454-kg cow, 225-d in gestation; or restricted (R), designed to meet 75% of predicted energy requirements. Cow BW and backfat (via ultrasonography) were measured every 28 d. Energy retention (RE) was calculated as the difference in body energy, which was estimated from BW and backfat thickness, at the beginning and end of each period. Predicted RE was calculated for each period by subtracting estimates of NEm requirements from NEm intake. Difference between observed and predicted RE was then estimated for each period. Data were analyzed using repeated measures. Diet did not affect change in backfat thickness (P ≥ 0.20). Backfat thickness decreased faster (P = 0.01) in cows fed R than those fed M. An intake × day interaction was observed (P < 0.01) for cumulative energy retention; RE was greater (P < 0.01) in cows fed M than cows fed R, and the difference between intake levels increased over time. No interactions were observed (P ≥ 0.19) for daily heat production. Heat production decreased linearly from 151 to 126 ± 2.22 kcal/kg EBW0.75 with increasing corn inclusion (P < 0.01) and from 148 to 126 ± 3.13 kcal/kg EBW0.75 with intake restriction (P < 0.01). No interactions were observed (P ≥ 0.26) for daily deviation from predicted RE. The deviation was more positive in cows fed R than those fed M (P < 0.01), increased linearly (P = 0.01) with corn inclusion, and increased over time (P < 0.01). Divergence in observed and predicted RE could result from either increased dietary energy utilization or reduced requirements. Effects of dietary energy density and intake restriction suggest that improvements in overall dietary energy utilization are realized by limit-feeding high-energy diets to gestating beef cows.