The effects of dietary energy density and intake restriction on apparent maintenance energy requirements of beef cows

Abstract Text:

Intensification of cow calf systems may offer a mechanism to increase land use efficiency of beef production. To determine effects of dietary energy density and intake restriction on energy requirements of beef cows, 32 crossbred cows were individually fed using Calan gates. Cows were fed either a high energy density (H; 1.54 Mcal NEm/kg) or low energy density (L; 1.08 Mcal NEm/kg) diet at each of two levels of intake to achieve 80 or 120 % of maintenance energy requirements as predicted by the NRC model. Cows were blocked by BW and treatments were applied in a 2 × 2 factorial to yield four treatment combinations (H80, H120, L80 and L120). After initial treatment application (d -7), cows were weighed daily for seven days to detect measurable fill change. To accommodate initial fill differences, BW gain was measured from d (-4) to d 42. On days (-7) and 42, 12^th rib fat thickness was measured via ultrasound. Equations from the NRC model were utilized to calculate heat energy (HE) for individuals. All responses were analyzed as a randomized block design with a factorial treatment arrangement. No energy density by intake level interactions were observed for any response (P > 0.14). While there was a difference (P < 0.01) in initial body weight between energy density groups due to differential fill loss prior to d -4, neither diet energy density (P = 0.09) nor intake restriction (P = 0.13) affected change in BW over the 46-d observation period. Similarly, change in rib fat was not affected by diet energy density (P = 0.48) or intake restriction (P = 0.24). Cows fed H had 11.6 % lower  (P < 0.01) HEd^-1 and 9.9 % lower maintenance requirements (daily HE/kg BW^0.75; P < 0.01) than cows consuming L. Cows fed 80 had 28.7 % lower (P < 0.01) HEd^-1 and 29.1 % lower maintenance requirements (P < 0.01) compared to cows fed 120. When predicted NEm requirements were based on the equation: 0.077Mcal NEm per BW^0.75, apparent maintenance requirement deviations were greater (more negative) in both H (P < 0.01) and 80 (P < 0.01). These results suggest that the additive effects of increasing diet energy density and restricting intake reduce apparent maintenance requirements of beef cows. Substantial gains in efficiency of maintaining beef cows in intensive systems can be achieved by limit feeding an energy dense ration.

Keywords: Cow Energy Requirements