Changes in calculated residual energy in variable nutritional environments: An indirect approach to apprehend suckling beef cows’ robustness

The major perturbation that beef cows have to face in extensive livestock systems is changes in feed resource availability. The ability of beef cows to face variable nutritional environments (robustness) involves adaptive processes that drive adjustments in the energy allocation toward life functions. This study proposes an indirect approach to quantify the modulation of energy allocation over a changing nutritional environment. The concept of residual energy (Eresid), defined as the net energy intake minus the energy secreted in milk and deposited in tissues, was used to investigate the variation in energy allocation priority for maintaining productive traits. In this study robustness was assessed by the difference in Eresid between cows experiencing either variable or non limiting nutritional trajectories and differing in body reserves at calving. Forty multiparous Charolais suckling cows, differing in their body condition at calving (moderate (M, n=19): BCSc=2.0±0.04 (scale 0–5)) vs fat (F, n=18): BCSc=2.8±0.08) were used. They were submitted to two energy levels during the first 120 days post-partum (P1): Control (MC (n=9) and FC (n=9)) vs Low (ML (n=10) and FL (n=9)). The average energy intake, expressed in net energy for lactation (NEL), was 90.7 and 54.7 MJ/d/cow for C and L cows, respectively. Subsequently (P2, 120–196 days post-partum) all the cows were turned out to a permanent pasture. BW, body condition and milk production were regularly measured in P1 and P2. Body lipid reserves were assessed at calving, end of P1 and end of P2 by measuring adipose cell diameter. The overall milk production was similar between groups of cows over the 2 phases of the changing nutritional trajectories highlighting the robustness of beef cows to achieve this function. During P1, L cows lost BW and body lipid reserves. During P2, BW and BCS gains were similar in FL and ML cows. At the end of P2, FL and ML cows weighed 20 and 10 kg less than FC and MC cows, respectively. Considering both experimental periods (P1+P2), Eresid was 23% lower in L than in C cows (P