Estimation of Energy Expenditure in Breeding Cows Grazing Rangelands with Different Herbage Allowance

The energy partitioning between total heat production (HP) and retained energy (RE) on the efficiency of energy use was evaluated throughout the annual production cycle (gestation-lactation) of 46 multiparous beef cows grazing on two herbage allowances (HA) of Campos grassland (4 vs. 7 kg dry matter/kg body weight; BW; LO vs. HI). Total RE was greater (P < 0.01) for HI than LO cows and presented minimum values during early gestation and maximum values during early lactation associated to the greater (P = 0.02) RE-milk in the former ones. Whole-animal HP, estimated by the heart rate-O2 pulse technique, and cow metabolized energy (ME) intake did not differ between HA treatments, but relative to BW0.75, HP was greater (P = 0.04) for LO than HI cows. Total HP and ME intake were minimum in gestation and maximum in early lactation and ME intake was greater during early lactation for HI than LO cows, and per unit of BW0.75, tended (P < 0.10) to be less for HI than LO cows in winter. The energy efficiency of the annual cycle was not affected by treatments (0.19 vs. 0.12 ± 0.02 for HI and LO respectively; P > 0.10) and there were no differences between treatments in terms of grams of calf per MJ of ME intake (P > 0.10). Management of grazing intensity of Campos grasslands with high herbage allowance improved energy balance of the beef cow-calf system through partitioning of cow ME intake towards RE instead of maintenance.