Energy balance of grazing beef cattle in Mediterranean pasture, the effects of stocking rate and season 2. Energy expenditure as estimated from heart rate and oxygen consumption, and energy balance

The energy expenditure (EE) of grazing cows and the metabolizable energy (ME) content of the pasture were measured over the course of 3 years under 29 representative treatments of grazing seasons, two different stocking rates, and two periods of confinement. Ten cows were used per treatment. Individual faecal output (FO) was measured for six of the grazing periods, and the metabolizable energy intake (MEI) was calculated from faecal output and in vitro digestibility. EE was estimated from measurements of daily heart rate (HR) and oxygen consumption, calibrated against oxygen consumption per heart beat (O ₂ pulse). The EE of lactating beef cows on lush pasture in the early spring reached 1260±31 kJ per (kg BW^0.75 day); with that of non-lactating cows in the summer decreased to 481±53 kJ per (kg BW ^0.75 day). The EE and O₂ Pulse increased with increasing MEI and ME. The diurnal and annual EE patterns were affected mainly by the herbage quality and intake. A high stocking rate resulted in a decrease in the cows' body condition score (BCS), associated with a reduction in the EE of the cows in confinement. The effects of animal and field conditions on HR, EE, retained energy (RE) and skin temperatures (T_sk) were studied for the grazing periods, and not those from the confinement. Skin temperature was negatively associated with BCS. The HR, EE and retained energy (RE) were highly correlated with MEI (P<0.001). These dependencies can be used to predict MEI and RE from HR measurements. The high dependencies of the energy balance variables on the herbage ME and biomass and on the cows' reproductive state offers a tool for predicting the cows' RE from these simple measurements.