Aharoni, Y., Department of Cattle and Genetic Sciences, Agricultural Research Organization, Newe Yaar Research Centre, PO Box 1021, Ramat Yishay 30095, Israel
Brosh, A., Department of Cattle and Genetic Sciences, Agricultural Research Organization, Newe Yaar Research Centre, PO Box 1021, Ramat Yishay 30095, Israel
Kafchuk, E., Kibbutz Beit Zera, Emek HaYarden 15135, Israel

The objectives were to demonstrate the potential of heat production measurements to characterize the gross and net efficiencies of dairy cows under commercial conditions and to compare the efficiencies of purebred Holstein and Montbeliarde × Holstein F1 dairy cows. The heat productions of seven Holstein (H) and seven Montbeliarde × Holstein (MH) cows were measured over two 10-day periods separated by a 75-day interval, during the summer of 2004, in a commercial high-yielding dairy herd in Israel. Energy expenditure was measured by monitoring heart rates and oxygen consumption per heart beat. Milk yield and composition were recorded for these cows and their investment of energy in the milk was calculated from the milk yield and composition. Live weight and body condition score were also recorded in parallel with these measurements. Metabolizable energy (ME) intake was estimated as the sum of heat production, energy in milk and body energy balance. The MH cows were heavier by 90 kg, had higher body condition scores by 0.9 units and secreted proportionately 0.19 and 0.38 less energy in their milk than H cows in the first and second periods, respectively. The gross energy efficiencies, expressed as the percentage of milk production plus body retention in ME intake were 48.3 and 43.4% in the first period and 45.6 and 32.8% in the second period, for H and MH cows, respectively. The milk production of MH cows in this study was lower than the potential of this cross, however, MH cows that expressed this potential would still be expected to require proportionately 0.10 greater intake of ME than H cows, per unit of energy in milk. © 2006 British Society of Animal Science.