RESUMEN
The aim of this study was to determine the energy metabolism and partition of lactating Gyr and F1 Holstein x Gyr (F1 HxG) cows in different planes of nutrition. Six F1 HxG and six Gyr cows with 130 days in milking (DIM) fed corn silage and concentrate were evaluated. The experiment consisted of four periods with different levels of feeding: 1st ad libitum dry matter intake (DMI) and the others with 5, 10 and 20% restricted DMI, related to the first one. An apparent digestibility assay was performed before measurements in the respiration chamber. Total feces were collected for three days. The cows were confined for 24h in the chamber in each period to determine methane and heat production (HP). F1 HxG had higher gross energy intake (GEI), metabolisable energy intake (MEI) and digestible energy intake (DEI). GE lost in feces was higher in F1 HxG (23.7% GEI) than in Gyr (20.5%) cows. Energy lost as methane and urine was similar between the groups. The metabolisability (q) was 0.67, and the efficiency of converting ME to NE (k) was 0.56. There was no difference in the energy requirements for maintenance between breeds (426.6 MJ/kg BW0,75 average value). The energy requirements for lactation were higher in F1 HxG animals due to the higher volume of milk produced, since there was no difference in energy requirements for production of one kg of milk.