Impact of defoliation intensity on energy balance and productive performance of dairy cows

The aim of this study was to evaluate the effect of defoliation intensity in Festuca arundinacea on the partitioning of energy intake and its efficiency in grazing Holstein dairy cows. Two experiments were conducted; in experiment 1 (Exp 1; spring), late-lactation dairy cows were assigned to three post-grazing sward heights: 9 (TC), 12 (TM), and 15 (TL) cm; in Experiment 2 (Exp 2; winter) mid-lactation cows were supplemented (70:30 forage:supplement) and assigned to two post-grazing sward heights: 6 (TC) and 12 (TL) cm. Heat production (HP) was estimated using the heart rate-O₂ pulse technique, and gross energy efficiency was estimated from milk energy production (RE-milk) and metabolizable energy intake (MEI). In Exp 1, TL showed higher RE-milk and greater efficiency than TM and TC, which was associated with higher MEI. While no significant differences in HP were observed between TL and TC, TM had higher HP and higher maintenance energy requirements (MEm), allocating a greater proportion of energy to costs associated with grazing activity. In Exp 2, TL also showed higher MEI, milk production, and RE-milk compared to TC, associated with greater selectivity and intake of higher quality forage. However, no differences were observed between treatments in HP or gross energy efficiency, suggesting that supplementation and earlier lactation attenuated the effect of defoliation intensity on energy partitioning. Additionally, analysis of daily dynamics in Exp 2 showed that energy efficiency decreased as the number of days of pasture occupation increased, due to an increase in HP and the MEm associated with grazing activity. In TC, MEI remained relatively stable, and net energy of lactation decreased as occupation progressed, while in TL, the increase in MEI allowed for sustained milk production. Grazing intensity was a determining factor in the gross efficiency of dairy systems, as it simultaneously determined MEI and MEm.