The aim of the study was to estimate the activity energy requirement of grazing dairy cows through the difference between the metabolizable energy intake (MEI) estimated measuring heat production (HP) + energy retained (ER) and the predicted MEI estimated by the NRC (2001) model. Twenty mid-lactation (101 ± 11 d in milk) multiparous Holstein cows (fall calving; 579 ± 53 kgBW and 2.6 ± 0.BCS) were assigned to 2 feeding strategies: cows grazing a Medicago sativa and Dactylis glomerata mix (21 kgDM/d/c herbage allowance above 5cm) in 2 grazing sessions between milkings (18 h/d) and supplemented with 7.5 kgDM/d/c of concentrate (18.5% CP; 13 MJ of ME/kgDM) (PMAX; n = 10) and cows grazing a herbage allowance of 9.7 kgDM/d/c of the same pasture after the afternoon milking (12 h/d) and supplemented with 15 kgDM/d/c of a mixed ration (50:50 forage to concentrate; 18% CP; 10.6 MJ of ME/kgDM) (P30; n = 10). The HP was measured using the O₂pulse technique. Predicted HP was calculated with the NRC (2001) model using BW^0.75, milk yield and composition (retained energy (RE) in milk), changes in BW and BCS (RE body tissue), and efficiencies (k) for maintenance, milk production, and loss/gain of body reserves. Data were analyzed with a mixed model. Milk production (33 ± 3 kg/d), RE in milk (924 ± 100 kJ/BW^0.75/d) and MEI (2025 ± 49kJ/BW^0.75) did not differ between feeding strategies but measured HP was greater (P < 0.01) for PMAX than P30 cows (1212 vs 1091 ± 36 kJ/BW^0.75/d). Residual HP was greater (P < 0.01) for PMAX than P30 cows (173 vs 43 ± 32 kJ/BW^0.75/d) and represented 8.4 and 2.1 ± 1.5% MEI, respectively. Residual HP was 2.4 fold greater than the estimated NRC requirement for activity of grazing cows for PMAX while it did not differ for P30 cows. Increased maintenance energy requirement associated with the inclusion of grazed forage in the diet may be related not only to activity but also to energy requirements associated with the gastrointestinal tract and excess nitrogen excretion. The NRC model underestimates the energy requirements of grazing dairy cows when pasture forage represents a large proportion of the diet (60% for PMAX cows).