Heat production and body composition of primiparous Holstein cows with or without grazing pastures in early lactation

The aim of the study was to compare energy partitioning between heat production (HP) and retained (milk and body reserves) energy as well as energy efficiency of dairy cows assigned to different feeding strategies (with or without pasture grazing) during early lactation. At calving, 18 primiparous cows (528 ± 40 kg body weight (BW); 3.2 ± 0.2 body condition score (BCS); fall calving) were assigned in a randomized block design, during the first 61 days postpartum, to either (G0) mixed ration (PMR) ad libitum (58% forage:42% concentrate) +4.0 kg DM/d of an energy-protein concentrate in the milking parlor or (G1) grazing of alfalfa (6-h grazing in 3 days strips; 20 kg DM/d of pasture allowance) + PMR at 70% of ad libitum intake +4.0 kg DM/d of an energy-protein concentrate in the milking parlor. Diets were composed by 77% PMR and 23% concentrate for G0 cows and 54% PMR, 22% concentrate and 24% pasture for G1 cows. Heat production (HP) was measured at 40 ± 3 days postpartum by the O₂ pulse technique and energy retained (RE) in milk and body tissue was estimate based on NRC equations for the period between 26 and 54 ± 3 days postpartum. In addition, body composition was determined using the urea dilution technique at −7 and 40 ± 3 days postpartum. Absolute body water, fat and protein mass, and gross RE decreased from −7 to +40 days but the decrease in fat mass and gross RE was 10% greater for G1 than G0 cows. In addition, during this period relative lipid mass and gross energy content decreased only in the G1cows. During the second month of lactation (from 26 to 54 days), the G0 cows tended to produce 6% more milk and had 0.3 units more of BCS than the G1 cows. Both RE in milk and in body tissue were greater for G0 than G1 cows (7% and 3-fold greater, respectively). No differences were found in metabolizable energy (ME) intake and HP measured at +40 days between the cow groups. However, residual HP (difference between HP measured and predicted HP calculated from BW⁰.⁷⁵ and total RE on the assumption of constant efficiency coefficients), expressed as percentage of ME intake, tended to be 10% less for G0 than G1 cows. The adjusted gross energy efficiency (total RE divided by ME intake) tended to be greater for G0 than G1 cows. The results indicated that 100% PMR fed cows were more efficient, secreting more energy in the milk and retaining more energy in the body tissue than grazing cows supplemented with PMR. This was probably due to an increase of about 10% in maintenance requirements associated to greater forage content in the diet and/or grazing and walking activities in grazing cows.