We compared the intakes of nutrients by high producing Holstein cows consuming pasture or a full nutrient positive control ration (total mixed ration; TMR) and identified nutrients that limited the milk production of cows consuming the high quality pasture. Cows (n = 8) were adapted to an all pasture diet by incrementally reducing the amount of TMR fed over a 4-wk period. A control group of cows (n = 8) remained in confinement and was fed a TMR. The performance of grazing cows differed significantly from that of cows fed the TMR in dry matter (DM) intake (19.0 vs. 23.4 kg/d of DM), milk production (29.6 vs. 44.1 kg/d), milk protein content (2.61% vs. 2.80%), live weight (562 vs. 597 kg), and body condition score (2.0 vs. 2.5). The high quality of the pasture permitted cows to consume the same daily intakes of neutral detergent fiber and crude protein (kilograms per day) as cows fed the TMR, but the pasture provided 19% less DM, organic matter, and net energy for lactation. Predictions using National Research Council estimates and the Cornell Net Carbohydrate and Protein System model indicated that the supply of metabolizable energy was first-limiting for the milk production of cows consuming high quality pasture rather than the supply of metabolizable protein or amino acids. Although a daily intake of 19 kg of DM was achieved on spring pasture, the significant mobilization of energy reserves indicated that supplemental energy is required to achieve milk production greater than 30 kg/d from high producing Holstein cows on intensive grazing systems.
