PERFORMANCE AND NUTRIENT INTAKE OF HIGH PRODUCING HOLSTEIN COWS CONSUMING PASTURE OR A TOTAL MIXED RATION

We compared the intakes of nutrients by high producing Holstein cows c onsuming pasture or a full nutrient positive control ration (total mix ed ration; TMR) and identified nutrients that Limited the milk product ion of cows consuming the high quality pasture. Cows (n = 8) were adap ted 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 c onfinement 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), mil k 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 p ermitted cows to consume the same daily intakes of neutral detergent f iber and crude protein (kilograms per day) as cows fed the TMR, but th e pasture provided 19% less DM, organic matter, and net energy for lac tation. Predictions using National Research Council estimates and the Cornell Net Carbohydrate and Protein System model indicated that the s upply of metabolizable energy was first-limiting for the milk producti on of cows consuming high quality pasture rather than the supply of me tabolizable protein or amino acids. Although a daily intake of 19 kg o f DM was achieved on spring pasture, the significant mobilization of e nergy reserves indicated that supplemental energy is required to achie ve milk production greater than 30 kg/d from high producing Holstein c ows on intensive grazing systems.