Effects of feeding time on nitrogen capture by lactating dairy cows grazing rye pasture

The objective of this experiment was to determine whether varying times at which a partial mixed ration was fed, either before or after grazing, affected N utilization from rye pasture and thus affected milk yield and components. Sixteen Holstein cows were fed a partial mixed ration (PMR) either at 0700, 0830, or 1100 h. Cows were milked at 0900 h and turned out to graze at 0930 h. Treatments represented feeding times 2.5 h and 1 h before grazing and immediately after grazing. The study was conducted as a 3 x 3 Latin square with three 17-d periods. There were no significant differences among treatments for pasture intake or yield of milk or milk components. Milk yield, fat %, and protein % were 29.4, 29.6, and 29.3 kg, 3.5, 3.5, and 3.4%, and 3.4, 3.5, and 3.4% for treatments, respectively. The milk urea levels were 15.6, 15.1, and 15.5 mg/dl, and were not different among treatments. Blood samples were collected on the last day of each period at 0645, 0845, 1045, 1200, and 1400 h. Blood urea nitrogen (BUN) was measured as an indicator of ruminal N capture. Concentrations were not significantly different among diets before grazing; however, they were significantly different among all treatments approximately 1 h after cows were removed from pasture. Cows fed at 0700 h, 2 h before grazing, maintained lower BUN levels across the 7 h during which the blood samples were collected. Cows that ate the PMR immediately after grazing maintained the highest BUN. Feeding a PMR to cows that graze at different times before and after grazing affected the capture of ruminal N, as indicated by differences in the levels of BUN, but there was no effect on yield of milk or milk components.