Presenter: Ronaldo E. Vibart
Advisor(s): Dr. Vivek Fellner
Author(s): Ronaldo E. Vibart, Vivek Fellner, Joseph C. Burns, James T. Green, Jr., and Marcia Gumpertz
Graduate Program: Animal Science, Animal Science, Crop Science, Crop Science, Statistics

Title: Partial Mixed Rations (PMR) as Alternative Feeding Systems for Lactating Dairy Cows in Southeastern U.S.

Abstract: Beginning March 2005, thirty lactating Holstein cows were used in a randomized complete block design to determine the effects of feeding PMR on intake and milk yield over an eight week period. Cows averaged 36.6 1.5 kg milk, 125.7 6.7 days in milk, 1.9 0.2 lactations, 607.3 11.9 kg bodyweight, and 2.88 0.06 body condition score at the initiation of the trial. Cows were assigned to either an all-TMR diet (100T, no access to ryegrass pasture) or one of the following three formulated (actual) PMR: 1) 85% TMR + 15% (10%) pasture, 85T; 2) 70% TMR + 30% (21%) pasture, 70T; and 3) 55% TMR + 45% (34%) pasture, 55T. Cows on pasture grazed as a single group for 7 h/d between the a.m. and p.m. milking. Pasture intakes were measured weekly on one cow selected randomly from each grazing group. Pasture organic matter intake was different among grazing groups and averaged 6.0, 3.7, and 1.9 kg/d for 55T, 70T, and 85T, respectively. Fat corrected milk yield was greatest for 85T (33.4 kg/d) and lowest for 70T (29.6 kg/d). By week eight, concentrations of plasma nonesterified fatty acids (185.3 vs. 92.5 μeq/L) and plasma urea nitrogen (12.4 vs. 6.7 mg/dl) were higher for 100T compared to the three grazing groups, respectively, but plasma glucose concentrations did not differ among treatments. Offering pasture did not replace TMR, but lowering TMR intakes lowered total dry matter intake. Grazing groups exhibited enhanced feed efficiency compared to 100T (1.72 vs. 1.40 kg milk/kg feed, respectively). Our data suggest that cows on a PMR with 10% pasture resulted in the best performance. In PMR feeding systems, pasture can be as high as 34% without affecting milk yield when compared to an all TMR ration.