Nutritional and Economic Aspects of Supplemented, Grazing Dairy Cattle
Joanne Knapp, John Bramley, Don Maynard, Doug Watkin, Ray Palmer and Sid Bosworth
Over the past ten years, we have conducted research in grazing dairy cattle at the UVM Spear St. Farm with both Holsteins and Jerseys, usually in mid to late lactation. The cows have averaged 75-80 lbs.milk/day at the beginning of each grazing season in May and finished at 35-45 lbs./day in mid to late September. Managed Intensive Grazing (Voisin system) has been utilized every year, with 70-75 cows grazing each 1 acre paddock for a 12 hour period. The farm crew mechanically clips the paddocks if necessary to maintain the forage mass at 2400 lbs./acre. We have estimated forage intake of pasture by cows by two separate methods, and dry matter intake has averaged 22 lbs./day for supplemented cows and 35 lbs./day for unsupplemented cows.
Much of the research during the past four seasons has focused on the level and composition of supplemental grain. Supplementation strategies have included no supplementation, grain, and grain + corn silage with NEl contents (Net Energy for lactation) ranging from 1.67-1.80 Mcal/kg or 0.76-0.82 Mcal/lb. and crude protein from 9-16%. Grain supplementation (14.7-18.0 lbs./cow/day) increases milk production by 10 lbs./day when compared to no supplementation, and results in 5 lbs. milk more than grain + corn silage supplementation. Although grain supplementation is more expensive than a mix of grain + corn silage, the higher milk production achieved results in greater income after feed expenses. Varying rumen undegradable protein had little effect on milk production.
Nutrient content of the pasture forage has also been measured. As expected, it follows the patterns of plant maturity observed for the mixture of grasses (brome, orchardgrass, timothy, fescue) and clovers that exist at the farm. In May, the pasture forage is greater than 1.54 Mcal/kg (0.70 Mcal/lb.) NEl and 30% protein. These values dip to a low point of 1.14 Mcal/kg and 16% CP during the second and third week of June, when the orchardgrass is in full bloom. After that, they rebound to 1.36 Mcal/kg and 25% CP and remain in that range for the remainder of the grazing season (through mid to late September) as long as the grasses are maintained in a vegetative state by either grazing or clipping. Grain supplementation increases dry matter and energy intake in the cows, allowing the Holsteins in particular to produce more milk. However, composition of the grain with respect to other nutrients (i.e. protein) has little effect on milk yield or composition. As with any dairy nutrition program, forage is the most important aspect. Thus, future research in grazing at UVM will focus on agronomic aspects, particularly fertilization of established pastures, in producing high quality pasture forage for grazing dairy cattle.