Mark Powell1, C. Alan Rotz2, Peter A. Vadas1 and Kristan F. Reed1
1 USDA, Agricultural Research Service, US Dairy Forage Research Center, 1925 Linden Drive, Madison, Wisconsin 53711 USA
2 USDA-Agricultural Research Service, Pasture Systems and Watershed Management Research Unit, University Park, Pennsylvania 16802 USA
Many dairy farms in the USA are growing and feeding more corn silage (CS) and less alfalfa silage (AS) to reduce feed costs. More corn grain (CG)-based concentrates are also being promoted to reduce enteric methane, a potent greenhouse gas. Whole farm simulations illustrate that growing more CS and less AS reduces the land requirement for feed production by approximately 27%, maintains milk production, increases animal N use efficiency (from 20 to 25%), and decreases manure N excretion (from 26.5 to 20.8 g N/kg milk). Growing more CS however, requires more fertilizer N (80 kg N/ha) and increases N losses (by 35 kg N/ha). Feeding more CG does not greatly impact milk production or animal N use efficiency, but requires about 40% more CG land area, more fertilizer N (23 kg N/ha), and increases nitrate leaching (by 10 kg N/ha). CS, AS and CG were labeled with stable isotope 15N and fed to mid-lactation dairy cows. Consumed 15N from AS and CS were distributed similarly into milk N and faecal N. Relatively more of the 15N contained in CG was transformed into milk N compared to 15N contained in AS and CS. After land application, more of the manure 15N from AS and CG was taken up by corn silage than manure 15N from CS. Trade-offs in N use and N loss need to be more fully considered when recommending more CS and CG in dairy cow rations.