Dairy cows in mid lactation were pen-fed ad libitum maize silage, ad libitum maize greenchop, or restricted maize greenchop. A cottonseed meal supplement was also fed at 20% forage maize DM. The maize greenchop was harvested daily over 5 weeks during February and March. Both forms of forage maize were fed to sheep in metabolism cages. Digestibility was always higher with maize silage. Digestibility of maize greenchop improved with time until 34-36% DM, after which it declined, this being associated with changes in structural carbohydrates in the crop. Cows fed maize silage produced more milk and gained less weight than cows fed both greenchop diets. In a concurrent field trial, cows grazed irrigated perennial pastures in late summer and were offered 7-8 kg DM/day of the silage or greenchop. Additional cows were allocated extra pasture with no supplement. Treatment differences in milk production and liveweight change were not significant. There were differences in rumen metabolism between diets in both pen-fed and grazed cows; rumen ammonia levels increased during the day in cows fed maize silage, but they decreased or remained constant when maize greenchop was fed. There was more maize grain in greenchop at physiological maturity than in silage, and this could have contributed to observed differences in performance and rumen metabolism. The low rumen ammonia levels, and the fact that forage maize was higher in energy than the pasture on offer, suggest that milk response would have improved with the inclusion of a rumen-degradable nitrogen source such as urea with the forage maize supplements.
Economics of Dairy Cow Feed Risk Management Strategies and Policy Analysis