Effect of the Dietary Ratio of Hay to Concentrate on Milk Production, Ration Digestibility, and Urinary Energy Losses

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Abstract
Lactation and balance studies were carried out with 12 Holstein cows in a randomized block design. Of the estimated net energy intake 20, 40, or 60% was supplied as hay; the remainder as concentrate. The intake of estimated net energy above maintenance requirements was the same for all cows, and was held constant during the 14-week experimental period. Average daily fat corrected milk production was approximately the same (45.8, 46, 44.3 lbs.) in the 3 groups. As the proportion of concentrates in the ration increased, the efficiency with which digestible energy was utilized for milk production increased in a rectilinear manner. Differences among treatments were significant (P<0.05). The apparent digestion coefficients for dry matter, ether extract, nitrogen-free extract and gross energy also increased significantly (P<0.01); while those for crude fiber and crude protein were not significantly affected by the ratio of hay to concentrate. Urinary energy losses were closely correlated (r = 0.96) with urinary nitrogen losses.