Apparent Digestibilities of Diets Varying in Ratios of Forage to Concentrate and Quality of Forage at Two Intakes by Dairy Cows

In four digestion trials we measured effects of feed intake and forage quality on apparent digestibility of dietary components. Trials 1 and 2 utilized four mixed diets containing either 30 or 50% concentrate blended with either early or normal cut alfalfa hay. Trials 3 and 4 utilized two straight hay diets of early and normal cut hay. High and low feed intakes were determined for lactating and dry cows. Cows in trial 1 digested diets to the same extent regardless of intake or ratio of forage to concentrate. Digestibility of dry matter for early cut diets was higher than for normal cut diets (67.4 versus 61.5%). In trial 2, digestibilities of dry matter were higher for low concentrate than for high concentrate diets (60.8 versus 56.9%) and for dry than for lactating cows (60.7 versus 57.6%). Age of cut had no effect in trial 2, which had unexpected slightly higher fiber in early cut diets. In trials 3 and 4, digestibility of dry matter was higher for early than normal cut hay (64.0 versus 61.2%) and for dry than lactating cows (63.4 versus 62.1%). Depression of dry matter digestibility with intake in trials 2, 3, and 4 ranged from .4 to 1.6 percentage units per intake above maintenance and was related to cell wall components. Age of cut was the most important factor affecting digestibility.