Effects of Chemical Composition on Forage Intake of Heifers Fed Hays of Different Species

Alfalfa, smooth bromegrass, and big bluestem hays harvested at two maturities differing by four weeks were fed at mature-to-immature hay ratios of 1:0, 2:1, 1:2, and 0:1 to yearling heifers in an experiment with a three 4 x 4 Latin square design with 14 day periods. Concentrations of in vitro digestible dry matter and crude protein were greater and concentrations of neutral detergent fiber, acid detergent fiber, and indigestible neutral detergent fiber (determined by either a manual method with a 96 hour incubation or an automated method with a 48 hour incubation) were less in alfalfa hay than in the two grass hays and in smooth bromegrass hay than in big bluestem hay. Concentrations of in vitro digestible dry matter and crude protein decreased whereas those of neutral detergent fiber, acid detergent fiber and indigestible neutral detergent fiber increased with increasing forage maturity. Consumptions of dry matter, digestible dry matter, in vitro digestible dry matter, and crude protein were greater for heifers fed alfalfa hay diets than those fed the two grasses. Consumptions of total neutral detergent fiber and indigestible neutral detergent fiber, determined by the automated method with a 48 hour incubation, were greater by heifers fed diets containing big bluestem than those fed alfalfa or smooth bromegrass diets. Consumptions of acid detergent fiber and indigestible neutral detergent fiber, determined by a manual method with a 96 hour incubation, were greater for heifers fed alfalfa or big bluestem hay diets than those of heifers fed smooth bromegrass diets. Consumption of dry matter, in vivo or in vitro digestible dry matter, crude protein, neutral detergent fiber, acid detergent fiber and automated indigestible neutral detergent fiber decreased as the mature-to-immature hay ratio decreased. Diet digestibility was not affected by forage species, but increased as the mature-toimmature hay ratio decreased. Fecal excretion of dry matter and neutral detergent fiber did not differ between forage species or mature-to-immature hay ratios. Forage dry matter intake expressed as a percentage of body weight was significantly related to the concentrations of in vitro digestible dry matter (r2=.14), crude protein (r2=.17), neutral detergent fiber (r2=.20), and manual indigestible neutral detergent fiber (r2=.18) of the hays and the concentration of digestible dry matter of the diets (r2=.43).