Utilization of energy and nitrogen by yearling Holstein cattle fed direct-cut alfalfa or orchardgrass ensiled with formic acid plus formaldehyde.

First-growth orchardgrass and alfalfa were harvested at two stages of maturity, treated with formic acid plus formaldehyde, and ensiled as direct-cut silage during 1978 and 1979. The 1978 silages were fed to eight yearling Holstein heifers (average BW 273 kg), and the 1979 silages were fed to eight yearling Holstein steers (average BW 264 kg) in replicated 4 x 4 Latin square experiments to measure total energy and N balance using the Beltsville open-circuit respiration calorimeters. Silage was offered daily at 70 g of DM/kg.75 BW, a rate that was essentially ad libitum for late-maturity orchardgrass, but restricted for the other three silages within each experiment. Cattle fed alfalfa used ME for growth with greater efficiency (55%) than did cattle fed orchardgrass (40%). Cattle fed orchardgrass achieved the same tissue N retention at a lower total N intake than did cattle fed alfalfa. Differences in tissue N retention were accounted for by differences in N intake insoluble in autoclaved ruminal fluid, but soluble in acid detergent, a fraction termed available N. At equal intake of ME and available N, cattle fed alfalfa gained more tissue energy than those fed orchardgrass and gained tissue protein similarly to cattle fed orchardgrass. Fractions composing digestible OM were different between forage types but similar within forage type between maturities at harvest. More efficient use of ME for growth by animals fed alfalfa compared with orchardgrass may be related to differences in digestible OM composition, load of digestive tract content, and composition of absorbed nutrients.