NET ENERGY VALUE OF TIMOTHY AND BROMEGRASS SILAGES FOR BEEF-CATTLE

Metabolizable energy (ME), net energy for maintenance and net energy f or growth of grass silages were evaluated by the comparative slaughter technique using a 2 x 2 x 3 factorial design. Sixty medium frame beef steers (259 +/- 29 kg BW) were divided in groups of five and fed duri ng 3 months either Timothy (T) or Bromegrass (B) harvested at stem elo ngation (S) of the first cut or at boot stage of the aftermath (A). Fo rages were fed at one of three levels of intake: ad libitum (FF), 80% of FF, or 65% of FF. Silages averaged 26.9% DM, 16.0% CP and 37.7% ADF . Regression of logarithm of heat production (HE) against ME intake we re similar for all silages (log HE = 0.00046ME + 2.4923; r(2) = 0.89) . From this equation, fasting HE of 311 kJ kg(-0.75) d(-1), ME for mai ntenance of 559 kJ kg(-0.75) d(-1) and efficiency of utilization of en ergy for maintenance of 56% were determined. Regression of ME intake a gainst retained energy (RE) were similar for all silages. Efficiency o f ME utilization for growth was 33% using the regression of ME over RE with a ME requirement for maintenance fixed at 559 kJ kg(-0.75) d(-1) . Net energy for maintenance and growth were similar for all silages, averaging 6.17 and 3.70 MJ kg(-1), respectively. The use of a predicti on equation based on ADF of forages underestimated ME values of silage s by approximately 25%. Moreover, NRC (1984) equations that estimate N E from experimentally estimated ME values tended to overestimate the n et energy of our grass silages.