189 - Effects of Hay Maturity on the Enteric Methane Emission, Intake, and Energy Metabolism by Beef Heifers

Abstract: Heifers (n=12; initial BW=296 ± 30 kg) were used to determine the effect of wheat (Triticum aestinum L.) hay maturity (HM) on energetics and gas emissions. Hay was harvested at 3 maturities and was fed for three 7-d periods. Heifers were randomly assigned to 1 of 3 HM levels including immature (cut at stem elongation, Zadok score [Tottman, 1987] = 3.1, 37% NDF), intermediate (cut at boot stage, Zadok = 3.9-4.1, 55% NDF), and mature (hay cut at milk stage Zadok = 7.9, 63% NDF). Hay was fed for ad libitum intake plus a daily supplement of 0.90 kg of pellets (12% CP) were offered via an automated head-chamber system that measured heifer CO2, CH4, and O2 fluxes. The experiment was designed as a split-plot with HM as the main plot, period the subplot, and beginning body weight as a covariate; main effects were compared using linear and quadratic contrasts. DMI was different for each level of HM with the most DMI for immature and the least for mature, but across all HM levels, DMI did not differ among periods (P ≥ 0.13). Neither HM nor period had an effect (P ≥ 0.16) on total enteric CH4 emissions. However, CO2 emission and O2 consumption had a positive quadratic response to both HM (P ≤ 0.02) and period (P ≤ 0.03). Heat production (HP) had a positive linear effect (P < 0.01) to HM and negative quadratic response to period (P < 0.01). Hay maturity and period had a significant negative linear effect (HM, P < 0.01) and quadratic (period, P < 0.01) effect on metabolizable energy (ME) intake (ME = HP + retained energy). These results indicate that as HM decreased it has a negative effect on DMI, CO2 emissions, O2 consumption, HP, and MEI, but did not have an effect (P ≥ 0.16) on total enteric CH4 emission.