ruMinanT nuTriTiOn v 0643 Methane emissions from lactating and dry dairy cows fed diets differing in forage source and NDF concentration

Methane emission factors are lacking from different livestock species in various production states fed varying diets. The objectives of the present study were to measure effects of physiological state, silage type, and supplemental NDF on DMI, DM digestibility (DMD), and methane emissions of lactating and dry dairy cows using two 4 × 4 Latin squares (5-wk periods) with four lactating (114 DIM ± SEM 3.30; square 1) or four dry and non-pregnant (square 2) ruminally cannulated Holstein dairy cows. Measurements included DMI and DMD during wk 4, and methane production (respiration chambers) in wk 5. Four isonitrogenous treatment diets were fed as total mixed rations (TMR) with 50% silage (DM basis) offered ad libitum to lactating cows and at 1.2 × maintenance ME to dry cows. Silage was comprised of either 25:75 (MS) or 75:25 (GS) grass silage:maize silage on a DM basis, without or with additional NDF from chopped straw and soy hulls (+ 47 g NDF/kg TMR DM). Data for each square were analyzed using mixed models for fixed effects of silage, NDF, and their interaction, and random effects of cow and period. Data from each square were combined to test the effect of physiological state. Lactating cows fed MS had a greater DMI (P < 0.02; 21.1 kg/d) and methane production (P < 0.10; 484 g/d) and lower methane yield (P < 0.02; 22.8 g/kg DMI) than when fed GS (17.7, 440, and 24.9, respectively), however there was no effect of silage type on DMD. Added NDF increased methane yield for lactating cows fed MS (22.8 vs. 23.7 g/kg DMI), but not GS (silage by NDF interaction, P < 0.02). Except for DMI, which was higher (P < 0.03) for MS compared to GS diets (12.9 vs. 10.6 kg/d), diet did not affect methane production or yield or DMD for dry cows. Compared to dry cows, lactating cows had greater DMI (19.7 vs. 11.0 kg/d; P < 0.02), higher DMD (749 vs. 725 g/kg; P < 0.01), and lower methane yield (24.0 vs. 28.0 g/kg DMI; P < 0.03). The difference in methane yield between lactating and dry cows may be due to differences in DMI and rumen function, including digesta dynamics such as rumen outflow and retention time. Such differences may also explain why silage type affected methane yield in lactating cows, but not dry cows.