Effects of grass silage quality and level of feed intake on enteric methane production in lactating dairy cows

The objective of this study was to determine the effect of level of feed intake and quality of ryegrass silage as well as their interaction on enteric methane (CH4) emission from dairy cows. In a randomized block design, 56 lactating dairy cows received a diet of grass silage, corn silage, and a compound feed meal (70: 10: 20 on DM basis). Treatments consisted of 4 grass silage qualities prepared from grass harvested from leafy through late heading stage, and offered to dairy cows at 96 +/- 2.4 (mean +/- SEM) days in milk (namely, high intake) and 217 +/- 2.4 d in milk (namely, low intake). Grass silage CP content varied between 124 and 286 g/kg of DM, and NDF content between 365 and 546 g/kg of DM. After 12 d of adaptation, enteric CH4 production of cows was measured in open-circuit climate-controlled respiration chambers for 5 d. No interaction between DMI and grass quality on CH4 emission, or on milk production, diet digestibility, and energy, and N retention was found (P >= 0.17). Cows had a greater DMI (16.6 vs. 15.5 kg/d; SEM 0.46) and greater fat-and protein-corrected milk (FPCM) yield (29.9 vs. 25.4 kg/d; SEM 1.24) at high than low intake (both P <= 0.001). Apparent total-tract nutrient digestibility was not affected (P >= 0.08) by DMI level. Total enteric CH4 production (346 +/- 10.9 g/d) was not affected (P = 0.15) by DMI level. A small, significant (P = 0.025) decrease at high compared with low intake occurred for CH4 yield (21.8 +/- 0.59 g/kg of DMI; -4%). Methane emission intensity (12.8 +/- 0.56 g/kg of FPCM; -12%) was considerably smaller (P <= 0.001) at high intake as a result of greater milk yields realized in early lactation. As grass quality decreased from leafy through late heading stage, FPCM yield and apparent total-tract OM digestibility declined (-12%; P <= 0.015), whereas total CH4 production (+13%), CH4 yield (+21%), and CH4 emission intensity (+28%) increased (P <= 0.001). Our results suggest that improving grass silage quality by cutting grass at an earlier stage considerably reduces enteric CH4 emissions from dairy cows, independent of DMI. In contrast, losses of N in manure increased for the earlier cut grass silage treatments. The small increase in DMI at high intake was associated with a small to moderate reduction in CH4 emission per unit of DMI and GE intake. This study confirmed that enteric CH4 emissions from dairy cows at distinct levels of feed intake depend on the nutritive value and chemical composition of the grass silage.