Methane emission, nutrient degradation and nitrogen turnover in dairy cows and their slurry at different milk production scenarios with and without concentrate supplementation

Methane emissions from dairy cows, including storage of their slurry, contribute significantly to the global greenhouse gas budget. Supplementation of diets with concentrate often diminishes enteric methane emissions from cows, but it may simultaneously enhance slurry methanogenesis because this is associated with extra amounts of undigested fibre which may be a substrate for slurry microbes. In the present study, the effects on feed degradation, methane formation, nitrogen turnover and body energy balance in cows and their 14 weeks stored slurry were examined. Cows on average producing 10 kg milk d-1 (group HG-10) were fed a forage-only diet composed of low-quality hay and grass silage (1.5:1, dry matter basis). Two groups of cows yielding 20 kg milk d-1 were either fed the same forage as the HG-10 group, but supplemented with concentrate in a ratio of 1:1 (group HG+20), or a forage-only mixture composed of maize silage and grass silage (1:1, group MG-20). The latter mixture was supplemented with concentrate in a ratio of 1:1 for a group of cows yielding 30 kg milk d-1 (group MG+30). The four treatment groups thus represented three milk production scenarios and included a direct comparison of two major feeding options at a given level of milk yield, which is why a certain confounding of milk yield and diet factors could not be avoided. Concerning substrate degradation, fibre digestibility was higher (P < 0.05) in the HG-10 group compared to the other groups. In the HG+20 group, a lower fibre digestibility was compensated by a higher degradation in the slurry, resulting in a similar overall fibre degradation compared to that of the HG-10 group. Relative to N intake, treatment differences in excreta-N amounts and gaseous N losses from slurry were not significant. The animals' daily enteric methane emission (313-441 g cow-1 d-1) correlated negatively (r = 0.491; P < 0.05) with the corresponding methane emission from the daily produced slurry stored over 14 weeks (25-63 g cow-1 d-1). Accordingly, concentrate-caused reductions in enteric methane emission per kilogram of dry matter intake (-18%, HG+20 versus MG-20) were thus diminished to -12% of total methane via the opposite trend in slurry methanogenesis. Changes in emissions of N and methane as depending on milk production scenario were as expected. The present study shows that more research needs to be done in combining enteric and slurry methane measurements to quantify the true effect of methane mitigation strategies.