Effect of substituting grass-clover silage with maize silage for dairy cows on nutrient digestibility, rumen metabolism, enteric methane emission and total carbon footprint

•Substituting grass-clover silage with maize silage for dairy cows led to.•15% lower methane emission per kg dry matter intake.•Only minor effects on digestibility of nutrients and no influence on rumen pH.•A higher carbon footprint from production of grass-clover silage than maize silage, but lower when change in soil carbon is included.•Slightly lower total carbon footprint when net carbon sequestration was included. The aim of this experiment was to determine the optimal ratio between maize silage and grass-clover silage in lactating dairy cow diets. The evaluation was based on nutrient digestibility, rumen metabolism and total carbon footprint (CF). Carbon footprint included enteric methane emission and CF from feed production, transportation, processing, and C sequestration in the soil, based on a Life Cycle Assessment (LCA) approach. Four diets varying in proportions of grass-clover silage and maize silage (100:0, 67:33, 33:67, and 0:100 on dry matter (DM) basis, respectively) were fed ad libitum to 4 lactating Holstein cows (a fifth cow was involved later in the study) fitted with ruminal, duodenal, and ileal canulae in a 4 × 4 Latin square design. Nutrient digestibility was measured using chromium oxide and titanium oxide as flow markers, and methane and CO2 emissions were measured in open-circuit respiration chambers on 5 consecutive days for each cow in each period. Data were analyzed with treatment and period as fixed effects and cow as random effect. DM intake (DMI) increased linearly (P = 0.01) with increased proportion of maize silage in the feed ration. Under the conditions in the present study, diets had only minor effects on digestibility of nutrients and there was no influence on rumen pH. Total VFA concentration decreased linearly with increased level of maize silage, but there were only minor effects on molar proportions of VFAs in the rumen. However, there was a linear increase in duodenal flow of microbial DM with increasing level of maize silage. The cows produced 30.7, 29.4. 28.4, and 26.2 L of methane/kg of DMI on diets 100:0, 66:33, 33:66, and 0:100, respectively (P = 0.02 for linear effect), which is equivalent to a 15% decrease, when all grass-clover silage was replaced by maize silage. Despite the significant reduction in enteric methane emission with increased proportion of maize silage, only a 4% decrease in total CF per kg DMI was found, when including CF from feed production, transportation, processing, manure, energy use a