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Repeatability and ranking of long-term enteric methane emissions measurement on dairy cows across diets and time using GreenFeed system in farm-conditions
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International audience. The aims of this work were to study on dairy farm conditions: i) the repeatability of long-term enteric CH4emissions measurement from lactating dairy cows using GreenFeed (GF); ii) the ranking of dairy cows accordingto their CH4 emissions across diets. Forty-five Holstein lactating dairy cows were randomly assigned to 3equivalent groups at the beginning of their lactation. The experiment was composed of 3 successive periods: i)pre-experimental period (weeks 1 to 5) in which all cows received a common diet; ii) a dietary treatmenttransition period (weeks 6 to 10); and iii) an experimental period (weeks 11 to 26) in which each group was fed adifferent diet. Experimental diets were formulated to generate more or less CH4 production: i) a diet based onryegrass silage and concentrates, low in starch and lipid, designed to induce high CH4 emissions (CH4+); ii) adiet based on maize silage and concentrates, rich in starch, designed to induce intermediate CH4 emissions(CH4int); iii) a diet based on maize silage and concentrates, rich in starch and lipid, designed to induce low CH4emissions (CH4-). Gas emissions were individually measured using GF systems. Repeatability of gas emissions,dry matter intake (DMI) and dairy performances measurements was calculated from data averaged over 1, 2, 4,and 8 weeks for each animal. Hierarchical cluster analysis was performed to rank individual animals according totheir CH4 emissions. No significant differences were observed for daily CH4 emissions (g/day) among diets,because of lower DMI of CH4+ cows. When CH4 emissions were referred to units of DMI or milk, the differencesamong diets emerged as significant and persistent over the observed period of lactation. Repeatability values ofgas emissions measurements were higher than 0.7 averaged over 8 weeks of measurement, but still higher than0.6 for CH4 g/day, CO2 g/day, CH4 g/kg milk, and CH4/CO2 even averaging only 2 weeks of measurement. Therepeatability of CH4 emissions measurement was systematically lower than those of DMI or dairy performanceparameters, like milk and FPCM yield, irrespective of the averaged measurement period. The dairy cow rankingwas not stable over time between all individuals or within any of the diets. In our experimental conditions, theGF performance in the long term can be considered reliable in differentiating dairy herds by their CH4 emissionsaccording to diets with different methanogenic potential, but did not allow the ranking of individual dairy cowswithin a same diet. Our data highlight the importance of phenotyping animals across environment in which theywill be expected to perform.
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