Effect of number of measurement days on variance in methane and carbon dioxide emissions measured using GreenFeed units in grazing dairy cows and growing heifers

Context The minimum number of days needed to measure gas emissions from cattle by using spot sampling methods is the result of the visit frequency, within animal variation and among animal variations. Aims To estimate (a) the effect of the length of the measurement period on the variation in methane (CH4) and carbon dioxide (CO2) emissions and (b) the number of animals required to detect a difference of 10% between two treatment means for CH4 and CO2. Methods Gas emissions from 72 dairy cows, supplemented with different concentrate diets, and 72 heifers, weaned at different ages, in two separate experiments, were measured for 3–5 weeks using GreenFeed units. In all four experiments, the animals grazed ryegrass-based pasture. The cows received various concentrate treatments twice daily during milking. The gas emissions in heifers were measured at 280 and 370 days of age. Data from 76 cows and 77 heifers were used in the data analysis. The coefficient of variation (CV) and number of animals required to detect a difference of 10% between the two means were modelled for periods of 3–36 days at 3-day steps. Key results The CV of CH4 emissions became stable between Days 12 and 18 of measurements in the cows and heifers, respectively (17–37 visits for cows and 43–73 visits for heifers) and then 13–19 cows and 9–11 heifers were required per treatment to detect differences of 10% between means. The CV of CO2 emissions became stable few days earlier than did the CH4 emissions and the variation was smaller. Conclusions A minimum of 12 and 18 measurement days are recommended to estimate CH4 emission in grazing lactating cows and heifers respectively, and 9–19 animals per treatment were required to detect differences of 10% between means for the conditions of the current studies. Implications The current analysis has provided information about among-animal variation of gas emissions when performing GreenFeed measurements with grazing cattle, within the experimental conditions of the data sets used for the current study, which can be used to design future cattle studies.