Investigation of cattle methane production and emission over a 24‐hour period using measurements of δ¹³C and δD of emitted CH₄ and rumen water

We have conducted a series of experiments to determine the effects different diets, including the addition of unsaturated fatty acids, had on isotopic signatures of CH 4 emissions from dairy cattle. Measurements of emitted δ 13 CH 4 , δ 13 CO 2 , and δ D ‐CH 4 were made on gases collected over a 24‐hour period from animal chambers containing individual Holstein cows. Some measurements of δ D ‐H 2 O from samples collected directly from the rumen were also made. We observed variation in δ 13 CH 4 values with time after feeding and a correlation in δ 13 C between emitted gases and diet. The average δ 13 CH 4 value, which includes all samples of emitted gases, was −70.6±4.9‰ ( n = 57). Measurements of δ D ‐CH 4 over the 24‐hour sampling period had an average value of −357.8±15.0‰ ( n = 56). These δ D measurements are among the lightest reported for CH 4 produced by cattle. Our cattle data indicate that hydrogen incorporated into the CH 4 produced by CO 2 reduction in high H 2 concentration environments is fractionated to a greater degree than that incorporated in systems with relatively low H 2 conditions, such as wetlands. Our results support bacterial studies that have demonstrated large hydrogen fractionation in high H 2 concentration systems during methanogenesis.