Direct measurements of methane emissions from grazing and feedlot cattle.

Abstract

Methane (CH₄) emissions from animals represent a significant contribution to anthropogenically produced radiatively active trace gases. Global and national CH4 budgets currently use predictive models based on emission data from laboratory experiments to estimate the magnitude of the animal source. This paper presents a method for measuring CH₄ from animals under undisturbed field conditions and examines the performance of common models used to simulate field conditions. A micrometeorological mass difference technique was developed to measure CH₄ production by cattle in pasture and feedlot conditions. Measurements were made continuously under field conditions, semiautomatically for several days, and the technique was virtually nonintrusive. The method permits a relatively large number of cattle to be sampled. Limitations include light winds (less than approximately 2 m/s), rapid wind direction changes, and high-precision CH₄ gas concentration measurement. Methane production showed a marked periodicity, with greater emissions during periods of rumination as opposed to grazing. When the cattle were grazed on pasture, they produced .23 kg CH₄·animal⁻¹·d⁻¹, which corresponded to the conversion of 7.7 to 8.4% of gross energy into CH₄. When the same cattle were fed a highly digestible, high-grain diet, they produced .07 kg CH₄·animal⁻¹·d⁻¹, corresponding to a conversion of only 1.9 to 2.2% of the feed energy to CH₄. These measurements clearly document higher CH₄ production (about four times) for cattle receiving low-quality, high-fiber diets than for cattle fed high-grain diets. The mass difference method provides a useful tool for “undisturbed” measurements on the influence of feedstuffs and nutritional management practices on CH₄ production from animals and for developing improved management practice for enhanced environmental quality.