Direct measurements of methane emissions from grazing and feedlot cattle

Methane (CH4) emissions from animals represent a significant contribution t o anthropogenically produced radiatively active trace gases. Global and nat ional CH4 budgets currently use predictive models based on emission data fr om laboratory experiments to estimate the magnitude of the animal source. T his paper presents a method for measuring CH4 from animals under undisturbe d field conditions and examines the performance of common models used to si mulate field conditions. A micrometeorological mass difference technique wa s developed to measure CH4 production by cattle in pasture and feedlot cond itions. Measurements were made continuously under field conditions, semiaut omatically for several days, and the technique was virtually nonintrusive. The method permits a relatively large number of cattle to be sampled. Limit ations include light winds (less than approximately 2 m/s), rapid wind dire ction changes, and high-precision CH4 gas concentration measurement. Methan e production showed a marked periodicity, with greater emissions during per iods of rumination as opposed to grazing. When the cattle were grazed on pa sture, 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 we re fed a highly digestible, high-grain diet, they produced .07 kg CH4.anima l(-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 (a bout four times) for cattle receiving low-quality, high-fiber diets than fo r cattle fed high-grain diets. The mass difference method provides a useful tool for "undisturbed" measurements on the influence of feedstuffs and nut ritional management practices on CH4 production from animals and for develo ping improved management practice for enhanced environmental quality.