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.
Generated Summary
This research presents a micrometeorological mass difference technique for measuring methane (CH4) emissions from cattle under grazing and feedlot conditions. The study aimed to assess the performance of models used to simulate field conditions by directly measuring CH4 production. The technique, developed to measure CH4 production, was tested and validated, then applied to cattle in pasture and feedlot environments. Measurements were conducted continuously under field conditions, semi-automatically for several days. The study examined the influence of feed quality on CH4 production. The researchers sought to determine CH4 production by cattle in pasture and feedlot conditions to assess the performance of common models used to simulate field conditions. The method allowed for the sampling of a relatively large number of cattle. Limitations included light winds and rapid wind direction changes, and high-precision CH4 gas concentration measurement.
Key Findings & Statistics
- Methane (CH4) emissions from animals are a significant source of anthropogenically produced radiatively active trace gases.
- The study found that CH4 concentrations have increased globally at a rate of about .7%, or 12 ppb/yr, during the decade preceding 1994.
- Measurements during grazing showed cattle produced .23 kg CH4-animal-1.d-1, corresponding 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.
- The study used four 19-mo-old, pregnant Murray Grey × (Charolais × Angus) heifers with a mean weight of 435.5 kg (SD of 21.1 kg).
- The botanical composition of the pasture included Yorkshire fog (Holcus lanatus) and phalaris (Phalaris aquatic) along with dead grass residue of phalaris, soft brome (Bromus mollis L.), and a small amount of subterranean clover (Trifolium subterraneum L.).
- The pasture experiment had a minimum daytime relative humidity between 20 and 50% and maximum solar radiation intensities near 900 W/m².
- Maximum air temperatures were between 18 and 31°C and minimum temperatures were between 5 and 13°C.
- During grazing, the mean production rate was .26 kg·animal−1·d−1, and for the feedlot study it was .066 kg·animal−1·d−1.
- The IPCC (1996) estimates of CH4 emission were .019 kg CH4·animal−1·d−1 when grazing the pasture and .009 kg CH4·animal−1·d−1 for the feedlot experiment.
- The predicted gross energy of CH4 as a fraction of feed intake ranged from 6.5 to 11.9% across the models.
- The field conversion was between 7.7 and 8.4%, which is 15 to 25% greater than predicted by the Blaxter and Clapperton (1965) model.
- The study found that the mass difference method provided on-site, unobtrusive measurements of methane emissions from cattle.
Other Important Findings
- Methane production showed a marked periodicity, with greater emissions during periods of rumination as opposed to grazing.
- Higher CH4 production was documented for cattle receiving low-quality, high-fiber diets compared to those fed high-grain diets.
- The study used a nonintrusive, micrometeorological technique to make direct measurements of CH4 production by cattle under grazing and feedlot conditions and to test for feed quality on CH4 production.
- The study compared CH4 emissions in grazing and feedlot settings, finding that feed quality has a larger influence on CH4 production than previously understood.
- The micrometeorological mass difference approach described has several advantages over other techniques used for measuring CH4 production by animals: it is a field technique; it is virtually nonintrusive; it can be operated continuously and semi-automatically over periods of several days; and it permits a relatively large sample size in terms of animal numbers and number of observation intervals.
- The study found that the developed method was tested and validated by constructing CH4 budgets for the test field when no cattle were present.
- The study’s results showed that the model of Giger-Reverdin et al. (1992) most closely predicted the observed emissions, whereas the IPCC (1996) and NGGIC (1996) methodologies underestimated, but not significantly, due to the large but real variability of the measurements.
- The feedlot models of Branine and Johnson (1990) and Blaxter and Wainman (1964) closely predicted the measured emissions.
Limitations Noted in the Document
- The mass difference method requires specific environmental conditions, including light winds (less than approximately 2 m/s) and minimal wind direction changes, which can limit its applicability.
- The study’s results showed that the method became unreliable when windspeeds were less than 2 m/s or when wind directions changed rapidly.
- The study’s data had a relatively high standard error (SE), which made statistical comparisons challenging with various estimating methodologies.
- The method’s effectiveness is limited by the need for specific environmental conditions, such as wind speed and direction, which restricts its use to certain situations.
- Measurements can be affected by the animals’ proximity to the fence line.
Conclusion
The study effectively employed a micrometeorological mass difference technique to directly measure methane emissions from grazing and feedlot cattle, providing valuable insights into the influence of feed quality on CH4 production. The findings revealed a significant difference in CH4 production between cattle grazing on pasture and those fed high-grain diets, with the latter exhibiting lower emissions. The research underscores the importance of feed management in mitigating methane emissions from livestock. The method itself is characterized by its field-based, non-intrusive nature, and its ability to measure CH4 production continuously over several days, thereby allowing a relatively large sample size. However, the technique’s reliability is contingent on specific environmental conditions, such as wind speed and direction, which could limit its broad applicability. The results of this study indicated that the model of Giger-Reverdin et al. (1992) was most closely aligned with the observed emissions. In contrast, the IPCC (1996) and NGGIC (1996) methodologies tended to underestimate, though not significantly, due to the considerable variability in the measurements. This study validates that feed quality has a larger impact on CH4 production than previously understood. This study’s findings can contribute to the development of improved management practices aimed at enhancing environmental quality and mitigating the effects of greenhouse gases. In conclusion, the mass difference method offers a promising approach for the study of methane emissions from ruminants. It provides the potential to explore the effects of various feedstuffs and nutritional management practices, particularly in grazing settings. Its limitations include the need for specific wind conditions, but the method offers considerable value in understanding and potentially reducing methane emissions.