NIST’s Agricomb Measures Gas Emissions from Cows

The “agricomb,” an optical frequency comb developed by researchers from the National Institute of Standards and Technology (NIST), is able to measure gas emissions from cow eructation and flatulence. The device may as a result aid in the optimization of agricultural processes to reduce the production of heat-trapping greenhouse gases.

NIST researchers, joined by a team from Kansas State University (KSU), set up the portable device adjacent to a beef cattle feedlot. The two-comb system’s laser light component was amplified and filtered to target specific gases, namely methane, ammonia, carbon dioxide, and water vapor, which it was able to measure simultaneously. The gases are identified based on the precise shades and quantities of infrared light absorbed by the atmosphere when the comb light is sent back and forth across open-air paths.

The experiment measured gases along two 100-m paths both upwind and downwind from pens containing about 300 cows. Methane and ammonia were the primary focal points of the work. Livestock emissions are the largest U.S. source of human-induced methane, a major greenhouse gas. Ammonia, also emitted from livestock, is a major atmospheric pollutant.

Diagram showing the approximate setup of the experiment. Courtesy of N. Hanacek, NIST.

Measurements looked at emissions from both the cattle’s digestive processes and physical waste on the ground. The agricomb measured both methane and ammonia concentrations at parts-per-million levels with a precision of 25 parts per billion. The results for methane proved comparable to those of a commercial sensor that sampled the air at multiple inlets along the edges of the feedlot. The researchers found the agricomb to be particularly useful for ammonia, as the gas is sticky and difficult to measure with systems that draw air into them. Additionally, the ability to measure many gases simultaneously is a difficult task to pull off with conventional systems.

Though the commercial sensors were able to measure precise background levels more quickly, the agricomb more precisely captured downwind plumes and could then better characterize the gas sources, the paper states. The increased accuracy may prove valuable in planned future measurements of methane from sparsely distributed cows in a pasture, a far more challenging task.

The agreement between the new and old techniques suggests the agricomb as a valuable tool for the accurate quantification of gases in an agricultural context, the paper notes. Advantages of the device include sensitivity to a broad range of infrared light, high precision, calibration-free detection of multiple gases simultaneously, and flexibility of the measurement setup.

“For the future our plan is to work with KSU to do a pasture measurement, where the cattle eat native grasses,” NIST physicist Brian Washburn said. “The different feed, plus microbial activity in grassland soils that consumes methane, may mean less atmospheric methane production in the pasture than in the feedlot.”

Variations in management practices and cattle characteristics make the estimation of emissions challenging. Feed, which is a major factor in emissions, is unaccounted for in national inventories, which creates large uncertainties in greenhouse gas emission models, according to the paper. The cattle at the Kansas feedlot ate a mix of hay and corn silage.

“The cattle spend about 75% of their life in the pasture, so this measurement would be more representative of the net methane production,” Washburn said. “This would also be a harder measurement, since it would take place over a larger area, about 500 meters by 500 meters, with fewer animals, about 40 head.”

The researchers suggest that the agricomb could prove useful in or even support precision agriculture, by measuring many gases simultaneously over large spatial scales, making it possible to design cleaner and more productive farms.

The research was published in Science Advances (www.doi.org/10.1126/sciadv.abe9765).