Methane emissions of beef cows grazing early, late, and dormant season tallgrass prairie

Authors: Todd, Richard; Moffet, Corey; Neel, James; Turner, Kenneth; Steiner, Jean; COLE, ANDY - Retired ARS Employee

Submitted to: American Geophysical Union
Publication Type: Abstract Only
Publication Acceptance Date: 10/1/2018
Publication Date: N/A
Citation: N/A

Interpretive Summary: Methane emissions from grazing cattle quantified. Methane is a greenhouse gas that is naturally produced during the digestive process of ruminants. Most of this enteric methane is emitted by cattle grazing on pasture. ARS researchers from Bushland, Texas and El Reno, Oklahoma measured the methane emitted by beef cows while grazing using several different methods. The amount of methane produced by cows increased as the forage they grazed became less digestible. When forage was green and fresh early in the grazing season, about three-fourths of a pound of methane was produced daily by each cow. When forage was old, dry and less digestible, each cow produced about a pound of methane per day. This methane production was greater than that from other studies on beef steers or heifers because beef cows were either pregnant or producing milk. The cows ate more because of this added demanded and produced more methane. Knowing how much methane grazing cattle produce will aid in assessing models and life cycle analyses.

Technical Abstract: Methane (CH4) is a greenhouse gas that is naturally produced during the digestive process of ruminants. Most of this enteric CH4 is emitted by cattle grazing on pasture. Grazing cattle are mobile point sources of CH4 and present challenges to quantify emissions. Our objective was to use multiple methods to quantify herd-scale and individual enteric CH4 emissions of beef cows during a grazing season with three distinct levels of forage quality. Methods used to measure enteric CH4 emitted from a herd of cows grazing on tallgrass prairie included multiple open path lasers coupled with forward and backward dispersion models, eddy covariance ratio method using carbon dioxide (CO2) as a tracer gas, and voluntary single animal open head chambers. We also evaluated two empirical models of enteric methane emission. Research was conducted on a tallgrass prairie in central Oklahoma during early season (summer), late season (autumn) and dormant season (winter) grazing periods, with forage quality highest in summer and lowest in winter. Individual cattle locations were tracked using global positioning system receivers and the locations used in dispersion modeling. Methane emission rate (MER) tended to increase through the grazing season as forage quality decreased; however the uncertainty of measurements, estimated at 15-25%, masked significant differences. Enteric MER ranged from 320 to 350 g d-1 cow-1 during the early season to 400 to 500 g d-1 cow-1 during the dormant season. Normalized by cow body weight (BW), MER ranged from 0.55 to 0.64 g kg-1 BW during the early season to 0.58 to 0.72 g kg-1 BW during the dormant season. Enteric MER in the study tended to be greater than values found in the literature. We partly attributed these higher MER to the lactating or gestating cows in our study, compared with beef steers or heifers in previous studies. The empirical models showed promise, but whenever possible, measured, rather than calculated, inputs are needed to provide best model estimates.