Technical Note: Effect of bait delivery interval in an automated head-chamber system on respiration gas estimates when cattle are grazing rangeland1

Agricultural methane (CH4) emissions worldwide account for approximately 43% of all anthropogenic CH4 emissions, and the majority of agricultural CH4 emissions are attributed to enteric fermentation within ruminant livestock. Therefore, interest is heightened in quantifying and mitigating this source. The automated head-chamber system (AHCS; GreenFeed, C-Lock Inc., Rapid City, SD) evaluated here can be placed in a pasture with grazing cattle to measure their CH4 and CO2 emissions and O2 consumption. However, improper management of an AHCS might have a significant effect on gas exchange estimates. One factor that may affect the quality of these estimates is the rate that bait is delivered and the length of time an animal has its muzzle in front of the intake manifold for sampling. During both experiments, at each visit to the AHCS an electronic ear tag triggered the delivery of 6-mm alfalfa pellets (bait; 32-g increments) at timed intervals up to 8 times per visit and a maximum of 4 sampling events/d. In Exp. 1, the AHCS was programmed to deliver feed at 18- (n = 2), 21- (n = 4), 24- (n = 4), or 27-s (n = 3) intervals for 73 d; in Exp. 2 the AHCS was programmed to deliver feed at 19- (n = 2), 27- (n = 4), 35- (n = 4), or 43-s (n = 3) intervals for 43 d. The AHCS was programmed to measure CH4, CO2, and O2 (Exp. 2 only) fluxes at each visit during the experiments. Time intervals were analyzed by ANOVA, and least squares means were compared using linear and quadratic contrasts. Carbon dioxide emission estimates were not affected by time interval in either experiment. Methane emission estimates and the ratio of CH4:CO2 linearly decreased (P < 0.01) with increasing time increment in Exp. 1 but was not different in Exp. 2. Time increment did not affect the O2 consumption estimate in Exp. 2. Increasing the time increment increased (P < 0.01) the time cattle spent in the AHCS but did not affect the amount of bait consumed. Cattle did not respond consistently to increasing time increment for bait delivery, and bait delivery interval had minimal effect on gas emission and consumption estimates.