Methane emissions and feeding behaviour of feedlot cattle supplemented with nitrate or urea

Nitrate may serve as a non-protein nitrogen (NPN) source in ruminant diets while also reducing enteric methane emissions. A study was undertaken to quantify methane emissions of cattle when nitrate replaced urea in a high concentrate diet. Twenty Angus steers were allocated to two treatment groups and acclimated to one of two iso-energetic and iso-nitrogenous finisher rations (containing NPN as urea or as calcium nitrate), with all individual feeding events recorded. A single methane measurement device (C-lock Inc., Rapid City, SD, USA) was exchanged weekly between treatments (2 1-week periods per treatment) to provide estimations of daily methane production (DMP; gCH(4)/day). A 17% reduction in estimated DMP (P = 0.071) resulted from nitrate feeding, attributed to both a tendency for reduced dry matter intake (DMI; P = 0.088) and H-2 capture by the consumed nitrate. NO3-fed cattle consumed a larger number of meals (14.69 vs 7.39 meals/day; P < 0.05) of smaller size (0.770 vs 1.820 kg/meal) each day, so the average interval between a feeding event and methane measurement was less in NO3-fed cattle (3.44 vs 5.15 h; P < 0.05). This difference could potentially have skewed the estimated DMP and contributed to the tendency (P = 0.06) for NO3-fed cattle to have a higher methane yield (gCH(4)/kg DMI) than urea-fed cattle. This study found short-term methane emission measurements made over 2 weeks (per treatment group) were adequate to show dietary nitrate tended to reduce emission and change the feeding pattern of feedlot cattle. Changes in feeding frequency may have confounded the ability of short-term methane measurements to provide data suitable for accurately estimating methane per unit feed intake.