Modifications of a gas production technique for assessing in vitro rumen methane production from feedstuffs

The objective was to modify an in vitro rumen gas production technique (GPT) so that CH4 production from feeds with contrasting compositions would better reflect in vivo findings. A phosphate-bicarbonate saline solution was mixed with citric acid and used as the buffer. Varying quantities (i.e., 0.3, 0.5, 0.7 g) of each of three contrasting feeds (i.e., barley grain, grass silage, barley straw) were incubated with 50 ml of varying ratios of rumen fluid to buffer (1:2, 1:4, 1:6). Effects on CH4 output/g feed dry matter (DM) incubated (CH4/DMi) and/g DM disappeared (CH4/DMd) and other associated changes were evaluated after 24 h of incubation. The in sacco degradability of barley straw, grass silage and barley grain DM was 212, 682 and 888 g/kg, respectively. Increasing the quantity of feed incubated in vitro led to a decline in CH4/DMi and CH4/DMd and a decline in pH (P<0.001). Increasing the proportion of rumen fluid in the mixture increased CH4/DMi and CH4/DMd and reduced pH (P<0.01). Results were consistent among feeds tested. However for grass silage, apparent DM disappearance declined (P<0.001) as the quantity of feed incubated increased, an outcome associated with a simultaneous decline in pH. CH4/DMd is a more appropriate unit for expressing in vitro rumen CH4 output than CH4/DMi and incubation of 0.3 g dried milled feed with 50 ml of rumen fluid and buffer mixture at a 1:2 ratio was a satisfactory combination with this buffer system. rticle is part of the special issue entitled: Greenhouse Gases in Animal Agriculture – Finding a Balance between Food and Emissions, Guest Edited by T.A. McAllister, Section Guest Editors; K.A. Beauchemin, X. Hao, S. McGinn and Editor for Animal Feed Science and Technology, P.H. Robinson.