In vitro rumen methane output of red clover and perennial ryegrass assayed using the gas production technique (GPT)

The in vitro rumen methane output of perennial ryegrass (receiving 0 or 150 kg of inorganic fertiliser N/ha/growth) and two red clover varieties (Merviot and Ruttinova) at three different harvests (early and late primary growths, and an autumn regrowth) was assayed using the gas production technique (GPT). Herbages were produced within a randomised complete block (n = 4) design experiment conducted over two consecutive years. The forage samples selected from this field plot experiment were arranged in a 4 (herbages) × 3 (harvests) factorial structure of treatments. Dried milled herbage samples were incubated at 39 °C in a buffered medium inoculated with rumen fluid obtained from fistulated steers. Effects on methane output, feed disappearance, volatile fatty acid (VFA) output and other fermentation variables were evaluated 24 h after inoculation. Red clover (mean of the two varieties) had a lower (P<0.001) methane output per g of feed incubated (CH4/DMi) than perennial ryegrass (mean of both treatments) but this effect was reversed (P<0.05) when methane outputs were expressed relative to feed disappeared (CH4/DMd). No differences in methane output were detected between the two red clover varieties (Merviot and Ruttinova) reflecting their similar chemical composition. The application of inorganic N fertiliser to ryegrass reduced (P<0.05) CH4/DMi resulting in similar output to the red clover. Mature herbage from the primary growth, and the autumn regrowth, had lower (P<0.05) CH4/DMi than immature herbage from the primary growth. The lowest (P<0.05) CH4/DMd was associated with the autumn regrowth and was probably due to the presence of non-fermentable soluble compounds in the sward. Overall, a reduction in the in vitro rumen methane output was observed with (1) red clover compared to perennial ryegrass, (2) nitrogen-fertilised compared to non-fertilised perennial ryegrass, and (3) mature primary growth or autumn regrowth compared to immature primary growth. The reduction in methane output was associated with a decline in the extent of fermentation of the herbage as indicated by reduced VFA production. In addition, an increase in the nitrate concentration of fertilised ryegrass could have played an important role in the reduction of methanogenesis by decreasing the availability of hydrogen.