Impacts of herbage mass and sward allowance of perennial ryegrass sampled throughout the growing season on in vitro rumen methane production

This study determined in vitro rumen CH₄ production of perennial ryegrass grown within a well managed Irish dairy production system. Four strategies, consisting of two pre-grazing herbage mass (HM; high ∼2400 and low ∼1600kg dry matter (DM)/ha) and two sward allowance (SA; high ∼20 and low ∼15kg DM/cow/d) treatments, were compared throughout the grazing season using an in vitro rumen gas production technique. Samples were collected during five 22 d sampling periods (SP 1-5) throughout the growing season and analysed for in vitro rumen CH₄ output, and total gas and volatile fatty acid production following 24h of incubation with rumen fluid and artificial saliva. High HM was associated with lower organic matter digestibility and crude protein concentration compared with low HM, whereas SA had no effect on herbage composition. Methane output as ml/g DM incubated or digested was higher (P<0.05) for the high HM treatment than for the low HM treatment (25.5 versus 24.6 and 32.2 versus 30.5, respectively). Sward allowance had no effect on CH₄ output, but CH₄ output/g DM incubated or digested was affected by sampling period. Sward allowance did not alter methanogenesis and, although HM affected CH₄ output in vitro, the biological scale of this effect was small. Thus, grass management strategy had little impact on in vitro rumen CH₄ output when herbage was consistently of high nutritional quality. This article 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.