This study determined in vitro rumen CH 4 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 1600 kg dry matter (DM)/ha) and two sward allowance (SA; high 20 and low 15 kg 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 CH 4 output, and total gas and volatile fatty acid production following 24 h 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 CH 4 output, but CH 4 output/g DM incubated or digested was affected by sampling period. Sward allowance did not alter methanogenesis and, although HM affected CH 4 output in vitro, the biological scale of this effect was small. Thus, grass management strategy had little impact on in vitro rumen CH 4 output when herbage was consistently of high nutritional quality.
