How does herbage mass effect voluntary dry matter intake and in vivo organic matter digestibility in sheep and the in vitro gas production of perennial ryegrass?

•Offering sheep perennial ryegrass (PRG) swards in excess of 3000 kg DM/ha will result in a decrease in voluntary dry matter intake (VDMI).•High herbage mass (HM) PRG contains higher indigestible fibre in the form of NDF (iNDFIV) which reduces VDMI and digestibility•Animals offered Low HM PRG(1000-1500 kg DM/ha) have higher rates of digestion for potentially digestible NDF (pdNDF)•Time of year (grass growth stage) significantly affects the ruminant animals DMI and digestibility of PRG with values lower in autumn than in spring or summer.•In situ iNDF can be used as a predictor of OMDIV for animals offered PRG swards•There is potential to use in situ iNDF to predict VDMI in small ruminants