Validating the n-alkane technique for determining herbage dry matter intake in sheep offered perennial ryegrass harvested at varying growth stages and seasons

•Estimating dry matter intake (DMI) is crucial in grazing research to the understanding of animal performance and efficiency.•The n-alkane technique can successfully be used to estimate DMI in small ruminants, specifically sheep.•Estimation of DMI using the n-alkane technique is most accurate when animals are grazing swards <1600 kg DM/ha.•High herbage mass (HM) PRG contains higher indigestible fibre in the form of NDF which reduces DMI and digestibility.•Time of year (season) significantly affects DMI and thus the ability to estimate DMI using the n-alkane technique.