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

Abstract Optimising nutrition for animals requires accurate estimates of feed intake. In research studies determining feed intake in grazing animals can be problematic with many methods requiring individual housing. The n-alkane technique whereby long chain alkanes are used as internal markers for predicting dry matter intake (DMI), facilitates estimation of feed intake from animals while grazing. The aim of this study was to determine the suitability of the n-alkane technique for estimating grass DMI from sheep grazing perennial ryegrass at three different herbage masses and across three seasons. A total of 242 in vivo DMI measurements were obtained from 50 Texel wethers across spring, summer and autumn from 2012 to 2014 using the total collection method; whereby herbage offered and refused plus total faeces produced was measured for each animal daily. Animals were offered perennial ryegrass ad libitium which was harvested at three different pre-grazing herbage masses (HM; low ( 3000 kg DM/ha), across three separate seasons (spring, summer and autumn). All animals were dosed once daily with a C32 n-alkane bolus for 12 days with a subsample of faeces collected for the final six days of each measurement phase. Lin’s concordance correlations were used to examine the relationship between each n-alkane pair when predicting DMI. The Lin’s concordance correlation coefficients between n-alkane estimated DMI using the ratio of C31 / C32 and C33 / C32 and measured DMI were 0.46 and 0.59, respectively. The r-squared value between measured DMI and n-alkane estimated DMI was 0.76 using the n-alkane pair C31 / C32 and 0.79 for the n-alkane pair C33 / C32. Herbage mass affected DMI with animals offered the low HM treatment having higher intakes (P