Estimation of roughage intake in sheep using a known daily intake of a labelled supplement

Abstract

AbstractA feeding trial with growing sheep was conducted on mixed roughage-supplement diets, to test whether roughage intake could be estimated from a known intake of a supplement and an alkane-based estimate of the supplement and roughage proportions in the diet (diet composition) without the need for separate dosing with external marker alkanes. Diets consisted of chaffed Trifolium subterraneum hay (SC), or a mix of SC and beeswax-labelled cottonseed meal (CSM) in the proportions of 7: 1, 6: 2, 5: 3 and 4: 4 (air-dry matter basis). Six sheep were given the pure SC diet and three sheep were given each of the mixed diets. Additionally, the CSM was also labelled with octatriacontane (C38) to investigate its use for labelling supplements. Due to unexpectedly low recoveries, C38 was not included in any estimation of diet composition or digestibility.The inclusion of beeswax-labelled CSM in a SC diet resulted in lower faecal alkane recoveries (as proportions of alkane ingested; P ≤ 0·05). Within mixed diets, recoveries decreased significantly with increasing proportion of CSM, but this effect was significant (P < 0·05) only for alkanes consisting of 25, 26, 27 and 29 carbon-atoms. Estimates of diet composition were close to measured values. Daily SC intake and organic matter digestibility (OMD) differed by −3·7 to 7·2% and −2·0 to 5·7% from measured values if a mean faecal recovery, across diets, was applied for each alkane. Accuracy increased greatly if estimates were based upon faecal alkane recoveries for individual diets, instead of mean recoveries across diets (−0·3 to 0·9% and −0·2 to 0·0% for SC intake and OMD, respectively).The results indicate that roughage intake can be estimated from a combination of known supplement intake and an estimate of diet composition, which obviates the need for separate alkane dosing to estimate intake. The method should be applicable in situations in which animals either normally receive supplements (e.g., dairy cows) or could readily be given supplements.