Effect of variable intake of alfalfa and wheat on faecal alkane recoveries and estimates of roughage intake in sheep

Abstract

The alkane technique allows estimates of intake and diet composition. For the latter a correction for incomplete recovery of faecal alkane concentrations has to be made. Feeding level and diet proportions have been discussed as factors possibly influencing recovery. A balance trial was conducted to study these effects on faecal alkane recovery rates and their consequences on estimates of roughage intake in sheep. The diets consisted of force-dried pelleted alfalfa meal (A) and wheat whole meal (W) in three levels each (360, 480, 600 and 270, 360, 450 g/day, respectively); the wheat was partly labelled with beeswax to provide a distinctive alkane pattern. Each diet was fed to four wethers. Despite significant effects (P<0·05) of A and W on organic matter digestibility, no such effects were observed for alkane recovery rates. Estimates of alfalfa intake were better if alkanes with a higher coefficient of variation of recovery were not included in diet composition estimates. Furthermore, in spite of the lack of significance of the dietary factors A, W and A×W, faecal alkane concentration correction based upon diet-specific faecal recoveries gave better estimates of alfalfa intake than based on overall recoveries. The estimate of alfalfa intake differed from known intake by −1·7 to −2·6%, depending on the method of estimation. The relative mean discrepancy was regarded as a tool to assess estimated intakes of individual animals. It ranged from 4·4 to 14·2% for dietary treatments and was 7·6% for all animals. The poor quality of estimates for individual animals may result from the variation of faecal recovery rates and needs further investigation.