Differences between Merino selection lines in microbial yield from the rumen and utilisation of protein for wool growth

Abstract

With most diets, microbial cells are the major contributor to total non-ammonia nitrogen flowing to the intestines, and the latter is the primary determinant of wool growth rate. Consequently, at constant intake, variation in the yield of microbial cells, more specifically microbial protein, from the rumen may account for a proportion of the between-sheep variation in wool growth rate. Recently, urinary excretion of purine derivatives has been suggested as a non-invasive method for estimating the yield of microbial protein from the rumen. However, some doubts exist as to the validity of this technique. Thus the studies reported in Chapters 8 and 9 examined the metabolism of, allantoin, the major purine derivative in the urine of sheep, in an attempt to determine if the urinary excretion of purine derivatives is a valid method for estimating the yield of microbial protein from the rumen. In the studies reported in Chapters 10 and 11, the urinary excretion of purine derivatives was used to examine whether divergent selection for clean fleece weight had produced animals that differed in the yield of microbial protein from the rumen. In addition, the importance of yield of microbial nitrogen from the rumen and the efficiency of utilisation of absorbed amino acids, in accounting for differences between the selection lines in wool growth rate was calculated. Finally, the suitability of lithium chloride as a marker of supplement intake in grazing sheep was assessed in the studies reported in Chapter 12. The studies reported in Chapter 13 used lithium chloride as a marker of supplement intake to determine whether genetic variation existed between fine-wool Merino bloodlines, grazing at pasture, in the response of wool growth rate, average fibre diameter and live weight gain to increasing amino acid intake.