The use of epidermal hairs of the legume Siratro recovered in the faeces to estimate the proportion of Siratro in the diet of ruminants

Abstract

The epidermal hairs of the tropical pasture legume Macroptilium atropurpureum cv. Siratro number 3-5x 106/g DM; the hairs pass through the digestive tract of sheep and cattle undigested. A method for counting the hairs in Siratro samples and in faeces from animals which had consumed diets containing Siratro was developed. This consisted of digesting 0.5 g of faeces in sodium hypochlorite solution and counting the hairs present in subsamples of the digest under a microscope. Compared with stems, leaves had more than twice as many hairs per unit of dry weight. Hair density varied with age of regrowth, but this was probably a function of growing conditions rather than age per se. At each regrowth age, hair density on leaves was strongly related to density on stems. In pen feeding studies with sheep, the percentage Siratro in the diet was linearly related to the epidermal hair counts in the faeces. The technique was capable of detecting as little as 1% Siratro in the diet. Mean recovery of ingested hairs from the faeces of sheep fed different amounts of Siratro was 100.2%. In a field study, the mean percentage Siratro in extrusa samples collected from oesophageally fistulated cattle at 3 times of the year was linearly related (r=0.989, P < 0.01, r.s.d.=5.3) to the mean hair counts in the faeces of cattle continuously stocked at 2 rates on the same Siratro-setaria (Setaria sphacelata) pastures. In order to accurately estimate the Siratro component in the diet of grazing animals it is essential that the hair density of the Siratro eaten and the digestibility of the diet are measured in addition to the hair density of the faeces. Hair numbers in the faeces may be adequate to determine the pattern of selection for Siratro throughout the year. The use of indigestible epidermal hairs as inert markers in grazing studies is discussed.