Abstract

The influence of dietary protein content, particularly the supply of metabolisable protein, on immunocompetence and on resilience to pathogenic effects during continuous exposure of young sheep to infective larvae is reviewed for the most important abomasal and small intestinal nematodes infecting young sheep in Australian grazing systems. In lambs infected with Haemonchus contortus supplementary protein improves both development of immunity and resilience in breeds of sheep that are susceptible to haemonchosis, but in relatively resistant breeds dietary supplementation appears unnecessary. Recent studies indicate that the increased protein requirements of Merino weaners parasitised by H. contortus would be met by diets containing about 190 g crude protein/kg dry matter, depending on the rumen degradability of the dietary protein source and level of feed intake. Some evidence exists for an enhanced immune response to Teladorsagia circumcincta in lambs receiving an increased supply of protein at the intestines.

Response to protein supplementation of lambs infected with Trichostrongylus colubriformis depends on the metabolisable protein content of the basal diet and on the period of exposure to incoming larvae. Where the basal diet only meets requirements for maintenance or low growth rates, increased supply of rumen undegradable protein enhances immune expression in terms of reducing faecal egg count and expelling adult worms, but does not appear to limit the initial establishment of incoming larvae. Effects on growth rate and wool production are most pronounced during the period of worm expulsion, indicating that this phase of the immune response competes with production when nutrient resources, and particularly protein, are limited.

Field trials in the Armidale region have shown that supplementation with protein meals of Merino weaners, exposed to natural infection from pasture, can reduce faecal egg count and improve resilience in terms of both growth rate and wool production. These effects may persist for some time after supplementation ceases. Further work is needed to determine the generality of these responses in other sheep-producing regions and genotypes and to establish cost-effective supplementation strategies.