Potential impact of nematode parasitism on nutrient partitioning forwool production, growth and reproduction in sheep

Abstract

Most ovine studies indicate that protein supply is a key determinant of the ability to mount a strong immunity-associated response to gastrointestinal parasites. This paper quantifies the demands placed on protein supply by various physiological states, to determine why this is so. Protein requirements in fast growing lambs and ewes in late pregnancy and early lactation are substantially increased, amounting to a 1.43-, 0.69- and 2.50-fold increase in maintenance requirement for dietary protein. Infestation with internal parasites may result in a total net loss of about 12 g protein/day, which consists of a net loss of about 6 g from the gastrointestinal tract and a further loss of about 6 g through increased amino acid oxidation in the body. This loss from the infection accounts for about 0.57-, 0.71-, 0.14- and 0.77-fold the protein requirements respectively for body growth, pregnancy, lactation and wool production, and creates strong competition for protein. Such considerable demands for protein by activation of immunity-associated response diminishes the animal's nutrient status in late pregnancy and particularly in early lactation. The total demand for protein cannot be met by pastures during the reproductive season in Mediterranean environments, and it is this nutrient deficiency, relative to the demand of sheep, which results in weak immune responses over these periods. The immunity-associated response requires more protein rather than energy, particularly sulfur amino acids such as cysteine and probably methionine. Therefore, parasite infection has a more severe effect on wool- than meat-producing sheep. Body condition of sheep could be a predominant factor in determining immune status and current intake and quality of pastures needs to be considered to further boost immunity-associated response. Wool- and meat-producing genotypes, and parasite-resistant sheep in particular, may have different interactions with nutrition in response to parasite infection. These factors need to be integrated with anthelmintic treatment to establish an effective strategy to control parasite infection in Mediterranean environments.