Principles of nutrient partitioning for wool, growth and reproduction: implications for nematode parasitism

Abstract

The capacity of sheep to withstand and repel intestinal parasites is a neglected component of effective parasite control. The immune response is strongly influenced by the nutritional status of the sheep. However, we are unable take advantage of this to develop effective control programs because we have neither an adequate understanding nor appropriate quantitative data on the impacts of protein and energy on sheep nutrition. This paper reviews some aspects of current knowledge about the impact on immune responsiveness of nutrient flows within the animal as well as hormonal partitioning mechanisms, and assesses research needs in this area. The availability of nutrients to the immune response in the gut is determined by the supply of nutrients to the sheep from both feed intake and body reserves, and the demands of other physiological processes such as growth, wool growth, pregnancy or lactation. Hormones coordinate nutrient flow among these processes. Breakdowns in immunity appear most severe when animals are faced with a demand for growth or lactation, but no single partitioning mechanism can explain all the observations in the field. Therefore, it is unrealistic to seek to establish a hierarchy of partitioning priorities. Protein appears to have a greater impact on immune responsiveness to parasites than energy. However, energy affects the availability of amino acids through a number of mechanisms including protein deposition and mobilisation, so protein supply cannot be considered in isolation. It is appealing to believe that specific limiting nutrients such as sulfur amino acids might explain the relationship between susceptibility to parasites and wool growth, but the experimental evidence for this view is still inconclusive. Rather, it appears that the total flow of nutrients from feed intake and body reserves is more important than specific partitioning mechanisms, or specific limiting nutrients. The potential conflict between role of the gut as a source of mobilisable protein reserves, and the need for protein in the gut to develop local immune responses, need to be explored experimentally. Practical applications of nutritional knowledge are likely to come through improved timing of management procedures rather than better supplements, which are rarely economic. The conclusions outline a number of research questions that must be answered before we can develop programs that integrate immune competence with drenching and other procedures in a holistic way.