The effects of protein and/ or energy supplementation have been well studied in sheep. However, the few pen studies conducted with goats have failed to identify the role of protein supplementation and combinations with energy supplements against H. contortus infection.
The general objective of this thesis was to investigate the effects of protein supplementation and plane of nutrition on resistance and resilience of Boer goats to infection from Haemonchus contortus. To achieve this the first experiment was designed to determine the appropriate amount of additional dietary protein (by having three groups of mature goats effectively fed three different levels of dietary protein) required by nematode free animals given a single relatively high dose of infective H. contortus larvae, to not become infected or at least develop resistance and/or resilience. It was assumed, based on the scientific literature, that a single relatively high dose of infective larvae would elicit strong pathogenic effects where additional dietary protein would give some assistance to the does with their defense to Haemonchus. As often is the case with research on goats the results were not unequivocally clear.
Based on these results it was clear that a single dose of infective larvae was insufficient to become established and that these animals had already developed some level of immune response and thus it was assumed, again based on the scientific literature, to elicit strong pathogenic effects the animals tested needed to be younger (more naïve to Haemonchus) and with greater i.e. continued exposure to infective larvae as would happen under grazing conditions (trickle infection) such that different quantities of dietary protein would give these animals different responses to infection by Haemonchus. Again the responses by the goats only indicated subtle differences in immune responses against trickle H. contortus L3 infection. These results, again unexpected, indicated that level of intake (or more specifically plane of nutrition, prior to or during exposure) may be at least as important as protein supplementation in reasonably well fed animals; and or that the method of infection (oral drenching with L3 larvae) was possibly an unreliable method of infection requiring a more consistent method of infection. As such the third experiment involved nematode free mature animals (the only animals available at that time) fed on very different planes of nutrition preceding and during the experiment given relatively high trickle doses of H. contortus larvae via intra-ruminal infection.
Overlaid across these experiments were constraints imposed on the amount of infective L3 larvae used to achieve infection; 100 L3 larvae per kg live-weight as a single dose was the maximum initially approved by the University Animal Ethics committee, as they had concerns about animals dying, they also stipulated the use of oral drenching for infective larvae. It was only after these methods failed to elicit a strong response was approval given by the University Animal Ethics Committee to trickle infect and then later use intra-ruminal injections of L3 larvae to infect the goats.
Therefore three experiments were undertaken where resilience was recorded by monitoring live-weight (LW), FAMACHA©, packed cell volumes (PCV), eosinophil percentage, haemoglobin concentration, total serum protein, globulin, and albumin concentration were recorded every 7 - 14 days after infection. Resistance was monitored by faecal egg counts (FEC), performed at days 0, 21 and at weekly intervals thereafter, and antigen-specific IgA, IgG, and IgM titres determined by ELISA tests on days 0, 28, and at the termination of experiments. Goats were removed if their PCV fell below 20%.
In Experiment 1, three groups of 2 year-old dry Boer does were used: a control group received 0.8 kg/d oaten hay, an amount calculated to meet maintenance requirement for energy; two protein supplemented groups were given diets containing 25% or 50% more protein by substituting lucerne hay for oaten hay. Protein supplementation significantly enhanced IgG titres (P<0.05) and significantly lowered FEC (P<0.05). In Experiment 2, protein intake was raised by similar proportions but the oaten hay was low in protein and allowances were raised to 1.3 kg/d to meet protein requirements. Protein supplementation gave little or no protection against a trickle infection that induced haemonchosis in 7 out of 23 6 month-old Boer wethers. In Experiment 3, plane of nutrition was varied by feeding goats different amounts of a mixed grass-legume hay to three groups of 2-6 year-old Boer does. Neither resistance nor resilience were improved by increasing food intake, which did not increase LW, arrest decreases in PCV and plasma albumin caused by the trickle infection. Failure of protein supplementation to improve resistance in Experiment 2 was probably attributable to use of low quality diets what failed to promote growth and thus compete with immune processes for protein. Plane of nutrition probably failed to influence resistance in Experiment 3 because the lowest food intake met any additional demands to resist infection. Nutritional responses in resistance and resilience to H contortus infection may only be obtained when un-supplemented animals are productive and fed diets that fail to meet protein and energy requirements. Overfeeding of goats infected with H. contortus did not improve immunity in these studies.