Is the anthelmintic effect of heather supplementation to grazing goats always accompanied by anti-nutritional effects?

To test the hypothesis that the beneficial anthelmintic effect of consuming moderate amounts of tannins may not always be accompanied by anti-nutritional effects in goats, two experiments were conducted. In the first, 48 Cashmere goats were randomly assigned to two treatments: supplementation with tannin-containing heather (6.4% total tannins) and non-supplementation. All goats grazed continuously from May to September under farm conditions in a mountainous area of northern Spain. The mean percentage of heather incorporated into the diet of the supplemented animals was 29.1%. Supplementation reduced the mean number of nematode eggs in faeces (P < 0.001) and the goat mortality rate (P < 0.05). The rumen ammonia concentration was markedly reduced in the goats receiving the heather supplement (160 v. 209 mg/l; P < 0.01), while volatile fatty acid (VFA) concentrations were significantly greater (63.0 v. 53.6 mmol total VFA/l; P < 0.05). The heather-supplemented goats also showed a lower loss of live weight (P < 0.01) and body condition score (P < 0.001). In the second experiment, batch cultures of rumen microorganisms with rumen fluid from nine goats whose diet included 29% heather – or not, were used to incubate three substrates (pasture, pasture + heather and pasture + heather + polyethylene glycol) to investigate in vitro ruminal fermentation. Differences (P < 0.01) among substrates were observed in terms of dry matter disappearance (DMD), in vitro true substrate digestibility (ivTSD), gas production and ammonia concentration, the greatest values always associated with the pasture substrate. Cultures involving rumen inoculum derived from goats receiving the heather-containing diet showed slightly lower DMD (46.9 v. 48.5 g/100 g; P < 0.05), ivTSD (64.6 v. 65.9 g/100 g; P < 0.10) and gas production (105 v. 118 ml/g; P < 0.001) values, but much greater total VFA concentrations (48.5 v. 39.3 mmol/l; P < 0.05), and suggest that the efficiency of ruminal fermentation in these animals was probably improved. Together, the results support the absence of a clear nutritional cost counteracting the beneficial anthelmintic effect of supplementing the diet of grazing goats with tannin-containing heather.