Effects of the use of heather as anthelmintic in goats infected with Trichostrongylus colubriformis on ruminal fermentation and digestibility

Previous field studies have supported the absence of a nutritional cost outweighing the beneficial anthelmintic effect of supplementing the diet of grazing goats with tannin-containing heather. In order to further research in this regard, an experiment was conducted indoors with 18 does artificially infected with Trichostrongylus colubriformis. The goats were offered lucerne hay for 6 weeks and then assigned to 3 treatments (diets): lucerne hay (L), 70% lucerne hay + 30% heather containing 64 g of tannic acid equivalents/kg DM (LH), and LH + polyethylene glycol (35 g PEG/animal and day; LH+PEG). Rumen fluid was obtained from each animal after 10 (period 1) and 36 (period 2) days, and afterwards total faecal output was collected for 5 days to assess gastrointestinal nematode egg excretion, and apparent digestibilities of DM and CP. Total daily faecal egg excretion was reduced in does consuming heather (491,216 for L vs 234,311 and 194,356 for LH+PEG and LH; P<0.05). Volatile fatty acid concentrations were greater in those animals (115 vs 102 vs 84 for LH, LH+PEG and L, respectively, P<0.05) but the use of PEG increased DM and CP digestibilities (P<0.05). In vitro gas production results suggest an adaptation of the rumen microbiota in goats supplemented with heather that was not reflected in differences between digestibility coefficients in periods 1 and 2. The fact that LH+PEG significantly improved the apparent digestibilities of DM (6%) and CP (13%) when compared to LH, but with both reducing egg excretion in the same proportion, might suggest that the threshold of tannins requested to obtain anthelmintic effects is probably quite low.