The flavour compounds indole and skatole (3-methylindole) are characteristic of products of pasture based production systems. The formation of these indoles by metabolism of the amino acid tryptophan in the rumen is favoured by high levels of readily-digestible protein in forage diets. However liver function is also a very important factor as indoles taken up into the animal are cleared by metabolism in the liver. An investigation of a persistent flavour taint from a dairy farm suggested a link between high milk indoles and facial eczema. As part of a genetic study, 1223 dairy cows in 4 herds in the Waikato and Bay of Plenty were screened for liver damage associated with facial eczema by measuring levels of the enzyme gamma-glutamyl transferase (GGT) in the blood. We examined levels of skatole in plasma from cows selected for high GGT (> 400 i.u. l-1, 47 cows) or low GGT (< 100 i.u. l-1, 43 cows) sampled from these herds. Blood indole and skatole levels overall were higher for the ‘high GGT’ cows compared to the `low GGT` cows (P < 0.001, Mann-Whitney test). The levels of indoles in milk collected from the same cows 4 days after blood sampling on one farm showed a highly significant (P < 0.001) non-linear relationship between levels in milk and blood, with milk fat skatole reaching extremely high levels (> 2000 ng g-1) in animals with very high blood skatole (> 20 ng ml-1). As the presence of relatively few animals with such extremely high levels of indole flavour compounds in milk is likely to raise the herd mean concentration to unacceptable levels, these findings suggest that facial eczema may have significant adverse effects on milk quality as well as on production.
Proceedings of the New Zealand Society of Animal Production, Volume 66, Napier, 315-319, 2006
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