White clover has high nutritive value, but contains excess protein relative to energy, leading to the wasteful excretion of nitrogen (N) by animals. Five cycles of divergent selection has resulted in white clover lines having high (HS) or low (LS) concentrations of water-soluble carbohydrate. A grazing trial was carried out to evaluate the effect of this selection on protein utilization, using skatole and indole concentrations in the milkfat as metabolic indicators for nitrogen utilisation in the rumen. Skatole and indole arise in the rumen from the degradation of the amino acid tryptophan, and rapidly accumulate in milkfat of lactating animals fed diets rich in soluble and degradable protein. Skatole and indole concentrations were both significantly (P <0.001) lower in cows fed HS clover (356 and 84 ng/g milkfat for skatole and indole, respectively) compared to those fed the LS white clover diet (800 and 420 ng/g milkfat, respectively) suggesting protein utilisation of the HS clover was improved. Although not statistically significant in this short-term study, the trends to lower concentrations of milk urea-nitrogen and urinary-N, and the nitrogen:creatine ratio for the HS clover were in the direction expected and warrant further investigation under longer-term feeding.
Proceedings of the New Zealand Society of Animal Production, Volume 70, Palmerston North, 23-28, 2010
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