The feeding value for dairy cows and the agronomic performance of white clover (Trifolium repens L.) selected for increased floral condensed tannin : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Plant Science at Massey University, Palmerston North, New Zealand
Legumes containing 20 to 40 g of condensed tannin (CT) per kg of dry matter (DM) can improve dairy cow milk production by reducing ruminal protein degradation to ammonia and preventing bloat. White clover (Triflium repens L.) produces CT in its flower heads. High tannin (HT) white clover, bred for increased flowering and increased floral CT concentration, was evaluated under dairy grazing in Hamilton, New Zealand. Its performance in monoculture was compared to that of Grasslands Huia white clover over two years, and five short-term grazing experiments determined its effects on Friesian dairy cows. Huia and HT had similar floral CT concentrations, ranging from 15 to 77 g/kg DM over two flowering seasons. HT clover had higher flower densities than Huia until the second summer after sowing, resulting in higher clover (leaf plus flower) CT concentrations. Clover CT peaked at 12.1 g/kg DM for HT and 5.7 g/kg DM for Huia. HT swards had lower stolon growing point densities than Huia swards and annual DM yields averaged 10.0 and 11.0 t DM/ha for the respective clovers. The ingress of non-sown white clover genotypes reduced treatment differences in the last 10 months of the experiment. Mild bloat occurred in cows grazing both clovers. Cows grazing HT white clover had rumen ammonia concentrations 5 to 26% lower than that of cows grazing Huia, indicating less proteolysis in the rumen of HT cows, but there were no consistent effects on rumen soluble protein or volatile fatty acids (VFA). Differences between treatments in dietary CT concentrations were too small to affect milk production or composition. Minced mixtures of 0, 25, 50, 75 or 100% of DM as white clover flower with the remainder as white clover leaf, were incubated in vitro and rumen metabolite concentrations determined at 0, 2 ,4, 8, 12 and 24 hours. Polyethylene glycol was added to one of the 50% flower treatments to inactivate CT. Clover flowers had less soluble protein than leaves at 0 hours, and increasing the percentage of flowers from 0 to 100% reduced the net conversion of plant-N to ammonia-N from 29 to 12%. The contribution of CT to these effects was small. Increasing percentages of clover flowers did not significantly affect total VFA production but increased acetate to propionate (A:P) ratios. White clover CT decreased A:P ratios. In another in vitro experiment perennial ryegrass leaf (Lolium perenne L.) was incubated either alone or with white clover flowers or birdsfoot trefoil (Lotus corniculatus L.). Clover flowers were more effective at reducing proteolysis than birdsfoot trefoil, due largely to less release of soluble protein, but birdsfoot trefoil treatments had the lowest A:P ratios. In conclusion, HT clover had higher forage CT concentrations than Huia because of increased flowering. Increased flowering reduced the agronomic performance of HT and lowered rumen ammonia concentrations, but did not increase milk production or prevent bloat. White clover flowers reduced rumen proteolysis in vitro, but this was mainly a result of their low protein concentration. White clover CT and birdsfoot trefoil forage benefited the molar percentages of VFA, but increasing the proportion of clover flowers did not. Further increases in white clover CT concentrations may benefit ruminant performance, but this should not be implemented through increased flowering.
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Burggraaf, V.T., Kemp, P.D., Thom, E.R., Waghorn,G.C., Woodfield, D.R. & Woodward, S.L. (2004) Performance of dairy cows grazing white clover selected for increased floral condensed tannin. Preliminary report from experiments presented in Chapter 4 published in the 2004 Proceedings of the New Zealand Grassland Association.