The hunt for the “holy grail”: Condensed tannins in the perennial forage legume white clover (Trifolium repens L.)

Forage legumes such as white and red clover improve the nutritional quality of grazed pasture but can cause bloat. Their rapid fermentation in the rumen also contributes to environmental issues through higher methane emissions and urinary nitrogen losses. Condensed tannins are known to reduce urinary nitrogen and methane production from grazing animals, reduce bloat, reduce internal parasite burden, and improve animal productivity. Several forage legumes including birdsfoot trefoil and sainfoin do have good levels of condensed tannins but unfortunately these species fail to persist in intensively grazed pasture systems. Conventional breeding approaches including mutagenesis and phenotypic selection have failed to deliver condensed tannins in legumes, such as white clover, red clover or lucerne, that do persist under grazing. A recent advance using a molecular biology approach has identified a transcription factor or master switch that can 'turn on' the condensed tannin pathway present in white clover allowing biologically significant levels of condensed tannin expression in leaf tissue. In vitro tests have demonstrated that the condensed tannins produced in white clover leaves can bind protein at a pH 6.5, as found in the rumen, and then release them at pH 2.5, the pH in the abomasum, before entering the small intestine for amino acid absorption. Additional tests have demonstrated that these condensed tannins can reduce methane production by up to 15.7% in the first 6 h of incubation. The journey to this point, and the challenges ahead to deliver white clover cultivars with condensed tannin expression, is described.