White clover (Trifolium repens) is an important part of New Zealand pastures. Like other legumes, white clover fixes nitrogen from the air, making it available (as nitrate) to other pasture grasses. The proportion of white clover in well-managed, fertile pastures can be up to 30 percent. But the downside is that white clover can cause bloat in some ruminant animals, and excess nitrate can contaminate waterways.
One way to avoid these unwanted effects is to increase the production of condensed tannins in the leaves of white clover. Condensed tannins are a complex group of chemical compounds that improve the protein uptake of grazing animals by binding to proteins in the plant matter and protecting them from being broken down by microbes in the rumen.
While there are some naturally occurring legumes that contain significant levels of tannins, white clover produces only trace amounts. Of the more than 250 different types of clover, only two are known to accumulate tannins in their leaves. Researchers at AgResearch in Palmerston North have used genetic techniques to identify a gene that is responsible for the high levels of tannins in Rabbitfoot clover (Trifolium arvense). A related gene in white clover remains switched off, but the team has developed transgenic cultivars that produce increased levels of tannins.
