The impact of cutting prior to goat grazing on variegated thistle (Silybum marianum)

Variegated thistle (Silybum marianum) is a prevalent weed of hill country in the East Coast of the North Island of New Zealand. Herbicide has traditionally been used to control large infestations but there is increasing demand for alternatives to chemical weed control. While defoliation techniques such as manual slashing/cutting have been advocated for control in steeper hill country, and mowing for flatter areas, cut thistle debris left lying in the paddock can shade and kill the pasture on which it lies. Goats may provide a novel management tool to remove this debris but little is known about how cutting thistles prior to grazing affects thistle consumption by goats. This study investigated the extent to which goats consume either uncut entire variegated thistle plants or cut thistles in which approximately half the leaves were removed from the plant 1, 2 or 3 days prior to grazing. Eight groups of 3 goats each were presented with thistle vegetation in each of two replicate 1-hour feeding sessions on 2 consecutive days. Averaged over both days, in the cut treatment, goats consumed 99% of the leaves that had been removed from the thistles and reduced the ground cover of the thistle plants by 68%. In the uncut treatment, ground cover of the thistles was reduced by 46%. Following the feeding sessions, all thistles in the cut treatments were excised at ground level, and at 5-day follow-up none survived. A combination of cutting and goat grazing is likely to be a useful tool in removing variegated thistles from infested paddocks and inhibiting seed setting, but further work is required to substantiate this at paddock scale on East Coast hill country.