Grazing of pasture weeds by goats and sheep. 1. Nodding thistle (Carduus nutans)

Abstract

Goats and sheep grazed replicate plots of perennial pasture infested with nodding thistle (Carduus nutans). Neither goats nor sheep ate the vegetative thistle until it reached ~30 cm in diameter at which time goats consumed outer leaves on some plants. Active goat grazing commenced at flowering as the capitula matured from green to purple, with subsequent preferential grazing of the purple capitula in the presence of abundant pasture (2500 kg green DM/ha). The stage of maturation of the thistle influenced the proportion of purple capitula eaten by goats. Over the extended (5 month) 1991–92 flowering period induced by above average summer rain, dispersal of viable seeds was prevented in goat-grazed plots as goats ate 100% of the flowering stems. In the short 1992–93 flowering season (1.5 months) dry conditions resulted in seedheads rapidly maturing from green to brown. Goat numbers were insufficient to eat 100% of capitula as they reached the palatable stage. Consequently viable seeds were dispersed. Sheep consumed the thistle capitula only when pasture became limiting, allowing viable seeds to be dispersed in both flowering seasons. Spraying with non-lethal doses of herbicide increased the proportion of vegetative thistles consumed by both sheep and goats. Goats selectively removed maturing capitula then grazed the stems of the nodding thistle. As goats consumed thistle capitula throughout the flowering period, the quantity of pasture in goat plots exceeded that in sheep plots, principally due to an increase in the proportion of clover. The stocking density of goats required to control nodding thistle is proportionate to the level of weed infestation and must be modified as a response to prevailing seasonal conditions and rate of removal of purple capitula. Grazing of the purple capitula of nodding thistle by goats in lieu of pasture indicates the potential for mixed animal species grazing of weed-infested pastures.