COSTS OF SOME WEEDS OF DAIRY PASTURE AND THEIR CONTROL

Scotch thistle and ragwort are weeds of dairy pastures and are commonly controlled wth broadcast herbicides. Information on the growth habits of populations of these weeds was used to estimate seasonal pasture shading and recovery of populations following herbicide application. These data were then integrated by a Computer model to provlde estimates of the effects of the weeds on milkfat production The economics of using broadcast herbicides for controlling these weeds could then be calculated, taking into account possible harmful effects of some herbicides on pasture clover content. Assuming that lost pasture production is directly proportional to the area covered by weeds, control of Scotch thistle is economic when populations reach one flowering plant per 6 sq m, using MCPB for control in spring. Ragwort is also economic to control but because its control is difficult with pasture-selective herbicides much higher populations can be tolerated. In this work, no account is taken of the poisonous nature of ragwort. Keywords: Scotch thistle. ragwort, MCPB, 2,4-D. weed economics, dairy production. Cirsium vulgare, Senecio jacobaea, dairy model.