Grazing intensity effects on weed populations in annual and perennial pasture systems.
A study was conducted to assess weed populations in annual and perennial forage grasses grazed at various intensities by cattle over a 4-year period (1994-97) at Lacombe. The perennial forages were Bromus inermis and Bromus riparius, and the annual forages were winter Triticosecale and a mixture of Hordeum vulgare and winter Triticosecale. With few exceptions, results from the two annual pastures could be adequately described as a group, as could the results from the two perennial pastures. The two most prevalent weed species were Capsella bursa-pastoris and Taraxacum officinale; other species encountered over the course of the study were analysed as a group. Tillage (seedbed preparation) in the annual system supported a proliferation of annual weeds in the spring. In the perennial pasture system, a lack of tillage and spring MCPA allowed T. officinale to increase as the study progressed, especially at the highest grazing intensity. In the perennial pastures, each unit increase in grazing intensity led to 51 more C. bursa-pastoris m-2 and 4 more T. officinale m-2. At lower levels of grazing intensity, C. bursa-pastoris and other species were most abundant in the annual pastures. Weed population shifts in response to grazing pressure in the annual pasture systems were restricted because of annual tillage and MCPA. Therefore, pasture managers may subject annual pastures to heavy grazing pressure with less negative weed population consequences than perennial pastures where herbicides are not applied.