Grazing intensity effects on weed populations in annual and perennial pasture systems

Few studies report animal grazing effects on weed populations. A study was conducted to assess weed populations in annual and perennial forage grasses grazed at various intensities by cattle over a 4-yr period. The perennial forages were Bromus inermis and Bromus riparius, and the annual forages wer e winter Triticosecale and a mixture of Hordeum vulgare and winter Triticos ecale. With few exceptions, results from the two annual pastures could be a dequately 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 analyzed as a group. Tillage (seedbed preparation) in the annua l system supported a proliferation of annual weeds in the spring. In the pe rennial pasture system, a lack of tillage and spring MCPA allowed T. offici nale to increase as the study progressed, especially at the highest grazing intensity In the perennial pastures, each unit increase in grazing intensi ty led to 51 more C. bursapastoris m(-2) and 4 more T. officinale m(-2). At lower levels of grazing intensity C. buna-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 pas tures to heavy grazing pressure with less negative weed population conseque nces than perennial pastures where herbicides are not applied.