Seasonal dynamics of ground cover in Cirsium arvense - a basis for estimating grazing losses and economic impacts

Analysis of the economics of managing Cirsium arvense in grazed pastures worldwide has been hampered by a lack of data on the within-year seasonal dynamics of the weed and its impact on the yield of palatable herbage. To redress this, the seasonal pattern in within-patch percentage ground cover of the weed was determined from measurements on 39 dairy and 66 sheep and/or beef cattle farms in New Zealand during 2009-2010 and 2010-2011. This pattern was then scaled using farmer estimates of peak whole-farm cover to derive mean monthly covers for dairy, beef, sheep/beef, sheep and deer farms. These monthly covers corresponded, respectively, to mean annual percentage covers of 2.7, 1.7, 3.0, 5.9 and 2.9% and to mean annual percentage losses in pasture growth (palatable herbage yield) of 3.6, 2.3, 4.0, 7.8 and 3.9%. The latter, in combination with 2011-2012 farm statistics, revealed that C.arvense caused a national loss in pastoral farm gross revenue in New Zealand in 2011-2012 of $685 million ($446m dairy, $233m sheep/beef, $6m deer). Beyond the scope of this paper, the monthly covers and their corresponding monthly losses in pasture growth provide a basis for modelling the economic impacts of C.arvense and its management at a farm scale. More generally, the analytical method that we have developed is appropriate for evaluating the economic impact of any weed in a grazed pasture, particularly those exhibiting pronounced seasonal patterns in occupancy, such as annuals and deciduous perennials.