Abundance and diversity of herbaceous weeds in sheep/beef pastures, South Island, New Zealand

This study compared species diversity, abundance and size of broad-leaved herbaceous weeds on 28 South Island sheep/beef farms that employed either organic, integrated management (IM) or conventional management (CM) systems. Three or six paddocks per farm were surveyed using walked transects in November 2005, and the presence and number of individuals of each weed species encountered were recorded. 39 broad-leaved herbaceous weeds were recorded on all the farms in the study, but 76.7% of occurrences were of just three species (Californian thistle (Cirsium arvense L.), dandelion (Taraxacum officinale) Weber and daisy (Bellis perennis L.)). The ten next most abundant species made up 21% of records and the remaining 26 species just 1.9%. Very few significant differences were found in the geographic distribution, species richness or Shannon diversity index, abundance, cover or size of broad-leaved herbaceous weeds present on farms employing the different management techniques. However, there were significantly fewer Californian thistle per m2 on CM than on organic or IM farms and cover of all herbaceous weeds averaged 5.0, 5.6 and 2.1 on organic, IM and CM farms, respectively. Weed infestation varied enormously by region and between individual farms, so the statistical power of the comparisons was relatively low. Until further research is reported, the authors caution against general and unquantified assertions that the sustainability of organic pastoral farming is, or is not, compromised by weed infestations.