US study finds waterhemp resistance indicators
A US study of glyphosate-resistant waterhemp (Amaranthus spp) in soybeans in Missouri has identified indicators for predicting future resistance incidences. Scientists at the University of Missouri (Columbia) found that cropping and herbicide use history, presence of other weed species and signs of herbicide survival served as the best indicators of glyphosate resistance. Tillage practices, field size, row spacing and weed infestation levels were not essential indicators. Glyphosate resistance was found in 69% of waterhemp samples from 144 soybean fields in 54 counties in Missouri in 2008 and 2009. Some 94% of the resistant populations had three field aspects in common: soybeans were the only crop planted in consecutive years; glyphosate was the only herbicide used for three or more years; and the field contained waterhemp that had survived herbicide application. The study was published in Weed Technology.