Density-effect and economic threshold of purple nutsedge (Cyperus rotundus) in soybean

Purple nutsedge (~nutsedge) is an important perennial weed, which infests soybean in India and causes high yield losses. Selective pre-emergence herbicides hardly control nutsedge. Post-emergent application of imazethapyr is effective against nutsedge with almost 70 % efficiency. Information on the interference effect of nutsedge across densities on soybean and its economic threshold (ET) is hardly available, but would be useful for its management, and saving herbicide treatments with lower densities. An experiment was designed to evaluate the interference of nutsedge in pure stands, and that of natural weed infestations on soybean. Moreover, it was aimed to determine ET of nutsedge in soybean. The dry weights of weeds in the treatments ‘natural weeds including nutsedge’ and the one of nutsedges in the pure stand density of nutsedge 200 plants/m2 were similar and higher than weed biomass in other nutsedge densities. The ‘natural weed infestation both including and excluding nutsedge’ and the treatment of 200 nutsedge plants/m2 caused greater reductions in soybean yields and were the most competitive. The ET of nutsedge in soybean was 19–22 (~mean 21) plants/m2, considering 70 % efficiency of the herbicide imazethapyr. It predicts that a density of 21 nutsedge plants/m2 can cause 9.1–11.5 % yield losses, which are an economic loss under this situation. This ET would help in making decisions for nutsedge management and fitting models and could be used for other similar sites with nutsedge dominance. This ET, considering several production factors, is more precise and reliable than the ET determined with only yield losses.