Effect of crop density on Palmer amaranth ( <i>Amaranthus palmeri</i> ) emergence and weed density interference potential in a furrow-irrigated rice system

Abstract There is a high demand for cultural weed management strategies targeting Palmer amaranth ( Amaranthus palmeri S. Watson) in furrow-irrigated rice ( Oryza sativa L.) production due to overreliance on herbicides and the lack of a continual flood to prevent weed emergence. Amaranthus palmeri has been shown to reduce corn ( Zea mays L.), cotton ( Gossypium hirsutum L.), and soybean [ Glycine max (L.) Merr.] yields when it interferes with the crop. However, minimal research has been conducted to assess the ability of this weed species to impact rice grain yield. The manipulation of rice seeding rate to enhance rice canopy formation and favor the crop over the weed has not been fully explored. Hence, research was conducted to (1) evaluate the effect of rice density on A. palmeri emergence, rice canopy cover, and relative yield; and (2) determine the impact of A. palmeri density and aboveground biomass on rice grain yield. A natural population of A. palmeri was allowed to emerge at varying densities throughout the growing season within furrow-irrigated rice. Amaranthus palmeri plants caused 12% to 87% yield loss at densities ranging from 1 to 20 plants m −2 , and yield loss was 45% to 80% for plants weighing 200 to 800 g m −2 . When furrow-irrigated rice was sown at various densities, crop canopy cover increased as rice plant density increased. Most A. palmeri emergence occurred within the first 4 wk after rice emergence, before canopy formation could have an effect. Amaranthus palmeri emergence beyond 5 wk after rice emergence decreased as rice plant density and canopy cover increased. These results indicate that A. palmeri has the potential to cause severe yield loss and that residual herbicides will be vital for A. palmeri management in a furrow-irrigated rice system, due to the continual emergence of weeds up to crop canopy formation.