Effect of tillage intensity and herbicide programs on changes in weed species density and composition in the southeastern coastal plains of the United States

Abstract

An experiment was conducted from spring 2001 to 2005 at Blackville, SC, to determine the impact of no-till and glyphosate-resistant crops on changes in the weed community. Glycine max (L.) Merr. (soybean) was grown in 2001 and 2003, and Zea mays L. (corn) was grown in 2002 and 2004. Tillage each year included no-tillage and conventional tillage (disking and field cultivation prior to crop sowing). Glyphosate-only and nonglyphosate programs were evaluated within each tillage system. Soil cores were collected at 0–5 and 5–10 cm depths prior to spring to estimate the seedbank. Weed biomass by species was determined in mid-summer each year, except in 2003. Twenty-seven species were found in soil collected from the test site throughout the study. Digitaria sanguinalis (L.) Scop. (large crabgrass), Amaranthus palmeri S. Wats. (Palmer amaranth), and Mollugo verticillata L. (carpetweed) were the dominant weeds in soil at initiation of the experiment. Summer annual grasses comprised more than 20% of total weed biomass in 2001, but less than 1% of total biomass in 2002 and 2004. D. sanguinalis seed density in soil was reduced rapidly in all management systems and was replaced by Dactyloctenium aegyptium (L.) Willd (crowfootgrass) as the dominant summer annual grass. There was an 80–99% reduction in the A. palmeri seed density (0–5 cm depth) during the first year of the study and M. verticillata became the dominant summer annual broad-leaved weed. Perennial weeds comprised <10% of the total weed biomass in 2001, but by 2004, >99% of the total weed biomass was made up of perennial weeds in nonglyphosate systems regardless of tillage treatment. These perennial weeds consisted of Cynodon dactylon (L.) Pers. (bermudagrass), Cyperus rotundus L. (purple nutsedge), and Solanum carolinense L. (Carolina horsenettle). The seed density in soil of most species declined over the four cropping seasons.