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Production Papers

Growth and Fecundity of Several Weed Species in Corn and Soybean

^a D.E. Clay, Plant Sci. Dep., South Dakota State Univ., Brookings, SD 57007
^b USDA-ARS, North Central Soil Conserv. Res. Lab., Morris, MN 56267
^c Dep. of Agric. and Biosyst. Eng., Iowa State Univ., Ames, IA 50011

^* Corresponding author (sharon.clay{at}sdstate.edu)

Received for publication June 10, 2004. Do weeds that emerge later in the season justify additional control costs? If crop yield is not reduced or few or no seeds are added to the soil seed bank, then no control may be needed. Eight weed species were sown in corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] (i) before crop emergence, (ii) at crop emergence, (iii) at V-1, and (iv) at V-2 stages of crop growth in 2002 and 2003. Weed seed was sown close to the crop row and thinned to 1.3 plants m^–2. Weed growth and fecundity were influenced by species, time of planting, and year. Only barnyardgrass (Echinochloa crus-galli L.), redroot pigweed (Amaranthus retroflexus L.), and velvetleaf (Abutilon theophrasti L.) survived to produce seed. Plants from the pre-emergence seeding had the largest canopy and produced the most seeds. Barnyardgrass had maximum canopy cover in early July in corn and late July in soybean but only produced seed in corn. Redroot pigweed and velvetleaf had maximum canopy cover in late August or mid-September, and some plants from most seeding dates survived and produced seed in both corn and soybean. However, plants that grew from seed sown at V-1 and V-2 crop growth stages did not reduce yield or biomass of adjacent crop plants, had low fecundity, and may not warrant treatment. Control may be necessary, however, to prevent yield losses if weeds are present at high densities or to prevent establishment of uncommon species.

Abbreviations: GDD, growing degree days • PAR, photosynthetically active radiation

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