Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/29/2007
Publication Date: 9/1/2007
Citation: Uscanga-Mortera, E., Clay, S., Forcella, F., Gunsolus, J. 2007. Common waterhemp growth and fecundity as influenced by emergence date and competing crop. Agronomy Journal. 99(5):1265-1270. Interpretive Summary: Common waterhemp is difficult to control even in crops tolerant to Roundup-type herbicides. When grown by itself, a single waterhemp plant can weigh more than two pounds (oven-dry weight) and produce one million seeds. However, the size and seed production of plants decreases with delayed seedling emergence and the presence of a crop. Competition from either corn or soybean reduced size and seed production of common waterhemp by 90% or more. Waterhemp plants in corn never were taller than the crop. However, they still produced seeds even if emergence occurred as late as the 10-leaf stage of corn. In soybean, waterhemp plants were taller than the crop when emergence occurred before the 4-leaf stage of soybean, and they produced more seeds than those in corn. However, waterhemp plants that emerged after the 5-leaf soybean stage produced no seed. These data indicate that corn and soybean canopies affect growth and seed production of common waterhemp differently. The data also suggest that control of late-emerging waterhemp in soybean may not be needed because these plants die on their own and produce no seeds. In contrast, late-emerging waterhemp plants in corn should be controlled, as they still produce abundant seeds that will plague future crops. Control options for late-emerging waterhemp in corn include residual soil-applied herbicides and late applications of postemergence herbicides. These results allow farmers, crop consultants, extension educators, and the agri-chemical industry to better understand common waterhemp and plan control options in the Midwest where this weed is of increasing concern.
Technical Abstract: Common waterhemp (Amarathus rudis Sauer) has become problematic in glyphosate-tolerant crops. Dry weight and seed production of this weed at different times of emergence and alone or in crops (corn, Zea mays L., and soybean, Glycine max [L.] Merr.) were examined in 2001 and 2002 in Morris, MN. Later planted weeds produced less dry matter and fewer seeds. Crop competition from either crop reduced dry weight and seed production potential of common waterhemp by 90% or more. Weeds in corn never were taller than the crop; however, they still produced seeds even if emergence occurred at V10 stage of corn growth. Weeds in soybean were taller than the crop when emergence occurred before V4 and produced more seed than those sown in corn. Common waterhemp plants that emerged after V5 soybean produced no seed. These data indicate that corn and soybean canopies have different effects on growth and fecundity of common waterhemp and while control of later emerging plants in soybean may not be needed, analogous plants occurring in corn may warrant control.