Influnce of Corn On Giant Ragweed and Common Sunflower Seed Production.

Giant ragweed (Ambrosia trifida) and common sunflower (Helianthus annuus) are troublesome weeds in agricultural fields of the United States; both compete heavily with corn (Zea mays) and major crops. However, crop interference may reduce growth and fecundity of weed species. This study examined weed seed production and growth when planted alone or with corn. Corn reduced fresh weight and circumference of both species.  Estimated seed production per plant for giant ragweed was 634 seeds when grown with corn and 1098 seeds when grown alone.  Common sunflower had 1625 seeds per plant when grown with corn and 2658 seeds per plant when grown alone.  The data demonstrated a 49% reduction of total fresh weight in giant ragweed and a 42% reduction in total seed when grown with corn versus grown alone.  Common sunflower losses were slightly less than those of giant ragweed, with a 26% loss of total fresh weight and 39% seed loss.  Common sunflower heads per plant and seeds per head decreased by 34% and 57%, respectively.  However, the total number of seed even in the corn plot was estimated to be 8,200 seeds per m² for giant ragweed and 33,700 seeds per m² for common sunflower.  Considering that 15% of the planted giant ragweed seeds produced a plant, estimated giant ragweed numbers in the second year would be about 1,000 plants. Sunflower germination was about 20%, therefore second year plant numbers may be around 6800 new plants.  Clearly, corn alone does not provide enough competition to effectively control either of these weeds.  Other control measures, such as mechanical (i.e. tillage), herbicides, or a combination of both are needed so that seed and plants from these first year plants do not continue the infestation.