Despite the abundance of common ragweed in crops and the potency of ragweed pollen as an allergen, pollen production in agricultural fields has hardly been evaluated. Our goal was to evaluate pollen and seed production of early- (i.e., plants missed by weed control) and late- (i.e., after weed control) emerging common ragweed growing in corn and soybean. Allocation and gender distribution were also evaluated. The experiment included 2 yr (2008, 2009), three competition treatments, two seeding/emergence dates, three densities, and four replicates. Competition treatments (main plots) included no crop or weeds (bare), corn, or soybean. Crops were glyphosate resistant. Subplots were seeded with common ragweed before or after glyphosate application at densities of 1 (4 m⁻²), 3 (12 m⁻²), or 6 (24 m⁻²) plants per plot. Ragweed plants were harvested in mid-October and measured (aboveground biomass, length of all male inflorescences, stem diameter, and seed production). Based on our estimates, mean (backtransformed from ln[x 1]) pollen production values were: 6.25 (bare), 0.74 (corn), and 1.13 (soybean) × 10⁸ pollen grains per ragweed. Biomass and diameter were good predictors of ragweed male and female fitness. Plant height was not correlated with maleness. In crops, ragweed gender distribution was shifted toward maleness. Estimations indicate early-emerging (June 18 to 23) ragweed produced three times more pollen than late (July 7 to 11) plants.

Nomenclature: Glyphosate; common ragweed, Ambrosia artemisiifolia L. AMBEL; corn, Zea mays L.; soybean, Glycine max (L.) Merr.