Weed growth in conventional and low-input cropping systems

We evaluated the effects of two rotational cropping systems in the soybean year on the growth of common waterhemp, common lambsquarters, giant ragweed, and velvetleaf in Central Iowa. The first experiment evaluated emergence of the four species. Common waterhemp emergence in the two-year system was more than twice that in the four-year system in 2008, whereas giant ragweed emergence was 1.7X greater in the four-year system in 2008. No other differences were observed in cumulative emergence.A second study evaluated the growth and biomass production of the four weed species when seeds were planted at soybean planting and when soybean was at the V2 stage. Velvetleaf in the first cohort in 2008 exhibited the only mature height difference and was 33 cm shorter in the two-year system compared with the four-year. Mature stem diameter was not influenced by cropping system in any weed species. The first cohort giant ragweed in the two-year system produced only 75% of the biomass of giant ragweed in the four-year system in 2008. First cohort velvetleaf in the two-year system produced 38% of the biomass than that in the four-year in 2008. No difference was detected in giant ragweed or velvetleaf biomass between the systems in 2009 or common waterhemp and common lambsquarters in either year. Height and stem diameter of soybean were greater in the four-year system compared to the two-year. Soybean yield in both years of the study was greater in the four-year system.