Controlling Palmer amaranth costs Georgia cotton growers more than $110 million each year, making this the most economically destructive weed in the southeastern United States. Many populations of Palmer amaranth have evolved resistance to glyphosate and other herbicides over the last decade, and additional control methods for this weed are now being evaluated.
A new study published in the journal Weed Science investigated the effect of delaying Palmer amaranth establishment on weed growth and seed production. In field trials conducted over two consecutive growing seasons in Georgia, Palmer amaranth was planted in cotton fields between 0 and 12 weeks after the crop, and compared with Palmer amaranth planted simultaneously without cotton.
Without crop competition, the earliest planting of Palmer amaranth produced 446,000 seeds per plant, with seed production falling 50% when plants were established 6 weeks later. When cotton plants were present and competing with the weeds for resources, the number of seeds per plant was reduced 30% to 312,000 for Palmer amaranth plants established at the same time as the cotton. The more time into the growing season the establishment of Palmer amaranth was delayed, the fewer seeds developed. Planting at 6 weeks after cotton establishment reduced the Palmer amaranth seed count by half, and for weeds established 9 and 12 weeks into the growing season, seed production was reduced by 89 and 99%, respectively, compared to weeds established at the same time as cotton.
Cotton yield was reduced 67% when Palmer amaranth was established at the same time as the crop, while delaying weed establishment until 6 weeks after crop planting reduced yield loss to below 30 %.
These results show that early season weed control programs that successfully delay Palmer amaranth establishment can have a large effect on crop yield, weed growth and weed seed production. In addition, reducing the soil seedbank is a vital preemptive measure against the next season’s weeds.
Full text of the article, “Glyphosate-Resistant Palmer Amaranth (Amaranthus palmeri) Morphology, Growth, and Seed Production in Georgia,” Weed Science, Vol. 63, No. 1, January–March 2015, is now available.