Submitted to: Weed Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/2/2000
Publication Date: 7/1/2000
Citation: Allen, J.R., Johnson, W.G., Smeda, R.J., Kremer, R.J. 2000. ALS-resistant common sunflower (Helianthus annus l.) interference in soybean (glycine max). Weed Science. 48(4):461-466.
Interpretive Summary: Field trials were conducted in Missouri to determine how soybean growth, measured as height, and seed yield were affected when the weed sunflower grew with the crop at different times in a field. In certain regions of the U. S., sunflower is a serious weed that competes with crops for nutrients, light, and water and can reduce soybean seed yield by 80%. It has become more of a problem recently because many sunflower populations are developing resistance to certain widely used herbicides. To manage these herbicide-resistant sunflowers, farmers must use approaches not reliant on the standard herbicides such as the use of herbicide- resistant crops like Roundup Ready soybeans that tolerate non-selective herbicide while competing weeds are killed. Results showed that soybean growth and yield are not affected by sunflower growing together with the crop for up to 8 weeks after planting. At that time, the sunflowers can be killed easily with Roundup sprayed over the tops of the soybean and sunflower plants, leading to optimum soybean seed yields. However, if sunflower is controlled too early (2-6 weeks after soybean planting), the weed can easily reestablish to compete with soybean and reduce yields 15- 80%. The results of this study are important to producers because a practical strategy for successfully managing herbicide-resistant sunflower was developed.
Technical Abstract: The impacts of early and early + late season ALS-resistant common sunflower interference on soybean and common sunflower growth and yield were studied at two Missouri field sites. Sunflower densities of 3/sq m were established shortly after soybean emergence in all plots except a weed-free check. To study early-season interference, sunflowers were removed with postemergence glyphosate (0.84 kg ae/ha) biweekly for 8 wks after planting and then kept weed-free during the growing season. A weed-free (no interference) treatment was included. Soybean yields were not affected by early-season interference at either location. To study early + late season interference, sunflower densities established at 3/sq m were removed periodically after planting with glyphosate and subsequently reestablished at the same density within 2 weeks after removal. Plants were allowed to remain in the field for the remainder of the growing season. This provided a weed-free period of 2 weeks during the growing season beginning 2, 4, 6, or 8 weeks after planting. A season-long interference (weedy check) treatment was included. Soybean yields were reduced up to 72% with season- long interference compared to the weed-free check. Soybean yields tended to increase as the weed-free period was delayed. Early-season weed-free periods (2-4 and 4-6 weeks after planting) allowed sunflower to reestablish before soybean formed a canopy and resulted in higher sunflower biomass and seed production and soybean yield losses of 15-80%, depending on environmental conditions. Re-establishment of sunflower in weed-free treatments 6-8 or 8-10 weeks after soybean planting resulted in sunflower surviving for only a few weeks after establishment, not producing seed nor reducing soybean yield.