Waterhemp control is an increasing challenge for soybean producers due to the evolution of multiple herbicide-resistant populations. With dwindling herbicide resources, there is a need to integrate nonchemical strategies into current weed management programs for soybeans. Thus, there’s interest in using cereal rye as a cover crop, along with planting soybeans in narrow rows, to manage herbicide resistant waterhemp.
Iowa State University Extension weed management specialists Prashant Jha and Bob Hartzler provide the following update on the latest ISU research on this evolving practice.
Cereal rye is the most common cover crop grown in the Midwest due to its winter-hardiness and short life cycle. The high carbon-to-nitrogen ratio of cereal rye compared to legume or brassica cover crops results in a slow degradation of the crop residues. This increases the duration of weed suppression provided by the cereal rye crop residue. Along with a greater biomass accumulation, it makes cereal rye an ideal cover crop candidate.
Another nonchemical, cultural strategy to suppress weeds and complement herbicide efficacy is the use of narrow rows vs. wide rows for soybeans. Growers need research-based information on how to best integrate these two strategies for managing herbicide-resistant waterhemp in soybean.
Field study compares 3 options
A field study was conducted in 2019-20 at the ISU Research and Demonstration Farm near Ames to quantify the impact of cereal rye cover crop and soybean row spacing (15 vs. 30 inches) on the glyphosate-resistant waterhemp seed bank. The previous crop was corn, with three levels of waterhemp control achieved by:
1. Marginal herbicide program. Two sites of action include 27 fluid ounces per acre Dual II Magnum PRE followed by 32 fluid ounce per acre Roundup PowerMax Post.
2. Aggressive herbicide program. Three sites of action include 2 fluid ounces per acre Sharpen plus 2.5 fluid ounces per acre Zidua SC PRE followed by 32 fluid ounces per acre Liberty SL plus 23 fluid ounces per acre Dual II Magnum POST.
3. Aggressive integrated program. This includes the three sites of action plus harvest weed seed control at corn harvest (no weed seed input).
The purpose of the three treatments was to manipulate the size of the seed bank, not to evaluate these particular herbicide programs. The intent was to demonstrate the importance of minimizing the size of the seed bank. So rather than add seeds or hand-weed the plots, three programs were selected that were expected to result in three different levels of weed seed production — zero, medium and high.
Indeed, the three programs resulted in three different levels of weed seed production.
After corn harvest, cereal rye was drill seeded (60 pounds per acre) in the second week of October. Soybeans (Enlist E3 beans) were planted into the standing rye cover crop at a 30-inch or 15-inch row spacing on May 22. On the same day, cereal rye (at anthesis stage) was terminated with 32 fluid ounces per acre Roundup PowerMax, and 27 fluid ounces per acre of Dual II Magnum was applied to provide early-season residual control of waterhemp.
Cereal rye biomass at the time of termination averaged 4,600 pounds per acre. To examine the potential of cover crop and narrow row soybeans on waterhemp control, no post-herbicide was applied in the soybean phase of the study.
Results show improved control
The aggressiveness of the prior year’s corn herbicide program had a strong impact on waterhemp infestation in the soybean crop. Waterhemp emergence in soybeans was reduced by 75% with the aggressive two-pass herbicide program (three sites of action) plus harvest weed seed control compared with the marginal herbicide program in the previous year.
The rye cover crop reduced waterhemp emergence (density) by 30% and waterhemp growth (size and biomass) by up to 75% through July. Reducing the soybean row spacing from 30 to 15 inches reduced waterhemp emergence by 15% and waterhemp growth by 50%.
The integration of these tactics resulted in a significant suppression of waterhemp even with limited herbicide inputs in the soybean phase of the rotation. For instance, an aggressive weed control program in corn followed by a rye cover crop and narrow-row soybeans showed 87% less waterhemp emergence, compared with the treatment that had marginal weed control in corn, no cover crop and 30-inch soybean row spacing.
Soybean yield will be recorded at harvest in the fall 2020 to determine the effects of cover crop, row spacing and weed competition.
The ISU Weed Science program is also researching cover crop termination timing by herbicide interactions to develop integrated weed management tactics and methods to integrate harvest weed seed control technologies to manage herbicide-resistant waterhemp in Iowa soybean production. For more information, visit ISU Extension.
Disclaimer: This article is for education purpose only. Mention of a specific product should not be considered as approval, nor should failure to mention a product be considered disapproval. Read the product label before using any herbicide.