Effective Waterhemp Control Programs and Compatibility with Interseeding in Corn: 2019 Trials

Project Leaders: Bryan Brown, NYS IPM Program; Venancio Fernandez, Bayer Crop Sciences; Mike Hunter, Cornell Cooperative Extension; Jeff Miller, Oneida County Cooperative Extension; Mike Stanyard, Cornell Cooperative Extension

Collaborators: Derek Conway, Conway Farms; Jaime Cummings, NYS IPM Program; Quentin Good, Quentin Good Farms; Antonio DiTommaso, Cornell University; Michael Durant, Lewis County Soil and Water Conservation District; Kathleen Howard, Cornell University; Grace Marshall, NYS IPM Program; Scott Morris, Cornell University; Ali Nafchi, Cornell Cooperative Extension; Jodi Putman, Cornell Cooperative Extension; Joshua Putman, Cornell Cooperative Extension; Matthew Ryan, Cornell University; Lynn Sosnoskie, Cornell University; Ken Wise, NYS IPM Program

Herbicide resistant waterhemp has spread into New York and caused yield losses for corn farmers. This project aimed to find ways to regain control of this weed in corn and determine the compatibility of more extensive herbicide programs with interseeded annual ryegrass. Our field trial included several treatments that effectively controlled waterhemp. One of the most effective treatments was an integrated program utilizing a reduced rate herbicide, row cultivation, and interseeding. This treatment was slightly more expensive than the other two-pass treatments but the cost may be offset by the benefits of cover cropping. Of the several residual herbicides that were compatible with interseeded annual ryegrass, Callisto provided the most effective control of waterhemp.

weed growing under corn
Figure 1. Waterhemp competing with corn at a farm in Seneca County, NY.

Background and justification:
In the past few years, herbicide resistant waterhemp has expanded into New York and is now present in 12 counties at the time of this publication. Corn farmers have reported yield losses of 20% due to this weed (Figure 1), even after herbicide applications. Our greenhouse spray chamber tests of waterhemp from three different locations in New York indicate that it is likely resistant to herbicides from WSSA groups 2, 5, and 9 (ALS inhibitors, photosystem II inhibitors, and EPSPS inhibitors, respectively). Effective control programs in other states have relied on herbicides from other groups as well as additional physical or cultural tactics. Pre-emergence applications of residual herbicides are often recommended in order to reduce both the burden placed on post-emergence applications.

However, residual herbicides can sometimes cause injury to succeeding crops. Cover crops interseeded into a corn crop are at particular risk of injury. Interseeding has grown in popularity as a way to include a winter cover crop, which can benefit soil health, reduce erosion, and provide weed suppressive residue. Interseeding typically occurs at corn growth stage V5 rather than waiting until after corn harvest, when it is oftentimes too late. Several prominent New York farmers have bought or built their own interseeders. Additionally, the Lewis County Soil and Water Conservation District and the Genesee River Coalition of Conservation Districts each have interseeders available for custom application.

Objective 1. Evaluate the effectiveness of several different programs in controlling waterhemp in corn.

Objective 2. Assess the compatibility of residual herbicides with an interseeded cover crop.

Objective 1.
The trial site was in Seneca County, NY on a field of Odessa silt loam soil where waterhemp had survived various herbicide applications and produced seed in 2018. In 2019, the ground was prepared for planting with a field cultivator on June 4, and planted on June 7. Pre-emergence applications were made after planting on June 7. Cultivation and interseeding occurred on July 12, while the other post-emergence treatments were applied on July 15. All treatments are listed in Table 1. For fertilizer, muriate of potash (0-0-60, 125 lbs K2O/A) was applied prior to tillage and urea nitrogen (46-0-0, 100 lbs N/A) was broadcast on July 12.

Plots were 25’ long and 10’ wide. Each treatment was replicated four times in a randomized complete block design. Spraying was conducted using a backpack CO2 sprayer with a 10’ boom. Spray volume was 20 gal/A applied at 40 psi. Row cultivation was achieved using a Double Wheel Hoe (Hoss Tools) with two staggered 6” sweeps (12” effective width). Two passes were made per row so that 24” of the 30” rows were cultivated. For Objective 1, interseeding was established by hand broadcasting annual ryegrass (Mercury Brand, “Ribeye”) at 20 lb/A.

Weed control was assessed on August 15 by collecting all aboveground weed biomass within a 2 ft2 quadrat. The quadrat was used four times per plot, placed randomly in the two middle rows of each plot. Weeds were placed in paper bags and dried at 113 degrees F for 7 days, then weighed. Control was calculated by subtracting the biomass of each treated plot from biomass of the untreated plots, dividing by the biomass of the untreated plots, and multiplying by 100. Waterhemp was the dominant species present in this trial. Other species did not provide enough data for comparison. All waterhemp was manually removed immediately after the weed control assessments in order to prevent it from producing seeds.

Table of weed herbicides for waterhemp in cornCorn grain yield was measured by first harvesting and weighing all ears in 10’ of a middle row of each plot on October 25. Weights were then adjusted based on the ratio of total ear weight to grain weight and then adjusted to 15.5% moisture based on subsamples that were completely dried (25 days at 113 degrees F). To provide an economic basis for comparison of each treatment, costs were estimated based on personal communications with several local custom applicators.

Objective 2.
This objective was conducted in Lewis County, NY on a field that did not contain any waterhemp. The field (Homer silt loam soil) was tilled June 9 and planted with silage corn (Pioneer, 95 day) on June 10 with 3 gal/A starter fertilizer (7-21-7). Pre-emergence herbicides were applied on June 12 and post-emergence on July 8. All treatments are listed in Table 2. Interseeding was conducted on July 10 using a 15’ interseeder (Interseeder Technologies) with three drills between each corn row operating at 0.5” depth. Annual ryegrass (Mercury Brand, “Ribeye”) was interseeded at 20 lb/A.

Table of pre-emergence herbicides for waterhemp with interseeded annual ryegrassWeed control of the pre-emergence herbicides was evaluated on July 7 by visually estimating the percentage of the ground covered by the most prevalent species or categories – common lambsquarters, velvetleaf, other broadleaf species, and monocot species. This was done using the same quadrat system described above and control was calculated in a similar manner.

Performance of the annual ryegrass was assessed on September 20 by collecting the aboveground biomass using the quadrat system and drying samples at 113 degrees F for 7 days before weighing. Although there would have been more cover crop biomass later in the fall, silage harvest would likely have altered the results.

Results and discussion:

Objective 1.
Waterhemp control was most effective in treatments that utilized herbicides from WSSA groups other than 2, 5, or 9, or treatments that integrated non-chemical tactics. The pre-emergence-only and two-pass treatments were more effective than the post-emergence-only treatments. It was unexpected that the treatment with a reduced rate of Callisto followed by row cultivation and interseeding would control 100% of the waterhemp since most in-row weeds would have been uncontrolled by cultivation and the competition from the interseeded annual ryegrass would have been minimal.

Table of effectiveness and cost for herbicide treatments of waterhempBoth the untreated control and the treatment of ResolveQ yielded 10% less than the treatments with more than one herbicide or tactic. Yield loss would likely have been greater in most treatments if waterhemp had not been manually removed in mid-August to prevent seed production. From personal communications with NY corn farmers who have waterhemp in their fields, a 20% yield loss can be expected in fields with poor control.

The two-pass programs were the most expensive, but were also the only treatments to offer 100% control of waterhemp. Several one-pass treatments offered 99% control with less expense, but the remaining 1% of uncontrolled waterhemp could likely produce enough seed to perpetuate the population.

Objective 2.
Early-season weed control was most effective for treatments containing Acuron or Callisto (Table 4) even though reduced rates were used. Weed control for the other treatments varied by weed species, which reflects their more common use in mixtures. Dual II Magnum and Warrant performed somewhat similarly, which was expected because they are both in WSSA group 15.

Annual ryegrass biomass of the grower standard (Treatment 2) was similar to several of the treatments containing residual herbicides (Table 4). Treatments with pre-emergence applications of Dual II Magnum, Sharpen, and Acuron affected annual ryegrass biomass, although the injury from Sharpen may have been confounded by the addition of ResolveQ in the post-emergence application. More injury to annual ryegrass was expected from Atrazine, but a heavy rain may have lessened its effect. A rainfall gage at the field showed that in the four weeks between the pre-emergence applications and the interseeding, the field received nearly 4” of rain, with 2” on June 20. Likewise, the post-emergence use of row cultivation in Treatment 10 may have lessened the effect of Acuron on annual ryegrass. Overall, Callisto stood out as the residual product that did not injure the annual ryegrass but also controlled waterhemp effectively in Objective 1.

Table of effectiveness of treatment on early season weed controlProject location(s):
Northern, western, and central New York.

Samples of resources developed:
Online articles:
Brown, B., DiTommaso, A., Howard, K., Hunter, M., Miller, J., Morris, S., Putman, J., Sikkema, P., Stanyard, M. Waterhemp Herbicide Resistance Tests: Preliminary Results. Cornell Field Crops Blog. May 15, 2019. https://blogs.cornell.edu/ccefieldcropnews/2019/05/15/waterhemp-herbicide-resistance-tests-preliminary-results/

Marshall, G., Brown, B. Waterhemp Control in Corn: 2019 Trials. NYSIPM. December 20, 2019. Accessed December 28, 2019. https://www.youtube.com/watch?v=8NQ6S39uQ-8&t=17s

Thank you to the New York Farm Viability Institute for supporting this project.

Disclaimer: Read pesticide labels prior to use. The information contained here is not a substitute for a pesticide label. Trade names used herein are for convenience only; no endorsement of products is intended, nor is criticism of unnamed products implied. Laws and labels change. It is your responsibility to use pesticides legally. Always consult with your local Cooperative Extension office for legal and recommended practices and products. cce.cornell.edu/localoffices


Print Friendly, PDF & Email