Herbicides and cover crops for improved corn establishment

Objectives:

• Determine the optimal rye termination practices prior to corn by examining:
  • The timing of fall rye cover crop termination prior to planting corn.
  • Whether the effect of herbicide providing fast burn would reduce the negative impact of planting green.
  • The impact of separating fall rye from corn when planting in twin rows versus solid seeded.
• Determine the potential of alternative herbicides to provide quicker rye desiccation than glyphosate.
• Determine the impact of residual corn herbicides on interseeded cover crops based on soil type and timing.

Expected Impacts:

• Knowing which combination of termination timing, herbicide type, and cover crop planting pattern will allow maximization of both fall rye biomass and corn establishment and yield will improve cropping system outcomes for Ontario’s grain farmers.
• Guidelines on the effect of PRE and early POST residual corn herbicides on interseeded cover crops allows growers to select cover crop species with the lowest probability of injury, ensuring increased establishment success.

Project Overview:

Cover crops are beneficial to cropping systems as they help improve soil health and contribute to crop protection. One challenge with cover crops is to maximise their growth without reducing the yield of the main crops. For example, delaying termination of a fall rye cover crop and planting green would maximize biomass production but may affect corn establishment. This may lead to variable results which have been attributed to various causes such as rye allelopathy, nitrogen (N) capture, or increased root diseases. Another factor that has not been considered is the possibility that corn plants emerging in the presence of a green rye cover crop may see their growth impacted because of altered light being reflected by the green tissues. Another approach to cover crops that warrants examination is the variable establishment of various species when interseeded into corn, which may be due to the use of residual herbicides in PRE- or early POST-emergence. Knowing which corn herbicides have enough residual to impact cover crops and determining species’ sensitivity to specific products allows for optimization of interseeding.

Part 1

In order to try to improve the establishment of corn into a rye cover crop, we isolated various factors to identify those that have the strongest impact.

• Timing of termination: Comparing termination at 2 weeks, 1 week, and 1 day before planting and 1 week after planting.
• Termination with glyphosate alone vs mixed with a fast-acting herbicide (Liberty) was chosen to determine if a more rapid kill would reduce the negative light signal that is perceived by corn compared to the slow kill due to glyphosate alone.
• Planting pattern: Rye planted in twin rows vs full coverage. Twin rows allow corn to be planted away from the rye to explore whether this would reduce any negative impacts this cover crop may have.

Part 2

As rapid rye termination is often needed by farmers, and because glyphosate is a slow acting herbicide, we tested whether a faster desiccation could be achieved by either adding other herbicides or adjuvants to glyphosate, or replacing it completely by other herbicides to improve this process.

Part 3

To determine which corn herbicides have enough residual to impact cover crops, various soil residual corn herbicides were applied PRE to corn prior to interseeding various cover crop species. The twelve herbicide treatments were: Acuron, Atrazine, Callisto, Converge XT, Dual II Magnum, Engarde, Frontier MAX, Integrity, Primextra II Magnum, Prowl H2O, XtendiMax, and Zidua SC. Six cover crop species were seeded at the V5-V6 stage of corn and were fall rye, oats, fall triticale, red clover, white clover  and oilseed radish.

To attain these objectives, field experiments were established at Elora and Woodstock in 2020; Elora, Woodstock, and Exeter in 2021; and Elora, Woodstock, and Ridgetown in 2022.

Results:

Part 1: Terminating the rye cover crop early (two weeks before corn planting) resulted in corn yield that was as high as corn grown without a cover crop. However, we observed that planting green tended to reduce yield, but this was environment specific. It is worth noting there was much variability around the mean values. Twin row planting compensated slightly for the negative effect of rye but there was still a potential for yield reduction when planting green. This effect can be explained in part by the alteration of the quality of the light reaching the emerging corn seedlings although other factors, likely acting in an additive manner, contributed to the yield outcome.

Part 2: None of the herbicides we tested (glufosinate, rimsulfuron, mesotrione, quizalofop, clethodim) were able to terminate rye quicker than glyphosate on its own. In addition, adding other herbicides to glyphosate did not speed up termination and was either neutral or antagonistic. The addition of UAN to glyphosate did not speed up rye desiccation.

Part 3: Six species of cover crops were interseeded into corn: oats, winter rye, winter triticale, white clover, red clover, and oilseed radish and twelve PRE herbicides were sprayed. All cover crops were able to establish under the corn canopy due to timely rainfall in 2020 and 2021. Conditions in 2022 were dryer and more challenging, especially for the earlier planted trial. Oats, triticale, and oilseed radish exhibited the highest level of tolerance when averaged across all herbicides, but the potential of severe injury still exists with some products. At the other end of the spectrum, red and white clover were most sensitive with average injury ratings ranging between 61 and 76%. There was variation among each crop for herbicide sensitivity but overall, Lumax, Acuron, and Primextra were the most injurious while the safest products were Integrity and Xtendimax. If a grower plans to grow cover crop under corn, the use of most pre-emergence residual herbicides would be contraindicated if clovers were to be grown.

Summary:

This research so far indicates that rye cover crop before corn can be managed such that the risks to corn yield could be mitigated while optimizing cover crop benefits. Over five site-years, we never observed any dramatic negative effect of rye on corn establishment. Overall, good yields could be attained when planting green although this timing increased the range in the actual results (increased variability). There was no benefit to planting rye in twin rows, and it seems that, with proper planting techniques, solid seeded rye provides the highest cover crop benefits.

There are many options to terminating rye, either with glyphosate alone or with glyphosate mixed with other herbicides. This means that a herbicide program could be designed that would aim at terminating a rye cover crop while also controlling herbicide resistant weeds without compromising any of the two aspects.

When interseeding cover crops in corn, the type of residual herbicide applied PRE matters. If deciding to plant a diverse mix of species, farmers would face more difficult choices of herbicides. Integrity appeared to be the safest overall, but this product may not provide the best weed control on any given site depending on the weed pressure. If sticking to cereal cover crops, a farmer would have a greater choice of product. There will always be risks though and one would have to balance the weed control benefits of a product with the risk of some cover crop injury.

Recommendations:

• If producers wish to maximize their corn yield potential, terminate the rye cover crop 1 to 2 weeks before corn planting.
• If maximizing cover crop biomass accumulation is your goal, then you can wait until one day before planting corn to terminate rye.
• High yields can be achieved when planting green, but the risks of a negative yield impact increase.
• Glyphosate is still the best choice as far as overall rye termination is concerned.
• Other herbicides can be added to glyphosate when terminating rye if specific weeds, especially those with glyphosate resistance, need to be controlled; this would not negatively impact the rye termination.
• Herbicides such as Integrity and Xtendimax could be sprayed PRE in corn with little negative effect on most interseeded cover crops; however, products such as Lumax, Acuron, Primextra and Converge would increase the risk of damage to most interseeded cover crop species.

External Funding Partners:

The project was funded in part by the Ontario Agri-Food Innovation Alliance, a collaboration between the Government of Ontario and the University of Guelph.

We acknowledge the support of the Natural Sciences and Engineering Research Council of Canada (NSERC) through the Alliance Grants program (ALLRP 570934 –

21).

Project Related Publications:

Noorenberghe, O.M., Sikkema, P.H., Cowbrough, M.J., Hooker, D.C., Soltani, N., Tardif, F.J. 2023. Enhancing winter rye termination by mixing glyphosate with other herbicides using water or UAN as the carrier. Weed Technology. 37: 489-493.