Soybean Response to Herbicides Used to Supplement Glyphosate

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Crop Insights by Dave Johnson, Pioneer Research Scientist

Summary

Prior to the introduction of glyphosate-resistant crops, growers had little choice but to use herbicides that in some cases had the potential to injure soybeans.
Over-reliance on glyphosate appears to have resulted in glyphosate-resistant biotypes of several weed species.
- Soybean growers must now consider alternative herbicides with other modes of action to supplement glyphosate to control these weeds.
With the increased necessity of supplementing glyphosate with other herbicides, soybean growers must be prepared for and be willing to accept higher injury potential of many of these products.
The extent of injury can depend on herbicide choice, use rate, application timing, weather conditions, and soybean genetics. Research has shown that most often this injury is transient, soybeans recover rapidly and yield is not affected.
Growers must be careful to follow product labels and recommendations on rates, timings, soils, and weather conditions to minimize injury potential.

Introduction

Glyphosate has been an important weed management tool for growers for several decades, but adoption of glyphosate-tolerant (Roundup Ready®) crops has dramatically increased its use in the past 15 years. The effectiveness, ease of use, and minimal crop response led many growers to broadly adopt glyphosate for weed control, and to phase out or eliminate other herbicides altogether. With this near-total reliance on glyphosate for weed control, especially in soybeans, several weed species have developed glyphosate resistance and have become increasingly hard to control. There are currently 21 weed species confirmed to have glyphosate-resistant biotypes worldwide. Of these, Palmer amaranth, common waterhemp, common and giant ragweed, horseweed (marestail), kochia, annual ryegrasses, and johnsongrass are found in the United States in areas where soybeans are grown (Heap, 2011).

To manage resistant weeds and help reduce their future occurrence, soybean growers are showing a renewed interest in herbicides with other modes of action to supplement glyphosate. Many of the herbicides used prior to the introduction of Roundup Ready crops are still available, or have been reformulated in premixes with other herbicides under new brand names. In considering these products, growers should be aware that many of them have potential to cause crop response under certain conditions. In most cases these crop symptoms are transient and soybeans recover rapidly, with little or no effect on yield. Nevertheless, understanding crop symptoms and causes will help avoid surprises and allow growers to take steps to minimize soybean injury.

Alternatives to Glyphosate for Weed Control in Soybeans

Prior to the introduction of glyphosate-resistant soybeans, growers relied on multi-pass programs that included soil-applied (preplant incorporated or pre-emergence) and foliar-applied products (Tables 1, 2 and 3).

Table 1. Soil-applied herbicides used prior to Roundup Ready introduction for weed control in soybeans.

Active Ingredient	Original Brand Name	Control
trifluralin	Treflan®	Annual grasses and some small-seeded broadleaf weeds
pendimethalin	Prowl®
metolachlor	Dual®

sulfentrazone	Authority®	Mostly broadleaf weeds
metribuzin	Sencor®	Mostly broadleaf weeds

Table 2. Postemergence herbicides used prior to Roundup Ready introduction for broadleaf weed control in soybeans.

Active Ingredient	Original Brand Name	Control
bentazon	Basagran®	broadleaf weeds
acifluorfen	Blazer®
lactofen	Cobra®
fomesafen	Reflex®

Table 3. Soil-applied or postemergence herbicides used prior to Roundup Ready® introduction for broadleaf weed control in soybeans.

Active Ingredient	Original Brand Name	Control
chlorimuron	Classic®	broadleaf weeds
cloransulam	FirstRate®
imazaquin	Scepter®
imazethapyr	Pursuit®

Herbicides introduced in the Roundup Ready era include flumioxazin (Valor®) and, in 2010, saflufenacil (Sharpen®), which are applied prior to soybean emergence for broadleaf weed control.

Future use of and potential soybean response to non-glyphosate herbicides: Herbicides in the PPO mode of action are effective on many of the broadleaf weeds that have developed resistance to glyphosate, and use will almost certainly increase in the next few years. Flumioxazin, sulfentrazone, saflufenacil, acifluorfen, lactofen, and fomesafen are all in this herbicide class. Additionally, metribuzin (a photosynthetic inhibitor) will probably increase in use, especially in the southern soybean areas.

Potential for Soybean Response to Metribuzin

Soybean response to metribuzin is characterized by interveinal chlorosis and necrosis of leaf tissue (Figure 1), stand loss and stunting, and is often influenced by soil and environmental factors. Coble (1973) and Ladlie (1976) observed higher metribuzin injury on sandy and low organic matter soils, and soils with higher pH. Soybean cultivars can also have different tolerance levels to metribuzin (Mangeot 1979, Hardcastle 1974, Barrentine, 1982). More recently, LSU reported differences in soybean variety injury and yield response to metribuzin (Stephenson, 2010). Variety tolerance ranged from poor to excellent, with those rated as poor having up to 18% yield reduction at a location with high rainfall after planting. However, these same varieties showed no injury or yield response at a location with low rainfall after planting.

**Figure 1**. Chlorosis and necrosis resulting from metribuzin application to soybeans.

Minimizing crop response to metribuzin

Carefully following label instructions can help growers minimize but not necessarily avoid potential crop responses to metribuzin application. The product label warns that:

Over-application or boom overlapping may result in stand loss and soil residues.
Uneven application or improper incorporation can decrease the level of weed control and/or increase the injury level.
Application is not recommended on certain sensitive soybean varieties (see product label).

Injury to soybeans may also occur:

when soils have a calcareous surface area or a pH > 7.5
when applied in conjunction with soil-applied organic phosphate pesticides
when applied to any soil with < 0.5% organic matter
when soil incorporation is deeper than recommended
when sprayers are not calibrated accurately
when heavy rains occur soon after application, especially in poorly drained areas where water may stand for several days
when soybeans are planted less than 1.5 inches deep, particularly in pre-emergence application

Potential for Soybean Response to PPO Herbicides

Soil-applied PPO Herbicides

Soil-applied PPO herbicides such as sulfentrazone, flumioxazin, and saflufenacil must be applied prior to soybean emergence. Soybeans that have reached the cracking stage (soybean hypocotyls cracking through soil surface) may be severely injured by these herbicides, so growers must apply these products prior to or as soon as possible after planting, but prior to emergence. Additionally, heavy rainfall after soybean emergence may splash soil particles containing these herbicides onto soybean foliage and cause injury.

Researchers have observed sulfentrazone and flumioxazin injury on soybeans (Swantek, 1998; Hulting, 2001; and Taylor-Lovell, 2001). Swantek (1998) and Hulting (2001) found a wide range of soybean variety tolerance to sulfentrazone, while Taylor-Lovell (2001) found a range in tolerance to both sulfentrazone and flumioxazin. In some cases this injury caused yield reduction. Injury symptoms include stand loss, stunting, necrosis, deformation of cotyledons or young soybean leaves, and girdling of soybean stems (Figure 2).

**Figure 2**. Soybean seedling injury caused by soil-applied PPO herbicides.

Minimizing crop response to sulfentrazone

According to the Spartan® product label, "the same processes that sulfentrazone affects in weeds can, under certain conditions, be affected in soybeans. These conditions include high pH (7.5 and above), cool weather, prolonged and excessive moisture, seedling diseases, and any other condition, including poor agronomic practices, that are unfavorable to vigorous crop growth. Such effects in soybeans are often observed as stunting and discoloration. The duration of these effects are somewhat dependent on the duration of the adverse growing conditions. These effects lessen and generally diminish with the return to normal growing conditions."

For these reasons, avoiding sulfentrazone use on high pH soils, and when cool, wet conditions are likely may help reduce soybean crop response from sulfentrazone. The label also warns to not apply after crop seed germination, and to not use on soils classified as sand, which have less than 1% organic matter.

Minimizing crop response to flumioxazin

According to the product label, pre-emergence application of flumioxazin must be made within 3 days after planting and prior to soybean emergence. Application after soybeans have begun to crack, or are emerged will result in severe crop injury. The label also warns that "crop injury may occur from applications made to poorly drained soils and / or applications made under cool, wet conditions. Risk of crop injury can be minimized by using on well drained soils, planting at least 1.5 inches deep, using high quality seed and completely covering seeds with soil prior to pre-emergence applications. Treated soil that is splashed onto newly emerged crops may result in temporary crop injury."

Foliar-applied PPO Herbicides

Foliar-applied PPO herbicides such as acifluorfen, lactofen, and fomesafen can cause leaf necrosis (burning) on the tissue that was exposed at the time of spraying (Figure 3). The extent of burn and subsequent recovery depends on growth stage and environmental conditions at and shortly after the time of application. Soybeans usually quickly outgrow the injury, but recovery can be slowed in dry, hot conditions (Hager, 2000). In Hager's (2000) studies, more than 95% of their treatments (nearly 300 over a three-year period) did not reduce soybean yield, and yield loss did not correspond to observed injury, but was related to environmental conditions after application that did not favor soybean growth. These herbicides should be applied to small, actively growing weeds for best control.

**Figure 3.** Soybean injury after foliar application of a PPO herbicide.

Minimizing crop response to foliar-applied PPO herbicides

To help minimize crop response from foliar-applied PPO herbicides, avoid overlapping spray swaths, which may increase application rates above those specified on the product label. Also, do not apply to soybeans that show injury from prior herbicide applications, such as leaf phytotoxicity or stunting, as injury may be enhanced or prolonged.

For all herbicides, growers should read and follow herbicide label instructions carefully to help minimize injury to soybean plants.

Conclusions

With the increase in glyphosate-resistant weed populations in the past several years, growers are considering using additional herbicides to supplement glyphosate in their weed management programs. Several of these products have the potential to injure soybeans, and growers must be prepared for this and be willing to accept some injury on their soybeans to control weeds. The extent of injury can depend on herbicide choice, use rate, application timing, weather conditions, and soybean genetics. In most cases this injury is transient, soybeans recover rapidly and yield is not affected.

References

Barrentine, W. L., E. E. Hartwig, C. J. Edwards, and T. C. Kilen. 1982. Tolerance of three soybean (Glycine max) cultivars to metribuzin. Weed Sci. 30:344-348.

Coble, H. D. and J. W. Schrader. 1973. Soybean tolerance to metribuzin. Weed Science 21:308-309.

Hagar, A., and C. Sprague. 2000. Postemergence soybean herbicide injury: is there a yield penalty? Accessed 03/24/2011.

Hardcastle, W. S. 1974. Differences in the tolerance of metribuzin by varieties of soybeans. Weed Res. 14:181-184.

Heap, I. The International Survey of Herbicide Resistant Weeds. Accessed 03/24/2011.

Hulting, A. G., L. M. Wax, R. L. Nelson, and F. W. Simmons. 2001. Soybean (Glycine max (L.) Merr.) cultivar tolerance to sulfentrazone. Crop Protect. 20:679-683.

Ladlie, J. S., W. F. Meggitt, and D. Penner. 1976. Effect of pH on metribuzin activity in soil. Weed Sci. 24:505-507.

Mangeot, B. L., F. E. Slife, and C. E. Rieck. 1979. Differential metabolism of metribuzin by two soybean (Glycine max) cultivars. Weed Sci. 27:267-269.

Owen, M.D.K, 2011. Herbicide Guide for Iowa Corn and Soybean Production. Iowa State University Extension Weed Science. Accessed 03/24/2011.

Stephenson, D., and D. Miller. 2010. Tolerance of eight MG IV and V soybean varieties to metribuzin. Agfax. Accessed 03/24/2011.

Swantek, J. M., C. H. Sneller, and L. R. Oliver. 1998. Evaluation of soybean injury from sulfentrazone and inheritance of tolerance. Weed Sci. 46:271-277.

Taylor-Lovell, S., L. M. Wax, and R. Nelson. 2001. Phytotoxic response and yield of soybean (Glycine max) varieties treated with sulfentrazone or flumioxazin. Weed Technol. 15:95-102.