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.
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
|sulfentrazone||Authority®||Mostly broadleaf weeds|
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.
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).
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.
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