Weed management is one of the most important, and often most expensive, components of a soybean production program. Weed management in soybeans is essential, as weeds compete with the crop for light, moisture, and nutrients; impact soybean growth through allelopathy; and decrease harvest efficiency and contribute to lower quality and expense of drying. According to a 2016 report published by the Weed Science Society of America’s Weed Loss committee, it is estimated that uncontrolled weeds in North Carolina soybean would reduce yields by 47.4%. This potential yield loss corresponds to an approximately $240 million loss in value. In addition, weeds can severely reduce harvest efficiency and serve as vectors and alternate hosts to a plethora of disease-causing plant pathogens. Knowledge of potential weed problems, proper identification of the weeds present, and timely weed control are all critical to successfully managing weeds in soybeans.

An effective weed management program uses a combination of cultural, mechanical, and chemical control strategies. Cultural practices such as planting date, planting rate, and row spacing improve weed control by enhancing the competitive ability of the soybeans. Mechanical practices such as cultivation provide a non-chemical control strategy by disrupting between-row weeds. Chemical practices involve a multitude of herbicides that are labeled for use in soybeans.

Herbicides are a necessity for profitable soybean production in most cases. They are, however, only one component of a weed management program. Weed management is most successful when several weed management tactics, such as crop rotation, crop competition, cultivation, and judicious herbicide use are combined in a planned and coordinated program.

Cultural Practices

Cultural practices can have a tremendous influence on the severity of weed problems in a soybean crop. Soybeans are very competitive with weeds once canopy closes over crop rows, but early emerging weeds can cause serious problems. Cultural practices that promote fast canopy closure will be most effective in controlling troublesome weed populations.

Crop Competition. Crop competition is an effective but often underused weed control tool. The basic strategy is to follow practices that result in rapid soybean growth and canopy closure so as to provide maximum shading of weeds. Soybeans should lap in the row middles as soon as possible and certainly before blooming begins. Planting in narrow rows (20 inches or less) is the most effective means of achieving rapid canopy closure. Other practices that enhance crop competition with weeds include good seedbed preparation, use of high-quality seed, proper seeding rate, early season disease and nematode management, proper fertilization and liming, and early planting.

Crop Rotation. Crop rotation should be an integral component of a weed management program. This practice allows the use of different types of herbicides on the same field in different years, which can prevent the buildup of problem weeds and help to keep the overall weed population at lower levels. Also, soybeans are generally healthier when planted in rotation with other crops, leading to better competition of the crop with weeds. Crop rotation and proper selection and rotation of herbicides are key components in a strategy to avoid evolution of weed resistance to herbicides. Some soybean herbicides may persist long enough to damage certain rotational crops. Before using any herbicide, check the rotational restrictions.

Mechanical Practices

Mechanical practices can be a cost-effective method of controlling weeds before and after planting soybeans. While cultivation alone rarely provides complete weed control, it can be used in conjunction with herbicides to reduce herbicide costs.

Cultivation. Soybeans obviously can be grown without cultivation. Cultivation, however, is an effective and economical way to supplement control achieved with herbicides when soybeans are planted in wide rows. Applying herbicides in a band over the row and cultivating the row middles can substantially reduce weed-control costs. In fields with a light weed infestation, cultivation alone may be sufficient. Cultivation should be shallow (1 to 2 inches) to avoid damaging crop roots and to avoid breaking through any residual herbicide layer and bringing up untreated soil and weed seed. To be effective, shallow cultivation must be done while the weeds are small. Weed control is the only benefit received from cultivation except where special soil problems such as crusting or poor drainage exist. In situations where a good burndown is not achieved, preplant tillage is recommended for proper weed control.

Harvest Weed Seed Control. Harvest weed seed control (HWSC) methods developed in Australia for managing resistant weed species have recently garnered attention in the U.S. Practices are still in various stages of research, but it is known that managing weeds at harvest and destroying seeds reduce the seedbank and the number of troublesome weeds that must be controlled in future seasons. Chaff lining, chaff carts, and combine-mounted HWSC units are all options to remove or reduce seed at harvest. An additional benefit of HWSC is the reduction of selection pressure placed on chemical weed control.

Chemical Practices

Herbicides, when used according to label instructions, provide a safe and effective method to control weeds in soybeans. It is important to remember that herbicides will not solve all weeds problems and should be considered one of many tools for effective weed management. A successful weed management program will include herbicide applications at different points in the growing season, including preplant, preemergence, and postemergence.

Early Preplant/Burndown (PPlant). Some herbicides can be applied several weeks before planting. These herbicides should be effective at controlling emerged weeds or for residual control of weeds that have yet to emerge. If weeds are emerged at treatment in no-till cropping systems, a “burndown” herbicide with foliar activity should be applied. A product with residual activity can be combined with a burndown herbicide to extend the weed control window.

Preplant Incorporated (PPI). In conventional tillage cropping systems, certain herbicides can be applied and incorporated into the soil before planting to control susceptible weeds. These herbicides need to be incorporated thoroughly for herbicide activation.

Preemergence (PRE). Some herbicides with residual activity can be applied to the soil surface after planting but before the soybeans have emerged. Rainfall is required after the application to move the herbicide into the soil where it can be absorbed by germinating weeds.

Postemergence (POST). With the widespread use of herbicide-tolerant soybean varieties, postemergence applications have become very popular with growers. As a general rule, postemergence herbicides should be applied when weeds are in the two- to four-leaf stage or 2 to 3 inches tall. This normally occurs 2 to 3 weeks after planting around the V2 growth stage. The most common cause of poor results with postemergence herbicides is application when the weeds have grown beyond the optimum size for treatment.

Weather conditions are also important to the success of postemergence herbicides. For best results, weeds should be actively growing and not under moisture stress. When weeds are under moisture stress, their physiological processes occur at a very slow rate. Such conditions can have a significant effect on the leaf shape and texture resulting in poor interception of herbicide droplets on to leaves, which essentially leads to poor weed control due to reduced herbicide absorption by plants.

In the current scenario of increased herbicide-resistant weeds, the most important concept in relation to herbicide application timings is that each herbicide application before canopy closure should include an overlapping residual herbicide. This overlapping residual herbicide application strategy will prevent weeds from having any escaping window before they can be shaded-out by canopy and can negatively influence crop growth and development to achieve maximum yields.

Several herbicides are registered for use on soybeans to control various weeds. Before using any herbicide, you need to learn the important capabilities and limitations of the various products labeled for use in soybeans. You need to know which weeds are controlled by a given product, what rate to apply and how best to apply it, the crop injury potential, rotational restrictions, and any special precautions. Product labels, manufacturers’ literature, and Extension publications are good sources for this information.

When selecting an herbicide, important factors to consider include: (1) weeds present, (2) stage of crop and weed growth, (3) crop rotation restrictions, (4) environmental considerations, and (5) herbicide costs.

Broadleaf Weeds: Annual broadleaf weeds are more competitive with soybeans than are annual grasses. Annual broadleaf weeds also vary greatly in their potential to reduce soybean yield. A zero-tolerance policy should be considered regarding the more competitive species such as cocklebur, lambsquarters, jimsonweed, and Palmer amaranth to reduce or maintain a static seed

bank. Annual broadleaf weeds can be controlled with combinations of preplant incorporated, preemergence, and postemergence herbicides. The best application method to use depends upon the weed species present and compatibility with your overall production system. This section discusses the various herbicide options.

Grasses: Annual grasses can be controlled with preplant incorporated, preemergence, and postemergence herbicides. The soil-applied broadleaf herbicides such as Canopy, Scepter, Valor, and Tricor give suppression of some annual grasses. With light infestations of susceptible species, one of these broadleaf herbicides plus cultivation may be adequate for annual grasses. Where moderate to heavy grass infestations or non-susceptible species are expected, or where you do not plan to cultivate, a grass control herbicide is recommended.

Common herbicides that can be used to control grasses and broadleaf weeds are listed below by application method (i.e., preplant incorporated, soil applied, and postemergence). Tables 7-1 and 7-2 provide an easy to use reference for targeting a specific weed species.

Preplant Incorporated Herbicides

Group 3

Prowl H2O and Treflan are very similar: both provide good to excellent control of all the common annual grasses. They also provide good control of certain small-seeded broadleaf weeds such as pigweed, lambsquarters, carpetweed, common purslane, and Florida pusley. Control is basically season-long, but carryover is not likely to be a problem unless high rates are used. Several generic brands of trifluralin (the active ingredient in Treflan) are available.

The labels for Prowl H2O and Treflan specify the maximum waiting period between application and incorporation (24 hours for Treflan, 7 days for Prowl H2O). It is best to incorporate these herbicides as soon as possible after application. A delay in incorporation can result in a significant herbicide loss. Incorporate about 2 to 3 inches deep.

Group 2—Acetolactate Synthase (ALS) Inhibitors

Scepter contains imazaquin and can be applied in various preplant incorporated or preemergence tank mixes. The most consistent and broadest spectrum control is obtained when Scepter is tank-mixed with Prowl (pendimethalin) or Treflan (trifluralin) and incorporated. For improved sicklepod control, a follow-up POST application is required.

More rainfall is required to activate a preemergence application of Scepter than is required for most preemergence herbicides. For good control, 3/4 to 1 inch of rainfall is needed within 7 to 10 days after application. For this reason, incorporated applications perform more consistently.

Soil Applied Herbicides

Broadleaf Control

Group 5—Photosystem II Inhibitors

This class of chemistry is typically reserved for the metribuzin-based products. They normally control many annual broadleaf weeds. Notable exceptions include cocklebur and morningglory. Tricor can be applied preplant incorporated or applied preemergence.

Soybeans have a narrow margin of tolerance for metribuzin, see the section on metribuzin sensitivity below.

Group 7—Photosystem II Inhibitors

Lorox (linuron) is a preemergence herbicide that controls common ragweed, pigweed, and lambsquarters but is inadequate for control of most other broadleaf weeds. Lorox should not be used on sand, loamy sand, or gravelly soils or on any soil with less than 1/2% organic matter. Lorox is somewhat safer than metribuzin on light-textured. However, injury may occur if excessive rates are applied or if heavy rainfall is received shortly after planting. Do not use on soils with more than 3% organic matter.

Group 13—DOXP Synthase Inhibitor/Diterpene Biosynthesis Inhibitor

Command (clomazone) controls prickly sida, tropic croton, jimsonweed, lambsquarters, smartweed, spurred anoda, and velvetleaf. Note that Command does not control pigweed, morningglory, ladysthumb, or sicklepod.

Group 14—PPO Inhibitors

Valor (flumioxazin) is widely used in soybeans in North Carolina for preemergence control of small and large seeded broadleaf weeds. Several premixes are commonly used for broad spectrum activity. Avoid applying with Group 15 herbicides to prevent potential soybean injury. Spartan (sulfentrazone) is more commonly used in tobacco; however, it provides effective control of many common broadleaf weeds found in soybean. Reflex (fomesafen) is commonly used postemergence for broadleaf weed control, particularly for Palmer amaranth or common ragweed. It is also an effective PRE herbicide and is commonly applied as a premix with Dual Magnum (Prefix).

Group 15—Long-chain Fatty Acid Inhibitors

Brawl, Cinch, Dual Magnum, Dual II Magnum, Medal, and Medal II (S-metolachlor) can control pigweed and can be tank-mixed with most broadleaf products. Do not mix with Valor (flumioxazin).

Zidua (pyroxasulfone) controls pigweed and nightshade when applied PRE. It can be combined with many broadleaf herbicides or purchased in a packaged mixture called Anthem that contains fluthiacet-methyl.

Outlook (dimethenamid) can be applied preplant and incorporated at least 2 inches deep or PRE to control small seeded annual broadleaf weeds.

IntRRo (alachlor) can be applied preplant and PRE to control small seeded annual broadleaf weeds.

Warrant (acetochlor) is an encapsulated formulation which can be applied PRE or early POST to provide residual control of small seeded broadleaf weeds such as pigweed.

Premixes

In most instances, a premix including multiple modes of action tends to render greater control over troublesome broadleaves. Gangster and Surveil both include Valor plus Firstrate (cloransulam—group 2). Envive contains Valor, Classic (chlorimuron—group 2), and Harmony (thifensulfuron— group 2) in a premix. Valor XLT contains Classic and Valor for a very effective residual. Fierce contains Valor plus Zidua (pyroxasulfone—group 15) and provides broad spectrum control of many annual weeds.

Boundary, Canopy, Authority MTZ, and Trivence are metribuzin-based mixtures that include Dual (S-metolachlor—group 15), Classic (clorimuron—group 2), Spartan (sulfentrazone—group 14), and Valor (flumioxazin—group 14) plus Classic (chlorimuron—group 2), respectively. Because Canopy contains a high percentage of metribuzin, the use restrictions discussed previously for Tricor also apply to Canopy. Canopy can be applied in various preplant incorporated or preemergence tank mixes. In contrast to metribuzin alone, Canopy provides better control of large-seeded broadleaf weeds such as cocklebur and morningglory. Canopy normally controls most annual broadleaf weeds. In fields heavily infested with cocklebur or morningglory, cultivation or a postemergence herbicide may also be needed. Some rotational restrictions apply when using Canopy. However, there have been no documented cases of Canopy carryover to agronomic crops in North Carolina. Carefully follow label directions for sprayer cleanout after using Canopy.

Optill is a premix that is made up of Pursuit (imazethapyr—group 2) and Sharpen (saflufenacil—group 14). Ziduda Pro is composed of Pursuit (imazethapyr—group 2), Sharpen (saflufenacil—group 14), and Zidua (pyroxasulfone—group 15). Zidua Pro is labeled for burndown through preemergence applications but can cause injury on course soils with less than 2.0% organic matter.

Sonic and Authority First are composed of Spartan (sulfentrazone—group 14) and Firstrate (cloransulam—group 2) and can be applied PPI, PPlant, or PRE for control of broadleaves and sedges.

Prefix contains Dual (S-metolachlor) and Reflex (fomesafen—group 14) and will effectively control most annual grasses and broadleaves except for sicklepod.

Warrant Ultra is a combination of Reflex (fomesafen—group 14) and Warrant (acetochlor—group 15) that provides effective control of most annual grass and broadleaf weeds, excluding sicklepod.

Authority XL and Authority Assist contain Spartan (sulfentrazone) plus Classic (chlorimuron—group 2) at different concentrations. Both are effective for broadleaf weed control in burndown, preplant, and PRE applications.

Broadaxe is a combination of Dual Magnum (S-metolachlor—group 15) plus Spartan (sulfentrazone—group 14) that provides control of annual broadleaf weeds plus yellow nutsedge.

Grass Control

Group 13

Command gives season-long control of annual grasses except shattercane and Texas panicum, and it controls several broadleaf weeds. Note, however, that Command does not control pigweed. Command may be preplant incorporated or applied preemergence.

Group 15—Long-chain Fatty Acid Inhibitors

Brawl, Cinch, Dual Magnum, Dual II Magnum, Medal, and Medal II (S-metolachlor) applied PRE controls annual grass species, with exception of Texas panicum, shattercane, and seedling johnsongrass. Do not mix with Valor (flumioxazin).

Zidua (pyroxasulfone) controls annual grass species when applied PRE. It may be combined with broadleaf herbicides or purchased in a packaged mixture called Anthem that contains fluthiacet-methyl. Texas panicum, shattercane, and seedling johnsongrass will not be controlled.

Outlook (dimethenamid) can be applied preplant and incorporated at least 2 inches deep or PRE to control annual grasses and sedges. Texas panicum, shattercane, and seedling johnsongrass will not be controlled.

IntRRo (alachlor) can be applied preplant and PRE to control grass species. Texas panicum, shattercane, and seedling johnsongrass will not be controlled.

Warrant (acetochlor) is an encapsulated formulation which can be applied PRE or early POST to provide residual control of most annual grass species except Texas panicum, shattercane, and seedling johnsongrass.

Mixtures

Boundary contains Dual Magnum (S-metolachlor—group 15) and Tricor (metribuzin—group 5). Annual grass and broadleaf weeds will be controlled; however, control of shattercane, seedling johnsongrass, and Texas panicum will not be acceptable.

Fierce is a combination of pyroxasulfone (group 15) and Valor (flumioxazin—group 14) that controls a broad spectrum of annual grass and broadleaf weeds. Texas panicum, shattercane, and seedling johnsongrass will not be controlled.

Warrant Ultra contains Warrant (acetochlor—group 15) and Reflex (fomesafen—group 15) and provides control of annual grasses and many broadleaf species. Exceptions include shattercane, seedling johnsongrass, and Texas panicum.

Prefix is a combination of Dual Magnum (S-metolachlor—group 15) and Reflex (fomesafen—group 15). Annual grasses and many broadleaf weed species will be controlled with the exception of seedling johnsongrass, shattercane, and Texas panicum.

Broadaxe is a combination of Dual Magnum (S-metolachlor—group 15) and Spartan (sulfentrazone—group 15). Annual grasses, sedges, and many broadleaf weed species will be controlled. Seedling johnsongrass, shattercane, and Texas panicum will not be controlled.

Zidua PRO contains Zidua (pyroxasulfone—group 15) plus Pursuit (imazethapyr—group 2) plus Sharpen (saflufenacil—group 14). Grass and broadleaf weeds will be controlled with the exception of Texas panicum, seedling johnsongrass, and shattercane.

Python + Treflan are packaged mixtures of grass and broadleaf herbicides. Python+ Treflan contains flumetsulam plus trifluralin (the active ingredient in Treflan). Salute contains trifluralin plus metribuzin (the active ingredient in Sencor). Turbo contains metolachlor (the active ingredient in Dual) plus metribuzin. Squadron contains pendimethalin plus imazaquin (the active ingredients in Prowl and Scepter, respectively). Tri-Scept contains trifluralin and imazaquin.

Postemergence Herbicides

Broadleaf Control—Herbicide Resistant Varieties

Herbicide-tolerant varieties have made POST applications much more effective and economical since their first release in 1996. Today, soybean cultivars have enhanced tolerance to ALS herbicides, glyphosate, glufosinate, dicamba, and 2,4-D. Understanding which cultivars are tolerant to which specific chemistries is crucial to developing an effective weed management program that staves off selection for resistance and maximizes crop safety.

Glyphosate-based products such as Roundup, Touchdown, and Glystar can be applied POST on varieties that possess the Roundup Ready trait. They can be regularly mixed with Flexstar GT, Extreme, and Warrant.

Similarly, Liberty (glufosinate) can be applied POST to Liberty Link varieties. These options, when used in rotation and in combination with tank mixes, have shown to be highly effective in controlling troublesome weeds in North Carolina.

Enlist soybeans allow farmers to make POST applications of Enlist One (2,4-D Choline) and Enlist Duo (2,4-D Choline plus glyphosate). Enlist E3 beans can be treated with glufosinate, glyphosate, and 2,4-D.

Xtend soybeans can be treated with POST applications of Xtendimax (dicamba), Engenia (dicamba), and glyphosate.

Xtendflex soybeans have tolerance to glyphosate, dicamba (Xtendimax and Engenia), and glufosinate (Liberty). Current labels do not allow the application of dicamba + glufosinate in a tank mixture, so plan weed management practices accordingly.

Off-target movement is a high concern for Group 4 herbicides and therefore extra precautions should be taken, including the use of buffer zones, drift-reducing spray tips, sprayboom height, sprayer speed, and environmental considerations. Carefully read the label prior to use to be sure all applications are conducted ethically and legally.

Broadleaf Control—Any Variety

Group 6

Basagran (bentazon) gives excellent control of yellow nutsedge, cocklebur, jimsonweed, and smartweed, and good control of prickly sida, spurred anoda, velvetleaf, and giant ragweed. Although control may be inconsistent, Basagran plus crop oil concentrate usually controls small common ragweed and lambsquarters. Crop tolerance is excellent.

Storm is a packaged mixture of Basagran (bentazon) and Blazer (acifluorfen—group 14) that can be added to many ALS-based herbicides (group 2) such as FirstRate (cloransulam), Pursuit (imazethapyr), and Raptor (imazamox).

Group 14

Ultra Blazer (acifluorfen) controls most broadleaf weeds. Exceptions include prickly sida, sicklepod, spurred anoda, velvetleaf, and volunteer cowpea. Blazer normally causes some soybean leaf crinkling and leaf bronzing or leaf burn. Addition of crop oil concentrate or higher rates of surfactant increase the amount of leaf burn. This injury is temporary and the soybeans recover quickly and grow normally.

Resource (flumiclorac) provides excellent control of velvetleaf when applied POST with 1 quart of crop oil concentrate.

Cobra (lactofen) controls most annual broadleaf weeds. Major exceptions are lambsquarters, sicklepod, smartweed, and spurred anoda. Morningglory control often is inconsistent. Cobra applied at the full labeled rate (12.5 ounces per acre) usually causes moderate to severe soybean leaf burn. Although the soybeans usually recover and grow normally, the injury often is unacceptable to growers. For this reason, Cobra at the full labeled rate is generally not recommended. Several tank mixes containing Cobra at low rates (6 ounces per acre) are registered.

Reflex (fomesafen) controls most common broadleaf weeds. Major exceptions are prickly sida, sicklepod, spurred anoda, and velvetleaf. Unless treated when smaller than ¹⁄₂ inch, lambsquarters will not be controlled. Soybean tolerance of Reflex is normally good. The Reflex label mentions rotational restrictions and injury on sorghum and corn has been observed in North Carolina. Additional fomesafen products include: Flexstar, Dawn, and Rythem.

Group 2

Classic (chlorimuron) will control many broadleaf weeds. Major exceptions are lambsquarters, prickly sida, and tropic croton. Control of spurred anoda and velvetleaf also may not be adequate. Classic may be tank-mixed with several other herbicides to expand the spectrum of control. Soybean tolerance of Classic is normally good, but some leaf distortion and yellowing and minor crop stunting may be noted, especially when applied to soybeans under stress conditions. The soybeans usually recover and grow normally. There are some rotational restrictions following Classic application; however, no documented cases of Classic carryover to agronomic crops have been observed in North Carolina. See the label for specific directions on sprayer cleanout.

FirstRate (cloransulam) can be applied twice per season for effective control of cocklebur, morningglory, and common ragweed. Sicklepod can be controlled only at the cotyledon stage.

Harmony (thifensulfuron)controls lambsquarters, pigweed, smartweed, and velvetleaf. Because the spectrum of control is limited, Harmony is normally tank-mixed with another broadleaf herbicide to expand the range of species controlled. Carefully follow label directions for herbicide and adjuvant rates when applying tank mixes containing Pinnacle.

Synchrony XP is a packaged mixture of Classic (clorimuron) and Harmony (thifensulfuron). It is important to use the recommended lower rate when applying to non-STS soybeans.

Scepter applied postemergence controls cocklebur and pigweed excellently. Control of other broadleaf weeds will be inadequate. See the section on sicklepod control.

Soybean tolerance of Scepter applied postemergence is excellent. See the most recent label for rotational restrictions.

Pursuit (imazethapyr) applied postemergence, controls cocklebur, pigweed, jimsonweed, and smartweed. Morningglory control is usually acceptable but can be inconsistent. Pursuit kills weeds very slowly, and it may take as long as three weeks to kill morningglory. In some cases, the weeds do not die completely but rather stop growing and the soybean canopy fills in above them. In addition to controlling certain broadleaf weeds, a postemergence application of Pursuit usually gives adequate control of broadleaf signalgrass, foxtails, seedling johnsongrass, and shattercane, and it sometimes gives adequate control of rhizome johnsongrass. Pursuit does not control common ragweed, lambsquarters, prickly sida, sicklepod, and certain other weeds. Soybean tolerance of Pursuit is good but there are rotational restrictions. Do not plant cotton or most vegetable crops the year following a Pursuit application.

Grass Control—Any Variety

Group 1

Select Max, Select, Arrow, Tapout, Volunteer (clethodim) controls grass species, including volunteer corn, when applied POST. Can be mixed with broadleaf products. See label for adjuvant instructions.

Poast, Poast Plus (sethoxydim) controls grass species, including volunteer corn, when applied POST. Can be mixed with broadleaf products. See label for adjuvant instructions.

Assure II (quizalofop) controls grass species, including volunteer corn, when applied POST. Can be mixed with broadleaf products. See label for adjuvant instructions.

Fusilade DX (fluazifop-p-butyl) controls grass species, including volunteer corn, when applied POST. Can be mixed with broadleaf products. See label for adjuvant instructions.

Metribuzin Sensitivity

Soybeans have a narrow margin of tolerance for metribuzin. Therefore, be extremely careful when selecting the application rate of a product that contains metribuzin. This can be a problem in fields with varying soil types. Miles Inc., makers of Salute, Sencor, and Turbo, offer a free soil testing program called SURE. The SURE program recommends a specific rate of Salute, Sencor, or Turbo based on soil texture, organic matter content, and application method (preplant incorporated or preemergence). SURE has proven to be very helpful in selecting the proper metribuzin rate to avoid crop injury.

There are several restrictions on the use of metribuzin. Specific restrictions vary depending on application methods and tank mixes, but in general this herbicide should not be used on sands or loamy sands with less than 1% organic matter or any soil with less than 1/2% organic matter. Certain varieties of soybeans, such as Asgrow 6520 and Coker 156, are very susceptible to metribuzin injury. Before applying a product containing metribuzin to a new variety, check with your seed dealer to determine the variety’s sensitivity to metribuzin. Increased soybean injury will also be observed when metribuzin is used in conjunction with soil-applied organophosphate insecticide/nematicides; see the product labels for details.

Herbicide Resistance

Herbicide resistance refers to the inherited ability of a biotype of a weed to survive an herbicide application to which the original population was susceptible. A biotype is a group of plants within a species that has biological traits (such as resistance to a particular herbicide) not common to the population as a whole.

Herbicide resistance is a very serious problem facing North Carolina growers. The widespread use of herbicide-resistant soybean varieties as well as the use of several herbicides with the same mode of action has created major concerns. Mode or site of action refers to the specific process through which an herbicide kills a susceptible plant. Today, herbicides having the same mode of action can be used on several crops that may be grown in rotation.

Resistance to both ALS-inhibiting herbicides and glyphosate is widespread across the state (Table 7-3). Based on a 2010 survey, 95% of the Palmer amaranth populations in the state contained individuals resistant to glyphosate and ALS inhibitors. Horseweed resistant to glyphosate is very common across eastern North Carolina and is becoming a problem in the piedmont. Common ragweed resistant to glyphosate and ALS-inhibitors exists in several northeastern counties. Italian ryegrass resistant to glyphosate occurs sporadically across the southern piedmont. Of particular concern is a population of common ragweed that was found to be resistant to ALS-inhibitors, glyphosate, and PPOs in the northeastern part of the state.

Growers can no longer rely on only glyphosate to control weeds in their Roundup Ready soybean systems. Although the technological advancement in genetic engineering gave rise to several other herbicide-tolerant soybean varieties that are either currently on market or will likely be introduced in the near future, the life of these technologies depends on their proper stewardship. If these newer technologies are not used judiciously and as only one component of a diverse integrated weed management approach, their fate will be no different than that of roundup-ready technology.

Resistance Management Strategies

Herbicides are used in crop production simply because they are more effective or more economical than other means of weed control. If resistance to a particular herbicide or family of herbicides evolves, suitable alternative herbicides may not exist. We must use herbicides in a manner that deters the development of resistance.

It is essential to understand how resistance evolves in order to understand how to avoid resistance. There are two prerequisites for herbicide resistance evolution. First, individual weeds possessing genes conferring resistance must be present in the native population. Second, selection pressure resulting from extensive use of an herbicide to which these rare individuals are resistant must be exerted on the population. Resistant individuals, if present, make up a very low percentage of the overall population. Typically, resistant individuals are present at frequencies ranging from 1 in 100,000 to 1 in 100 million. If the same herbicide or herbicides with the same mode of action are used continuously, the susceptible individuals are killed but the resistant individuals are unharmed and produce seed. If the selection pressure continues for several generations, the resistant biotype will ultimately make up a high percentage of the population. At that point, acceptable weed control can no longer be obtained with the particular herbicide or herbicides.

Rotation of herbicides having different modes of action is the single most important component of a management strategy to avoid evolution of herbicide resistance. Tank mixes or sequential applications of herbicides having different modes of action are often touted as components of a resistance management strategy. If the components of the tank mix or sequential applications are chosen wisely, this strategy can help delay resistance evolution. Unfortunately, many of the requirements of tank mix or sequential applications to avoid resistance are not met with commonly used mixtures. To be most effective at preventing resistance evolution, both herbicides used sequentially or in tank mixtures should have the same spectrum of control and should have similar persistence.

To the extent possible, integrate nonchemical control practices such as cultivation into your weed management program. Maintain good records of herbicide usage in each field for future reference.

Detecting herbicide-resistant weeds

Most weed control failures are not due to herbicide resistance. Before assuming that weeds surviving an herbicide application are resistant, eliminate all other possible causes of poor control. Potential causes of a weed control failure may include misapplication (such as inadequate rate, poor coverage, poor incorporation, or lack of an adjuvant); unfavorable weather conditions for good herbicide activity; improper timing of herbicide application (in particular, applying postemergence herbicides after weeds are too large for good control); and weeds emerging after application of a short-residual herbicide.

After eliminating all other possible causes of poor control, the following may indicate presence of an herbicide-resistant biotype: (1) all species normally controlled by the herbicide except one are controlled well; (2) healthy plants of the species in question are interspersed among plants of the same species that were killed; (3) the species not controlled is normally very susceptible to the herbicide in question; and (4) the field has a history of extensive use of the herbicide in question or herbicides with the same mode of action.

If resistance is suspected, immediately stop using the herbicide in question and other herbicides having the same mode of action. Contact your local Extension agent and a representative of the chemical company for advice on alternative control strategies. Follow an intensive program that relies upon herbicides with a different mode of action and nonchemical control practices to reduce weed seed production as much as possible. Avoid spreading weed seed to other fields by cleaning equipment and plan your weed management program for subsequent crops carefully.

Spray Adjuvants

Most postemergence herbicide labels recommend use of a spray adjuvant. Spray adjuvants are any non-pesticidal material added to an herbicide to improve the effectiveness of the herbicide.

A surfactant reduces the surface tension between the leaf surface and the spray droplet and allows the spray droplet to spread out and cover a greater area of the leaf surface. This effect results in greater absorption of the herbicide into the leaf and a greater probability of killing the weed. Use of surfactants may also increase crop injury with some herbicides. Brands of surfactants may differ in their concentration of active ingredients (referred to as surface-active agents). The concentration of surface-active agents should be listed on the label. A good agricultural surfactant should contain at least 75% surface-active agents.

Recently, several silicone-based surfactants have entered the market. These surfactants have excellent wetting properties but generally are more expensive than regular nonionic surfactants. Only a limited amount of research has been conducted comparing herbicide efficacy with silicone surfactants to that with nonionic surfactants. Research to date generally shows that silicone surfactants work well but are not sufficiently better than nonionic surfactants to justify the extra cost.

Crop oil concentrates are a mixture of 80 to 85% nonphytotoxic petroleum-based oil plus 15 to 20% emulsifier (surfactant). The function of crop oil concentrates is to reduce surface tension between leaf surfaces and spray droplets, to promote herbicide penetration into leaves, and to prolong droplet drying time which allows more herbicide to be absorbed. Use of crop oil concentrates may increase crop injury with some herbicides.

Vegetable oil concentrates are similar to crop oil concentrates. A vegetable oil concentrate normally contains 80 to 85% once-refined soybean or cottonseed oil plus 15 to 20% emulsifier. The function of a vegetable oil concentrate is basically the same as that of a crop oil concentrate. Research has shown that the effectiveness of vegetable oil concentrates may equal that of crop oil concentrates but is rarely greater. Some herbicide labels allow substitution of a vegetable oil in place of a crop oil concentrate, while other labels prohibit use of vegetable oils.

Some postemergence herbicide labels specify the use of only nonionic surfactants, some specify the use of only crop oil concentrates, and some specify the use of either. The best herbicide performance will be achieved by following the label directions for adjuvant usage.

There are several types and numerous brand names of adjuvants on the market. In most cases, the buyer cannot determine the exact composition of the particular adjuvant. In addition, because there are so many brands available, neither university scientists nor herbicide manufacturers can evaluate all adjuvant-herbicide combinations. Without this unbiased evaluation, adjuvant manufacturers are free to make whatever claims they wish for their particular brands.

Be skeptical of incredible claims or flashy demonstrations. If the sales representative shows you data on a particular adjuvant, make sure legitimate comparisons are being made. As a general rule, if the claim sounds too good to be true, it probably is not true. Be suspicious of anyone claiming that an adjuvant allows use of greatly reduced herbicide rates, improves activity of soil-applied herbicides, improves water infiltration into soil, controls insects or diseases, or breaks soil hardpans.

Here are some guidelines from the American Soybean Association for the use of spray adjuvants with postemergence herbicides:

1. Follow label directions for adjuvant usage. If the label specifically states to not use an adjuvant, then do not use one. If the label does not mention the use of an adjuvant, think twice before using one. It may or may not improve performance and it might result in unacceptable crop injury.
2. Whenever possible, use the exact adjuvant(s) specified on the label. When the specific brand is not available, ensure that the substituted product has the same general characteristics. In the case of surfactants, select one that is of the same type specified on the label (usually nonionic) and has the same percentage of surface-active agents as specified on the label. For oils, select one that contains the type oil specified on the label (petroleum-based or vegetable oil) and has the same amount of emulsifier.
3. If the label specifies only the type adjuvant but not the brand, buy a reputable brand from a reputable dealer.
4. Be careful when mixing adjuvants. When more than one adjuvant is added to the spray tank, there could be an unexpected interaction.
5. Be careful with adjuvant rates. Adding more than is recommended on the label is not always better, and adding less may give poor results.

The key to weed management is to correctly identify and treat weeds while they are small. A good weed management plan involves field scouting to determine the size and kind of species present. By understanding what is present in a field, treatment decisions can be made to protect crops from weed competition and prevent selection of herbicide-resistant weeds. If weeds can be effectively managed until the middles are lapped, soybeans can typically compete well with weeds that emerge later in the season. Scouting should also occur after an herbicide application to effectively identify escapes or potentially species developing new types of resistance.

Broadleaves

Asteracea (Sunflower) family.

Description. Summer annual.

Height: Up to 7 feet tall.

Leaves: Thick, waxy, ovate cotyledons. The first true leaves are opposite while later leaves are alternate. Leaves feel like sandpaper, are triangular in shape, have serrated edges, and have three prominent veins arising from the same point.

Stems: Erect and branched, with dark spots and short, stiff hairs.

Flowers: Inconspicuous, green flowers occur in clusters in leaf axils and at the ends of stems. Fruit is encased in an egg-shaped, brown bristly burr with two chambers.

Management Considerations. Common cocklebur is very competitive and germinates from deep in the soil. Control with an ALS, PPO, or PSII inhibiting herbicide preemergence is ideal. Glyphosate and glufosinate are effective postemergence options in soybean.

Asteraceae (sunflower) family

Description. Emerges from March through November so considered both a winter and summer annual.

Height: 1 to 9 feet tall.

Leaves: Alternate and crowded along the stem with simple blades.

Stems: Erect, simple, with stiff hairs. Branched above the inflorescence.

Flowers: Small white pink or yellow disc flowers.

Management Considerations. Horseweed can germinate 10 months out of the year and can be a challenge to control. Once it begins flowering, it becomes very difficult to control with postemergent herbicides. Treat when the weed is small, typically with an effective burndown program. Deep tillage can help combat horseweed.

Convolvulaceae (morningglory) family.

Description. Annual.

Height: Vine up to 6 feet long.

Leaves: Butterfly shaped cotyledons. Alternate, ivy- or heart-shaped leaves up to 4 inches long/wide. Depending on the species, leaves may have few hairs or be densely pubescent.

Stems: Climbing, hairy vines.

Flowers: Multicolored funnel shaped flowers with colors including red, blue, purple, white, and variegated.

Management Considerations. The vining/climbing nature makes this a difficult weed to control. Control when plants are four-leaf or smaller prior to developing runners can be achieved with glyphosate, Liberty, ALS inhibitors, and PPO inhibitors. Once runners develop, control is greatest with FirstRate or Resource.

Amaranthaceae (pigweed) family.

Description. Annual.

Height: Up to 8 feet tall.

Leaves: Alternate, ovate shaped, hairless leaves with long petioles, prominent veins, and a white, V-shaped watermark.

Stems: Erect, branched, and hairless. May be tinged red.

Flowers: Arranged in thick spikes. Male and female flowers are produced on separate plants. Male flowers have soft, thin, triangular bracts that shed pollen. Female flowers have bristly, stiff, sharp bracts.

Management Considerations. This is the biggest weed problem in North Carolina because it has developed resistance to glyphosate as well as both ALS and PPO inhibiting herbicides. Combat with multiple modes of action and supplement herbicide control with hand weeding if needed. A single plant can produce a million seeds, so take all measures to prevent these weeds from going to seed.

Asteraceae (Sunflower) family.

Description. Annual.

Height: Up to 3 feet tall.

Leaves: Thick oblong cotyledons. Lacey, finely divided, and slightly hairy leaves up to 4 inches long. Lower leaves are alternate while upper leaves are opposite.

Stems: Erect, branched, and hairy.

Flowers: Greenish-yellow flowers about 1/8-inch long. Male flowers are produced in racemes at the end of stems, and female flowers are produced in the upper leaf axils.

Management Considerations. Ragweed is very competitive. Resistance to glyphosate as well as the ALS and PPO inhibitors has been documented in North Carolina. Residual herbicides are critical for early season control of common ragweed. PPO inhibitors, ALS inhibitors, and PSII inhibitors such as metribuzin and linuron should be considered where infestations occur. Combat with effective preemergence herbicides followed with effective postemergence herbicides.

Fabaceae (bean/pea) family.

Description. Annual.

Height: 1 to 6 feet tall.

Leaves: Rounded cotyledons with 3 to 5 distinct veins. Alternate leaves with four to six egg-shaped leaflets.

Stems: Erect, branched, and hairless.

Flowers: Distinctive yellow petals that arise from the leaf axils. Seeds are housed in long, sickle-shaped hairless pods.

Management Considerations. The critical time to control sicklepod is the first four weeks after planting. Seeds can be an issue when harvesting, so manage escapes before plants go to seed.

Grasses

Poaceae (grass) family.

Description. Semi-prostrate summer annual.

Height: Up to 3 feet tall.

Leaves: Seedlings have hairless blades that may be maroon-tinged and hairy, maroon-tinged sheaths. True leaves are short, wide, hairless blades that are rolled in the bud. Margins, collar, ligule, and sheaths are hairy.

Flowers: Slender 12-inch long seed head with two to six smaller branches. Spikelets are somewhat flattened.

Management Considerations. Emerges from April through July, making it difficult to manage. Control with both a preemergence and postemergence application.

Poaceae (grass) family.

Description. Annual.

Height: 1 to 3 feet tall.

Leaves: Seedlings have shiny leaves. True leaves are flat, glossy, hairless blades that range from 2 to 10 inches long. Leaves are rolled in the bud and ligules are membranous. Auricles are usually well developed, but sometimes lacking.

Flowers: Small, stalkless spikelets alternate one another along the 3 to 12-inch-long main flowering stem. Needlelike awns are on the individual flowers.

Management Considerations. The best time to control ryegrass is in the fall with tillage or herbicides. If plants emerge in the spring, treat when plants are small as larger plants become difficult to control because of profuse vegetative growth and a dense root system.

Poaceae (grass) family.

Description. Spreading summer annual.

Height: May be erect and reach up to 2 feet tall or grow close to the ground.

Leaves: Seedlings have soft hairs that cover sheaths and blades, and the first leaves are relatively broad for a grass. True leaves are pale or yellowish-green, velvety hairy leaves that extend to leaf sheaths and nodes. Auricles are absent and ligule is membranous with a fringe of hairs.

Flowers: Simple, narrow spike seed head up to 10 inches long. Each spike has two rows of spikelets.

Management Considerations. Texas millet germinates late in the season. Residual herbicides applied with postemergence applications will provide the greatest control.

Sedges

Cyperaceae (Sedge) family.

Description. Perennial.

Height: Up to 2 feet tall.

Leaves: Shiny, green, and hairless with a noticeable ridge along the midvein. Leaves occur in groups of three from the base of the plant.

Stems: Three-sided, erect, and unbranched.

Flowers: Yellow or brown spikelets that occur at the ends of the stem in a cluster.

Management Considerations. Nutsedge is hard to control because it emerges from tubers throughout the growing season. Tillage will only temporarily disrupt the tubers. Glyphosate provides only marginal control. Basagran is an effective option for control.

Adapted from 2017 Cotton Information

The performance of any pesticide depends upon many things, not the least of which is proper application at the correct rate. Failure to apply the correct rate uniformly can lead to poor pest control, crop injury, or unnecessary expense.

Every sprayer should be thoroughly calibrated before the first use of the season, and the calibration should be checked periodically during the season. Additionally, the sprayer should be recalibrated every time nozzles, pressure, or travel speed are changed.

Before Calibration

Remove nozzles and strainers, including in-line strainers. Using a soft brush, wash nozzles and strainers in soapy water. Be sure to remove all deposits. Do not clean nozzles with any hard object (such as a knife or wire) because this will destroy the nozzle.

Thoroughly wash out sprayer and flush lines using a strong detergent or commercial tank cleaner.

Check hoses and connections for leaks or signs of aging or damage. Replace defective hoses.

Check components such as the pressure gauge, pressure relief/regulating valve, control valves, and agitator. Replace defective parts.

Select proper size and type of nozzle for the particular pesticide application planned. Consult nozzle manufacturers’ catalogs or pesticide labels for guidance. Replace nozzles at least once a year. If the sprayer is used on a large acreage, nozzles may need replacing more frequently. Remember that brass nozzles wear more quickly than stainless steel or ceramic nozzles.

Make sure every nozzle on the sprayer is the same type and size (an exception may be hooded sprayers; see discussion below). Then check for proper spray pattern. Replace nozzles that do not produce the proper pattern. Next, check for uniformity of nozzle output. This needs to be done even if new nozzles are installed.

To check for uniformity of output, partially fill sprayer with clean water. Adjust pressure to the level desired during the spraying operation. Catch and measure output from each nozzle separately for a given length of time. Replace any nozzle having an output of 10%more or less than the average of all nozzles.

Calibration Procedure

The procedure uses the equation below:

GPA = [(GPM)(5946)] ÷ [(MPH)(W)]

GPA = gallons per treated acre
GPM = gallons per minute
MPH = travel speed, miles per hour
W = effective coverage per nozzle, inches

Step 1. Determine effective coverage per nozzle, or W.

W = nozzle spacing for broadcast application

W = band width when banding with one nozzle per band

W = band width divided by number of nozzles per band if banding with more than one nozzle per band

Step 2. Determine travel speed.

Measure off a distance of at least 200 feet in a field with surface conditions similar to fields to be sprayed. Engage any equipment to be used during the actual spraying operation (such as a disk or planter), choose the gear and throttle setting you plan to use during actual spraying, and determine the time required to drive the designated distance. You can improve your accuracy by doing this several times and taking the average.

MPH = [(feet traveled)(60)] ÷ [(seconds to travel)(88)]

Step 3. Determine nozzle output.

Partially fill the tank with the desired liquid carrier, but do not add pesticide. Adjust the pressure to the level that will be used during the actual spraying operation. Catch the output from several nozzles separately for one minute. Average the output over nozzles. It is best to catch the output as ounces per minute (OPM) and then convert to gallons per minute (GPM).

GPM = OPM ÷ 128

Step 4. Determine sprayer output, as gallons per treated acre (GPA).

GPA = [(GPM)(5940)] ÷ [(MPH)(W)]

Step 5. Determine amount of pesticide to add to tank.

Amount to add = [(pesticide rate per acre)(gallons spray solution)] ÷ GPA

Examples

Broadcast Application: Preplant, Preemergence, or Postemergence Overtop

Assume you plan to broadcast 1.5 pint per acre of Dual Magnum as a preemergence application. Your sprayer has nozzles mounted 19 inches apart along the boom, hence W = 19. The tank holds 240 gallons. It takes 20 seconds to drive 200 feet. You catch an average nozzle flow of 42 oz per minute (OPM).

MPH = [(ft)(60)] ÷ [(sec)(88) = [(200 ft)(60)] ÷ [(20 sec)(88)] = 6.82

GPM = OPM ÷ 128 = 42 ÷ 128 = 0.3281

GPA = [(GPM)(5940)] ÷ [(MPH)(W)] = [(0.3281)(5940)] ÷ [6.82)(19) = 6.82

Amount to add = [(pesticide rate per treated acre)(gallons spray solution)] ÷ GPA

Amount to add = [(1.5 pt per treated acre)(240 gal solution)] ÷ 15.04 gal per acre = 23.94 pt or 3 US gal

Banded Application Using One Nozzle Per Row: Preemergence or Postemergence Overtop

Assume you plan to apply Boundary behind the planter at the broadcast rate of 2.4 pints per acre. You want to make a 14-inch band of Boundary over the row using a single nozzle per band. In this case, W = 14. Your soybeans are planted on 30-inch rows. The tank holds 240 gallons. It takes 28 seconds to drive 200 feet. You catch an average of 26.5 oz per minute (OPM).

MPH = [(ft)(60)] ÷ [(sec)(88) = [(200 ft)(60)] ÷ [(28 sec)(88)] = 4.87

GPM = OPM ÷ 128 = 26.5 ÷ 128 = 0.2070

GPA = [(GPM)(5940)] ÷ [(MPH)(W)] = [(0.2070)(5940)] ÷ [(4.87)(14)] = 18.03 gallons per treated acre

Amount to add = [(pesticide rate per treated acre)(gallons spray solution)] ÷ GPA

Amount to add = [(2.4 pt per treated acre)(240 gal solution)] ÷ 18.03 gal per acre = 31.95 pints or 4 gallons

Banded Application Using Two Nozzles Per Row: Postemergence-directed

Assume you plan to direct Aim in a 16-inch band under soybeans. Your soybeans are planted on 36-inch rows, you have two nozzles per row on your directed sprayer, and your sprayer tank holds 250 gallons. In this case, W = 8. The Aim label suggests 1 oz per treated acre. It takes 25 seconds to drive 200 feet. You catch an average nozzle output of 19 oz per minute (OPM).

MPH = [(ft)(60)] ÷ [(sec)(88) = [(200 ft)(60)] ÷ [(25 sec)(88)] = 5.45

GPM = OPM ÷ 128 = 19 ÷ 128 = 0.1484

GPA = [(GPM)(5940)] ÷ [(MPH)(W)] = [(0.1484)(5940)] ÷ [(5.45)(14)] = 20.22 gallons per treated acre

Amount to add = [(pesticide rate per treated acre)(gallons spray solution)] ÷ GPA

Amount to add = [(1 oz per treated acre)(250 gal solution)] ÷ 20.22 gal per acre = 12.36 oz or 0.39 quarts

NOTE: When banding, always think of application rates in terms of “rate per treated acre,” which is the rate given on labels. Obviously, you are not treating the whole acre, so the pesticide rate per planted acre will be less. But you have calibrated your sprayer output on the basis of “gallons per treated acre,” and you want to calculate the amount of pesticide to add to the tank on that same basis.

Herbicides are an integral part of any weed management system. In most cases, target weeds are controlled without compromising the soybean plants, but sometimes injury does occur. Many different factors can affect soybean herbicide tolerance including environment conditions, variety, herbicide active ingredient, soil properties, tank mixtures, or additives.

The first step to diagnosing potential herbicide injury is to document what has occurred in the field. Things you should take note of include:

• Date injury first appeared
• Plant parts affected and to what extent
• Speed and degree of plant recovery
• Drift patterns across the field
• Recent prevailing winds
• Overlapped areas at the ends of rows or across the field
• Injury symptoms corresponding to spray tank loads
• Uniform injury strips caused by application equipment
• Injury differences across soil types
• Symptoms on weeds present
• Crop damage pattern versus field equipment/spray patterns
  • *From Iowa State University Soybean Field Guide.

Each herbicide mode of action (MOA) group has different injury symptoms that may mimic other biotic or abiotic stresses. Refer to Table 7-4 for common herbicide injury symptoms as well as other stresses the symptoms often mimic.