8. Managing Weeds in Winter Wheat
By Drew Lyon & Case Medlin
Weeds compete with winter wheat for water, light, space, and nutrients. Weed competition reduces wheat yields and profitability, and also slows harvest and increases combine repair costs. Growers may be docked at the elevator for having excessive moisture or weed seeds in their grain. Weeds also may serve as hosts for insects or diseases that can harm winter wheat plants and reduce yields. An effective weed control program considers all aspects of the cropping system, including tillage program, rotational crops, rotation of herbicides used, soil fertility, disease and insect management programs, and the complex of weeds targeted.
Integrated Weed Management uses a combination of different practices to manage weeds. By reducing the reliance on one or two specific weed control techniques (for example, relying solely on the use of herbicides), weeds are less likely to adapt to these methods. The objective of integrated weed management is to maintain weed densities at manageable levels while preventing weed shifts to more difficult-to-control species. This objective is met by preventing weed problems before they start, helping the crop gain the competitive advantage over weeds, and making it difficult for weeds to adapt to a cropping system. All of these factors contribute to a healthy, competitive crop.
Preventive Weed Control
The best way to control weeds is to keep them out of fields in the first place. Prevention, or stopping the advancement of weed infestations, is an important part of an integrated weed management program. It requires time and diligence from the grower, but offers an effective, low cost control.
Planting crop seeds contaminated with weed seeds has been the most common method of spreading weeds for centuries. Drill box surveys in Kansas, Nebraska, and Oklahoma have shown that many growers are planting unacceptable levels of weed seeds with their crop. Using trashy wheat seed will not only increase the weediness of a field, but it also reduces the seeding rate, resulting in a lower wheat population and a less competitive crop. At the very least, farmers should have their seed cleaned at certified seed conditioners. To ensure you are planting high quality, weed-free seed, purchase certified seed. The benefits, which include increased forage and grain yields, far outweigh the cost.
Other Preventive Control Methods
Clean tractors, implements, trucks, and combines before moving them from weed-infested fields to clean fields. This should include inspecting equipment of hired contractual operators before they enter your fields, especially harvesting equipment that may introduce weed seed from other counties or states.
Keep uncropped areas (fence lines and field borders) weed-free by establishing a good stand of a perennial grass or spraying annually with herbicides. A typical 20-foot long fence surrounding a section of wheat amounts to less than 10 acres. Compare the cost of preventing weed establishment on the 10-acre border (with relatively low-cost options) versus the long-term control of weeds on 630 acres of cropland (with typically more expensive measures).
Do not allow livestock to move directly from infested to clean areas. It can take 7 to 10 days for ingested weed seeds to pass through most livestock. Prevent weed seed production in all areas. To control annual and biennial weeds, you must control their seed production, while to control perennial weeds you must control both seed production and vegetative reproduction structures.
Cultural Weed Control
Cultural weed control involves manipulating the crop-weed environment so that conditions are more favorable for crop plants than weeds. Crop rotation and crop competitiveness are important cultural control practices in winter wheat production.
Crop rotation is an important component of integrated weed management. The use of diverse crops with different life cycles, seeding dates, herbicide options, and competitive abilities will prevent weeds from adapting and thriving in fields and will help prevent weed shifts as well.
Since growing conditions vary across regions, crop rotations also vary across regions (refer to Chapter 4-“Diversified Cropping”). Common crop rotation sequences used when growing winter wheat in the northern Great Plains include winter wheat-fallow, winter wheat-corn-fallow, winter wheat-grain sorghum-fallow, winter wheat-corn-soybean, and continuous winter wheat. Proso millet and sunflower also are used commonly in rotations with winter wheat. On the other hand, the most common crop rotation in the southern Great Plains is continuous winter wheat, although rotations of wheat with canola, corn, sorghum, and soybean do occur. Continuous winter wheat (monoculture) is problematic for weed control because too often the same class of herbicide is continually used, often resulting in weed resistance and/or species shifts.
Weeds with the same life cycle as the crop tend to increase under monoculture. Winter annual weeds, particularly the grasses, tend to be the most common weeds in winter wheat. Downy brome, hairy chess, cheat, jointed goatgrass, feral rye, Italian ryegrass, and volunteer wheat are most troublesome when winter wheat is grown continuously or every other year on the same land. Winter annual broadleafweeds also increase but can be readily controlled in the growing winter wheat with herbicides.
Inserting a warm-season crop such as corn, grain sorghum, proso millet, soybean, or sunflower into a winter wheat-fallow rotation can break the life cycle of these economically important winter annual weeds. Any regionally adapted, warm season crop will suffice and serve as an important weed management tactic. Inserting a cool season spring crop such as spring wheat or oat is not as effective as a warm season crop at disrupting the life cycle of winter annual weeds. These weeds can emerge as late as mid-April, after the cool season crops are established and still have enough time to produce seed. A rotation of winter wheat-corn-fallow is excellent for the management of winter annual weeds in winter wheat and for improving wheat yield.
A reduction in the duration of the pre-wheat fallow period (for example, by planting winter wheat immediately after a summer annual crop like corn for silage or soybean), often results in wheat stands of reduced vigor due to limited soil water or late planting. This winter wheat is less competitive, resulting in increased weed growth (Table 8.1). However, this is less of a problem in the southern Great Plains where planting dates are more flexible.
Table 8.1 Effect of crop rotations on winter wheat yield and weed density following winter wheat harvest in 174 Nebraska fields.
Continuous winter wheat has been the dominate crop rotation in the southern Great Plains since the 1930s. Much of this is dual purpose wheat grown for both grazing and grain production (see also Chapter 6-“Dual Purpose Wheat”). However, continuous, dual purpose wheat rotation has led to major winter annual weed infestations. In the southern Great Plains (Oklahoma and northern Texas), successful summer crop production is difficult due to high temperatures and limited rainfall. A winter broadleaf crop, such as winter canola, would be a better fit for rotation with winter wheat. This crop would allow application of several different herbicide modes of action that are not typically used in wheat. Other benefits may include the breaking of disease cycles that normally plague continuous winter wheat and improving certain soil characteristics with the deep tap-rooted crop.
An important reason for rotating winter annual and summer annual crops is to deplete the soil weed seed bank. With two or more years between winter wheat crops, soil weed seed banks decline to levels of low competition and may be more easily managed. However, for this to occur weeds must be managed during the fallow season.
Fallow Weed Management
Weed management during fallow is critical to preserve soil water, eliminate weed seed production, and disrupt insect and disease pests. Herbicides and tillage may be used to achieve weed control during the fallow period. Herbicides maintain greater residue cover than tillage, which helps to reduce soil erosion and increase soil water storage (see also Chapter 5-“Wheat Fertility Management”). If tillage is used, it should preserve as much residue on the soil surface as possible.
Volunteer wheat is host to the wheat curl mite (which is the vector of a complex of three wheat diseases), the Russian wheat aphid, and several other pest problems. Volunteer wheat should be controlled throughout the fallow period and must be completely eliminated at least a 10-day period between wheat harvest and wheat seeding. This is known as “breaking the green bridge,” which prevents the carryover of these insects from one wheat crop to the next by depriving the insects of a key host. Controlling these insects and disease pests will improve wheat health, which results in a more vigorous wheat stand.
High temperatures during July and August often stress volunteer wheat and weeds, and reduce the efficacy of herbicides. Additionally, broadleaf weeds that have had their tops cut off by the combine are difficult to control with herbicides. Although tillage can work well at this time of year, it must commence shortly after harvest, or soils may become too hard for tillage equipment to be effective. The number of tillage operations needed depends on precipitation, weed species present, slope, susceptibility to erosion, and the amount of crop residue the drill can handle. Sweep tillage maintains crop residues on the soil surface and can provide very effective weed control when soils are dry and air temperatures are warm enough to cause rapid desiccation of weeds. More aggressive tillage (for example a tandem disk), may be needed when soils are moist. Herbicides are typically a better option for weed control than tillage when soils are moist.
A firm seedbed enhances wheat seed germination and seedling growth. Residues should be maintained on the soil at seeding to help prevent wind and water from silting under the winter wheat seedlings or burying the seeds too deep. If tillage was used during the previous fallow periods, a rodweeder should be used to control weeds and create a firm seedbed during the final two to four weeks before seeding. In areas where winter annual weeds are a problem, rainfall prior to wheat seeding can cause weed seeds to germinate. Following rain, rodweeding and wheat seeding should be delayed at least one week to aid in controlling winter annual weeds. This delay, followed by a burndown herbicide prior to planting, effectively controls the winter annual weeds and leads to lower infestation levels during the cropping season. Research indicates a 69 percent yield savings by using this technique for downy brome control prior to seeding winter wheat. Downy brome is more of a problem in early planted fields than in later planted fields. However, it is important not to delay wheat seeding much beyond the optimum planting date or yields will be reduced.
Select adapted competitive winter wheat varieties. Research and field surveys have shown a large difference in weed suppression characteristics of winter wheat varieties (Table 8.2). Tall varieties competed with weeds better than short varieties in two out ofthree years. Other factors that may improve wheat's competitiveness with weeds include rapid early fall growth, good tillering, winter hardiness, and extensive leaf display. The same weed suppression characteristics have been observed in wheat varieties commonly grown in Oklahoma in studies conducted with feral rye (Table 8.3).
Table 8.2 Effect of winter wheat varieties on summer annual weed density at North Platte, Nebraska
Table 8.3 Effect of wheat cultivar on percent wheat yield loss due to feral rye infestations in four experiments during the 1997-1998 and 1998-1999 wheat growing seasons in Chickasha, Perkins, and Orlando, Oklahoma.
1Adapted from Roberts et al., Weed Technology 15:19-25.
*Cultivars 2180 and Karl 92 were not included in the 1998-1999 experiments.
Table 8.4 Effect of winter wheat planting date on density of summer annual grasses in wheat and the following grain sorghum crop at North Platte, Nebraska .
1Adapted from Wicks et al., Weed Science 43:434-444.and yield may be reduced.
Table 8.5 The effects of wheat seeding rate, row spacing, and seeding date on the density and biomass of cheatgrass in April at Lahoma, Oklahoma 1.
Seed at the optimum time to ensure the most advantageous growing conditions for wheat and a healthier wheat stand. For example, at North Platte, Nebraska, the optimum seeding period is September 15 to 25. In the southern Great Plains the optimum planting date typically ranges from September 15 to October 30 when the intended use of the crop is wheat grain production; however, seeding generally occurs two to four weeks earlier when it will be used as a forage crop for grazing cattle.
Planting wheat earlier than the optimum seeding date may result in lower winter wheat yield because it is more vulnerable to crown and root rot infection. Additionally, weeds are more prevalent in wheat that is seeded before the optimum date. Even the following summer crop, for example grain sorghum, was found to have more weeds when it was planted into early seeded winter wheat residues rather than winter wheat seeded near the optimum date (Table 8.4). Similarly, winter wheat seeded too late may not tiller enough to suppress weeds in the spring, and yield may be reduced. If one cannot seed at the optimum seeding time, the competitive edge can still be achieved by altering other factors such as seeding rate and row spacing (Table 8.5). By increasing seeding rate and decreasing row spacing, wheat competitiveness can improved even when seeding date is less than optimum.
Table 8.6 Optimal wheat seeding rate (pounds per acre) derived for alternative wheat seed and grain prices for three levels of cheat infestations 1.
1Adapted from Epplin et al. 1996, J. Prod. Agric. 9:265-270.
Table 8.7 Effect of row direction on weed density when an 11 to 14-month fallow period precedes winter wheat 1.
1Adapted from Wicks et al., Weed Technology 17:467-474.
2Numbers in columns followed by the same letter are not significantly different at the 5% level.
Adjust seeding rates to improve weed control. In the northern Great Plains, winter wheat is seeded at 45 to 120 pounds per acre depending on location and planting date. The 45 lb/ac rate is more common in the west while 60 to 75lb/ac is more common in the east. When winter wheat is planted at the optimum time, the appropriate seeding rate is 18 seeds per foot of row. This is about 60 lb/ac with average seed size. Planting fewer seeds may result in increased weed growth. Generally; seeding rates need to be increased when seeding is delayed beyond the optimum dates to compensate for reduced tillering. Higher seeding rates are used when winter wheat is planted late, such as after soybean harvest, or when wheat is irrigated. Seed treatments should be considered to control seedling diseases.
In the southern Great Plains, winter wheat seeding rates range from 30 to 150 lb/ ac depending on location, planting date, availability of irrigation, and whether or not the crop will be used for forage production. A 60 lb/ac seeding rate is common for dryland, grain-only wheat, while 90 lb/ac is a minimum for wheat that is intended for grazing or where irrigation will be used. Under intense grazing pressure, seeding rates of 120 to 150 lb/ac are still economically viable and will improve weed control by helping the crop canopy fill back in after grazing.
Seeding rates should also be optimized based on seed costs, expected weed infestations, and the potential selling price of the harvested grain (Table 8.6). These effects are possible due to the improved weed suppression, crop yield, and reduced weed seed production brought about by increased seeding rate.
Row spacing affects competition with weeds. Winter wheat is planted in row widths from 6 to 14 inches. Generally, row spacings are wider in the west, where soil moisture is more limited. Wide rows are advantageous when soil moisture is limited because hoe openers can move dry soil to the inter-row without excessive seed coverage. The wheat seeds then are placed into firm moist soil, thereby improving wheat germination, seedling vigor, and crop competitiveness with weeds.
When moisture is not a limiting factor, however, narrow rows and increased crop density help with weed control by shading the ground and suppressing further weed germination and development. Narrow row spacing can improve weed control during the fallow periods because weeds are smaller and more easily controlled with herbicides than they are in wide row spacings.
Row direction may influence weed densities. In fields where rows run in a north south direction, weed control following wheat harvest is better than where rows run in an east-west direction (Table 8.7). It is hypothesized that the north-south rows shade the ground better than east-west rows and reduce weed emergence. In fields where soil erosion is not a concern, north-south rows also are preferred.
Table 8.8 Effect of fertilizer application timing on wheat yield, tiller density, and weed density in three regions of Nebraska 1.
1Adapted from Wicks et al., Weed Technology 17:467-474.
2Within a region, numbers in columns followed by the same letter are statistically similar (µ =0.05).
Table 8.9 Influence of phosphorus on winter wheat yield, stem density, and weed density when banded in a farmer’s field at winter wheat seeding time in west central Nebraska.
1Adapted from Wicks et al., Weed Technology 3:244-254.
Figure 8.1 Wheat grain yield loss due to weed competition as a function of rainfall received during the first 10 days after planting across Oklahoma (data pooled over five winter annual grasses seeded at five densities).
Seeding at an optimum soil depth can result in earlier germination, better stand establishment, and, thus, a more competitive crop. Establishment conditions within the first two weeks after planting are very important for weed management throughout the growing season. Ideally, the crop will emerge and establish itself before weeds emerge. Depth of planting should vary based on soil texture, soil moisture at the time of planting, and anticipated rainfall soon after planting (Figure 8.1). Clean-tilled, finetextured soils tend to crust under warm conditions following rainfall. Under these conditions, using a higher seeding rate may help the wheat seedlings push through the crust.
Ideal wheat planting depths range from V2 inch to 2 inches, but a general rule of thumb is 1 to 1.5 inches in medium to fine textured soils and 2 inches in coarse textured soils. If one must seed deeper to reach soil moisture, a long coleoptile wheat variety must be used. Never cover wheat seed with more than 3 inches of soil. If the top 2 to 4 inches of soil is dry at planting, a hoe drill is preferred over a disk drill to place the winter wheat seeds into firm moist soil. The openers must have proper tension to ensure the wheat is planted deep enough, especially in the tractor wheel tracks. Weed density is often greater in wheel tracks because the wheat does not emerge well due to improper seeding depth.
Fertilize to increase crop competitiveness with weeds. A good fertilizer program based on soil tests and appropriate application timing will increase the vigor and com petitiveness of the winter wheat crop. In general, weed control is better when nitrogen is applied in the fall rather than in the spring (Table 8.8). Fall fertilization improves the competitiveness of winter wheat and reduces summer annual weed growth. However, nitrogen applied in the fall is more susceptible to leaching than spring-applied nitrogen, especially in areas of higher rainfall and courser soils.
Spring-applied nitrogen requires adequate and timely rain to be moved into the root zone. If rainfall is not adequate and timely, late germinating weeds can take advantage of the nitrogen. Weeds may be larger after harvest and more difficult to control where nitrogen was applied late in the spring. The excess weed growth is due to incomplete utilization of nitrogen by the wheat as a result of late application. Spring applications of nitrogen are best applied as early in the spring as possible. Do not wait to apply nitrogen until the optimum time for herbicide application, or some of the potential yield benefits may be lost. When nitrogen is to be applied in spring, apply phosphorus in a band at planting to stimulate crop growth.
Fertilizer placement is very important with phosphorus. Phosphorus applied as a band when wheat is seeded can increase early season forage production which increases the wheat’s competition with weeds, increase wheat yield, and reduce weed density after wheat harvest (Table 8.9). Row-applied phosphorus is very beneficial to wheat seeded after the optimum planting date, even for soils containing high levels of phosphorus. Never put ammonium thiosulfate (12-0-0-26) with the seed.
Managing insect and disease pests throughout the growing season improves the vigor of the wheat stand. This results in a more competitive wheat stand that is more likely to compete with weeds (reference Chapter 7-“Arthropod Pests of Wheat” and Chapter 9-“Disease Management of Wheat”).
Chemical Weed Control
Herbicides have provided excellent control of broadleaf weeds in winter wheat for many years. In more recent years, herbicides have been developed to selectively control winter annual grasses in winter wheat. In order to get the best weed control with the least crop injury, be sure to:
1. Correctly identify the problem weed(s).
2. Apply herbicides when weeds are small and actively growing.
3. Use proper spray equipment that is in good condition and not contaminated with previously used herbicides.
4. Calibrate the sprayer to ensure application accuracy.
5. Read and follow directions on the herbicide label.
6. Know your rotational plans to avoid herbicide carryover problems to sensitive crops. Be aware that crop disasters such as winter injury, hail, or disease occur, and previously applied herbicides may limit the choices for recropping.
7. Check current local weed management recommendations for options in addi- tion to those mentioned, because new herbicides are continually entering the market.
Winter Annual Grass Weeds
Only in the last few years has it been possible to selectively control winter annual grass weeds in winter wheat. Control of these weeds is best when herbicides are applied in the fall, shortly after emergence, when plants are growing rapidly but before they become well tillered. Winter wheat fields that look like a lawn probably have winter annual grassy weeds filling in between the rows of wheat.
Maverick®, Olympus®, and OlympusFlex® herbicides provide selective control of downy brome and other Bromus species in winter wheat. Maverick and Olympus provide very similar control of downy brome when applied in the fall. Downy brome control with both of these products when applied in the fall has ranged from about 70 to 95 percent control in University of Nebraska trials. Spring applications have been less consistent, ranging from 35 to 85 percent control. Plant growth rate and stage of development at the time of application, and weather conditions following application, influence the level of control.
All three products have important rotation restrictions. Olympus Flex has a little less soil residual than Olympus, which allows a few rotational crops, such as soybean, to be planted a little sooner than is the case with Olympus. However, the differences are small and may be of little practical significance in non-soybean production regions.
Growers who have seeded a Clearfield., wheat variety can use Beyond., or ClearMax herbicide to selectively control downy brome, jointed goatgrass, feral rye, cheat, wild oat, and minor populations of ltalian ryegrass. Of these weeds, feral rye control has proven to be the most difficult and least consistent. The best control of feral rye has been achieved by applying five ounces per acre of Beyond in the early fall before rye plants have formed a tiller. It is recommended that UAN and surfactant be added to the spray mixture for improved control. Fall control of feral rye with Beyond has ranged from 70 to 90 percent, while spring applications of Beyond have been very inconsistent and are not advised in most situations.
Unlike feral rye, the control of jointed goatgrass with Beyond has been very effective and consistent. Fall and spring applications of Beyond at 4 ounces per acre have generally ranged from 85 to 100 percent control. Surfactant and UAN should be added to the spray mixture. Herbicide resistance is a concern with jointed goatgrass, so growers should be careful not to overuse this technology or it may soon lose its use fulness. We do not recommend that growers use Beyond herbicide more than twice in six years. Although downy brome control with Beyond is usually good, downy brome can be controlled more economically with the previously discussed herbicides.
Winter Annual Broadleaf Weeds
Common broadleaf winter annual weeds in winter wheat include blue mustard, tansy mustard, tumble mustard, field pennycress, and shepherd’s-purse. Unfortunately, many growers are unaware of these weeds in their fields until they start to bloom in the spring. By this time, control is difficult and most crop damage has already occurred. The sulfonylurea herbicides Ally®XP, Amber™, Finesse™, or Peak™ can be applied alone, without 2,4-D, in the fall to control winter annual broadleaf weeds. Herbicide applications made in late winter or early spring must be applied before weeds begin to bolt, or stems elongate, for effective control.
Blue mustard is perhaps the most difficult of the winter annual broadleaf weeds to control because it bolts very early. If timed correctly, 2,4-D (8 oz/ ac of LV4 ester or 16 oz/ac of 4 lb/gal amine) provides low cost and effective control of winter annual broadleaf weeds. Wheat should have at least four tillers before applying 2,4-D or serious crop injury may occur. The addition of a sulfonylurea herbicide, such as Ally®Extra or Amber to 2,4-D, may improve control, particularly after these plants have bolted. If the sulfonylurea herbi- cide is used after bolting, but prior to weed seed production, it may be useful to reduce the amount of weed seed produced, but such late control may not prevent yield loss.
Warm Season Broadleaf Weeds
Many broadleaf weeds in winter wheat can be controlled at a modest price with amine or ester formulations of 2,4-D. Generally, ester formulations of 2,4-D provide better broadleaf weed control than amine formulations because they are oil soluble and readily penetrate plant foliage. Amine formulations are water soluble and do not penetrate foliage as easily, resulting in reduced control of weeds such as kochia and Russian thistle. However, amine formulations provide greater crop safety than ester formulations.
To reduce injury with 2,4-D, use low rates and apply in early spring to fully tillered wheat, prior to stem elongation (jointing). Winter wheat is considered fully tillered when it has six to nine tillers; however, the number of tillers depends on the seeding rate and date. Wheat injury and yield loss can be significant if 2,4-D or other herbicides are misapplied.
Dicamba (Banvel™, Clarity™, Sterling™, etc.) and 2,4-D are combined to control a wider spectrum of broadleaf weeds, including wild buckwheat, which is not controlled by 2,4-D alone. Dicamba plus 2,4-D must be applied to well tillered wheat, but before jointing, to avoid crop injury.
Sulfonylurea herbicides have soil persistence and will control germinating broadleaf weeds for about four weeks after application. A surfactant (at 0.25 % v/v) should be added to the spray solution whenever the sulfonylurea herbicides are used, unless liquid fertilizer is being combined with the herbicide.
Among the weeds that may or have become resistant to the sulfonylurea herbicides are kochia, Russian thistle, and prickly lettuce. The use of 2,4-D (4lb/gal) at Y2 pint per acre applied with one of the sulfonylurea herbicides and a surfactant improves weed control and helps prevent resistant weed development. Higher rates of 2,4-D and surfactant may injure the wheat.
The sulfonylurea herbicides have rotational restrictions of one to 36 months that limit their use in areas where susceptible crops are grown in rotation with wheat. This is especially important when the crop is lost to hail or other crop failures. The degradation of sulfonylurea herbicides in soil is slowed by high soil pH. Some of the sulfonylurea herbicides should not be applied to soils with a pH greater than 7.2 to avoid the risk of rotational crop injury. Growers should follow label directions carefully and determine rotational plans before using these products.
Wild buckwheat has become an increasing problem in winter wheat fields. Wild buckwheat is best controlled when herbicides are applied before it produces vines. Herbicides with short residuals applied before wild buckwheat germinates will not provide adequate control. Dicamba and aminopyralid (Cleanwave®) can be combined with 2,4-D for improved control of wild buckwheat.
Weed control in winter wheat requires an integrated system that relies on numerous management decisions related to maximizing crop growth and minimizing weed growth. The use of multiple cultural practices for weed control frequently provides synergistic benefits greater than the added effects of using just one or two cultural practices. Timely field scouting is essential in good weed management.