Wooly Cupgrass and Wild Proso-Millet Management

James A. Mickelson, Chris M. Boerboom, and R. Gordon Harvey

Woolly cupgrass (Eriochloa villosa) and wild-proso millet (Panicum miliaceum) are two of the most difficult weeds to control in Wisconsin and other parts of the north central U.S. Woolly cupgrass and wild-proso millet share several characteristics that make them especially troublesome weeds. Therefore, greater management is required to avoid problems from these weeds.

Woolly Cupgrass

Woolly cupgrass is an annual grass weed found in many fields throughout southern Wisconsin, southern Minnesota, Iowa, and northern Illinois. It was identified in Wisconsin in the 1960's, but is now reported in over 35 counties and continues to spread to uninfested fields. Controlling woolly cupgrass is especially difficult because it is naturally more tolerant to many herbicides and has the potential to emerge late in the season. The risk of crop yield loss and weed management costs are often greater for fields infested with woolly cupgrass.

Description

Woolly cupgrass has plump, oval seeds that are pointed at one end and blunt at the other. The large seeds are about 3/16 inch long and about half as wide. The color of the seed can vary from light green to purple to tannish-brown. After seeds have weathered in the soil they become dull tan in color.

The first true leaf of the seedling has a short, wide, pointed leaf blade that lies flat to the ground. To assist with seedling identification, carefully dig up the plant to find the seed hull attached to the roots.

The adult plant is dark green in color. The leaf sheath is open near the collar and overlapped at the base. The collar lacks auricles and the ligule is a fringe of fine hairs. The leaf blade surface is covered with short dense hairs that are difficult to see but feel velvety. One edge of the leaf blade is typically crinkled. Plants commonly tiller, especially with early emerging plants at low densities. If lodging occurs, the plant can root at nodes that lie on the ground.

The woolly cupgrass seed head has 2 to 8 racemes that branch from one side of the main stem. Seeds hang in two rows from the raceme. A "woolly" tuft of hair exists where the seed attaches to the raceme. Seeds shatter from mid-August through September.

Woolly cupgrass seedling emergence

About 5 to 25% of the woolly cupgrass seeds in the seedbank (top 6 inches of soil) emerge during a single growing season. Most seedlings (about 85%) emerge from seeds located in the top 2 inches of the soil, but seedlings can emerge from as deep as 4 inches.

Woolly cupgrass begins to emerge early in the spring, around late April to early May, depending on soil temperature and moisture. It is usually the first annual grass weed to emerge in the spring. A majority (60 to 90%) of the season’s total emergence occurs during the initial two weeks of emergence. After this period, woolly cupgrass emergence declines but some cupgrass continues to emerge through May and June. By mid to late June, emerging seedlings typically account for less than 5% of the season’s total, but this can be a substantial number of plants if the seedbank population is high. Plants that emerge after an application of a nonresidual postemergence herbicide, will not be controlled. In fields with low seedbank populations, very few plants may emerge late in the season. Environmental conditions, especially rainfall, play a large role in stimulating late season emergence.

Woolly cupgrass seedbank depletion

Preliminary results from woolly cupgrass seedbank research indicates that if woolly cupgrass seed production is completely prevented, the seedbank can be reduced by up to 65% per year. This reduction in the seedbank population is due to seeds that germinate, seeds that decay, and seeds on or in the soil that are eaten by birds, rodents, and insects.

Wild - Proso Millet

Wild-proso millet is an annual grass weed found throughout the north central region of the U.S. and parts of Canada. It was first recognized as a potential weed threat in 1970 and quickly spread to many fields throughout the north central region. It is a vigorous competitor with many row crops, especially sweet corn, field corn, and soybeans. Controlling wild-proso millet is difficult because many soil-applied herbicides are not highly effective and wild-proso millet continues to emerge late in the season. The risk of crop yield loss and weed management costs are often greater for fields infested with wild-proso millet.

Description

Wild-proso millet has smooth, shiny, oval seeds that are pointed at each end. The seeds are about 1/8 inch long and about half as wide. The seed color ranges from olive-brown to brownish-black.

The first true leaf of the seedling has a wide, pointed leaf blade that lies flat to the ground. As a seedling, wild-proso millet resembles volunteer corn but has narrower leaves that are considerably hairier. To aid in seedling identification, carefully dig up the plant to find the shiny seed hull attached to the roots.

The adult plant has hair on the upper and lower leaf blade surfaces. Leaf sheaths are open and densely covered with stiff hairs. The collar lacks auricles and the ligule is a fringe of dense hairs, fused at the base.

The wild-proso millet seed head is an erect, spreading panicle, about 6 to 12 inches wide. Seeds shatter early in the season, usually before field corn or soybeans are harvested.

Seedling emergence

About 5 to 10% of the wild-proso millet seeds in the seedbank (top 6 inches of soil) emerge throughout the growing season. Most seedlings emerge from seeds located in the top 2 inches of soil, but seedlings can emerge from greater depths.

Wild-proso millet starts to emerge around mid-May, depending on soil temperature and moisture. This is about 2 weeks later than woolly cupgrass. The majority (60 to 80%) of the season’s total emergence occurs during the initial two weeks of emergence. After this period, wild-proso millet emergence declines, but some millet continues to emerge through May and June. By mid to late June, emerging seedlings typically account for less than 5% of the season’s total, but this can be a substantial number of plants if the seedbank population is high. Plants that emerge after an application of a nonresidual postemergence herbicide, will not be controlled. In fields with low seedbank populations, very few plants may emerge late in the season. Environmental conditions, especially rainfall, play a large role in stimulating late season emergence.

Wild-proso millet seedbank depletion

Research conducted on wild-proso millet seedbanks suggests that if wild-proso millet seed production is completely prevented, the seedbank can be reduced by up to 70% per year. This reduction in the seedbank population is due to seeds that germinate, seeds that decay, and seeds on or in the soil that are eaten by birds, rodents, and insects.

Woolly Cupgrass and Wild-Proso Millet Management Strategies

Prevention

Fields where woolly cupgrass and wild-proso millet are most easily controlled are fields where they were never introduced. Because seeds of these weeds are spread by contaminated field equipment, till and harvest infested fields last to prevent moving seed to noninfested fields. After tilling, chopping silage, or combining infested fields, thoroughly clean the equipment. Birds and animals can also spread woolly cupgrass and wild-proso millet seeds. If small new patches are found in a previously clean field, use aggressive control measures to try to prevent their establishment and spread.

Crop competition

Woolly cupgrass and wild-proso millet emerge and grow early in the season, therefore they are aggressive competitors with most crops. Thus, it is important to plant competitive crops and manage them favorably so they maintain as much of a competitive advantage over these weeds as possible. Plant corn and soybeans soon after the final seedbed tillage so the crop has an even start with the weed. A delay in planting gives the advantage to the weed. Crop rotation is also an important tool. Alfalfa is competitive against woolly cupgrass and wild-proso millet and the frequent harvest of alfalfa prevents cupgrass or millet seed production. After a few years o  alfalfa, the weed seedbank will be greatly reduced but not eliminated. Winter wheat is also a very competitive rotational crop against woolly cupgrass and wild-proso millet.

Mechanical control

Rotary hoeing kills a large percentage of the first flush of woolly cupgrass or wild-proso millet if properly timed. Rotary hoe about 7 to 10 days after the last tillage for seedbed preparation when weed seedlings have sprouted, but before they emerge. If a postemergence herbicide program is planned, rotary hoeing gives greater flexibility for the timing of postemergence herbicide application. In row crops, cultivation is low cost and substantially improves woolly cupgrass and wild-proso millet control. This option should be considered when woolly cupgrass or wild-proso millet emerges after soil-applied herbicides have lost their activity or after the application of nonresidual postemergence herbicides.

Soil-applied Herbicides

Soil-applied herbicides containing Harness, Surpass, Frontier, Dual, Lasso, and Prowl provide early season suppression of woolly cupgrass and wild-proso millet, but rarely provide excellent season long control (Table 1). Woolly cupgrass naturally has more tolerance to these soil-applied herbicides than most other annual grass weeds. Therefore, as soil-applied herbicides break down in the soil as the season progresses, woolly cupgrass can emerge and escape control relatively early in the season. Wild-proso millet is also less susceptible to soil-applied herbicides because of its unique growth and development during seedling emergence. During emergence, the shoot and growing point quickly emerge from the soil, limiting the time these tissues are exposed in the soil for uptake of soil-applied herbicides. Receiving adequate rainfall is critical for soil-applied herbicides to provide good early season control of woolly cupgrass and wild-proso millet. Usually, late season control of these weeds with soil-applied herbicides is fair to poor, regardless of rainfall.

Postemergence Herbicides

Woolly cupgrass. Postemergence grass herbicides such as Accent in corn or Assure II, Fusion, Poast Plus, or Select in soybean are effective at controlling emerged woolly cupgrass (Table 1). However, these herbicides do not always completely kill all woolly cupgrass plants. Although some treated woolly cupgrass plants may appear severely injured after application, new shoot  can appear several weeks later. Liberty (for use in Liberty Link crops only) and glyphosate (for use in Roundup Ready crops only) are two very effective postemergence herbicides for woolly cupgrass control. Glyphosate and Liberty usually completely kill woolly cupgrass plants that have emerged at the time of application. The postemergence herbicides listed above have little or no soil residual activity. Therefore, any plants which emerge after application will not be controlled.

Wild-proso millet. Postemergence grass herbicides such as: Accent in corn; Assure II, Fusion, Poast Plus, or Select in soybean; Liberty in Liberty Link crops; and glyphosate in Roundup Ready crops, are all very effective at controlling emerged wild-proso millet (Table 1). However, these postemergence herbicides have little or no soil residual activity. Therefore any plants that emerge after application will not be controlled.

Lightning, for use in imidazolinone tolerant (Clearfield) corn hybrids only, is a postemergence herbicide that provides excellent control of emerged woolly cupgrass or wild-proso millet and has soil residual activity as well. Research has also been conducted using nonresidual postemergence herbicides tank-mixed with soil-applied herbicides in corn. Late season control provided by the soil residual activity of Lightning or the soil-applied herbicide in the tank-mixes has been inconsistent. In many cases, good late season woolly cupgrass or wild-proso millet control has been reported if timely rainfall is received to activate the soil residual herbicide and if the weed and crop canopy is not too dense at the time of application.

Sequential Herbicide Applications

Single herbicide applications rarely provide consistent season long control of woolly cupgrass or wild-proso millet in any crop. Therefore, sequential herbicide applications (soil-applied herbicide followed by a postemergence herbicide or two postemergence herbicide applications), are often necessary to prevent crop yield loss and minimize weed seed production, especially with severe cupgrass or millet infestations.

Reduced Herbicide Rates

It is legal to apply less than the labeled rate of a herbicide, but the grower assumes all risks involved. Reduced herbicide rates work best when: weeds are small, growing conditions are favorable, the herbicide is very effective on the targeted weed, full rates of adjuvants are used, and there is no risk of antagonism from a tank-mixed herbicide.

If a single herbicide application or a single herbicide application followed by cultivation is planned for woolly cupgrass or wild-proso millet control, you should not use a reduced herbicide rate. If sequential herbicide applications are planned, using a ½x rate for the second herbicide application will provide adequate control to prevent crop yield loss. However, depending on the effectiveness of the ½x rate herbicide application, this treatment may or may not lower weed seed production to a level that decreases the seedbank population.

If sequential herbicide applications are followed by cultivation, using a ½x rate for the second application will provide adequate control to prevent crop yield loss and minimize weed seed production to a level that decreases the seedbank population.

Applying Herbicides

For specific information on recommended rates, adjuvants, tank-mix partners, and proper application timing and conditions, read and follow the herbicide label. Additional weed control recommendations are available in the UW-Extension Publication A3646, "Pest Management in Wisconsin Field Crops".

Special Report: Woolly Cupgrass Management in Field Corn

The number of fields infested with woolly cupgrass in Wisconsin has increased rapidly throughout the 1990's. Woolly cupgrass has been more difficult to control in corn than in soybean because the corn canopy is not as dense as the soybean canopy, which allows some light to penetrate late in the season. Therefore, it is important that farmers with woolly cupgrass infested corn fields pay special attention to weed management. Ideally, woolly cupgrass should be managed so that: 1) corn yield loss does not occur, and 2) the woolly cupgrass seedbank population decreases over the long term. Additionally, we want to achieve these goals while minimizing weed management costs. Therefore, a second herbicide application for woolly cupgrass should only be applied if it is necessary to prevent crop yield loss or to prevent the woolly cupgrass seedbank population from increasing. To determine if and when this is necessary, research was conducted at the University of Wisconsin from 1997 to 1999. The following information, based on results from these experiments, explains specifically how to manage woolly cupgrass in field corn.

Woolly Cupgrass Competition with Corn

A number of factors influence the competitiveness of woolly cupgrass with corn. In general, early season weed control is most important to prevent corn yield loss.

Weed density. Woolly cupgrass density plays a large role in determining corn yield loss. With high (compared to low) weed densities: 1) there is a greater risk of corn yield loss, 2) the amount of corn yield loss will be greater, and 3) corn yield loss will occur earlier in the season if weeds aren’t controlled. High weed densities in a field are the result of a high seedbank population. Weed seedbanks will build up if weeds were not adequately controlled in previous years.

Time of weed emergence relative to the crop. Woolly cupgrass seedlings that emerge early in the season (prior to or at corn emergence) are much more competitive than woolly cupgrass seedlings that emerge later in the season (well after corn emergence). Because the majority of woolly cupgrass seedlings emerge at about the same time as corn, early season woolly cupgrass control is very important to prevent corn yield loss.

Weather and management practices. Under optimal growing conditions (adequate moisture, warm temperatures, adequate fertility) corn will be affected less by weed competition. Therefore, when conditions are not optimal, early season woolly cupgrass control is even more important.

Implications for management in corn. For fields with high woolly cupgrass densities (severe infestations), especially if growing conditions are not optimal, woolly cupgrass should be controlled prior to the V3 corn stage (3 collared leaves present). Application of a soil-applied grass herbicide (followed by adequate rainfall) or application of an effective postemergence grass herbicide prior to the V3 corn stage will prevent corn yield loss. If woolly cupgrass is allowed to compete with the corn until the V3 stage or later, corn yield loss may occur.

For fields with low to moderate woolly cupgrass densities (light to medium infestations), especially if growing conditions are optimal, woolly cupgrass control (postemergence herbicide application) can be delayed to the V3 corn stage or later, without suffering corn yield loss. However, you should not apply a herbicide to woolly cupgrass or corn that is larger than the  maximum size listed on the herbicide label.

Woolly cupgrass seedlings that emerge later in the season - after the corn has reached the V2 stage, will generally cause little or no corn yield loss. This is because the corn has a competitive advantage due to its head start, and fewer seedlings will emerge after this time because it is past the time of peak woolly cupgrass emergence.

Woolly Cupgrass Seed Production in Corn

Woolly cupgrass seedlings that emerge late in the season do not need to be controlled to prevent crop yield loss. However, plants that emerge late and are not controlled can survive to produce seed. This seed production and the addition of these seeds to the seedbank population must be considered to determine whether a second herbicide application or cultivation is needed to prevent the weed seedbank from increasing. High seedbank populations can increase weed management costs and the risk of greater yield loss in future years.

Two factors that have a large influence on woolly cupgrass seed production are: 1) the woolly cupgrass density and 2) the time of woolly cupgrass emergence relative to the crop. Table 2 shows how woolly cupgrass seed production in corn is influenced by woolly cupgrass density and time of emergence. In general, seed production decreases as density decreases and as time of emergence relative to the crop is delayed. Therefore, early season weed control is very important because plants that emerge early produce the most seeds and because peak emergence occurs early to create high woolly cupgrass densities.

Recommendations for Woolly Cupgrass Management in Corn

Severe infestation. If a field has a high woolly cupgrass seedbank population, application of a preemergence herbicide or an effective postemergence herbicide applied prior to V3 is recommended to prevent crop yield loss due to early season competition. Following this herbicide application, you can: 1) plan to apply an effective postemergence herbicide at the V4 to V5 corn stage, 2) plan to cultivate at about the V5 stage, or 3) scout for woolly cupgrass emergence and use WCMANAGER, a woolly cupgrass seedbank management decision aid (see below), to help determine if a second herbicide application or a cultivation is actually necessary to decrease the following year’s seedbank population.

Light infestation. If a field has a low woolly cupgrass seedbank population, a single application of an effective postemergence herbicide can be applied at the V3 to V4 corn stage with little risk of reducing crop yield or increasing the following year’s seedbank population.

For fields with high seedbank populations, two herbicide applications may often be necessary to prevent crop yield loss and reduce the woolly cupgrass seedbank population. However, if the field receives several years of proper management, the high seedbank population will be reduced to a low population, at which point only one herbicide application should be necessary.

WCMANAGER:*
A woolly cupgrass seedbank management decision aid

The following information describes how to scout for late season woolly cupgrass and use WCMANAGER, a woolly cupgrass seedbank management decision aid, to determine if an additional herbicide application is necessary to prevent the woolly cupgrass seedbank population from increasing.

This decision aid is intended to help manage woolly cupgrass in corn. The decision aid assumes that a preemergence herbicide or a postemergence herbicide prior to the V3 corn stage has been applied to provide early season control. By counting woolly cupgrass that emerge following this initial herbicide application, the decision aid can be used to determine if a second herbicide application is necessary to control late emerging plants in order to decrease next year’s woolly cupgrass seedbank population.

Directions

1. At the first sign of woolly cupgrass emergence following initial herbicide application, mark a 30 by 30 inch area in the field between two corn rows with flags or stakes. From just outside of the marked area, take 10 soil cores (3/4 inch diameter by 6 inches deep). Put all 10 samples in a plastic bag or clean bucket. Be careful not to lose any soil. Within the marked area count and pull newly emerged woolly cupgrass plants. Record the number of plants from this area and the date.
   
2. Repeat step 1 for two more areas so that you have three replicates. Be sure to label each marked area separately and keep all soil samples and counts separate and labeled.
   
3. Extract the woolly cupgrass seeds from the soil samples. Use metal window screen fastened around a metal or wooden frame to make a sieve. Use a water hose and the homemade sieve to wash the woolly cupgrass seeds from the soil. Be careful not to lose any soil or seeds during the washing process. Count and record the number of viable seeds. Only firm seeds are viable. Use a knife point to press each seed to determine viability.
   
4. For each replicate, enter the number of seeds collected (per 10 core sample) under the "Initial spring seedbank size" box. Also, enter the number of woolly cupgrass plants counted (per 30 by 30 inch area) in the "Emergence Data" box marked "woolly cupgrass density" for week 1.
   
5. Enter the number of corn growing degree days that have accumulated since the date of corn planting in the "Emergence Data" box marked "GDD (F)" for week 1.
   
6. Default values are used for the "Seedbank Emergence" and "Seedbank Mortality" information unless other information is available.
   
7. When all this information is in, the decision aid will predict next spring’s seedbank population and give the percent change in next year’s seedbank population (pink boxes).

If the "% Change in Next Year’s Seedbank Population" is positive then the decision aid predicts an increase in next year’s seedbank. Therefore, a second herbicide application should be applied between the V4 and V5 corn stages to prevent the seedbank from increasing.

If the "% Change in Next Year’s Seedbank Population" is negative then the decision aid currently predicts a decrease in next year’s seedbank. Woolly cupgrass emergence should continue to be counted on a weekly basis. Keep the data for week 1 and add the weekly counts in week 2, week 3, etc. Continue to count each week until corn reaches the V5 stage or the decision aid predicts an increase in the following year’s seedbank population. If an increase is predicted, a second herbicide application should be applied between the V4 and V5 corn stages to prevent the seedbank from increasing. If the decision aid never predicts an increase in the following year’s seedbank, then a second herbicide application is not necessary. Table 3. WCMANAGER predicted changes in next year’s seedbank under low and high seedbank densities with woolly cupgrass emerging at different times and densities. If seedbank density is low, early emerging woolly cupgrass will cause a large increase in next year’s seedbank because the seedbank density is low. Later emerging woolly cupgrass produce fewer seed and may not cause an increase in the seedbank. If the seedbank density is high, some woolly cupgrass can emerge at later dates without increasing the seedbank density.

* WCMANAGER is available as a Microsoft® Excel 97 spreadsheet file. A copy of this file can be obtained from Dr. Chris Boerboom, Department of Agronomy, 1575 Linden Drive, Madison, WI 53706. Phone: 608-262-1392. e-mail: boerboom@facstaff.wisc.edu