Category 100: General Farming APPENDIX – Weed Management

Weed Management

When traveling through any crop production area, one is impressed with the variation in weed control from one field to another. Some fields are almost weed-free while others are infested to varying degrees with weeds of every sort and variety. Why the difference? Effective weed management doesn’t happen by chance. It requires that you:

• Have a thorough knowledge of the weed problem,
• Prepare a well-designed plan using all appropriate weed control strategies to deal with that problem,
• Implement the plan appropriately, and
• Evaluate the effectiveness of the plan and adjust it accordingly.

Your Weed Problem

Growers who use this approach to weed management in their crops are going to have the best success. For many people, there’s little interest in identifying weeds, but a lot of interest in controlling them. However, effective weed control must begin with accurate weed identification and some assessment of the magnitude of the problem. Knowing whether you are dealing with an annual, biennial, or perennial weed is essential in determining which control methods will be most effective.

Annual Weeds

Annual weeds are abundant and prolific seed producers and, therefore, costly to control. For example, a single redroot pigweed plant can produce over 100,000 seeds. Seeds are spread from one area to another on tillage and harvest equipment and in the harvested crop. While many germinate the following year, some remain dormant in the soil for many years. Thus, seeds entering the soil form a seed bank, which maintain an annual weed threat.

Biennial Weeds

Biennial weeds infest untilled areas such as pastures, no-till fields, or noncropland. Both first- and second year plants can be in the same area. First-year plants, which are usually easier to control, are less visible and may escape treatment. Second-year plants, though more noticeable, are often much less susceptible to herbicide treatment. For example, musk and plumeless thistles are quite easily controlled with 2,4-D in the rosette stage (in their first growing season) but become
quite tolerant as they grow larger. Mowing to remove the seed-bearing flower stalks gradually reduces a biennial weed infestation (as long as seed sources from adjoining land are also eliminated).

Perennial Weeds

Perennial weeds are usually the most persistent and difficult to control. The vegetative plant parts can be transplanted by tillage equipment and begin new infestations. Infestations are especially noticeable in annual crops.

A few isolated perennial weeds might cause much concern. But they spread rapidly—a single, healthy quackgrass rhizome can produce over 400 feet of rhizomes in less than a year. So it’s important to map fields, noting areas infested with particularly troublesome perennial weeds. Designate infested areas for specific perennial weed control treatment.

Predicting Your Weed Problem

If using preemergence herbicides, the most accurate way to identify your weed problem and assess its severity is to leave small areas in the field where you make no effort to control weeds. Next year’s problem will be similar to the present year’s weed problem as long as a large number of weeds don’t escape and produce seed. Base next year’s weed control program on the predictions you make this year. If you predict a serious weed problem, implement special weed control measures
next year. Where weed pressure is only moderate, the field is a candidate for a reduced herbicide use rate plus timely rotary hoeing and row cultivation. And where weed pressure is very light, timely rotary hoeing and row cultivation may be the only weed control strategy you need.

After completing the weed problem evaluation, destroy weeds in the check areas with a nonselective herbicide (e.g., glyphosate) applied with a hand sprayer.

Planning Your Weed Management Program

Once you’ve determined your weed problem, you
can customize your weed management program
for that particular field. Use all prevention, cultural,
and mechanical weed control practices that fit your
crop management plan. Acquaint yourself with the properties
and capabilities of the various herbicides that you
might use. Both herbicide choice and rate are influenced
by your weed problem, soil characteristics, crop growth
stage, and future cropping plans. Special rules related to
environmental protection may also limit herbicide choice
and rate. Your crop management practices will also limit
your choice of weed control strategies.

Methods of Weed Control

There are, of course, many methods you can use to control weeds in your crops. We outline some of these techniques below.

Prevention
Weeds often get their start in fields from a few seeds that were accidentally planted along with the crop. Growers can prevent such introductions by planting only tested and tagged seed. Certified seed is high quality and is free of noxious weed seeds. Weed seeds and vegetative parts are also spread from field to field on farm equipment. Clean your tillage and harvest equipment before moving from an area of known weed infestation, even within the same farm.

A percentage of most weed seeds that contaminate livestock feed can germinate after passing through the animals. Growers may inadvertently introduce these seeds into their fields when they spread manure or let livestock out on pasture. Thus, weed control in homegrown forages and grains is an important part of a total farm weed management program. Likewise, purchased hay, feed grains, and livestock bedding may contain new weeds that aren’t currently on a farm.

Keep in mind that it is often easier and less costly to keep weeds out of a field than it is to control them once they become established. In addition, watch for new weeds while scouting weeds. The earlier a troublesome new weed is found, the better chance of preventing its spread to the rest of the field or to other fields.

Mechanical Control
Tillage for seedbed preparation, rotary hoeing, and cultivating are mechanical practices that fit into many cropping systems. Tillage controls weeds by burying the weeds, severing the shoots from the roots, or uprooting plants so they dry out. Small annuals and biennials can be killed by tillage, but this won’t work for most perennials (except seedlings) unless done repeatedly. For best results, till when the soil is dry so roots will have little chance of becoming reestablished.

For many years, cultivation between rows of a growing crop was a standard weed control practice. The use of effective herbicides has reduced the need for inter-row cultivation. However, many growers use both herbicides and row cultivation as their weed control strategy.

If you routinely rotary hoe and/or row cultivate, there’s a good chance that you can reduce your herbicide rate and still get good weed control.

Mowing is effective against tall weeds. It helps reduce the weeds’ ability to compete with the crop and also limits seed production. It is feasible, but difficult, to control certain tall perennials by mowing; you would need to mow frequently to deplete the weeds’ food reserves to the point that the plants cannot grow back. For example, the repeated mowing associated with harvesting forages can effectively control tall perennials.

Other mechanical control practices include flame cultivators and mulching and can be used in certain vegetable crops. The propane burners on flame cultivators are effective on weed seedlings. Black plastic mulch or other types of mulches control annuals and biennials by preventing the seedlings from getting light. Perennials are more difficult to control with mulches because they may grow through the mulch (depending on the material) or beyond the edge of the mulch.

Cultural Control
As stated earlier, weeds compete with crops, but remember that the reverse is also true: crops compete with weeds. By selecting the best variety and using the best production practices, you ensure that the crop grows well that it either shades out smaller weeds or is a vigorous competitor. In essence, anything you do to help the crop will help it compete with weeds. Keep in mind that most vegetable crops are much less competitive  than field crops.

Several practices help the crop gain a competitive advantage over weeds. For example, planting immediately after the tillage for seed bed preparation, which destroys any weeds that had germinated or emerged, gives the crop an even start with the weeds. Weeds that emerge before the crop are much more competitive than weeds that emerge after the crop. Planting crops in narrower rows or at higher densities also gives the crop an advantage; the crop canopy closes sooner and most weeds do not grow well in the shade under a full crop canopy. Likewise, cover crops like oats provide a quick canopy to shade weeds until alfalfa has become established. And planting oats at 3 bushels per acre, rather than at 1½ bushels per acre, makes the crop substantially more competitive with weeds.

Crop Rotation
The farming practices for each crop favor certain weeds and discourage others. Thus, weed problems often differ from crop to crop. Rotating crops tends to limit the populations of weeds associated with each crop, especially if the crops have different life cycles or growth habits. For example, many summer annual weeds associated with corn or vegetables do poorly under the farming practices used to grow fall-planted small grains. Likewise, the winter annuals that thrive in fall planted
small grains do poorly in row crops. Similarly, annual weeds won’t become established in vigorous perennial forages. On the other hand, perennial weeds often gain a foothold in forages because they aren’t disturbed by annual tillage operations. Where applicable, crop rotation can be a powerful weed control measure.

Crop or Variety Selection
With a weed that is especially tough to control, you
may want to grow a crop that allows you to use a specific
herbicide that is highly effective on that weed. You
might also grow one of the herbicide-tolerant crops that
allows the use of the desired herbicide.

Biological Control
Biological control agents are natural pests of weeds. Specific insects, fungi, and bacteria are the most common examples of biological control agents. Unfortunately, these natural enemies are often absent or rare in crop fields. This is partly due to our crop production practices. For example, we discussed in earlier chapters how tilling the soil can help control some crop pests; at the same time, however, the biological control agents may also be killed.

Natural enemies are also rare because most of our major weeds were introduced from other countries. Once in the U.S., these weeds flourished because they were free of natural enemies. Biological control specialists hope to correct this problem by finding natural enemies and releasing them where a given weed is a pest. Biological control agents won’t completely control a weed, but they may reduce the weed’s ability to compete and make it more susceptible to other control measures.

The success of biological control is also limited in field and vegetable crops because the agent would generally only attack one weed species, leaving other weeds to compete with the crop.

Biological control could have long-range effects on certain problem weeds, but to date there have been few success stories and most have been in range or pasture situations where there is less disturbance.

Chemical Control
Though herbicides are an important asset to sound crop management, don’t rely on them for a complete program. Herbicides are often only the initial step in weed management. Shading from the crop canopy often provides much of the weed control for the rest of the season.

Fitting Weed Management Into Your Crop Management Plan

Regardless of which weed control options are available, they must fit your crop management plan to be practical. Where a specific weed is particularly troublesome, you may need to modify the crop management plan to accommodate the proper weed control practice. For example, the control of woolly cupgrass in corn may become so difficult and costly that it is to your advantage to switch to another crop to bring this weed under control. It may be desirable to plant narrow crop rows to make crops more competitive with weeds and to increase crop yield. This requires a costly equipment change but should be considered when worn equipment is being replaced. Check rotational crop guidelines on herbicide labels to ensure that you use a product that allows you to rotate crops according to your crop management plan.

The increased use of conservation tillage sometimes calls for an increase in herbicide rate to compensate for the intercept by the surface debris. An application of livestock manure will similarly reduce the effectiveness of a surface-applied herbicide treatment and require a herbicide rate increase. Soil-incorporation of at least some of this surface debris before herbicide application improves the effectiveness of soil-applied herbicides. Any reduction in soil tillage also enables
deep-rooted perennial weeds to proliferate. Often these perennials can be controlled with systemic postemergence herbicide treatments. Row cultivation also helps to control escaping perennial weeds.

Plan all details of your weed management program well in advance of the cropping season to make certain that they fit your crop management plan. If you modify your crop management practices, make certain that the weed management plan is still appropriate. Don’t hesitate to ask questions of others who may have experienced situations or problems similar to yours.

Your Weed Control Program in Action

Having a well-thought-out weed management plan that fits well with your total crop management plan should be very reassuring as you approach the busy cropping season. It may not be the perfect plan, but it certainly will be a lot more reliable than the hitor-miss weed control used by some.

Review your plan as the planting season nears. Most importantly, stick to the plan. Too often in the rush of the planting season, growers sacrifice weed control or crop safety to save a little time. If your plan appeared the best route to follow earlier, it’s probably still the most effective way to handle your weed problem. Slight modification due to unforeseen circumstances, such as equipment breakdown or a rainy period, may sometimes be necessary. But don’t let hurried, poor judgment jeopardize your weed management plan.

Protecting Environmentally Sensitive Areas

Review your cropping and herbicide use plan as it relates to adjacent sensitive areas that might be adversely affected by the herbicide application. Consider neighboring fields as well as the one you’re treating.

For example, grapes, beans of all types, tobacco, and many other broadleaf crops are especially sensitive to dicamba and 2,4-D. Don’t use these herbicides adjacent to fields or gardens where sensitive crops are growing. Consider the presence of lakes, streams, or ponds in the vicinity of fields you wish to treat. Leave an untreated buffer strip adjacent to such bodies of water to catch any runoff from the field that might contain herbicide. Both direct application and runoff water containing
herbicide can be harmful to established vegetation on grass waterways and terraces. Keep the applied herbicide in the field you are treating for the greatest environmental safety and for maximum efficiency of the herbicide treatment.

Several agricultural herbicides have been found in groundwater. Atrazine in Wisconsin groundwater has become so widespread that rates are restricted or use is prohibited, depending on the location in the state. Be aware of the status of your farmland. Minimize atrazine use on sandy soils, especially where the water table is near the soil surface.

Monitoring Your Weed Control Program

You’ve done the best possible job of assessing your weed problems, considered all practical strategies of weed control, and developed and implemented a management plan to the best of your ability. Now you need to monitor the plan to see if it was as good as you thought. Do you need to adjust or modify the plan to better suit your needs?

Weed Scouting

Monitor the success of your weed management program plan as the growing season progresses. Watch for seedling weeds that escape and decide whether a follow-up treatment or cultivation is necessary to control them. Don’t let escapes get beyond the stage for effective control. Mark the location of escaping patches of perennials on a weed map so that you can consider them when planning future weed management programs. Finally, note any new, unfamiliar weeds and have them identified.

Minimize Crop Injury

If your herbicide choice and rate were appropriately matched to soil conditions or crop stage, herbicide injury shouldn’t be a problem. However, adverse environmental conditions like heavy rainfall or unusually cold or hot weather following herbicide application sometimes causes even relatively safe herbicides to cause crop damage. Crop seedlings generally recover from minor, early season injury. Nonetheless, you may wish to modify your herbicide rate or use a different herbicide
next year.

Weeds Adapt

Weeds have a tremendous capacity to adapt to both cultural and chemical weed control practices. With selective herbicides, there are always some weed species that are not controlled. The initial escapes may not even be noticed, but they will reproduce and multiply. Continued use of the same herbicide treatment results in a proliferation of those initial escapes until eventually you have a different weed problem. One example of such a weed shift is the increase in crabgrass in corn
caused by the frequent use of Accent-based herbicide programs (or other herbicides similar to Accent).

Resistance to Herbicides

Similarly, repeated use of herbicides with the same site of action selects for herbicide-resistant weed biotypes. Weed resistance to the triazine herbicides is widespread in Wisconsin. It has occurred due to repeated use of triazine herbicides over the past 40 years. Weed resistance to the ALS-inhibiting herbicides has occurred in many different weed species in Wisconsin in the last several years. The frequent use of glyphosate in glyphosate-resistant crops has selected for glyphosate resistant giant ragweed, common ragweed, waterhemp, Palmer amaranth, and horseweed in many places. Rotating use among different herbicide families with successive crops is the best way to delay the development of weed resistance to herbicides. Monitoring crop fields enables you to detect escaping weeds early and to modify your control program before weeds get out of hand.

Refining Your Plan

With assessing, planning, implementing, and scouting, you have all of the ingredients for a successful weed management program. Your plan may not work perfectly each year and may require some changes. Modify your plan to deal with escaping weeds. Although weeds will adapt to your best control efforts, continued refinements of your plan should ensure effective weed management in the future.