Weed Management: Am I on the Right Track?

Jerry Doll

Such a title perhaps suggests there is only "one track" to be on in terms of weed management. This is obviously not true as producers can build effective, efficient and successful programs in different manners. But it is true that any practice needs to be evaluated periodically and in light of current economic and environmental concerns in agriculture in general and weed management in particular, it is appropriate to review what you have been doing and see if modifications can lead to improvement.

For starters, let's consider cultural weed management practices. This includes anything you do to have a healthy, vigorous crop: seedbed preparation, plant spacing and density, variety selection, plant nutrition program, crop rotation, etc. Most of you are right on top of these considerations. If I had to single out one area for closer monitoring it would be crop rotation. Some of our hard to control weed problems, herbicide resistant weeds, and groundwater contamination has been linked to too much continuous corn. While this has been perhaps the best system economically, in biological sustainability terms it is shaky.

Have you reviewed your mechanical weeding program lately? Is it time for a new cultivator? Do/should you have a rotary hoe? Are your weeding tools appropriate for your soils, weeds, tillage program, row crop acreage? Integrated control programs require the availability and use of mechanical weeding if and as needed.

Herbicide use is under close scrutiny from people such as the Public Intervenor, the Town of Casey, environmental groups, and others. Have you reviewed your herbicide practices lately? Do you understand the atrazine rule? Do you have a pesticide/crop record keeping system in place? Do you check fields and make treatment decisions based on need? Do you know the weed species and densities on a field by field basis? Can/should you reduce your herbicide use by banded applications or reduced rates of broadcast sprays?

With atrazine use declining rapidly, how will you manage quackgrass? Our suggestion is to use glyphosate (Roundup) in situations where all or most of a field needs treatment and the cropping systems allows for timely application. This is particularly appropriate when old hay fields are being rotated to corn. A fall application of glyphosate in reduced water volumes almost always gives excellent quackgrass control and often kills alfalfa, dandelions and other perennials that might be in the field. Spring applications of glyphosate still kill quackgrass but usually fail to give full season suppression of the broadleaves. When only localized areas of fields have quackgrass or where the cropping system makes glyphosate application difficult, consider a selective postemergence herbicide. These include Accent or Beacon in corn and Fusilade or Assure in soybeans.

Accent and Beacon are new postemergence options available for use in corn. Both give excellent suppression of quackgrass and control of shattercane. Accent has very good activity on foxtails and wild proso millet but is weak on crabgrass and lambsquarters. Beacon has some activity on foxtails but is gives better control on annual broadleaves than Accent. They both offer potentially better control for particular weeds in certain situations but they probably will not replace current control alternatives for most producers. Crop injury may occur if these products are applied to corn taller than the label allows (20 inches for Beacon and 24 inches for Accent) or if Counter insecticide is applied for root worm protection.

Recently we have been talking about "weed pressure". This is a measure of the density of weeds that a field has. University of Wisconsin weed scientists have developed a method to estimate how weedy a field is and shows how you can use that information to make sound weed management decisions. Many times the weed control programs used give excellent results, but the level of weeds present is much less than realized. In fields with few weeds, reduced rates and perhaps mechanical only weeding programs should be considered with little risk of poor results.

Conservation Compliance Plans and other incentives are moving us rapidly away from the moldboard plow. Can we manage weeds effectively in reduced tillage systems? It appears that in most situations, weed management in chisel plowed or disked fields is similar to that in conventional tillage systems. This is probably contrary to what many of us have been taught: that as tillage decreases more herbicides are needed. In no-tillage systems, this may be true. But in conservation tillage, reduced rates of soil-applied herbicides, banded applications and row cultivation generally perform as well as in moldboard plowed fields. Most of the banded application, reduced rate and cultivation trials we have conducted in recent years were done in chisel plowed fields whit corn as the previous crop.

However, when no herbicides are use in fields with heavy weed pressure, weed management needs to be monitored very closely. We find that chisel plowing generally results in more weeds than no-tillage or moldboard plowing systems, and carefully timed rotary hoeings and subsequent cultivations must be planned and carried out.

Perennial weed management in no-till systems usually requires special attention. Quackgrass is particularly aggressive in these systems. Our research has shown that no-tillage + no quackgrass control = no crop. Both the chisel plow and moldboard plow seem equally effective in giving corn a fighting chance to compete effectively with quackgrass. Quackgrass in old hay fields that will be no-till planted next year should be treated with Roundup in the fall. Accent and Beacon do not give acceptable quackgrass control in no-tillage systems and atrazine rates are too low to give effective suppression unless complimented with two cultivations.

Reduced tillage does not mean weeds will be more difficult to manage. But as with any significant change in a production system, tillage changes may cause a change in either weed species, their density, or from annuals to perennials over time. The key to success is a planned monitoring program that detects these changes and gives producers the information needed to make appropriate changes in their weed management program.

In summary, take time to review your weed management program. Discuss areas you think need changing or attention with your agricultural extension agent, crop consultant or other ag professional. This will keep you abreast of changes that are occurring and allow you to systematically modify your weed management program in light of current concerns and to plan for many more years of sustainable farming.

Prepared for the 1992 season.