Weed Control in Maize Farming

Herbicides in the past were highly effective, cheap and easy to use. But reliance on herbicides alone has contributed to the widespread herbicide resistance problems that we are seeing today. If you look at the problem simply, herbicide resistance is nature’s way of telling us herbicides alone are not sustainable and introducing more diverse weed control methods is required to disrupt the weed’s life cycle.

Weed identification is the key to an effective maize weed management program. Incorrect identification can mean the difference between profit and loss. Although a weed’s life cycle, including its method(s) of reproduction, is the most important identifying characteristic, it is sometimes necessary to know the exact species before selecting weed management measures.

Maize growers should make a weed inventory to aid in the selection of weed control programs. By tailoring control programs to fit the problems in each field. Growers can minimize weed control costs while maximizing yields and profits. An inventory can be made by scouting fields two or three times during the year and recording the types (such as broadleaf annuals or annual grasses) of weeds present in each field.

The first observation should be made by the time maize is 3 or 4 inches tall. These early-season observations reveal how effective preplant or preemergence herbicides used and suggest the possible need for cultivation or for postemergence herbicide applications. A second look at the fields is before the maize is waist high and can provide information on the overall effectiveness of weed control practices and provide clues on how the program might be adjusted in future years. This also is a good time to record the types and numbers of weeds present and to map the location of special problem areas in the field. Additional notes on weed types and numbers can be taken at harvest to complete the weed inventory.

Weed Management Methods
Although herbicides can provide effective weed management, maize growers should not depend on herbicides alone. Growers should use good cultural practices so the maize is competitive with any weeds and should integrate chemical control programs with cultivation, especially with difficult-tocontrol weeds or when weather conditions reduce herbicide effectiveness.

The first step in cultural weed control is the selection of a seed variety that has adapted to local growing conditions. Timely planting along with proper fitting in tilled situations or proper adjustment of no-tillage planters ensures rapid germination and a competitive advantage for the maize. Another cultural practice that favors rapid establishment of maize is proper band application of fertilizer at planting.

All primary (plowing) and secondary (fitting) tillage operations help provide a weedfree seedbed. Cultivation of row crops is an effective way to control annual weeds between maize rows. Band application of herbicides over the row at planting, combined with one or two cultivations, provides good control of annual weeds. Although rotary hoes effectively destroy weed seedlings in small maize, a row cultivator adjusted to minimize pruning of maize roots should be used after maize is 5 or 6 inches tall. Creeping perennials are not adequately controlled by one or two cultivations. These weeds regrow from rhizomes (underground stems) following cultivation and are controlled with tillage only if the operations are repeated over long periods. Biennial and simple perennial weeds do not persist in fields that are plowed but can be a problem in reduced and zone/no-tillage fields.

A variety of herbicides are available for preplant, preemergence, and/or postemergence weed control in maize. These herbicides vary in their effectiveness in controlling different weeds and in the length of time they remain active in the soil. Some maize herbicides can carry over to affect triazine-sensitive rotational crops such as legumes, small grains, and soybeans. Knowledge of the weeds present, herbicide effectiveness, and rotational plans should be considered when selecting herbicides.

Cost of chemical weed control dictates that herbicides be applied when they will provide maximum return. Label guidelines for the timing of herbicide applications are based on research and are geared for maximum weed control and minimum crop injury.

Factors Affecting Soil-Applied Herbicides To be effective, soil-applied herbicides must be available for uptake by the roots and/ or shoots of germinating weed seedlings. This means that they must be dissolved or suspended in the soil solution. Soil properties that affect the availability and activity of soil-applied herbicides include soil texture, organic matter level, and pH. All should be considered when determining herbicide rates.

Soil texture is determined by the relative percentages of sand, silt, and clay in a soil. Names describing texture such as loamy sand and clay loam are assigned to soils depending on these percentages. Clay particles are negatively charged and have a large surface area. As a result, soils high in clay content (heavy soils) have the capacity to adsorb or tie up herbicides and generally require higher herbicide rates than coarsetextured or light soils.

Organic matter content also affects adsorptive capacity of soils. Although un-decomposed plant and animal residues can influence herbicide performance, the well-decayed, fine organic matter particles known as humus are of greatest importance. Like clay particles, humus particles are negatively charged but exhibit an even greater capacity to adsorb or tie up herbicides than clay. Consequently, herbicide rates also have to be adjusted to the soil organic matter level.

Rates for soil-applied herbicides are moatly based on for medium-textured (loam) soils with organic matter levels of 3 to 4 percent. Fine-tuning the rates for other soils can be done by consulting the herbicide label for different soil textures and for varying organic matter levels.

Soil pH can also affect the availability of some soil-applied herbicides. This is important for the triazine herbicides . These herbicides are most strongly adsorbed (tied up and unavailable for uptake by weeds) on clay and organic matter particles at low pH levels. Although the amount of triazine adsorption increases at all pH levels below 7.0, adsorption is most dramatic at pH levels of 6.0 and below. This is an important consideration for continuous zone/notillage fields where the surface inch of the soil profile may have a lower pH than is reflected in the results of a normal soil sample analysis. It may be helpful to check the pH in the top inch of the soil profile with a soil pH kit in fields that have been in zone/ no-tillage for extended periods.

Soil pH also affects availability of some herbicides. These herbicides should not be applied to areas where soil pH is greater than 7.8 as this may result in unacceptable crop injury. In addition, soil applications of these herbicides should not be made to soils with more than 5% organic matter if soil pH is less than 5.9 as reduced weed control will result.

Herbicide Resistance Management Herbicide resistant biotypes are common. Populations of these weeds were originally controlled with one or more of the herbicides at normal use rates. However, shifts to weed populations dominated by the resistant strains have occurred in many locales. These resistant strains are not controlled with extremely high herbicides use rates. In addition, there is crossresistance among the different herbicides used in maize.

This situation has prompted refinements in the control guidelines for annual broadleaf weeds in maize. Herbicides have played, and will continue to play, an important role in maize weed control programs; however, effective control programs for these herbicide-resistant strains will involve the use of crop rotation and cultivation along with herbicide rotation and/or use of herbicide combinations that include herbicides with different sites of action (how they affect weeds). These practices will also delay development of weed populations that are resistant to the herbicide.

Rotating herbicides with different sites of action and the use of tank mixes or sequential applications that involve herbicides with different sites of action are key elements in herbicide resistance management plans. To do this most effectively, everyone involved in decisions about weed management must have site of action classification for herbicides readily available. The Weed Science Society of America (WSSA) has approved a numbering system to classify herbicides by their site of action (Mallory-Smith, C.A. and Retzinger, E.J. 2003. Revised classification of herbicides by site of action for weed resistance management strategies. Weed Technol. 17:605-619). In this system, a group number is given to all herbicides with the same site of action. To further efforts in management of existing herbicideresistant weed populations and to delay or avoid development of new herbicideresistant weed populations, these “GROUP NUMBERS” are included in the “Chemical weed control tables” in each crop section of this guide.