When tillage is either significantly reduced or eliminated, herbicides become one of the most relied-upon strategies for weed control, but they must be used in conjunction with appropriate cultural practices. There is no single “big hammer” solution to weeds; rather, a strategy involving “many little hammers,” or cultural weed control practices such as cover crops, crop rotations and tactics that improve a crop’s competitive advantage, are essential to ensuring successful weed management . Switching to conservation tillage requires increased management intensity to develop an integrated weed control plan suitable for the farm.
Herbicide effectiveness is reduced when it is intercepted by surface residue or not incorporated through tillage. This is especially true for pre-emergent and pre-plant herbicides. With the loss of many soil-active herbicide options in high-residue systems, residual weed control is also lost. This can result in reliance on post-emergent herbicide applications. Repeated applications of post-emergent herbicides may be necessary to successfully reduce weed competition.
In many cases, the cheapest and easiest way to achieve good weed control is to grow herbicide-resistant corn, cotton and soybeans, and to use non-selective herbicide for post-emergent weed control. However, with the increased use of the related herbicides, herbicide-resistant weed populations have been noted across the Southeast.
To avoid herbicide-resistant weed infestations, different herbicides are incorporated into the weed management plan along with cultural weed control practices. The use of post-emergent herbicides with different modes of action reduces the risk of developing herbicide-resistant weeds. A diverse crop rotation expands the range of herbicide options, including modes of action, because herbicide recommendations vary by crop. An herbicide’s mode of action describes the way in which it controls weeds; it usually refers to the biological process that is interrupted in susceptible plants but can also include a description of the injury symptoms the herbicide causes. The Weed Science Society of America organizes herbicides into 30 groups (as of 2016) based on their mode of action. Information about a product’s mode of action and its group may appear on the product label, or it can be found by contacting a local Extension office or by visiting the Weed Science Society of America online at www.wssa.net.
Although pre-emergent herbicides can sometimes have reduced efficacy, sequential applications of different pre-emergent, soil-applied herbicides can reduce the need for multiple post-emergent applications. Pre-emergent applications on fields without high residue levels, such as fallow systems, are fairly effective. If there is residue on the surface, the application rate is increased to account for herbicide interception by residue. When strip tillage is used, the herbicide can be banded over the row. If heavy cover crop residue is left on the soil surface, weed seed germination may be suppressed. But again, it is still advisable to use a pre-emergent herbicide to help reduce the need for herbicide applications later in the season. In areas with high amounts of residue, early post-emergent applications may not reach emerging weed seedlings, so scouting for surviving weeds is important.
Early control of weeds can help reduce problems later in the season and prevent infestations in the following years. Control of weeds before they go to seed, through either a spot-spray application (if they have been missed with a broadcast herbicide application) or through hand removal, can reduce the risk of a larger problem the following year. If a weed population is suspected to be resistant, it is even more critical to remove plants early to avoid future infestations that cannot be controlled with herbicides or hand removal.
Herbicide-Resistant Weeds and Resistance Management
Weed resistance to herbicide modes of action develops when only one herbicide is used for weed control. This has recently been proven true for the non-selective herbicide glyphosate and to a lesser degree with 2,4-D and dicamba. After the introduction of glyphosate-tolerant crops, implementation of conservation tillage became much easier. Successful weed control could be achieved with a single herbicide and relatively little planning was needed. However, an overdependence on glyphosate in both conventional and conservation tillage systems has resulted in resistant weed populations. For example, a number of weeds commonly found in conservation tillage systems have demonstrated resistance to glyphosate, including Palmer amaranth, Italian ryegrass and horseweed (Table 11.2). These and other weeds that have developed resistance to glyphosate can develop resistance to other modes of action as well. With the release of crops that are resistant to 2,4-D and dicamba herbicides, there is concern that resistance to these will also increase, reducing their ability to be used as alternatives to glyphosate . For reduced-tillage systems, this is especially serious since other effective herbicide alternatives are limited, and it underscores the important role of non-chemical strategies in a successful weed management plan.
To prevent the development of herbicide resistance or to manage resistant weed populations, weed management tactics should be diversified to include cultural management practices in addition to a rotation of herbicide modes of action. Cultural practices such as crop rotation and cover crops can aid in resistance management and are discussed in the following sections of this chapter. Crop rotation, especially a multi-year rotation, ensures that a variety of herbicides can be used for weed control and limits a weed population’s repeated exposure to a single herbicide. Including pre-emergent and post-emergent herbicides, as well as selective and non-selective herbicides, in a weed management plan further reduces the risk of over exposure to similar modes of action. Extension and research staff have developed herbicide recommendations for crops and weeds in states affected by herbicide resistance. These recommendations provide growers with effective, alternative plans that can reduce dependence on one herbicide. Additionally, scout fields early and often to find and properly identify weeds that have escaped an herbicide application. Maintain clean equipment, since machinery can carry weed seeds from one field to another. Remedial strategies for restoring weedy, unproductive fields are outlined later in this chapter.
There is a great deal of information available about the benefits of cover crops. In addition to reducing water runoff and erosion, increasing soil organic matter and providing a means to sequester carbon, cover crops can also reduce weed seed germination and growth. Winter cereal crops, legumes and brassicas are typical cover crops in the Southeast. They provide weed control through a mulch effect as well as through the release of chemicals that inhibit plant germination or growth, referred to as allelopathic compounds. Many times, a grass cover crop like rye or black oats is the best option if the primary goal is weed control. These covers produce high amounts of biomass that break down less quickly than some broadleaf cover crops such as legumes. This helps to suppress weed growth longer into the season. Since some problem weeds such as pigweed can easily grow under high-shade conditions, high amounts of cover crop residue are preferred to provide a barrier to weed growth. Planting winter cover crops early allows time for sufficient biomass production and increases weed suppression potential. In Alabama, a conservation tillage system using rye or black oat cover crops eliminated the need for post-emergence herbicides in soybean and cotton. Including rye or black oats increased yields of non-transgenic cotton in two of three years, compared to conservation tillage without a cover crop .
There are drawbacks to growing cover crops if they are not managed properly. One drawback related to weed management is that the release of allelopathic compounds poses an injury risk to the cash crop as well as to weeds. Temperature and rainfall can change the impact on weeds and subsequent crops, and not all cover crops contain allelochemicals. The risk of crop injury from allelopathic compounds may be reduced by terminating cover crops early, but early termination reduces the amount of biomass and therefore the mulch effect on weeds. Additionally, if cover crops are not completely killed, they compete with the primary crop for light and nutrients. Because of this, non-selective herbicides are used for cover crop termination to ensure complete kill. Since this practice adds to the risk of developing herbicide resistance, research continues with the use of rollers/crimpers for cover crop termination (see Chapter 9). Although concerns have been raised as to whether cover crops reduce the efficacy of pre-emergence herbicides, it has been suggested that any loss in weed control due to herbicide interception is offset by the control that cover crop residue provides.
Regardless of tillage practices, crop rotation can be employed to help control weed populations. In crop rotations, the life cycle of problematic weeds is disrupted as the growing environment changes due to the timing of both field operations and crop growth. This reduces weed infestations that might otherwise result if the same crop were continually planted. By rotating crops, the environment and herbicide plans are modified enough to keep problematic weeds in check. Select crops that combine with herbicides with different modes of action. Otherwise, the risk for developing herbicide resistance increases. Additionally, rotations can help control weeds through competition. For example, wheat and other fall-seeded cereal grains will be well established when spring-germinating weeds begin to emerge, causing those weeds to suffer from severe competition . Crop rotation over several growing seasons has been shown to increase yields when compared to monoculture systems, so the benefits are not limited to weed control alone .
When including pasture or forage crops in a rotation, mowing can be an effective strategy for managing some weed species. Mowing every 30–60 days can reduce competition from perennial weeds and can prevent many types of weeds from producing seeds. Mowing can also be used to keep some cover crops from becoming weeds that compete with the primary crop. To provide control, mow legume cover crops such as hairy vetch after the first flowers appear, and mow cereal grains such as cereal rye after heading . However, in humid climates, mowed residues break down faster, negating some of the residue benefits of conservation tillage .
Other Cultural Practices
Other planting practices can be manipulated to help reduce weed competition. Improving a crop’s competitive advantage is especially important when weeds are abundant and more likely to escape other control methods . Using narrow row spacing allows the crop canopy to close more quickly than when using rows with normal spacing. As the canopy closes, shading from the crop hinders weed seed germination. Yields with narrow row spacing have been shown to be similar to yields with normal row spacing. However, late-season weed control may be hampered if it is unfeasible to cultivate and apply herbicide with a shielded sprayer . As a general rule, crops are more competitive against weeds the closer they are planted to a square grid arrangement. In addition to row spacing, row orientation may play a helpful role in managing weeds. Mathematical models have shown that a crop’s exposure to sunlight during the growing season can be maximized when rows are planted in a north-south orientation rather than east-west. Maximizing the amount of light a crop captures will minimize the amount that reaches weeds growing near the soil surface. The effect increases as one moves farther south in latitude. In contrast to the growing season, winter crops would receive more light exposure if planted in an east-west orientation, due to the sun’s lower rise in the sky. However, determining row orientation based on the potential for weed control would probably never outweigh the importance of planting across slopes for the sake of soil conservation .
Planting date also affects weed pressure in row crops. Delayed planting of some crops allows for control of early-germinating weeds with cultivation or herbicide applications. Early planting so that a crop becomes established prior to weed germination could be possible if frosts are not a threat. Soil temperatures will likely be cooler under heavy residue, and this may initially slow crop growth.
The use of banded fertilizer applications or subsurface drip irrigation can also limit the germination and growth of weeds by directing nutrients and water toward the crop, making them less likely to reach weeds. Nutrient sources can also influence the competitiveness of crops over weeds. Highly available forms of nutrients, such as chemical fertilizers and organic fertilizers that decompose rapidly, tend to favor weeds. On the other hand, green manures and compost provide slow-release nutrients that tend to favor crops. Aim to achieve the right balance of nutrients for each stage of crop growth; a low level of nutrients may slow crop growth while allowing weeds to dominate, and excessive nutrients may accelerate crop growth without benefiting crop vigor .
Carefully choosing varieties and ensuring uniform establishment can also give the crop an edge over weeds. While factors such as yield, market demand and disease resistance play a major role in variety selection, consider any features that could contribute to weed suppression. Characteristics to look for include vigorous early growth, speed of canopy closure, height and foliage density. If growing several varieties of a particular crop, consider planting varieties with competitive characteristics on weedier fields and planting varieties with fewer competitive characteristics on fields with fewer weed problems .
Although every practice described here may not be suitable for every farm, valuable weed management tools may be found by evaluating each practice. Tactics that reduce weed problems by only a small amount may prove to be vital parts of an overall control plan if they are cheap and easy to implement .
In reduced-tillage systems, tillage can be used to control weed germination on a limited area of the field, such as with strip tillage. When combined with herbicide applications, this practice can control weeds that thrive in reduced-tillage environments. Ridge tillage, in which crops are planted on a ridge or raised bed, is another option. Cultivation to maintain the ridges and control inter-row weeds can decrease the amount of herbicides needed. At planting, the top two inches of the ridge are scraped into the inter-row area by an attachment ahead of the planter. This eliminates small weeds growing immediately near the row and moves their seeds to the inter-row area where they can be controlled more easily through cultivation and ridge-building upon their emergence. A winter cover crop can slow weed growth in the spring, increasing the ability of ridge-till planting to eliminate them. The system is typically used for crops planted in 30-inch rows [1, 12].
Less-aggressive cultivation using a high-residue cultivator is also an option. This implement is a sweep that runs underneath cover crop residue in the row middle. This disrupts the upper soil layer while leaving cover crop residue on the soil surface intact. Two passes with the cultivator may be necessary when attempting to reduce the number of herbicide applications. When using a cultivator, make sure that the crop’s roots are not disrupted along with the weed’s roots. Cultivation in combination with other weed control strategies can help to control small-seeded annuals and disrupt the growth of perennial weeds.