Weed Control Guide for Ohio Field Crops

Burndown Herbicide Programs for Corn and Soybeans

In almost all no-till corn and soybean fields, it is essential to apply herbicides with foliar activity before crop emergence to control existing weeds. Depending upon the herbicide approach used in the field for that year, herbicides used to control weeds at planting may include glyphosate or Gramoxone Max, or it may be possible to rely solely on PRE herbicides with foliar activity (atrazine, Canopy XL, Sencor, etc) in combination with 2,4-D. The latter approach will be most suitable when weeds in the field are fairly small, sometimes several weeks before planting.

Glyphosate (Roundup UltraMax, Touchdown, Glyphomax, etc)

Fields with quackgrass, Canada thistle, and other cool-season perennial weeds will almost always require the use of glyphosate around the time of planting. This treatment can reduce the population of perennial weeds that reach an appropriate size by the time of application, such as quackgrass. For other perennials such as Canada thistle, the level of long-term control with glyphosate will be variable, since they may be small at the time of application. Application of glyphosate to small perennials often controls existing foliage, but regrowth occurs later in the growing crop. Cool-season perennials, legumes, and cool-season grasses are more effectively controlled with glyphosate the previous fall if crop rotations allow. Glyphosate is also effective on most annual broadleaf and grass weeds, although the addition of 2,4-D ester greatly improves control of marestail (horseweed), giant ragweed, mustard species and a few other key no-till weeds. Activity of glyphosate on established dandelion and some winter annuals can be extremely slow when applied in the spring under cool conditions, and use of fall application or alternative herbicides should be considered in fields where chickweed, purple deadnettle, and dandelion have been problematic (see later section on fall applications). Glyphosate is most effective when applied alone or with 2,4-D, in spray volumes of 10 gpa or less, and when ammonium sulfate is included in the spray mix. Ammonium sulfate helps maintain glyphosate effectiveness in hard water, or when tank-mixed with residual herbicides. Glyphosate activity can be reduced when tank-mixed with certain residual herbicides (Sencor, for example), and when applied using 28% nitrogen solution or a similar material as the spray carrier. For this reason, glyphosate labels often specify that these types of mixtures should be used only for control of small annual weeds.

2,4-D ester (Weedone, Salvo, etc)

In many no-till fields, vegetation up until early May consists primarily of broadleaf weeds, and 2,4-D ester is an economical and effective tool for control of these weeds. While 2,4-D alone can provide adequate control of many broadleaf weeds, there are restrictions on rate and timing of application to minimize the risk of crop injury. In addition, 2,4-D is weak on chickweed, purple deadnettle, henbit, and a few other weeds.

Consequently, 2,4-D ester is most often used in combination with other herbicides to ensure that complete control of emerged weeds is achieved. Weeds not well-controlled by other herbicides that 2,4-D helps out on include marestail, prickly lettuce, mustards, giant ragweed, Pennsylvania smartweed, and dandelion. When applied too close to soybean or corn planting, 2,4-D can potentially reduce crop stands and cause injury to new seedlings. With regard to soybeans, restrictions are as follows: rates up to 1 pint/A must be applied at least 7 days before planting; rates between 1 and 2 pints must be applied at least 30 days before planting. With regard to corn, some labels suggest that 2,4-D be applied at least 7 to 14 days before planting or 3 to 5 days after planting. Other labels allow application anytime after planting. The risk of corn injury seems to be primarily when 2,4-D is applied around the time of corn planting, and application is followed by enough rain to move 2,4-D into the soil where seeds are germinating. There is also risk of injury when seed furrows fail to close completely and rain washes herbicide into the seed furrow where direct contact with seed is possible.

Paraquat (Gramoxone Max)

Use of Gramoxone in no-till systems has declined greatly over the past decade due to reductions in the price of glyphosate and the greater versatility of glyphosate across a range of weed life cycles (perennial, biennial, etc) and sizes. Gramoxone is most effective on small annual weeds, and when combined with photosynthetic inhibitor-type residual herbicides (atrazine, Sencor). Mixing 2,4-D ester with Gramoxone also results in more complete control of broadleaf weeds. Gramoxone is probably most useful when rapid dessication of weeds is essential, in order to allow tillage or planting. For example, a combination of Gramoxone plus atrazine or Sencor will result in more rapid death and dessication of chickweed or purple deadnettle, compared to glyphosate, when applied in the spring under cool conditions. Gramoxone is also useful for control of established dandelions in the spring when applied with Canopy XL (or Sencor plus 2,4-D ester if not too close to planting). Otherwise, Gramoxone should not be used for control of perennial or biennial weeds, legumes, or cool-season grasses. Gramoxone is most effective when applied with crop oil concentrate in relatively high spray volumes (at least 15 gpa), and spray volume should be increased as weed density increases.

Residual herbicides with foliar activity (atrazine, Sencor, Canopy XL, etc)

A number of residual herbicides also have foliar activity, and will control or help control small annual weeds. Herbicides in this category include atrazine, Sencor, Canopy XL, Python, FirstRate, and Hornet. These herbicides have activity primarily on small weeds, and the spectrum of control varies by herbicide. All have activity on broadleaf weeds, but some have little or no activity on emerged grasses. The most effective strategy when using one of these herbicides, in order to minimize the need for glyphosate or Gramoxone, is to apply a few weeks or more prior to planting with 2,4-D ester. If emerged grasses are present, consult the product label to make sure it will provide adequate grass control, and supplement the spray mix with glyphosate or Gramoxone as needed. Mixtures of residual herbicides with 2,4-D and/or Gramoxone should generally be applied with crop oil concentrate. Mixtures with glyphosate should include only ammonium sulfate and possibly nonionic surfactant, depending upon the glyphosate product used.

Fall herbicide applications

Some winter annual and perennial weeds have been difficult to manage in no-till systems with spring herbicide treatments. For example, control of well-established dandelions in the spring with 2,4-D and/or glyphosate has been inconsistent and extremely slow at times. Common chickweed and purple deadnettle can prevent the soil from drying and interfere with crop planting if not controlled by late winter or early spring. Application of herbicides in the fall can be an effective strategy for managing some perennials and winter annual weeds. The timing of application and herbicide choice varies with the weeds present and the goal of the fall treatment. OSU weed scientists have conducted several years of research on management of winter annual weeds with fall and spring herbicide applications. This information is available at the OSU Weed Science website ­ www.oardc.ohio-state.edu/weedworkshop/.

Back | Forward | Table of Contents