Preharvest herbicide applications are an important part of direct-harvest dry bean production
Function, timing and rotation restrictions are important to consider when selecting a pre-harvest herbicide for dry beans.
Over the next two and a half months, many Michigan dry bean fields will be treated with pre-harvest herbicides. Preharvest herbicides, also known as “harvest aids,” are used to desiccate or dry down “green” stem and leaf tissue that can hinder dry bean harvest. The main intention of preharvest herbicide applications is to desiccate weeds; however, many growers use these herbicide applications to hurry along or even out the maturing process of dry beans.
Currently in Michigan, there are four different herbicide options labeled for preharvest applications in dry beans. These herbicides are Gramoxone SL 2.0 and other formulations of paraquat, glyphosate (Roundup and several other formulations), Valor 51WG (flumioxazin) and Sharpen 2.85L (saflufenacil). Differences in these products include the speed of activity, re-cropping restrictions and effectiveness. Understanding the strengths, weaknesses and restrictions of each of these products will be important in selecting the herbicide that best fits your operation. If the guidelines below are not followed, yield can be reduced, rotational crops can be injured and illegal herbicide residues, such as glyphosate, can be found in the marketed product. Below are the benefits and use precautions for herbicides that can be used for preharvest herbicide applications in dry beans.
Gramoxone SL 2.0
Gramoxone SL 2.0 (paraquat) was one of the first products registered as a harvest aid in dry beans. The primary use of Gramoxone is to desiccate uncontrolled weeds that may interfere with harvest. However, Gramoxone will also help desiccate dry beans that may have some green leaves or stems. In Michigan State University trials, Gramoxone has been one of the herbicides with the quickest speed of activity, showing greater control of weeds and desiccation of dry beans at three days after treatment than glyphosate or Valor. However, by seven days after treatment, dry bean desiccation with Valor and 14 days after treatment weed control and dry bean desiccation with glyphosate (Roundup) have been similar to Gramoxone. Gramoxone is a contact herbicide, so desiccation is dependent on good spray coverage.
The use rate of Gramoxone is 1.2 to 2 pints per acre. In MSU trials, we have generally applied 2 pints per acre. A non-ionic surfactant (NIS) at 0.25 percent volume for volume (v/v) must be applied with Gramoxone. The application timing for Gramoxone is when the dry bean crop is mature, at least 80 percent of the pods should be yellowing and mostly ripe and no more than 40 percent (bush-type beans) or 30 percent (vine-type beans) of leaves still green. Gramoxone can be applied as a split application if weed and dry bean growth is lush and vigorous, but the total application rate cannot exceed 2 pints per acre. There is a seven-day preharvest restriction between application of Gramoxone and dry bean harvest. Gramoxone is also a restricted-use pesticide, so a private or commercial pesticide applicator’s license is required for use of this product. There are no crop rotation restrictions with Gramoxone.
Glyphosate (Roundup PowerMax and several other formulations) provides the most consistent and effective weed control of the preharvest herbicides labeled for use in dry beans. However, glyphosate is the one product that if not used properly can lead to illegal residues in the marketed product and has led to buyers rejecting dry bean exports. To avoid illegal residues, glyphosate applications need to be made to dry beans after they are in the hard dough stage (30 percent seed moisture or less). Additionally, in MSU trials we have also observed a loss of black color in canned black beans if preharvest applications of glyphosate are made too early. While the intentions of most growers are to make these applications according to the label, the unevenness in dry bean maturity of some varieties makes it difficult to have an entire field all at the same stage for preharvest herbicide applications. In these cases, growers should either wait until the entire field of dry beans is in the hard dough stage (30 percent moisture or less) or they should consider using a different product.
The maximum use rate of glyphosate as a preharvest treatment is 0.75 pounds acid equivalent (a.e.) per acre – equivalent to 22 fluid ounces per acre of Roundup PowerMax. Not all glyphosate products are registered for preharvest use in dry beans, so it is important to consult product labels for legal applications. We recommend to always include 17 pounds of ammonium sulfate (AMS) per 100 gallons of spray solution for applications of glyphosate. Remember there is also a seven-day preharvest interval for glyphosate. Glyphosate provides the slowest activity of all the preharvest herbicides labeled and it generally takes seven to 14 days after treatment for complete weed and dry bean desiccation. There are no crop rotation restrictions with glyphosate.
Valor 51WG (flumioxazin) has provided similar desiccation of dry beans as Gramoxone, by seven days after treatment in several MSU trials. However, weed control has not been quite as effective. There is not an initial application timing listed on the label, but I would recommend using similar guidelines as Gramoxone. These guidelines are when the dry bean crop is mature, at least 80 percent of the pods are yellowing and mostly ripe and no more than 40 percent (bush-type beans) or 30 percent (vine-type beans) of leaves still green.
Valor should be applied at 1.5 to 2.0 ounces per acre with 1 quart per acre of a methylated seed oil (MSO). In MSU trials, 1.5 ounces per acre of Valor with MSO has provided similar desiccation as 2 ounces per acre of Valor. Dry beans can be harvested within five days of application, but in MSU trials it generally takes seven to 14 days after treatment to reach maximum dry bean desiccation. Depending on your crop rotation, the residual activity of Valor activity can be a draw back or benefit. If your intended rotation is corn or soybeans, Valor can provide some residual control of winter annual weeds prior to planting these crops. However, if you are planning on planting winter wheat after a desiccation application of Valor, there needs to be one month and 1 inch of rain before planting this crop. For sugarbeets, the labeled rotation restriction is four months if the soil is tilled and eight months if the soil is not tilled with a bioassay using a maximum application rate of 2 ounces per acre of Valor.
There have been some concerns with these rotation restrictions prior to planting sugarbeets. We have conducted two years of research that looked at planting sugarbeets following Valor desiccant applications to dry beans. In our research, we observed reductions in sugarbeet stands that were not reflected in yield or recoverable white sugar when the tilled rotation restriction was followed. Even though we have not observed a reduction in sugarbeet yield, I would still caution growers when using Valor as a desiccation treatment prior to planting sugarbeets. If a grower intends to plant sugarbeets after Valor has been used as a desiccant treatment, a successful soil bioassay planted with sugarbeets needs to be conducted and tillage is essential. Even with tillage there is a high probability that sugarbeet stands may be reduced. Another thing to keep in mind is Valor residues can be trapped in poly-tanks and hoses if the spray equipment is not adequately cleaned. There are special sprayer cleanup procedures listed on the label. It is important to follow these procedures so there is not a problem with tank-contamination in the following spray loads.
Sharpen 2.85L (saflufenacil) is the newest herbicide labeled for use as a harvest aid in dry edible beans and has been an effective dry bean desiccant in MSU trials and grower fields. Sharpen has similar speed of activity as Gramoxone and is quicker than glyphosate and Valor. Sharpen is a contact herbicide so desiccation is dependent on good spray coverage. Sharpen can be applied at rates up to 2 fluid ounces per acre. However, in MSU trials, 1 fluid ounces per acre of Sharpen with MSO (1 percent v/v) + AMS has provided similar desiccation as 2 fluid ounces per acre, so we recommend the 1 fluid ounce per acre rate. The application timing for Sharpen is when the dry bean crop is mature, at least 80 percent of the pods should be yellowing and mostly ripe and no more than 40 percent (bush-type beans) or 30 percent (vine-type beans) of leaves still green. There is a two-day preharvest restriction between application of Sharpen and dry bean harvest, however it generally takes seven days to reach maximum desiccation activity.
Crop rotation should also be considered when using Sharpen as a preharvest herbicide. Winter wheat can be planted immediately after dry bean harvest. However, if rotating to sugarbeets, special precautions need to be taken. The rotation restrictions for sugarbeets are four months for a 1 fluid ounce per acre application rate of Sharpen and five months for a 2 fluid ounces per acre rate, this excludes months where the ground is frozen. So in many cases, if you are rotating to sugarbeets you should consider using Gramoxone or glyphosate. Furthermore, there are still a few maximum residue levels (MRLs) not established for export of dry beans to Taiwan and only MRLs are established for kidney beans for export to Korea.
Combinations of different preharvest herbicides
Several growers have felt they have had better results with combinations of the aforementioned harvest aid products. In MSU trials, we have rarely observed a benefit to these combinations for dry bean desiccation. However, if weeds are a problem, the addition of glyphosate or Gramoxone to either Valor or Sharpen would improve weed desiccation. Michigan State University Extension would like to remind growers that if one chooses to use a combination of any of these products, it is important to follow use precautions of the most restrictive product.
Dr. Sprague’s work is funded in part by MSU’s AgBioResearch.