Preharvest herbicide applications in dry edible beans can lead to illegal residues if you are not ca
Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.
Even though there is still plenty of time until dry bean harvest, the impending late harvest combined with the unevenness of dry bean growth in many fields will probably lead to a number of preharvest herbicide applications being made in dry bean fields. Over the past several years, preharvest herbicide applications in dry edible beans have become more popular as have growers switched to direct harvesting techniques. These preharvest herbicides applications, 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 are using 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 Inteon 2SL (paraquat and other formulations), Valor 51WG (flumioxazin), Aim 1.9EW (carfentrazone), and glyphosate (Roundup and several other formulations). Differences in these products include the speed of activity, recropping restrictions and effectiveness. But more importantly, if not used properly one of these herbicides, glyphosate, can lead to illegal herbicide residues in the marketed product.
Over the last several months, there have been reports of exported dry beans being rejected from buyers due to illegal glyphosate residues in the shipped product. Glyphosate residues can be found in dry beans if the glyphosate label is not followed and applications occur prior to thehard dough stage (30 percent moisture or less). This coupled with the extreme sensitivity of current residue testing equipment makes the detection of small amounts of herbicide residues exceedingly easy in exported products. If residues are detected above acceptable levels this can render exported product shipments worthless and can put future roadblocks in the exportation of Michigan dry beans. 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. Below are the benefits and additional use precautions for herbicides that can be used instead of glyphosate as a preharvest herbicide application in dry beans.
Gramoxone Inteon 2SL(paraquat) was one of the first products registered as a harvest aid in dry beans. The primary use of Gramoxone Inteon is to desiccate uncontrolled weeds that may interfere with harvest. However, Gramoxone Inteon will also help desiccate dry beans that may have some green leaves or stems. In MSU trials, Gramoxone Inteon has been the herbicide with the quickest speed of activity, showing greater control of weeds and desiccation of dry beans at three days after treatment. 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 Inteon. Gramoxone Inteon is a contact herbicide so desiccation is dependent on good spray coverage. The use rate of Gramoxone Inteon is 1.2 to 2 pt/A. In MSU trials, we have generally applied 2 pt/A. A non-ionic surfactant (NIS) at 0.25 percent v/v must be applied with Gramoxone Inteon. The application timing for Gramoxone Inteon 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 Inteon 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 pt/A. There is aseven-day preharvest restriction between application of Gramoxone Inteon and dry bean harvest. Gramoxone Inteon is also a restricted-use pesticide, so a private or commercial pesticide applicator’s license is required for use of this product.
Valor 51WG (flumioxazin) is the newest herbicide labeled as a preharvest treatment in dry beans. Valor has provided similar desiccation of dry beans as Gramoxone Inteon, 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 Inteon. 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 oz/A with 1 qt/A of a methylated seed oil (MSO). In MSU trials, 1.5 oz/A of Valor with MSO has provided similar desiccation as 2 oz/A 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 soybean 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 one inch of rain before planting this crop. For sugarbeets, the rotation restriction is four months if the soil is tilled and eight months if the soil is not tilled with a maximum application rate of 2 oz/A of Valor. Because we have very little experience with these rotation restrictions to sugarbeets from fall applied Valor, we initiated a trial to examine these rotation restrictions on Michigan soils. Until the sugarbeet harvest results are available, I would caution growers on using Valor as a desiccation treatment if you are intending to plant sugarbeets the following spring. 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.
Aim 1.9EW(carfentrazone) has consistently been the least effective preharvest herbicide for weed and dry bean desiccation in MSU trials. However, if this is the herbicide that you choose to use there are a few guidelines that need to be followed. The application timing is similar to Gramoxone Inteon. These guidelines are to apply Aim 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. The Aim use rate is 1 to 2 fl oz/A and Aim should be applied with one percent v/v MSO. The preharvest interval for Aim is three days.