Preharvest herbicide use in dry edible beans: Caution needs to be taken to avoid illegal residues
Dry beans will most likely need preharvest herbicide applications in the 2012 season.
The lack of good dry bean weed control this year (2012) combined with the unevenness of dry bean maturity, often observed with increased moisture near the end of the season, will require the use of herbicide applications prior to harvest. Preharvest herbicide applications have become more popular as dry bean growers have switched to direct harvesting techniques. Herbicides used prior to harvest, 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 five 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), glyphosate (Roundup and several other formulations) and Sharpen 2.85L (saflufenacil). Differences in these products include the speed of activity, recropping restrictions and effectiveness. But, more importantly, if not properly used one of these herbicides – glyphosate – can lead to illegal herbicide residues in the marketed product.
Additionally, the newest registered product, Sharpen, does not have clearance for use on dry edible beans being exported outside of North America. While we have had good results with Sharpen as a harvest aid, we are currently not recommending the use of Sharpen until MRLs (maximum residue levels) are established for the export market; since much of Michigan’s dry edible bean crop is exported.
As mentioned earlier, caution needs to be taken if using glyphosate as a harvest aid. Over the last several years there have been an increasing number of 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 the hard 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. The maximum use rate of glyphosate as a preharvest treatment is 0.75 lb a.e./A (equivalent to 22 fl oz/A of Roundup PowerMax). Remember, there is also a seven-day preharvest interval (PHI) for glyphosate.
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
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 Michigan State University trials, Gramoxone Inteon has been the herbicide with of 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 a seven-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) 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 1-inch of rain before planting this crop. For sugarbeets, the labeled rotation restriction is four months if the soil is tilled and 8 months if the soil is not tilled with a bioassay using a maximum application rate of 2 oz/A of Valor.
There have been some concerns with these rotation restrictions prior to planting sugarbeet. We have conducted two years of research that looked at planting of sugarbeet following Valor desiccant applications to dry bean. In our research, we have observed reductions in sugarbeet stand 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 sugarbeet needs to be conducted and tillage is essential. Even with tillage, there is a high probability that sugarbeet stand 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.
Aim 1.9 EW (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 1 percent v/v MSO. The preharvest interval for Aim is three days.
Combinations of different preharvest herbicides
Several growers have felt that they have had better results with combinations of the aforementioned harvest aid products. In MSU trials, we have rarely observed a benefit to some of these combinations. However, 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.
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