No. 18 Article 7/July 22, 2005

Late-Season Herbicide Applications in Soybean

In the not-so-distant past, mid- to late July was a time when most applications of postemergence soybean herbicides had been completed for the year. Nowadays, with the seemingly ubiquitous occurrence of waterhemp in Illinois soybean fields, the widespread adoption of glyphosate-resistant soybean, and a generally low tolerance for weeds poking through the soybean canopy, late-season herbicide applications are much more commonplace. However, such applications are not without risk.

Obviously, larger weeds can be expected to be more difficult to control because they are older plants and because spray coverage can be limited. Application rate, volume, and spray additives are important factors to keep in mind, especially if you are attempting to achieve good spray coverage on larger weeds. The possibility of herbicide drift that can injure sensitive vegetation is ever-present.

Almost every postemergence soybean herbicide has a preharvest interval specified on its label. Preharvest intervals indicate the amount of time that must elapsebetween the herbicide application and crop harvest. These intervals are established to allow sufficient time for the herbicide to be broken down or metabolized in the plant. Additionally, the preharvest interval reduces the likelihood of herbicide residue remaining on the harvested portion of the crop. Failure to observe the preharvest interval may result in herbicide residue levels in the crop in excess of established limits.

In addition to preharvest intervals, there are restrictions on many post-emergence soybean herbicide labels about whether the soybean crop may be used for livestock feed or if treated fields may be grazed as forage. Table 2 addresses preharvest intervals and grazing restrictions for a number of postemergence soybean herbicides.

Another interval that is important to observe is the rotational crop interval. Nearly all herbicide labels (soil-applied or postemergence) have rotational crop intervals that specify the amount of time that must elapse between herbicide application and planting a rotational crop. This becomes particularly important with late-season herbicide applications. These intervals are established to reduce the possibility that sufficient herbicide residues will persist in the soil that could adversely affect the rotational crop. Some herbicide rotational restrictions are based solely on time, while other factors, such as soil pH and the amount of precipitation received after herbicide application, can influence the length of the crop rotational intervals for other products. For example, the Classic label indicates that field corn may be planted 9 months after application; however, the interval is extended 2 additional months if applications containing chlorimuron are made after August 1. Rotational intervals for many soybean herbicides can be found in Table 5b of the 2005 Illinois Agricultural Pest Management Handbook .

Another potential risk of late-season postemergence soybean herbicide applications is crop injury. Most of the soybean crop across Illinois is in full bloom, and a significant percentage of plants have begun setting pods. Many postemergence soybean herbicide labels caution against making applications after the crop has begun to bloom, as injury during this time could potentially reduce soybean yield. Two types of soybean injury we have been asked about recently are soybean leaf cupping and yellowing of upper trifoliolate leaves.

Soybean leaf cupping is not unique to the 2005 growing season. We have addressed this phenomenon in past issues of the Bulletin, covering many of the proposed theories as to why soybean leaves exhibit cupping. If you would like to read the article we wrote last year on soybean leaf cupping, it appeared in issue number 15 (July 2, 2004).

Perhaps the most recent research on soybean exposure to plant growth regulator herbicides and soybean leaf cupping was conducted at the University of Illinois by Kevin Kelly, a graduate student, under the direction of Dr. Dean Riechers. Kevin's research included a field project where soybeans were treated at either the V3 or V7 growth stage with various post-emergence soybean herbicides, alone or "contaminated" with a low rate of 2,4-D, clopyralid, or dicamba. The many interesting findings from this research included these: (1) soybean yield reduction was more likely if exposure to plant growth regulator herbicides occurred at a later stage of soybean development (i.e., later in the growing season, especially after soybean entered reproductive development) compared with exposure during early vegetative development, and (2) a larger soybean yield reduction was possible if soybean was exposed to certain postemergence soybean herbicides and dicamba compared with exposure only to dicamba or the soybean herbicide. Much of this field research was sponsored by funding provided through the Illinois Soybean Program Operating Board and is summarized in a new fact sheet that can be downloaded from the University of Illinois Weed Science Web site--click on the link titled "Fact Sheet on Plant Growth Regulator Injury to Soybean").

We've also been asked about yellowing of uppermost trifoliolate leaves following a recent application of a glyphosate-containing product. This also is not unique to Illinois soybean fields or to the 2005 growing season. Weed scientists in Illinois and other states have observed this response since the early years of glyphosate-resistant soybean adoption, but there hasn't been widespread consensus as to what exactly causes this to occur. Many soybean farmers have become accustomed to seeing very little soybean response, if any, following an early-season application of glyphosate, so observing this yellowing after later-season applications does tend to raise questions. Some observations made across the years related to this yellowing of uppermost trifoliolate leaves include these: (1) the frequency of this response is much higher when applications of glyphosate-containing products are made after soybeans have entered the reproductive stage of growth compared with applications made during vegetative development; (2) yellowing may be more intense on headland rows or other areas of the field where spray overlaps occurred; (3) not all soybean varieties have the same propensity for this response; and (4) the effect is usually transitory and green color returns to the leaves within a couple weeks.--Aaron Hager

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