No. 20 Article 6/August 5, 2005

Impact of Late-Season Glyphosate Applications on Weed Seeds

Late-season applications of glyphosate-containing herbicides in soybean have become more common in recent years. These applications occur for a variety of reasons, one of which is the desire to reduce the amount of weed seed produced by late-emerging weeds. Do late-season applications of glyphosate actually reduce the amount of weed seed produced? The simple answer is that success depends to a great extent on timing. Dr. Bob Hartzler, a professor and weed science extension specialist at Iowa State University, recently summarized research conducted in Louisiana and Iowa that examined the effects of late-season herbicide applications on weed seed production. We thank Dr. Hartzler for granting us permission to reprint his work, which constitutes the remainder of this article.

Each year in mid- to late July weeds begin to poke their heads through the soybean canopy, therefore tempting farmers to load up the sprayer for one last trip across the field. Although they realize much of the impact of the weeds on crop yields may have already occurred, they feel they may still be able to reduce weed seed production and reduce harvest problems. This article summarizes research investigating the impact of late-season applications on weed seed production (Clay, P. A. and J. L. Griffin. 2000. "Weed seed production and seedling emergence responses to late-season glyphosate applications." Weed Science 48:481-486).

Field and greenhouse experiments were conducted with cocklebur, hemp sesbania, and sicklepod (the latter two species are problem weeds for soybean growers in the south). Only results of cocklebur will be presented here, since results were fairly similar among species. Glyphosate was applied at three stages: initial seed set75% of blooms had set burs; midseed fillburs fully elongated; physiological maturityburs initiated dry down. Roundup D-Pak was applied at 0.38, 0.56, and 0.75 lb ae/A (equivalent to 1,1.5, and 2 pt per acre of original Roundup). The results of two field experiments are presented in Table 2. Data are averaged over years and glyphosate rates, since there were no interactions between these factors and application timing. The earliest application of glyphosate reduced bur production by 75%, and burs averaged only 20% of the weight of burs produced by control plants. When application was delayed until burs were fully elongated or had reached maturity, the effect on seed production and viability was minimized. Similar results were found in the greenhouse, in that glyphosate had little impact on seed production once seed fill had initiated.

Researchers at Iowa State University studied the effect of late-season 2,4-D treatments in corn on the seed production of cocklebur and velvetleaf (Elizabeth Green Lorch and Dick Fawcett, 1983, Iowa State University). 2,4-D was applied at 0.5 to 1 qt/A at several corn stages, from V11 to the dent stage. Little effect was observed on weed seed production or viability when applications of 2,4-D were delayed until seed capsules or burs were present at application timing. If applications were made prior to seed set, the productivity of the weeds was greatly reduced. Seed production was reduced more effectively in cocklebur than velvetleaf, because cocklebur initiated flowering at later dates than velvetleaf.

The adoption of Roundup Ready technology has made late-season treatments more attractive because of minimal potential for crop injury (except for wheel tracks) and the ability of glyphosate to control large weeds. However, the potential value of these late-season treatments should be carefully evaluated prior to committing to spraying. Keep in mind that much of the impact on yields will have already occurred, so little yield benefit is likely to be achieved. If the weeds have not initiated seed set at the time of application it should be possible to reduce seed production. However, if the fruiting structure is visible, it is unlikely that killing the weeds at this late date will influence seed production or viability of the seed. Many people think that late-season treatments will reduce the viability of seeds that are produced, but research has consistently shown that seeds that have been initiated at the time of application are unlikely to be greatly influenced. The other possible benefit of the late-season treatments is harvesting efficiency, and there may be situations where this may make the treatment worthwhile.--Aaron Hager and Dawn Nordby

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