No. 21 Article 2/September 7, 2012

Considerations for Fall-Seeded Small Grains

The dry conditions of the 2012 growing season did little to improve the performance of many herbicides used in corn and soybean. Of perhaps comparable consequence is that the degradation of many herbicides has also been reduced by the exceptionally dry soil conditions. Concerns about herbicide carryover were discussed in issue 17 (July 27) of the Bulletin, and recent results from greenhouse bioassays have sustained some of these concerns.

About two weeks ago we collected soil at Urbana, Brownstown, and Perry from plots used for weed control experiments this year to initiate greenhouse bioassays. Our goal for the assays was to improve our understanding of how the dry conditions have impacted herbicide degradation in soil. Using our research plots allowed us to sample from areas where we knew what herbicides had been applied, when, and at what rates as well as the precipitation amounts received before and after application. Soil samples were also collected from nontreated control plots.

The soil collections were placed in greenhouse pots; half the pots were subirrigated for 7 days before planting corn, soybean or wheat, while the other pots were subirrigated only after planting.

Wheat growing in soil previously treated with a soybean herbicide (right) demonstrates herbicide injury symptoms.

Crop species emerged within 7 days after planting. Wheat growing in soil collected at Perry began to demonstrate symptoms of injury to some herbicides within a few days after emergence, suggesting that residues remained high enough to injure the crop. Injury symptoms have not developed on emerged wheat growing in soil collected from Urbana and Brownstown.

Based on these early observations, we offer the following comments:

In fields where there is no risk of herbicide carryover, be sure to control any existing weed vegetation before planting wheat. Products such as glyphosate, Clarity, Gramoxone, and Sharpen can be applied before planting wheat (but be sure to wait the required time after applying Clarity--15 days at 8 fluid ounces and 30 days at 16 fluid ounces).--Aaron Hager

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