Issue No. 3, Article 6/April 20, 2012
Weed Science Odds and Ends
It's challenging to write a single article encompassing the possible weed management scenarios of a cropping season ranging at present from emerged corn to corn at the 5-leaf stage to fields not yet planted. That said, here are a few topics that may interest weed management practitioners.
- While conditions in March and early April were conducive for planting, these same conditions were not conducive for good performance of soil-residual herbicides. Many surface-applied herbicides received neither timely precipitation nor mechanical incorporation to move the applied herbicide into the soil solution.
Herbicide effectiveness can be significantly reduced when a soil-applied herbicide is sprayed onto a dry soil surface with no incorporation (mechanical or by precipitation) for several days following application. How much precipitation is required to move herbicide into the soil and how soon after application it is needed are difficult to define and can vary by herbicide, but for optimal incorporation, surface-applied herbicides generally need 1/2 to 1 inch of rain within 7 to 10 days. Factors including soil condition, soil moisture content, residue cover, and the herbicide's chemical properties influence how much and how soon after application precipitation is needed. Weed control can be reduced if enough rain is not received in the needed timeframe.
- A few callers have reported broadleaf and grass weeds emerging in fields where a soil-residual herbicide was applied to the surface with no subsequent precipitation or mechanical incorporation. Several wondered if the soil-residual herbicide will control these emerged weeds once enough rain is received to move the herbicide into the soil solution. The simple answer is that there is no simple answer.
In a few instances the soil-residual herbicide might "reach back" to control small emerged weeds after precipitation. For example, in research plots treated with isoxaflutole we have seen emerged velvetleaf (up to about 1 inch tall) turn white after precipitation. More often, however, emerged weeds are likely to survive a recently "activated" soil-residual herbicide. Annual grass weeds that have emerged in fields treated with soil-applied chloro¬acetamide herbicides will not be controlled by that herbicide and will have to be controlled with a postemergence product.
It is advisable to take immediate steps to control emerged weeds taller than 1 inch rather than wait several additional days after precipitation to see whether a soil-applied herbicide will control them. Even though the soil-applied herbicide might not control emerged weeds following the next precipitation event, it could still provide residual weed control once enough rain is received.
- In fields not yet planted it is advisable to control emerged summer annual weeds before planting. Established weeds can be quite competitive with emerging crop plants. In fields with emerged waterhemp that will be planted to soybean, controlling emerged weeds before planting is especially important. If the field will be planted with a non-GMO or glyphosate-resistant soybean variety and the emerged waterhemp is resistant to glyphosate and PPO and ALS inhibitors, there are no herbicide options to control the waterhemp after the soybean plants emerge. Waterhemp began to emerge at Urbana the second week of March, and across areas of central Illinois some producers have reported emerged waterhemp as tall as 3 or 4 inches.
- Horseweed (marestail) is growing rapidly in no-till fields that have yet to be treated with a burndown herbicide. Be sure to select the appropriate herbicide or herbicide combination and application rate to ensure good control of these plants. If you suspect glyphosate-resistant horseweed, consider adjusting your burndown herbicide program before making an application to increase the likelihood of achieving complete control before planting. Whether or not a horseweed population is resistant to glyphosate, it is advisable to control horseweed before plants exceed 4 to 6 inches in height.--Aaron Hager