Fall herbicide application: Risk vs. benefits
by Bob Hartzler

September 21, 1999 --  In recent years, several companies have promoted fall applications of preemergence herbicides . ISU has not recommended these treatments due to concerns over performance and possible environmental risks. Our concerns on performance are based on the fact that herbicides applied in the fall are subject to the environment for a longer period of time than spring applications, therefore, increasing the risk of control failures due to early degradation of the herbicide. This is supported by ISU research (Table 1.).

Table 1. Comparison of foxtail control with fall and spring applications of Dual II

Data is the summary of four experiments conducted during 1995 and 1996 at Ames and Nashua. Dual II was applied at 3 pt./A on undisturbed soybean residue and corn planted during the year of the study. Minimum tillage included a single pass with a field cultivator prior to planting.
Source: Owen, Lux, and Franzenburg. Iowa State University.

These data show that fall applications were less consistent than spring applications (reduced weed control in 6 out of 8 studies) and also resulted in an overall reduction in grass control. It should be recognized that these studies represent a near worst-case scenario: Planting dates in these studies were later than for most growers in the area, the experimental areas had heavy weed pressure, and herbicide treatments were not supplemented with cultivation.  Thus, many growers have had more consistent results than shown in our studies.  Selecting fields with low to moderate weed infestation should help reduce some of the risks associated with fall applications; however, fall applications inherently will have a greater likelihood of late-season escapes.

Until recently, Dual was the only grass herbicide marketed for fall applications in corn. However, their success in capturing this market has driven other companies to consider entering this marketing opportunity. The primary competitors for the preemergence grass market are acetochlor (Harness, Surpass), dimethenamid (Frontier) and flufenacet (Axiom). How do other products compare to Dual for fall applications?

Research was conducted by weed scientists at the University of Illinois to evaluate the influence of application timing on the performance of metolachlor, acetochlor and dimethenamid. These products were applied at 15 day intervals from 60 days before planting until planting. The results found that metolachlor was the most consistent product with early application dates (Table 2.) For example, at DeKalb all products provided greater than 95% foxtail control when applied at planting. However, only metolachlor provided greater than 90% control when applied 30 DBP or earlier. The other two products provided less than 80% control with early application dates. While these tests did not evaluate fall applications, they illustrate the risks associated with applications made well ahead of planting.

Table 2. Influence of application timing (DBP = days before planting) on giant foxtail control at two Illinois locations during 1995. Giant foxtail control ratings taken approximately two months after corn planting.

Source: Wax, Hart and Maxwell. 1995 NCWSS Research Report. pp. 392-395.

It is important to consider the advantages and disadvantages of fall applications when determining whether this strategy is appropriate for specific situations. The primary advantage of fall applications is time management. Completing herbicide application in the fall eliminates an operation in the spring and may allow growers to initiate planting earlier. This may be an important consideration for growers managing large acreages. Fall applications also reduce the risk of herbicide failures in years with a lack of rain following preemergence applications.

The primary disadvantage of fall applications is the potential for late-season weed escapes caused by early degradation of the herbicide. Using this strategy in fields with moderate weed infestations and the use of cultivation as a supplementary weed management strategy reduces this risk and should result in satisfactory control for many growers. Research has shown that metolachlor (Dual) is more persistant than the other amide herbicides, thus it is the best candidate for this type of application. However, it is our belief that spring applications will result in more consistent control with all products, and the risk of control failures should be weighed against the convenience of fall applications.

Current fall labels:

Axiom:  Apply after October 15 when the sustained soil temperature at the four inch soil depth is less than 50 degee F.   Apply before soil freezes.  Apply at 19 to 23 oz/A on medium textured soils with >2.5% OM and at 23 oz/A on fine textured soils with >2.5% OM.

Dual II:  North of Highway 30 apply after September 30 whereas south of Highway 30 apply after October 15.  Apply after soil temperature at the four inch depth is less than 55 degree F and falling.  Do not apply to frozen ground. 

(This article is an updated version of article appearing during 1997).

Prepared by Bob Hartzler, extension weed management specialist, Department of Agronomy, Iowa State University