Integrated Crop Management

Application timing of preemergence herbicides

The optimum application timing for preemergence herbicides varies depending on both the characteristics of the herbicide and environmental conditions. Applications made at or close to planting usually are more consistent than applications made several weeks ahead of planting, except in situations where no significant rainfall events occur in the first few weeks following planting and herbicide application. During dry springs, early preplant applications may be more consistent than at-planting applications due to the greater likelihood of the herbicide being moved into the soil profile by rain. However, early preplant herbicides may fail to provide full season control in years with above average rainfall or delayed planting due to the greater time period between application and crop canopy formation.

Early preplant applications also are used to eliminate the need for burndown herbicides in no-till fields. Several weed species have emerged around Iowa (e.g., giant ragweed, sunflower, lambsquarters). Because many preemergence herbicides will not control emerged weeds, early preplant applications may not eliminate the need for burndown herbicides in fields with weeds present.

Research conducted by weed scientists at the University of Illinois reported differences in giant foxtail control with several acetamide herbicides applied at different intervals ahead of planting. Metolachlor (Dual), acetochlor (Surpass, Harness), and dimethenamid (Frontier) were applied at 15-day intervals from 60 days before planting until planting. Metolachlor was the most consistent product with very early application dates (Table 1) For example, at DeKalb all products provided greater than 95 percent foxtail control when applied at planting. However, only metolachlor provided greater than 90 percent control when applied 30 days before planting (DBP) or earlier. The other two products provided less than 80 percent control with early application dates. At the Urbana location, no product gave acceptable giant foxtail control, regardless of application timing. Metolachlor again was the only product that did not show decreased performance with applications made 30 DBP or earlier.

Early applications may reduce the risk of poor performance due to lack of rain, especially for growers who are unable to supplement preemergence herbicides with rotary hoeing. Growers willing to rotary hoe their fields when necessary are likely to achieve the most consistent control with applications made at or close to planting. Applications made more than two weeks ahead of planting should be viewed primarily as a convenience, rather than as a method to improve herbicide performance. Table 2 lists application restrictions for several herbicides used for grass control in corn. Due to the greater persistence of metolachor, this product is usually better suited for applications made more than four weeks ahead of planting. Differences in persistence among these herbicides is less of a factor with applications made within four weeks of planting.

Generally, Iowa State University recommends that herbicide preplant and preemergence applications be made as close to planting as possible. Supplemental weed management such as rotary hoeing and cultivation is a critically important component of weed management programs and lessens the variability of herbicide performance attributable to unfavorable rainfall patterns.

Table 1. Influence of application timing (DBP, days before planting) on giant foxtail control at two Illinois locations. Control ratings taken approximately two months after corn planting.

Application timing
Location Herbicide 60 DBP 45 DBP 30 DBP 15 DBP 0 DBP
DeKalb 2.4 lb acetochlor 80 70 68 80 98
1.5 lb dimethenamid 68 77 77 94 95
3.0 lb metolachlor 96 93 95 96 97
Urbana 2.4 lb acetochlor 38 60 65 70 77
1.5 lb dimethenamid 55 57 65 70 72
3.0 lb metolachlor 77 68 67 78 73

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

Table 2. Restrictions concerning early preplant applications for several corn herbicides.

Herbicide Maximum interval ahead of planting for single application Maximum interval ahead of planting for split application
Dual II 30 45
Frontier 30 45
Harness 30 45
Surpass 30 30
TopNotch 40 40

This article originally appeared on pages 42-43 of the IC-480 (5) -- April 13, 1998 issue.

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