Integrated Crop Management

Delayed preemergence herbicides for corn

Each year some fields have corn emerge prior to the application of preemergence herbicides. Many preemergence products registered for corn allow for application after corn emergence; however, Axiom, Balance, and Epic do not include early postemergence applications on the label. Consult labels to determine specific restrictions concerning delayed preemergence applications.

The performance of preemergence herbicides is less consistent when application is delayed, primarily due to weeds emerging prior to exposure to the herbicide. Researchers at Michigan and Wisconsin conducted research from 1993 to 1995 to investigate the impact of application delays on the performance of tank mixes of Banvel and acetamide herbicides (alachlor, metolachlor, and acetochlor). The three acetamide herbicides performed similarly across application dates, thus data presented are means of the three herbicides (Table 1). The at-planting application provided better or equal giant foxtail control than applications made 7 or 14 days after planting (DAP) in all experiments. Applications made 7 days after planting resulted in reduced giant foxtail control in one out of six experiments, whereas a 14-day delay resulted in reduced giant foxtail control in four experiments.

Application timing did not affect common lambsquarters control in five of the six experiments. At Wisconsin in 1995, both the 7- and 14-DAP applications provided better control than the at-planting treatment. The difference in response to application timing between foxtail and lambsquarters is due to the postemergence activity on common lambsquarters provided by dicamba. Addition of Accent to the combinations eliminated the reduction in giant foxtail control caused by application delays (data not presented).

Preemergence herbicides applied within a few days of planting will provide more consistent control than applications that are delayed 1 or 2 weeks after planting. In situations where weather conditions or other factors cause application delays, preemergence herbicides may still provide acceptable control. However, these fields should be monitored closely to determine the need for additional control tactics. Where delays in application occur, rotary hoeing prior to or after application can reduce the likelihood that additional control tactics will be needed later in the season.

Table 1. Influence of application timing on performance of acetamide-dicamba tank mixes in corn (1993-1995).

Timing MI - 93 WI - 93 MI - 94 WI - 94 MI - 95 WI - 95
% Giant foxtail control
At planting 93 95 70 91 95 94
7 DAP 92 93 61 93 97 90
14 DAP 93 56 58 68 97 90
LSD NS 16 10 10 NS 3
% Common lambsquarters control
At planting 93 97 96 96 97 69
7 DAP 93 94 97 98 99 88
14 DAP 95 98 96 96 99 89
Rainfall (inches)
AP to 6 DAP 0.16 1.26 0.47 0.08 0.39 0
7 to 13 DAP 0 0.39 0 1.02 0.12 2.4
14 to 27 DAP 2.91 1.97 0.63 0.51 2.20 3.4

Source: Spandl, Rabaey, Kells, and Harvey. 1997. Weed Technol. 11: 602-607.

This article originally appeared on page 65 of the IC-482(10) -- May 17, 1999 issue.

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