In recent years, several companies have promoted fall applications of preemergence herbicides (e.g. Dual, Command, Treflan). 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 conducted over the past two years (see table, below).

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 extremely heavy weed pressure, and herbicide treatments were not supplemented with cultivation.

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. However, it is our belief that spring applications will result in more consistent control, and the risk of control failures should be weighed against the convenience of fall applications.

1 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.

2 Owen, Lux, and Franzenburg. Iowa State University.