Before selecting one or more herbicides, you should know what weeds are present or expected to appear, the soil texture and organic matter content, the capabilities and limitations of the various herbicides, and how best to apply them. You can now use the North Carolina WebHADSS (Herbicide Application Decision Support System) for help with weed management strategies.

Weed Survey and Scouting

The first step in a weed management program is to identify the problem. Every field should be surveyed for weeds each fall. On a field map or other suitable place, record the species present and the general level of abundance. The Weed Science Society of America website has excellent resources available for weed identification. Species present in the fall can be expected to be the predominant problems during the following year. By knowing ahead of time what species to expect, you can better plan a herbicide program and avoid costly mistakes. Considering weed maps over a 2- or 3-year period will improve your accuracy in predicting problems and will also help you to detect shifts in the weed population.

Scouting to determine the need for postemergence herbicides or cultivation should begin about 10 to 14 days after planting. Fields should be scouted at least weekly for the first 4 to 6 weeks after planting. It is important to detect and identify the problem early so the proper postemergence treatment can be applied at the proper time. Applying postemergence herbicides after weeds are beyond the optimum size for treatment is one of the most common causes of poor results with these herbicides.

The NCSU soybean weed scouting website provides more information.

Soil Characteristics

Application rates for most soil-applied herbicides must be adjusted according to the soil texture and organic matter content. Failure to do so may result in poor weed control or crop injury. In addition, soil texture and organic matter content may determine whether or not certain herbicides can be used on a given field.

Reports on soil samples submitted to the North Carolina Department of Agriculture (NCDA), Agronomic Division, list the percentage of humic matter, not percentage of organic matter. Herbicide activity is probably more closely related to humic matter content than to organic matter content. However, herbicide labels specify application rates based upon organic matter content, and there is not a simple correlation between humic matter content and organic matter content. In addition, reports on soil samples submitted to NCDA do not specify soil textural classes. Herbicide application rates cannot, therefore, be based upon soil sample reports from the NCDA.

When using a herbicide with a narrow margin of crop tolerance, such as a product containing metribuzin, it is advisable to have an organic matter and textural determination made by a private laboratory. Except on newly cleared land, one such soil test every five years is sufficient. Some agrichemical companies offer a free soil testing and herbicide rate recommendation program. Check with a local pesticide dealer or your county Extension Service agent for details.

Herbicide Selection

Before using any herbicide, you need to learn the important capabilities and limitations of the various products labeled for use in soybeans. You need to know what weeds are controlled by a given product, what rate to apply, how best to apply it, the crop injury potential, rotational restrictions, and any special precautions. Product labels, manufacturers’ literature, and Extension publications are good sources for this information. Tables at the end of this publication contain much of that information. In addition, your Extension agent has computer software to help you select herbicide programs. This software is available to growers.