Tips for a Successful Post-Emergence Herbicide Application

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By Dairyland Seed Agronomy Team

Recent warm temperatures and moisture resulted in widespread rapid weed growth. This has several people considering a post-mergence (POST) herbicide application. For a successful POST application, growers should take into consideration things like application timing, weed size, crop size, weather conditions, specific herbicides and adjuvants.

Application Timing

Timely weed control is key for preventing yield loss and weed escapes. Be sure to scout your fields so you have an idea of weed pressure and size, along with the size of the crop. Ideally, targeting weeds before they are four inches tall will result in better weed control. Weeds larger than four inches are much more difficult to control and are more likely to have escapes. Different herbicides have different crop cutoffs so be sure to check the label for application restrictions prior to use.

Weather Conditions

Weather conditions both before application and after application can greatly influence the absorption of herbicides and the crop tolerance. If rain is predicted around the application timing, then it’s best to check the label for the rain free period prior to making the application. Cooler temperatures will slow plant growth and reduce the absorption of herbicides, while warm and humid conditions will promote rapid growth and greatly increase herbicide absorption. For best results we aim to apply herbicides to actively growing plants, but if applications need to be made during periods of stress, then a more aggressive adjuvant can help mitigate the lower absorption. On the flip side, if applications need to be made during rapid growth, then a more conservative adjuvant package should be considered to help with crop tolerance.

Herbicides and Adjuvants

When choosing herbicides, it’s best to use a combination of herbicides that still have efficacy on tough-to-control weeds. Selecting herbicides with multiple sites of action will help to combat herbicide resistance in weeds and prolong the effectiveness of products. Always be sure to check the label for each herbicide and follow the label for the most restrictive product in the tank mix. The label will also give instructions for adjuvant requirements and rates, along with allowable tank mix partners and mixing order.

Cooler temperatures are predicted this week for much of the Dairyland Seed footprint so it is important to keep in mind that cooler weather can greatly reduce the effectiveness of herbicides. Weed control is best achieved when herbicides are applied to actively growing weeds. Systemic herbicides like glyphosate and 2,4-D need to be absorbed by the plant and move through the plant (translocation) to work properly. When temperatures drop below 60°F, absorption and translocation are reduced which causes herbicides to act slowly. As a rule of thumb herbicide application should be avoided if nighttime temperatures drop below 40°F.

If nighttime temperatures are below 40°F, it is best to wait for warmer weather to make the application. If herbicides must be applied before ideal conditions return, then make sure you are using the full rate and appropriate spray adjuvants. This is especially important if the water you are using as a carrier is coming out of the well and/or is very cold. Colder water temperatures can have a dramatic negative impact on herbicide performance.