Choosing the Right Post-Emergence Herbicide Applications for Your Farm
Once your crop is in the ground, there are still many critical decisions to be made that can help you protect the yield potential you planted at the beginning of the season. One of the primary decisions you face is which post-emergence herbicide(s) to use on your crops.
Hopefully, you were able to start out with clean, weed-free fields.
But depending on conditions at and after planting, it’s possible that you could see some weed escapes that you’ll need to control in order to bridge the gap between planting and canopy.
Before making any decisions about herbicide applications, you’ll want to make sure you know how to read a pesticide label and take some time to scout your fields in order to assess crop growth and pest pressure.
How to determine which herbicide(s) to use in your fields
There are two main factors that should play into your herbicide selection:
1. Crop growth stage or height
As you know, many crop-related factors play into your herbicide decision. Most of the efficacy and timing for a majority of herbicides is based not only upon the crop in the field, but also on its growth stage or height. For example:
- Glyphosate* can be broadcast on up to 24-inch corn, or, if drop nozzles are used, 30-inch corn.
- Atrazine can be used pre-emergence, pre-plant or post-emergence, but only on up to 12-inch tall corn.
- Aim® is labeled for use in corn at the V8-V14 growth stages.
- Armezon® can be used up to the V8 stage of corn.
*Please note: Never use a post-emergence application of glyphosate on crops that are not traited for glyphosate tolerance.
2. Weed presence and characteristics
The offensive weeds themselves will also help you determine which herbicide will work best. You’ll want to consider whether you’re dealing with broadleaf or grass weeds (different herbicides are labeled for each), if the weeds have emerged, how tall they are and whether or not it’s possible they have developed resistance to a particular herbicide.
Putting together a complete package
Incorporating multiple modes of action is key to fighting weeds and accounting for herbicide resistance.
Furthermore, different herbicides will impact plants in different ways—through the roots, shoots or leaves—which is why including compatible herbicides in your post-emergence spray is vital.
It’s also important to overlap your pre-emergence application with a residual and to use chemicals with residual control in your clean-up spray as well.
Here’s an example of this strategy: Using glyphosate in your pre-emergence application along with a product such as Bicep II MAGNUM®—which incorporates atrazine and s-metolachlor—gives you three modes of action. Then using Callisto® and dicamba in your post-emergence spray can clean up any escapes and provide some late residual control.
In total, that’s five modes of action to combat weeds in your field.
A weed that is up is harder to kill, so you’ll likely want to use a herbicide that can be picked up by the roots. Dicamba, for example, can be picked up by the roots, shoots or leaves, while Prowl® is only taken in by the shoot.
Determining your application rate
Each ag chemical product label will display a range of application rates. The best rate for your farm and fields can be determined based on weed size, population density, weather and method of application.
As a general rule, the larger the weed or scope of the outbreak, the higher the application rate you’ll need. This is especially true if it’s hot and dry, which will cause the plant to slow down its processes and therefore absorb the herbicide more gradually.
Under moderately normal conditions a medium rate will work, while a lower rate simply allows for weed escapes all too often.
PRO-TIP: In an effort to maximize the efficiency of your post-emergence spray(s), make sure you consider which adjuvants to use in your tank mix.
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ALWAYS READ AND FOLLOW LABEL DIRECTIONS. It is a violation of federal and state/provincial law to use any pesticide product other than in accordance with its label. The distribution, sale and use of an unregistered pesticide is a violation of federal and/or state law and is strictly prohibited. We do not guarantee the accuracy of any information provided on this page or which is provided by us in any form. It is your responsibility to confirm prior to purchase and use that a product is labeled for your specific purposes, including, but not limited to, your target crop or pest and its compatibility with other products in a tank mix.