The first half of April was cool, but in the last week the daytime temperatures have really started to warm up.

I have observed many winter annual weeds such as common chickweed, henbit, and deadnettle in our no-till fields that are now flowering. Common ragweed and lambsquarters are emerging with the foxtails and velvetleaf not far behind. It’s a good time to remember that environmental conditions and the size of the weeds greatly affect the activity of burndown herbicides and weed control. Even though the daytime temperatures have increased, nights can still be cool, which impacts active weed growth and herbicide uptake and activity. Also, larger annual weeds and perennials that have overwintered tend to be more difficult to control under cold conditions. There is often a tradeoff in getting good control of the winter annual weed populations like chickweed and deadnettle (where smaller is better) and trying to suppress perennials like Canada thistle and quackgrass (where bigger is better). It’s difficult to do a good job controlling both of these types of weeds with a single application.

In general, herbicides such as glyphosate and 2,4–D provides the best control when annual weeds are 6 inches tall or less and still in the vegetative stage of growth. Winter annuals that are flowering may require higher rates or different combinations of products. Perennials should be at least 6 to 8 inches tall and preferable more when a systemic herbicide is applied. Perennials that are beginning to flower (bud to bloom stage) are most susceptible to control with systemic herbicides.

Remember — do not plant corn or soybean into live green vegetation without making a burndown application or follow the planting operation ASAP with an effective knockdown herbicide program.

Corn in particular is very susceptible to early season competition for light, nutrients, and water. Planting into green vegetation is challenging for any crop seed and can lead to additional problems such as hair pinning, open slits, poor seed to soil contact, insect and bird problems, etc. which all result in reduced stands.

Here are a few additional suggestions to help improve burndown herbicide activity:

• After a cold spell, wait until a few days of warm, sunny weather have occurred before applying herbicides. Herbicides work slower under cool conditions.
• Use ester formulations of 2,4–D, instead of the amine (2,4–D LV4). Esters are usually more effective under cool conditions and on perennial weeds. Also, esters usually penetrate the waxy leaf surface (cuticle) better than amines.
• Add ammonium sulfate (AMS) or a similar N product to glyphosate.
• Increasing the rate of the burndown herbicide may be necessary if weeds are stressed by cold and drought conditions or are larger in size.
• Spray as soon as possible — fields with a high density of winter annuals may benefit from the burndown now to conserve soil moisture which almost always is a problem later. This also will reduce the amount of vegetation that is present at planting time.
• Most importantly, scout fields prior to planting and spraying to ensure you use the correct herbicide program for the problem.

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