The wet weather has continued to prevent application of burndown herbicides in many parts of the state. Most of the information presented in the April 26 article on burndown herbicide options still applies. Keep in mind that herbicide rates may need to be increased as application continues to be delayed. Growth of summer annual weeds (ragweeds, lambsquarters, foxtails, etc) has generally been slow but they are getting larger, and have the potential for rapid growth with sun and heat.
Winter annuals that have flowered and gone to seed are dying anyway, and therefore may be not an essential target for burndown treatments. Control of emerged marestail continues to be a primary concern. In our experience, soybean burndown herbicide mixtures that can adequately control herbicide-resistant marestail are likely to be effective on most of the other emerged weeds in a field.
We did ignore corn burndown situations somewhat in the April 26 article. Effective burndown is generally easier to achieve in corn than soybeans, due to the burndown activity of atrazine, higher rates of saflufenacil, dicamba, SureStart, Hornet, Lumax/Lexar, etc. Application of 2,4-D too close to corn planting does occasionally cause crop injury, and most labels advise applying either at least 7 days before or 7 days after planting. Dicamba products can be used at the time of planting. Dicamba product labels allow the use of up to 1 pint/A on medium to fine-textured soils with at least 2% organic matter, but state that corn should be planted at least 1 ½ inches deep with adequate seed furrow closure to reduce risk of injury. Gramoxone can have more utility in corn than in soybeans, because it works well in combination with atrazine and a growth regulator herbicide.
We have received questions about whether PRE herbicide rates in PRE+POST programs can be reduced when planting is delayed into late May or June. The answer is – maybe. The result of crop planting in late May or early June is an overall compression of the period of time for which weed control is needed. Crops planted in late April often grow extremely slowly for a while, and a late April planting will be subject to the full season of summer annual weed emergence. The result is that the crop can require 8 weeks or so of weed control. Crops planted in late May typically emerge and grow faster than those planted in April. A late May planting occurs well into the period of summer annual weed emergence, so the weeds that emerge after planted is lower.
The net result of these factors is that only about 4 weeks of weed control may be needed. So, it may be possible to reduce PRE herbicide rates where a POST herbicide will be applied. Keep in mind that reducing PRE herbicide rates reduces the longevity of weed control, but it also reduces the initial active rate of herbicide. This can mean a reduction in control in tougher broadleaf weeds that require a higher herbicide rate to be controlled, such as giant ragweed, cocklebur, and morningglory. For these weeds, reducing the PRE herbicide rate could result in a higher population of weeds, and larger weeds, at the time of the POST application.