Why Do I Have So Many Weeds?

 
Red root pigweed

Many growers have complained about weeds escapes this year, and many fields are showing vigorous weed growth. There are several reasons that contributed to the lack of weed control this year.

First and foremost is the lack of rainfall in the early season. Most preemergent herbicides need at least ½" of rain to activate the product. If rain does not arrive before weeds begin to germinate, these early weed escapes grow and tower over the crop by fall. It is amazing how large a single weed can grow from this early start. Some growers tried irrigation to activate herbicides with mixed results. The best control I've seen from this method was with Devrinol (which requires incorporation within 2 days) that was overhead irrigated immediately after application with great results. Perhaps waiting a few days for rain before deciding to irrigate does not work as well.

The second reason is the warm temperature. With high heat units accumulated this season, weeds benefited as much if not more than crops. Late season heat units also pushed weeds to grow late in the summer.

Postemergent herbicides to control weed escapes did not perform as some growers wanted. In some cases, weeds were too large when the rescue treatment was applied. Remember that postemergent herbicides are designed to control small weeds. Also, products need to be absorbed by the plants and work best on actively growing weeds. By June, weed escapes were beginning to feel the drought as well as the crops, and were not actively growing when postemergent herbicides were applied.

Rainfall patterns also promoted late weed escapes and lush weed growth. Moisture finally came to most areas in late July or August. Although welcomed by the crop, this moisture also gave life to weeds that had escaped weed control treatments. By late August, weeds appeared tall and healthy above many crops. Although there weren't many weed escapes, the level of weed control appeared much lower than it truly was. Generally, these late weeds have little impact on yield, but are giving harvesting challenges in many crops. One exception is the escape of Eastern black nightshade in beans, which may reduce yields and bean quality.

In general, 2005 will be remembered for its unusual weather patterns and poor weed control results. As you plan your weed management strategy for 2006, remember the reasons for weed escapes this year, and note the effects on yield and quality before making drastic changes for next year. If you choose to plough, remember you will be turning under this year's weed seeds and turning up last year's. What was your weed control like in 2004? This may have an effect on what you will see next year. For no-till and conservation till fields, weed control results from this year should be your guide for next year.

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