No. 1 Article 6/March 22, 2012

Considerations with (Very) Early-Season Weed Management

Much informal discussion has been happening about the relative mildness of the winter just past. Several articles in this first issue of the Bulletin discuss possible implications of a mild winter for crop production and protection. This article also touches on how the mild winter and current above-normal air temperatures might impact weed management in 2012.

An obvious implication is the abundance of winter annual weed vegetation growing in many fields across the state. Survivorship over the winter of weeds that emerged last fall was undoubtedly higher than usual, and the recent stretch of warm air temperatures has prompted these weeds to resume growth several weeks ahead of average. Windshield observations found that henbit began to flower over two weeks ago in many areas of central Illinois.

It’s a challenge to list many reasons not to begin managing these dense populations of winter annual weeds. The higher-than-normal air temperatures and abundant sunshine we are experiencing are advantageous for good activity of burndown herbicides. Soil conditions across much of the state are conducive for preplant tillage operations that can effectively control winter annual weeds. Whatever tactic is used, a primary objective of effective winter annual weed management is to control existing vegetation before it produces viable seeds--and the current growing conditions mean seed production is likely to occur sooner than normal. Also, the early elimination of winter annual weeds reduces the oviposition sites of insects such as the black cutworm.

The suite of winter annual species is a familiar one. One species of particular concern this spring is horseweed, also known as marestail. Like other annual plant species, horseweed completes its life cycle in one year. Unlike many other annual species, however, it may exist as a winter or summer annual. Populations of winter annual horseweed typically emerge in the fall, within a few days or weeks after seed is dispersed from the parent plant. Summer annual populations can emerge in early or late spring--as late as early summer in some instances. In northern areas of Illinois, most horseweed demonstrates a winter annual life cycle, whereas south of about Interstate 70, there is a substantially higher proportion of spring emergence. Both winter and summer annual life cycles can be found across central Illinois.

Seedling horseweed plants.

Mature horseweed plants in soybean fields were common late in the 2011 growing season.

Glyphosate-resistant populations of horseweed are common across the southern portion of Illinois, and we are concerned that they are becoming increasingly common across much of central Illinois. Horseweed was seen in many soybean fields across central Illinois during the late summer months of 2011. Although we did not screen any of these populations for resistance to glyphosate, we encourage weed management practitioners across central Illinois to be aware that glyphosate resistance does occur in Illinois horseweed populations.

Burndown of glyphosate-resistant horseweed will require using alternative herbicides or tank-mixing other effective herbicides with glyphosate. If you suspect glyphosate-resistant horseweed, you might consider adjusting your burndown herbicide program before making an application to increase the likelihood of achieving complete control before planting. Whether or not a horseweed population is resistant to glyphosate, it is advisable to control horseweed before plants exceed 4 to 6 inches in height.

With soil conditions conducive to preplant tillage operations, many farmers are taking advantage of the unseasonable situation to prepare fields for planting. Tillage coupled with the increasing soil temperatures can promote seed germination of summer annual weed species. If tillage is done in mid-March and no soil-residual herbicides are applied within a few days of the last tillage operation, be sure to scout for emergence of summer annual weed species prior to planting. If summer annual species have emerged before corn is planted, consider including a herbicide that has burndown activity when applying a soil-residual herbicide.

Reduced rates of soil-residual herbicides applied in mid-March are unlikely to provide much residual control of summer annual weed species following crop emergence. If a soil-residual herbicide is applied this early to fields where corn will be planted, consider applying the full recommended rate for the particular soil type. Another way to increase the duration of residual weed control following planting is to split the application of the soil-residual herbicide, applying perhaps 50% or 60% now or sometime before planting and the remainder before summer annual weed species emerge. It is beneficial to control winter annual weeds in fields to be planted to soybean as soon as possible, but extended residual control of summer annual weeds can be achieved when soil-residual herbicides are applied closer to planting.--Aaron Hager

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