By Dwight Lingenfelter
Now is the time to think about controlling summer annual weeds, prior to seed set, in certain cropping systems.
Preventing seed production is important for driving down the weed seed bank and reducing the need for weed control inputs (i.e. herbicides). In particular, it is easy to prevent weed seed production following a cereal grain such as wheat, barley, or oats as well as some vegetable crops such as sweet corn or snap beans. Proper timing of the control practice is essential in preventing seed production.
Several years ago, we conducted an experiment at Rock Springs looking at giant foxtail seed “rain” (when seed are shed) as influenced by date of emergence. Foxtail emerged from the middle of May to the middle of June in the experiment. In the end, the date of emergence did not matter. The later emerging grasses flowered at the same time as the earlier plants. With foxtail, this phenomenon is controlled more by day length (short day plants) than temperature. In both years, mature foxtail seeds were not produced until late August and peak seed rain occurred from late September through the month of October. Other species can be different. For example, we included yellow foxtail in the experiment. Yellow foxtail seed rain began in early August and continued into late October. Pigweed species can begin to produce mature seed by mid-August, while lambsquarters and ragweed generally do not mature until the month of September.
To prevent seed production, fields can be sprayed with an effective herbicide or mowed once or twice. Glyphosate is particularly effective at stopping grass growth and reproduction. The plant growth regulators (2,4-D and dicamba) would probably be a better choice for broadleaf weeds. With giant foxtail, even treating the field by mid-September would have greatly reduced seed production in our experiment. If seed heads are present, check suspect fields to determine how advanced flowering and seed rain are and time control practices accordingly. Taking the time to prevent seed production this year can make a big difference next year. In most cases, up to 80% of annual weeds in the field are the result of last year’s seeds.
In addition, many perennial broadleaves are evident in these same small grain stubble fields. The challenge with perennial weeds at this time of year is the fact they are still in the vegetative and reproductive phases. Therefore, most of the plant sugars are not being significantly transported to the roots and a herbicide application now will mostly only impact the top-growth. One consideration would be to mow those fields soon to prevent seed production and allow regrowth to occur. Then apply an effective systemic herbicide (ie, glyphosate, 2,4-D, dicamba) in late September or early October so the herbicide will be transported to the roots for more effective control.
As a reminder about burndown considerations prior to small grain establishment, the options are limited. Glyphosate and Gramoxone are the most commonly used nonselective products and can be used before establishing most of the various small grains. Clarity 4S or other dicamba equivalent products can be used but be aware of the rate structure --- 4 oz/A allows immediate planting but for each pint of Clarity applied delay crop planting 20 days. Harmony Extra can be used in the burndown before fall establishment of wheat and barley. Sharpen at 1-2 oz/A can be applied early preplant to preemergence for wheat, barley, triticale, and rye. Valor can be sprayed 30 days before planting wheat, so this may limit its utility in this crop in our area.
One more thing to think about is the establishment of a cover crop. If herbicides such as glyphosate, 2,4-D, or dicamba are being used prior to cover crop establishment remember to consider the replanting interval. For glyphosate, most cover crop can be planted after application. However, depending on the rate of 2,4-D applied, at least a week to four-weeks waiting period is necessary before establishing cover crops. If small grain covers are being established, some research suggests that a minimum delay of 7-10 days after application at rates of 1 pint/acre of 2,4-D ester. Also other studies have shown that certain clovers (eg, red and crimson) and alfalfa may be established within the same constraints with 2,4-D ester. To be certain, a longer waiting period and rainfall help to reduce the potential for injury. Dicamba usually requires at least a couple weeks or more before certain cover crops can be planted and this also is directly related to the amount applied.