By Ed Lentz
Farmers have to consider fall weed control for next year’s crops because of winter annuals.
Winter annuals are plants that germinate in the fall, go through the winter, and produce seed next spring or summer — completing their life cycle in one year.
Some of these winter annuals provide a food source for other pests, such soybean cyst nematodes. This allows the nematode population to increase even though soybeans were not grown, causing problems in future soybean crops.
Other winter annuals, such as cressleaf groundsel, are toxic to livestock.
Purple deadnettle, henbit, common chickweed, marestail, field pennycress, shepherd’s purse, and cressleaf groundsel can easily be found in fields at this time.
All of them have leaves in a rosette pattern in the fall, except for purple deadnettle and common chickweed, which look like the mature plant at this time. The other plants will bolt next spring and produce a stalk with flowers and seeds.
The only way to eliminate seed production of these winter annual weed species before planting next spring is to apply herbicides this fall. If marestail is not controlled in the fall, it becomes very difficult to control next spring, requiring at least four active ingredients to obtain good control.
Cressleaf groundsel also is important to control in the fall. It is a yellow flowering weed in the spring. If planting is delayed, this plant will produce seed and blow seed to other fields.
This past spring, lots of cressleaf groundsel went to seed because of the late planting. There is so much of this weed around that it is becoming prevalent in hay, pastures, and wheat fields. This weed is poisonous to livestock when it is fresh and when it is dried, making it a serious problem for livestock producers.
Winter annual weeds begin to emerge in early August and continue to emerge until the soil freezes. New emergence is likely after each rain in the fall.
This wide window of emergence can make control more difficult. Because of this difficulty, farmers have to decide when to apply a fall herbicide and which herbicide products should be used.
The window of application is quite wide, but must be done before soils freeze or become saturated with water, making it difficult to spray.
The most common time to apply fall herbicides is late October, especially if glyphosate and 2,4-D are applied. To be effective, the weed must be growing enough to absorb these herbicides.
It is always best to spray on warm and sunny days, but spraying under cool conditions is better than doing nothing. It may take longer for the herbicide to kill the weeds when applied during cold weather and the control may be somewhat less.
However, even if the weeds are not killed, the injured plants will be easier to control next spring.
Farmers have many products to consider in their fall weed program, but a mixture of at least two different herbicides should be used. The most cost-effective herbicide combination should be the one applied.
One program is to apply 22 to 32 fluid ounces of glyphosate (Roundup) mixed with 1.5 pints of 2,4-D ester. Ester formulation of 2,4-D is favored over the amine version because it has more activity in cool conditions. Farmers do not want to apply too early or new weeds may emerge later.
Another popular combination is a premixture of 2,4-D and dicamba, such as Brash or Weedmaster, applied at 1 quart per acre plus metribuzin at 6 ounces per acre. This program is fairly effective, but less under cold weather conditions because it uses 2,4-D amine.
The rate of the 2,4-D plus dicamba premixture should be increased to 1.25 quarts per acre if dandelions are present. Dandelions are a perennial plant rather than a winter annual.
The metribuzin is important to control common chickweed and provides some residual control for the fall (not the spring), allowing this treatment to be applied earlier. Chickweed is not controlled by 2,4-D.
These two programs allow any crop to be planted next spring, while most other combinations available lock you into a specific crop next season. For example, a program using Canopy will only allow soybeans to be planted the following spring.
Farmers also have to consider whether the fall weed program will kill a cover crop. Most grass cover crops will not be affected by these programs if glyphosate is not used, such as cereal rye.
However, all broadleaf cover crops, such as radishes, clovers, and brassicas, will be injured or killed with these programs.
Controlling weeds and growing a crop is always a challenge for farmers. In many situations, a fall weed control program has to be implemented to successfully control weeds in next year’s crop.
However, farmers have to consider many things in choosing a program such as the future crop to be grown and the use of cover crops.
Lentz is extension educator for agriculture and natural resources for the Ohio State University Extension Service in Hancock County. He can be reached at 419-422-3851 or via email at firstname.lastname@example.org.
Lentz can be heard with Vaun Wickerham on weekdays at 6:35 a.m. on WFIN, at 5:43 a.m. on WKXA-FM, and at 5:28 a.m. at 106.3 The Fox.