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The united States Department of Agriculture records hundreds of thousands of corn, soybean, and wheat acres as unplanted every year. Additional acres might have been idle in cover crops or conservation programs for one or more years. Several factors should be considered before planting corn or soybeans into previously unplanted fields.
While corn may not have been planted this year, there may still be a risk for corn rootworm (CRW) pressure in the field the following growing season. Fields planted with small grain cover crops or left fallow with weeds such as waterhemp and ragweed, may still have had beetles present in July or August. If beetles were present in these fields, eggs could have been laid.
As you consider your CRW management plan in a field that has been out of production, fields with a history of CRW pressure should be evaluated carefully. Weedy fields can support a CRW population. It is important to evaluate the risk of CRW pressure in these fields the same way you would following a corn crop. If you identified CRW beetles on weedy plants, you can assume some CRW laid eggs in the field and they could cause problems in the following corn crop.
Here are some questions to consider as you evaluate the right product choice for below ground insect protection in the next season:
If it is determined that a CRW risk exists for the next growing season, you may consider purchasing a corn product such as Genuity® SmartStax® RIB Complete® corn blend with two modes of action against CRW. Additionally, you could use a soil applied insecticide with Genuity® VT Triple PRO® RIB Complete® corn blend products. If you are planting a refuge-in-a-bag corn blend outside the Corn-Growing Area, planting a structured refuge is required. Contact your local representative to guide you in selecting the most appropriate products for your field.
Even if there is a history of effective weed control in your field, uncontrolled weeds that have emerged in fields where row crops have not been recently planted might cause significant yield reduction. In addition, thriving and uncontrolled prolific seed producing weeds, such waterhemp, Palmer amaranth, redroot pigweed, and common lambsquarters, may result in long term weed challenges, by increasing the weed seed bank.
Furthermore, seeds of tough-to-control or herbicide-resistant weeds may have moved from neighboring fields to your area via water and flooding during heavy spring rains and/or by air, introducing new weeds/biotypes to your field. Therefore, additional attention and scouting for herbicide resistance and invasive weeds is recommended in prior unplanted acreage.
It is important to manage weeds before they produce seeds on previously unplanted acreage. Managing weeds will help minimize the growth of the weed seed bank and prevent depletion of nutrients that are important for crop growth and development. Herbicides, tillage, and mowing are control options that can be used either alone or in combination prior to weeds setting seed.
If a cover crop was planted, a burndown and possibly an in-crop herbicide application may be needed. If no cover crop was planted, then additional herbicide options are available. Roundup® brand glyphosate-only agricultural herbicides, dicamba, and 2,4-D are common herbicide options for controlling weeds on recently unplanted acreage. All three herbicides can be used as a burndown, but Roundup brand glyphosate-only agricultural herbicides have the least plant-back restrictions.
Select the herbicide and use rates based on timing, plant growth stage, and plant-back restrictions. This is especially important if late herbicide applications were applied this summer to either control weeds in the fallow fields or over the top of a cover crop. Because higher rates or repeated applications of the same chemistry may have been needed to control larger weeds this year, be sure you know which herbicide options are still available to use next spring and summer; given that your normal application timing and rates may have been altered this past season.
Tillage can be used in conjunction with a herbicide burndown to control larger weeds. For most perennial weeds, waiting 5 to 7 days after the herbicide application to perform tillage can help improve weed control by allowing time for herbicide translocation.
The key to a successful mowing is making sure it is conducted before weeds set seed. Mowing can be used in conjunction with tillage or herbicides. For most perennial and stressed weeds, waiting to mow 5 to 7 days after the herbicide application can help improve weed control. The risk for erosion and fallow syndrome would likely be less with mowing versus tillage.
During the growing season, you might have applied nitrogen (N) for the planned corn prior to deciding not to plant the field. Under normal conditions, there should be some leftover N in the soil, which means a reduction in spring N fertilizer applications may be warranted next season. However, too much rain and saturated soil conditions can reduce last season’s remaining N. When soils are saturated, significant N loss can occur because of denitrification and leaching. Nitrogen loss should be evaluated and supplemental N may be needed. One method to help determine if supplemental N is required is the pre-sidedress N test (PSNT). Pre-sidedress N soil samples should be collected and tested. If significant N has been lost, more N should be applied via sidedressing.
Cover crops can help minimize N losses. Legumes can build soil N by fixing atmospheric N, while non-legume cover crops scavenge or trap N, thereby preventing leaching and denitrification. However, if the cover crop was planted later than usual due to a wet spring, some of the nutrient benefits typically provided by cover crops may have been diminished. A soil test can assist in determining the remaining nutrients available for the coming season. If a field was left fallow, soil compaction and erosion, reduction in organic matter, and N losses can occur.
Saturated soil that prevents row crop planting can also reduce vesicular arbuscular mycorrhizal fungi (VAM) colonization of plant roots, which can result in fallow syndrome. Under normal conditions, there is a symbiotic relationship between the plant and the fungi, where VAM fungi provide the plant with nutrients such as phosphorus (P) and zinc (Zn) and the plant provides sugars for the fungi to survive.2
Symptoms of fallow syndrome include reduced plant growth and P and/or Zn deficiency. A significant decrease in VAM population in the field is generally due to lack of host plants in prior unplanted fields. Soil testing for P and Zn is highly recommended before planting the previously unplanted acres. Corn is more susceptible to fallow syndrome than wheat because wheat has a more fibrous root system and is more efficient in P uptake. Even if soil test results indicate a high amount of P, a band application of P is strongly recommended when planting corn. Applying 60 to 80 lbs P/acre as a starter fertilizer can help overcome the effect of fallow syndrome.3 The starter should be applied in a 2 x 2 band, not in furrow. It is not feasible to use VAM fungi inoculants due to availability and cost.
Soybean roots under normal soil conditions are very efficient in absorbing soil nutrients; however, poor nodulation and reduced N-fixation can occur in flooded or saturated soil conditions. This may occur where saturated soil leads to a decrease in N-fixation bacteria populations. Under these conditions, soybean seeds should be inoculated with N-fixing bacteria at the time of planting. Sorghum is another viable option for recently unplanted acres because it has less negative response to low VAM populations.4 Please work with your local brand agronomist to determine the appropriate management strategy for your prior unplanted field conditions.