Continuous Corn Considerations

From an agronomic perspective, a continuous corn cropping system is fraught with a multitude of negative yield-influencing factors. A recent review of crop rotation research literature indicated an average yield loss of 9% for continuous corn, with yield losses ranging from 2% to 23%. Of the 26 studies reviewed, only two cited yield advantages to continuous corn.

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Key issues to consider with continuous corn include fertilizer needs, increased disease and insect pressures, and better weed management. Here’s a review of some of these factors.

Most agree that optimum nitrogen (N) fertilizer rates for corn following corn are higher than for corn following legumes, and range from 30 pounds to 50 pounds additional N required per acre. A major shift of soybean acres to corn acres nationally may greatly increase the difficulty for some grower-customers in arranging for their corn N fertilizer needs.

Another consideration for grower-customers who routinely sidedress most or all of their N fertilizer is the fact that more days will obviously be required to complete this operation if more corn acres are planted. However, sidedressing must be completed with­in a certain time period because of plant height limitations imposed by traditional ground-driven sidedress applicator tools. High-clearance N applicators (e.g., Hagie) help extend the sidedress window by allowing for N application to taller corn.

Corn removes more soil phosphorus (P) and less soil potassium (K) per acre than soybeans. A one-time move to second-year corn will have negligible effects on P and K soil fertility levels. Over a number of years of corn following corn, however, grower-customers should obviously monitor soil P and K levels and adjust fertilizer rates accordingly.

Disease, Insect Risks

The risk of some corn diseases is greater when corn follows corn, especially when some form of reduced tillage is practiced. Two such diseases that can devastate susceptible hybrids are gray leaf spot and northern corn leaf blight.

During the past few years, there has been a lot of talk about substantial yield increases in field corn sprayed with strobilurin fungicides. However, the decision to use a foliar fungicide should be based on known susceptibility of the hybrid to gray leaf spot and northern corn leaf blight and the likelihood that disease will develop.

Greater levels of surface corn residues in a corn-corn system can delay corn emergence and early seedling development, resulting in a lengthier exposure of corn seedlings to secondary soil pests (wireworms, seedcorn maggots, and white grubs) and soilborne seedling diseases. That in turn may result in weakened plants and/or stand reductions.

Mitigate the insect risk in second-year corn by the judicious use of soil-applied insecticides, insecticide seed treatments (high-rate formulations), or transgenic resistance (Bt rootworm) for rootworm. Scout fields during seedling emergence for cutworm and armyworm damage to leaves and stems to determine the possible need for rescue treatments of foliar insecticides.

Weed Management Issues

A continuous corn cropping system limits grower-customers to fewer herbicide options than a corn-soybean crop rotation. In addition, the higher crop residue levels associated with continuous corn can decrease the efficacy of many soil-applied herbicides and favor certain weed species that thrive in an environment of greater soil moisture (certain annual grasses, johnsongrass, and some small-seeded broadleaf weeds).

If using soil-applied herbicides, use full rates to compensate for the effects of greater residue to best manage weeds in continuous corn. If plans include greater reliance on postemerge herbicide applications, ensure that weeds don’t exceed 6 inches in height before making the applications. Weed escapes supply new weed seed to the soil weed seed bank. Furthermore, giant ragweed, burcucumber, waterhemp, and crabgrass have relatively long emergence periods and two-pass weed control programs are always more successful on these weeds.

Mitigate the risk of poor giant ragweed and burcucumber control by adjusting weed management plans to include the use of postemergence herbicides that provide residual activity on these weeds. Shifting atrazine use from preplant to postemergence will extend the residual window of activity and reduce late season weed emergence. Products containing Callisto, Hornet, and Peak (Spirit) also provide foliar and residual activity treatments, unless the giant ragweed is ALS-resistant.

For better control of late emerging grass weeds and some small seeded broadleaf weeds, consider adding a reduced rate of an amide, acetochlor, or flufenacet to the post­emergence herbicide treatment. Amide herbicides will not control emerged grass weeds, however. So if grass weeds have emerged, a post­emergence grass herbicide will be required for control.

Mitigate the risk of glyphosate-resistant weeds by including a variety of herbicide modes of action, especially on weeds that are most problematic for control with glyphosate alone. If glyphosate-resistant corn was grown in a particular field in the previous year, one should also strongly consider using herbicides with other modes of action to prevent additional selection pressure for glyphosate-resistant weeds.

This is particularly important in fields where growers have found it increasingly difficult to control giant ragweed and common lambsquarter. Marestail, lambsquarters, and giant ragweed are effectively controlled by many postemergence herbicides. Products containing atrazine provide the most effective control of these weeds, provided they are applied before weeds are 6 inches tall.

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