When Corn Comes After Corn
By Dr. Paul Tracy

High corn prices push growers to plant more corn. Here are a few agronomic and cultural issues to consider as corn acres increase.

One of the hottest topics this winter is continuous corn production in the Midwest. The two primary reasons for continuous corn production have been the whirlwind associated with ethanol production and the relatively high predicted long-term prices for corn. Soybean rust to our south and soybean aphids to our north have also stimulated discussions toward converting planned soybean acreages to corn production.

Agronomically, I always recommend crop rotation into any long-term crop management program. Continuous mono-cropping commonly leads to reduced yields, higher input costs, increased pest pressure and less flexibility in marketing options compared to well-planned crop rotations. However, some growers have successfully grown continuous corn (or soybeans) for many years.

Most success with continuous corn has occurred in the northern or western Corn Belt.

In our region, a few years of continuous corn will not present insurmountable problems. However, those who are considering continuous corn need to modify management practices accordingly.

Must we mono?
Can we increase corn acreages without going to continuous corn? Using numbers from the USDA, it appears we can. The USDA statistics since 2000 have shown Missouri to average 2.76 million acres of corn compared to 4.98 million acres of soybeans. Assuming corn/soybeans to be the dominant crop rotation, my mathematics estimate that Missouri growers could convert 1.1 million acres of traditional soybeans to corn and still maintain a 50/50 corn/soybean crop rotation. However, personal experience tells me that growers currently involved in continuous soybean production will not necessarily be the ones converting soybean acres to corn acres in 2007.

I have heard Kevin Bradley,University of Missouri extension weed scientist, attribute glyphosate-resistant weed issues in Missouri to the fact that we grow so many acres of continuous soybeans.

Converting some of those acres to corn may reduce current glyphosate-resistant weed selection pressure. It could also help produce greater volumes of corn—without depending upon continuous corn as a base cropping system for our region.

Iowa, Nebraska, Illinois and Kansas all grow more corn than soybeans. If additional corn is produced on traditional soybean acreages in those states, there will be much more pressure on a continuous corn production system.

My perspective is those states are somewhat better suited to continuous corn than Missouri. Continuous corn seems to fit better into higher organic matter soils and irrigated corn production environments that are prevalent west and north of MFA’s trade territory.

Money talks
Obviously, short-term economics drive the interest in continuous corn. Since 2000, average corn yield in
Missouri has been 129 bushels per acre. Average soybean yield during this time period has been 37 bushels per acre. Using estimated cost of production calculations taken from the University of Missouri guides developed by Melvin Brees, the estimated cost of producing 37-bushel soybeans would be approximately $200 per acre and the cost of producing 129-bushel corn would be approximately $345 per acre. I estimate the actual costs to be somewhat higher based upon current seed, fuel and fertilizer prices.

Currently, the relatively high commodity prices for corn ($3.50 per bushel) and soybeans ($7 per bushel) would provide an estimated corn gross income of $451.50 with a net of $106.50. Similarly, the estimated gross income for soybeans would be $259 with a net of $59 per acre.

The above numbers look relatively positive for corn compared to soybeans in today’s economic environment. However, as you move toward continuous corn, inputs will probably increase and yield potential may decrease. Recent estimates from
Purdue University show that continuous corn has an average 9 percent yield reduction compared to corn rotated with soybeans. This range of yield reduction is almost universally accepted by agronomists across the Corn Belt.

Soybeans supply between 20 and 50 equivalent units of nitrogen to a following corn crop. Using $0.30 per unit of nitrogen fertilizer applied, that becomes $15 more spent on nitrogen alone for continuous corn compared to corn following soybeans.

Cost is not the only consideration with nitrogen fertilizer in continuous corn. Heavy corn residue may tie up surface-applied nitrogen. Also, urea volatilization is greater when urea fertilizer is intercepted by heavy residue before contacting the soil. Knife placement, surface banding and the use of urease inhibitors like Agrotain are more justifiable for continuous corn compared to corn following soybeans.

Nitrogen is not the only nutrient that may need tobe modified when changing to a continuous corn production system. Phosphorus and potassium become more important as residue increases and soils become wetter and cooler early in the spring. Due to being closer to young roots and less subject to fixation, starter fertilizer and fertilizer band placement may become more important for continuous corn acreages.

Invitation to pests
Monocropping almost always puts more insect, disease and weed pressure on the crop being grown. For example, continuous corn provides a host home for corn rootworms, most of which do not flourish during the alternate host soybean year. Disease and insect pests that flourish in continuous corn include corn rootworm, seed corn maggot, gray leaf spot, corn blight and anthracnose. Continuous corn residue also attracts mobile insects like thrips, cutworms and armyworms, especially if winter annual weeds are present.

An example of added costs associated with insect pressure in continuous corn would be the move toward corn rootworm-resistant hybrids, especially in regions where corn acreage outnumbers soybean acreage. The additional cost can be $45 to $75 per bag of seed corn, or approximately $15 to $25 per acre, to plant corn rootworm-resistant hybrids. Without these hybrids, additional costs associated with in-furrow applied insecticides would be necessary.

Weed control needs to be modified when switching to continuous corn. Continuous cropping increases pressure on the few herbicides available for any given crop. It also limits usable multi-year weed control arsenal. For example, johnsongrass is a major problem in continuous corn. When soybeans are grown in the rotation, we have many good grass control products that can kill johnsongrass.

Greater residue in continuous corn also can have a negative impact on the efficacy of soil-applied herbicides. Therefore, continuous corn may require higher rates of residual herbicides and increase the effectiveness of and requirement for two-pass herbicide programs.

Rise in residue
Continuous corn generates considerably greater amounts of residue compared to corn/soybean rotations. This residue affects many aspects of your farm practices. In addition to the previously mentioned crop fertility and environmental aspects, high amounts of residue influence a number of management issues such as equipment requirements, planting operations, soil compaction, crop harvests, tillage management and erosion protection. I personally believe the positives associated with higher amounts of residue counterbalance the negatives.

Personal experience tells me that residue trash management (especially even distribution after harvest) and good planting practices (strip tillage, row cleaners and properly maintained, high quality planters) will greatly reduce the negatives associated with extremely high surface residues in continuous corn.

Another issue with continuous corn that many forget to consider is the heavier harvest load. Grain transportation, fuel costs and grain storage will increase along with the overall higher grain volume of corn compared to soybeans. This can be a major factor for growers, grain handlers, marketers and end users.

If you make the move
I would advise growers to make the choice toward continuous corn only after evaluating the total program. Consider the risks and rewards carefully. In our region, added pressure toward increased corn acreages may actually help break the continuous soybean cycle that many of our producers have adopted. If you decide to grow continuous corn, be prepared to modify traditional corn/soybean rotation management practices accordingly.

An excellent review of continuous corn was written in November 2006 by Dr. Bob Nielson at Purdue University. This publication can be found at: www.kingcorn.org/news/articles.06/CornAfterCorn-1121.pdf

Additionally, visit your local MFA Certified Crop Adviser to get local recommendations for successful continuous corn production. They will spend some of this winter gearing up to deal with this issue. MFA is poised to help with continuous corn production systems should you decide to use them.