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Mitigate the downside risks of corn following corn

Indiana corn growers planted an additional 1.1 million acres of corn in 2007 compared to the previous season, for a total of 6.6 million acres. Essentially all of the additional corn acres came at the expense of a decrease in soybean acres. Consequently, the number of acres planted to second-year corn and/or continuous corn increased markedly. Farmers' planting intentions for 2008 are yet unknown, but the amount of aggressive tillage being conducted in corn stubble fields this fall would suggest that many farmers plan to continue planting corn following corn.

From an agronomic perspective, a continuous corn cropping system is fraught with hazards and typically yields less than corn in a crop rotation system. Most growers understand this. However, some are equally concerned that soybean rust, soybean aphid or other major soybean stresses in coming years may result in unacceptably low soybean yields and/or high production costs.

Consequently, some growers are willing to accept the known risks associated with growing corn following corn in order to avoid the uncertain risks associated with soybean production. While most agronomists certainly do not encourage monoculture of any kind, they can at least offer suggestions for mitigating the downside risks of corn following corn for those growers who feel pressured to do so.

Indiana corn growers planted an additional 1.1 million acres of corn in 2007 compared to the previous season, for a total of 6.6 million acres. Essentially all of the additional corn acres came at the expense of a decrease in soybean acres. Consequently, the number of acres planted to second-year corn and/or continuous corn increased markedly. Farmers' planting intentions for 2008 are yet unknown, but the amount of aggressive tillage being conducted in corn stubble fields this fall would suggest that many farmers plan to continue planting corn following corn.

Most agronomists agree that optimum nitrogen (N) fertilizer rates for corn following corn are higher than for corn following legumes, with estimates ranging from 30 to 50 additional pounds of N required per acre. Coupled with the oft-cited seven to 10% lower yield potential of continuous versus rotation corn, the higher required optimum N rates for continuous corn "adds insult to injury."

Higher levels of corn residue associated with continuous corn cropping systems on poorly drained soils in Indiana can create difficult stand establishment conditions due to slowed warming and drying of the soil. High levels of surface residue often also physically interfere with the furrow opening and closing functions of the corn planter's row units.

The risk of some corn diseases is greater when corn follows corn, especially when some form of reduced tillage is practiced that leaves greater amounts of non-decomposed, inoculum-bearing residue on the soil surface. Two such diseases that can devastate susceptible hybrids are gray leaf spot and, as some experienced in 2004 and 2005, northern corn leaf blight. Other diseases that may become more prevalent in corn following corn are stalk and ear rots.

The major insect threat to corn following corn in Indiana is the Western corn rootworm. The yield and production cost consequences for corn following corn is particularly meaningful for growers in areas of the state where crop rotation remains a viable control option for this insect pest.

Good hybrids for rotation corn tend to be good hybrids for continuous corn. Therefore, growers should first seek out hybrids that demonstrate consistent high yield performance across multiple environments. Consistent performance across multiple sites is important because multiple sites represent possible weather patterns your farm may experience in the future.

Glyphosate-resistant marestail is widespread in southeast Indiana and southwest Ohio and effective postemergence control of marestail with glyphosate alone in this region is unlikely. In addition, glyphosate-resistant marestail has now been documented in 15 states in the U.S. In 2006 and 2007, we observed frequent giant ragweed and lambsquarter control problems with glyphosate in soybean and corn. Lambsquarter biotypes with elevated tolerance to glyphosate have been reported in Indiana and Ohio.

The decision to switch significant soybean acres to second-year corn acres should be made cautiously with careful attention to both the economics and agronomics of such a choice. While short-term economics may favor second-year corn over soybean production, long-term economics are very much dependent on the economic assumptions made when calculating comparative crop budgets.

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