This summer I was "cussing and discussing" field crop problems and opportunities with another agronomist and the topic of continuous corn came up. One of this person's clients was growing continuous corn, and the question was how long this practice could continue without encountering problems with soil organic matter and soil structure. Not a simple question to answer.
A key factor is that every acre of corn on this farm is harvested for grain, resulting in several tons per acre of corn stover being added to the soil each year. We agreed that the farmer could grow continuous corn for more years than if he harvested the crop for silage, but because so few farmers in northern New York grow continuous corn for grain, we were in "by guess and by gosh" territory. We also agreed that planting a cover crop would be a fine idea, but the late date at which corn grain is harvested complicates the issue.
Mother Nature hates monoculture
Soil structure issues and maintaining adequate organic matter may be of long-term concern, but other issues may arise long before these become problems. The more years that corn is planted in a field, the more likely it is that insects, primarily northern and western corn rootworms, will become a serious problem. We've learned that while seed treatments will control moderate rootworm infestations, they may not be powerful enough to control serious infestations. Genetic solutions (the several corn hybrid Bt "events" on the market) do a great job of controlling corn rootworms - maybe too good, given the resistance problems now being encountered in the Midwest.
Resistance problems are resulting in a modest comeback for granular insecticides. There's never been a confirmed case of insect resistance with these products, which kill some but not all of the corn rootworms because they're applied in a narrow band. This is key, since the surviving rootworms mate with other rootworms, including (hopefully) the exceedingly rare one that's resistant to one or more of the Bt toxins. This mating would produce offspring that are susceptible to Bt toxins.
Weed infestations often become more serious when corn is grown for many years in the same field, especially if the farmer continues to use the same herbicide, or an herbicide with a similar mode of action, each year. Unfortunately, farmers do this all too often, thinking: "If it worked last year, then it should work this year." By the time they realize that the herbicide program is missing some weeds, some yield potential has already been lost and a large reservoir of weed seeds is now in place to cause problems in the future.
Continuous use of a single herbicide or herbicide family is also a setup for the development of herbicide-resistant weed biotypes, something we've been seeing a lot more of in recent years. Triazine-resistant lamb's-quarter has been around for 40 years now, but the list of herbicide-resistant weeds is long ... and growing. The recent focus has been glyphosate resistance, but this is only one of several herbicides some weed species have developed resistance to. These problems have resulted in a whole new area of crop science: resistance management.
The simple fact is that Mother Nature hates monoculture, particularly of annual crops. Plant wheat year after year and diseases such as take-all will soon take their toll; similarly, disease problems also occur with continuous years of soybeans. That's why a typical Corn Belt crop rotation is one year of soybeans followed by one year of corn in an effort to break up the insect and disease life cycles. The combination of higher pest control costs and (often) lower yields makes continuous corn more expensive both per acre and per ton or bushel compared to corn grown in rotation with non-row crops such as alfalfa, clover and grass.
Soil fertility considerations
If you followed the suggestion in my September Farming column, you've already completed most soil sampling and have the results back from the soil testing lab. Check soil pH levels on row crop land, since a fall lime application can be incorporated where needed by tillage (unless you're practicing no-till). Low application rates of lime can be applied on the surface, but rates of over 2 tons of lime per acre should be plowed down or at least worked into the top few inches of soil by a chisel or similar tillage implement.
If the soil is very acid - much below 6 - half the lime should be incorporated by tillage this fall, with the other half applied next year. Soil pH doesn't change much from year to year, so unless lime or high annual rates of nitrogen have been applied, soil analyses that are two or three years old are still OK for this purpose.
The price of lime has increased much less in recent years than the price of fertilizer and it's a good buy. Acid soils decrease the availability of many nutrients, so it's important to maintain adequate pH levels. A pH of 6 to 6.2 is OK if corn and grass are the only crops grown, but if alfalfa is included in the crop rotation, aim for a soil pH of 6.8 to 7.
Unless soils are subject to excessive leaching, potassium fertilizers don't move much in the soil, staying close to where they're applied. If you have gravelly, sandy loam or otherwise highly leachable soils, talk to your crops consultant or extension educator before applying potash in the fall. However, for most fields fall is a good time to apply 0-0-60 and similar potassium fertilizers. A good crop of corn silage removes a lot of potassium, much more than corn harvested for grain does, as long as the stover remains in the field. With good corn silage yields, unless manure is applied on a regular basis at moderate to high rates, soil potassium levels can soon become depleted. High soil test phosphorus levels often restrict the rate of manure application on CAFO farms, making fall-applied 0-0-60 a good way to supply potassium.
Ev Thomas has worked as an agronomist in New York for 45 years, first with Cornell University Cooperative Extension, then with the William H. Miner Agricultural Research Institute in Chazy, N.Y., including managing its 680-acre crop operation. He continues to work part-time for Miner Institute and is now an agronomist at Oak Point Agronomics. He has written our Forages column for 15 years and has been an expert contributor on a number of other topics.