Some of the major league baseball teams that were hot in May are falling by the wayside, while the teams built for the long run are rising to the top. Much the same is happening in your legume-grass fields: Because of a high population of grasses, some fields may have looked good this spring, but now that soils have warmed and dried, the proportion of alfalfa and other forage legumes is more obvious. We no longer evaluate alfalfa fields by the number of plants per square foot, but by the number of stems, since as stands thin the plants produce more stems per crown. Stands averaging at least 40 stems per square foot are generally considered worth keeping, but that's for clear alfalfa stands: Alfalfa-grass stands may remain productive with fewer stems than this. A trip through your fields at this time of year should give you a good idea of which are (using sports parlance) contenders and which are the pretenders.
Problems in the cornfield
Late summer is an opportune time to decide which legume fields are candidates for rotating to another crop, in many cases corn. Crop input costs and pest problems combine to make a good crop rotation important. A Farm Credit study comparing the top and bottom 25 percent of dairy farms by profitability found that the biggest determinant of profit was feed and crop expense. The bottom 25 percent of farms spent a lot more per hundredweight of milk sales for feed and crop expense. A good crop rotation can reduce crop input costs in several ways: Reduced nitrogen fertilizer cost, especially during the first year of corn production; reduced insect control costs during the first and second years of corn; and by allowing you to maximize the use of livestock manure.
Another reason for crop rotation is the recent discovery in the Midwest of corn rootworms that have developed resistance to the toxin produced by one of the major Bt "events" in genetically modified corn hybrids. It's strongly suspected that one cause of resistance was too many farmers not following insect refuge requirements, which dictate that a certain percentage of corn acreage must be planted to hybrids without the rootworm resistance trait. Maintaining a proper refuge allows some rootworms to complete their normal life cycle and mate with the "one-in-a-million" rootworm that is resistant, resulting in progeny with no resistance to the Bt toxin. As entomologists examined the cases of rootworm resistance, one commonality was that every field with resistant rootworms had a history of corn-on-corn cropping. Fields rotated with other crops didn't develop resistant rootworms.
Lower input costs, higher yields
Crop rotation may be able to prevent the development of insect resistance, but it also has a more immediate effect, one that you can put in your pocket through lower input costs. After evaluating your legume and legume-grass fields, choose the worst fields and rotate them to corn or another non-legume crop. First-year corn needs little or no supplemental nitrogen, since the amount contained in the starter fertilizer is usually sufficient, and because corn rootworms aren't a problem in first-year corn, it eliminates the need for any chemical control measures. It's no coincidence that "sod ground" corn is usually the greenest, healthiest-looking corn on the farm, and it's often the highest yielding as well. That's because as the sod breaks down it releases nitrogen to the growing crop. Corn needs a lot more nitrogen midseason and later than early in its growth. As the sod breaks down it provides "time-release" nitrogen, and the conditions favoring nitrogen release - heat and moisture - are the same ones that promote the rapid growth of your corn crop.
Another benefit of crop rotation is increased flexibility in manure application. A good field of alfalfa needs a lot of potassium, a fertilizer nutrient that's become expensive in recent years. A single application of livestock manure is often inadequate to replace the potassium that alfalfa removes from the soil each year. Therefore, as alfalfa fields age the soil potassium status often declines, contributing to lower yields as well as the loss of plants through decreased winter survival. Potassium is antifreeze for perennial legumes. Therefore, by the time alfalfa fields are in their fourth or fifth year of production the supply of plant-available potassium is often running low. By rotating these fields to corn you not only release nitrogen from the sod, but you're also able to apply a higher rate of manure, thereby supplying more "free" potassium. A reasonable goal: Apply enough manure during the several years of corn production to increase soil phosphorus levels so that you don't have to apply any phosphorus fertilizer during the years the field is in alfalfa or alfalfa-grass. Liberal manure applications during the years the field is in corn may also increase soil potassium levels enough to minimize potassium applications during at least the first couple years of alfalfa.
There's a potential problem in using moderately high rates of manure to increase soil potassium levels: If the manure contains a lot of phosphorus, manure application could result in excessively high soil test phosphorus levels, wreaking havoc with nutrient management programs on CAFO (concentrated animal feeding operation) farms. That's why it's important to monitor the phosphorus content of dairy rations; high phosphorus rations result in high phosphorus manure. Several years of research and practical experience at Miner Institute concluded that the only way we could maintain a reasonable whole-farm phosphorus balance was to keep the phosphorus concentration in dairy rations under .4 percent.
Putting it all together
The ideal crop rotation differs according to soil type, acres of cropland per animal unit and available crop storages. However, you need a very good reason for keeping legume and legume-grass stands longer than five years, and for growing continuous corn for more than four or five years. In fact, economic analyses show that on many farms the best option may be no more than three years of each crop. For farmers who till their land this means more time and equipment expense (and in many cases stone picking), but this is more than offset by increased crop yield and quality plus lower fertilizer and pest control costs.
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 a written our Forage column for over 13 years and has been an expert contributor on a number of other topics.