Crop rotations that included grass-legume sod used to be a common agricultural practice in the Midwest. Gradually, on many farms, these sod crop rotations were abandoned in favor of higher-profit continuous corn production.
More recently, there has been a marked shift from continuous corn cropping to corn-soybean rotations. At the same time, many farmers are altering their tillage methods (e.g., traditional fall moldboard plowing with numerous spring secondary tillage trips) to a range of practices referred to as conservation tillage' (chisel plowing, discing, no-till planting, etc.).
What effects do these changes in cropping sequence and tillage systems have on crop yields? What soil and other environmental conditions are being brought about by these changes, and how might such conditions affect crop yields over the long run? The purpose of this publication is to answer these questions, so that you, the farmer, can make the right decisions concerning your particular cropping program.
Research and farmer experience show that corn yields are likely to be higher when corn follows any other crop but corn. This has been illustrated on the century-old Morrow plots at the University of Illinois, where corn after soybeans consistently yielded better than continuous corn and usually by more than 10 percent (Table 1). Trials at Purdue University's Agronomy Farm confirm these results and also show an even greater yield increase for corn following a good hay crop vs. corn following corn (Table 2).
Continuous Corn-soybean Increase over Year corn rotation continuous corn ---------------------------------------------- bu./acre bu./acre bu./acre 1969 136 145 9 1971 148 169 23 1973 146 190 32 ---------------------------------------------
Yield avg., Yield avg., Rotation 1966-71 Rotation 1972-77 ------------------------------------------------------- bu./acre bu./acre Continuous corn 135 Continuous corn 137 Corn-corn-oats- 154 Corn-soybeans 150 hay (1st yr. corn) Corn-soybeans- 156 soybeans Corn-corn-oats- 147 hay (2nd yr. corn) Corn-oats-hay-hay 159 ------------------------------------------------------- *Research by S. A. Barber
In both the Illinois and Purdue tests, the crops were well fertilized with nitrogen, phosphorous and potassium. Therefore, yield differences between rotations were not the result of nutritional differences.
Long-term studies conducted by the Ohio State University have compared yields of corn in three cropping sequences (continuous corn, corn-soybeans and corn-oats-meadow) grown under no-till and conventional tillage systems on two widely different soil types (Table 3). Here are the results:
Hoytville silty Wooster silt loam clay loam ----------------- --------------- Rotation No-till Plowed No-till Plowed ------------------------------------------------------ bu./acre bu./acre Continuous corn 150 134 109 128 Corn-soybeans 151 139 126 132 Corn-oats- meadow 167 155 130 134 ----------------------------------------------------- * From "Long-Term Influence of Tillage Rotation and Soil on Corn Yield" D.M. Van Doren, G. B. Triplet and J.E. Henry. Ohio Report, September-October 1975
At the Purdue Agronomy Farm, a study is continuing that compares yields of corn and soybeans in three cropping sequences (continuous corn, alternate corn-soybeans and continuous soybeans) produced under three different tillage systems (fall plow, fall chisel and no-till). The study is being conducted on a poorly-drained Chalmers silty clay loam which has been tile drained. The 6 years of information collected so far and summarized in Table 4, reveals the following;
Cropping Sequence and tillage system Corn yield Soybean yield ---------------------------------------------- bu./acre bu./acre Corn as previous crop Plow 159 51 Chisel 153 52 No-till 152 48 Soybeans as previous crop Plow 63 49 Chisel 162 No-till 158 44 ---------------------------------------------
Unusually warm, dry weather occurred during planting and the early growing season in 2 of the 6 years; in those 2 years, no-till yields were as good as yields in the plowed plots. In fact, yields on no-till and chisel treatments were better for continuous corn than for corn following beans, because the continuous corn sequence provided greater moisture-holding surface residue. Three of the 6 years had cool, wet planting and early growing seasons; in those 3 years, the tillage systems leaving appreciable surface residue did have lower corn yields.
Several factors influencing corn and soybean yields are significantly affected when cropping sequence and tillage practices are changed. The major one is the type, amount and placement of crop residue, which affects soil moisture content, soil temperature, soil structure, nutrient availability and pest problems, which in turn determine yield potential. It's important to understand these inter-relationships in assessing the likely results of a cropping sequence change under various tillage systems.
Different crops produce different amounts of residue. For example, 130-bushel-per-acre corn provides approximately 7300 pounds of above- ground residue, whereas a comparable 40-bushel-per-acre soybean crop on that same land would produce less than half that amount (3600 pounds).
In addition, different crops vary in chemical makeup, which determines their rate of decomposition and, thus, their effect on soil physical properties and nutrient availability. Although a very complex matter, a general guide to estimating decomposition rate is the carbon to nitrogen (C:N) ratio in the residue. The lower the C:N ratio, the more rapid the decomposition. Thus, alfalfa and soybean residues, with C:N ratios of 20:1, break down more quickly than corn residue (C:N 70:1).
Also influencing decomposition is placement of residue--i.e., whether it is left on or near the surface, partially incorporated with a chisel or disc, or plowed under.
Some researchers feel that reduced yields of continuous corn or soybeans (monoculture) vs. corn or beans grown in rotation are due, in part, to toxic effects of that crop's residue on the next planting. There is good evidence that this is true for wheat following wheat. However, in the case of row crops, it would be difficult to separate the effects of plant disease from that of residue toxicity.
Crop residues and their placement influence soil water content in at least three ways:
1. A soil surface that is protected by crop residue will not surface-seal or crust readily, thereby allowing rainfall to enter the soil rather than run off.
2. Surface residue can significantly reduce evaporation, thus retain more water in the soil for plant growth. For example, no-till cropland with heavy residue cover may have as much as 20 percent more available soil water.
3. Large amounts of residue returned to the soil increase slightly both the soil's organic matter and its water-holding capacity. In an Iowa study, researchers found that 2.25 tons per acre per year of residue were needed to maintain the soil's organic matter content. Increasing to 7.2 tons per acre the amount of residue returned per year increased organic matter content from 3 percent to 4 percent after 11 years.
Soil water content is higher under a continuous corn cropping sequence than with a corn-soybean rotation, because more residue is produced. Thus, in years when moisture is deficient, continuous corn cropping can result in higher yields; but it can markedly reduce yields when excess moisture is a problem.
Soil temperatures will be lower with increased residue amounts. More residue means higher soil moisture levels; and it requires more energy to heat water than air. Also, surface residue insulates the ground, thereby slowing the rate of soil warm-up. For instance, soil temperatures in the spring are often 2 or more degrees colder at a 4-inch depth under a heavy cornstalk residue mulch than under essentially bare soil (as would be the case following soybeans).
Lower soil temperature can reduce the rate of seed germination and plant growth early in the season, as well as slow microbial activity (and thus residue decomposition). In the northern half of the Corn Belt, the result can cause a major reduction in crop yields, particularly on poorly-drained soils and in cooler-than-normal growing seasons. Under these conditions, a rotation of corn following soybeans has a distinct yield advantage over continuous corn, and especially if the tillage system leaves most of the previous crop residue on the soil surface.
The term `soil aggregate' refers to a cluster or mass of soil particles. The higher the level of soil aggregation, the higher the quality of soil structure.
Purdue research agronomist S. A. Barber has shown that soil is more highly aggregated after alfalfa than after corn, and after bromegrass than after alfalfa. His and other studies have also found that soil aggregation on land where the crop rotation includes a sod is always highest the first year after sod and decreases rapidly in the next 2-3 years of row cropping.
Both cropping sequence and tillage system are of major importance to soil structure over the long run. Here are some of the effects that certain crops and tillage practices are likely to have on structure:
* Deep, taprooted sod crops like alfalfa produce root channels that extend several feet down into the soil, improving drainage and aeration. Annual row crops, such as corn and soybeans, are less effective in this regard.
* Earthworms and other soil life also benefit soil from aeration and nutrient standpoints. Generally. they are more active in rotations that include appreciable sod rather than just row crop cultures.
* Decomposed residues from sod crops (grasses and legumes) seem to stabilize soil aggregates more effectively than decomposed corn and soybean residues.
* Crop residues left on the surface or partially incorporated improve aggregation in the upper 2-3 inches of the soil, which is important to minimize surface sealing and crusting. The benefit is lost when residues are turned under by moldboard plowing.
* The seedbed following a soybean crop is looser and in better filth than one following corn. This may be one reason why corn yields are better in a corn-soybean rotation than from continuous corn, particularly on poorly-drained soils.
Plowing under heavy amounts of crop residue that has a high carbon-to-nitrogen (C:N) ratio, such as corn, tends to limit the availability of nutrients, especially nitrogen, for the succeeding crop. This is because the microorganisms that decompose the residue are competing for these nutrients.
Plowing under alfalfa or soybean residues, on the other hand, does not have this adverse affect, because of their narrow C:N ratios. There is plenty of nitrogen available within these residues both for active decomposition and for feeding the growing crop.
Residues concentrated near the soil surface keep the soil temperature cooler. This reduces the rate of residue decomposition, thus the rate of nutrient release from the residue. Cooler soil temperatures also reduce nutrient uptake by the crops.
In summary, nutrient availability will likely be higher either when there is less residue present (regardless of whether on the surface, incorporated or turned under) or when the residue has a low C:N ratio.
When residue of continuous corn or soybeans is left on the soil surface year `round (as would be the case with conservation tillage), it is apt to harbor weed seeds, insects and diseases. And serious crop damage can occur if these pests are not adequately controlled. Crop rotation, on the other hand, reduces the potential for serious infestations of pests associated with a specific crop residue.
Weeds. There are two main reasons for increased weed problems when crop residue remains on the ground: (1) herbicides are partially absorbed by the residue, preventing them from coming in contact with the soil; and (2) mechanical cultivation is more difficult as the amount of surface residue increases.
The amount, type and placement of residues affect the method of weed control required. Conservation tillage systems do not `turn over' the plow layer as moldboard plowing does. Thus, weed seeds are not buried, which makes herbicide action more crucial.
Herbicides are much more effective in controlling weeds in corn than in soybeans where conservation tillage is used. Even though no-till soybean yields are higher in a corn-soybean rotation than in continuous soybeans, they are usually not as high as yields from plowed and chiseled fields. Growing a winter rye cover crop has been shown to improve weed control in both soybeans and corn.
Insects. Many insects require residues of one crop species continuously to carry out their cycle. Crop rotation often breaks that cycle, lessening the severity of insect problems. This is perhaps partially responsible for some of the yield advantage of rotations over monocultures. However, it should be mentioned that certain insect species and rodents can cause serious problems where no-till corn or soybeans follows a sod crop.
Disease. Changing crop species usually breaks the cycle of a disease organism, which is a major reason why yields are normally higher in rotations than in monocultures. For example, diseases that overwinter in corn residue, such as blights and anthracnose, are effectively controlled by growing soybeans using conventional tillage. With conservation tillage, rotation becomes even more important as a means of disease control, because of the overwintering protection that surface residues would provide.
Farmers not presently using crop rotation may want to consider adopting the practice as a way to increase yields or reduce weed, disease and insect problems. This is especially true if you are using conservation tillage systems. Here is a review of the likely effects on yield of crop rotation in relation to conventional and conservation tillage. This summary can assist you in decisions regarding your cropping practices.
* The yield advantages of corn-soybean rotations (vs. continuous corn or soybeans) in Indiana can certainly be supported by research data. Those who produce these row-crops in sod-based rotations can expect even higher yields, particularly in the first year out of sod.
* There is evidence that conservation tillage systems which leave much of the prior crop residue on the soil surface are much better adapted to crop rotations than to monocultures of corn or soybeans.
* Rotating crops (vs. continuous corn or soybeans) provides some yield increase under conventional moldboard plow tillage, but even greater yield advantages when using some form of conservation tillage.
* The degree of yield advantage from rotation practices depends, in large part, on how the prior year's crop residues influence various crop yield factors. For example:
Single copies of the following related Purdue Extension Publications are available free of charge to Indiana residents from their county Cooperative Extension Service office or from the Media Distribution Center, 301 South Second Street, Lafayette, IN 47901-1232.
"Adaptability of Various Tillage-Planting Systemsin Indiana" (AY-210)
"Soil Erosion in Indiana-An Overview" (AY-228)
"Soil Compaction in Indiana" (AY-221)
"Types and Uses of Soils Information in Indiana (AY-215)
Cooperative Extension Work in Agriculture and Home Economics, State of Indiana, Purdue University and U.S. Department of Agriculture Cooperating. H.A. Wadsworth, Director, West Lafayette, IN. Issued in furtherance of the Acts of May 8 and June 30, 1914. It is the policy of the Cooperative Extension Service of Purdue University that all persons shall have equal opportunity and access to our programs and facilities.