Fortunately, a key method of reducing soil erosion on cropland—conservation tillage—is generally well suited to southeastern Minnesota. If properly managed, conservation tillage can reduce average soil erosion by up to two-thirds. It often lets a farm maintain or enhance profitability through production cost savings. When combined with other common conservation practices—such as grassed waterways, buffers and contour planting—it can help to retard erosion even on the region's steeper, longer slopes where erosion of unprotected cropland is severe. Generally, the conservation tillage benchmark of 30 percent surface residue after planting provides significant erosion control. This is easy to achieve following corn production, but can be a difficult target to attain for crops planted in soybean stubble. Somewhat less surface residue still gives substantial erosion control. Therefore, fields meet crop residue targets if they have greater than 30 percent residue following corn, and at least 15 percent residue cover following soybeans.
Crop residue surveys indicate that farmers' adoption of conservation tillage varies greatly throughout the basin (Table 1). In five counties, no more than one-third of the fields surveyed had adequate crop residue to control erosion, while in three counties two-thirds or more of fields were adequately protected. There appears to be considerable opportunity for increased adoption of conservation tillage across the basin, particularly in areas where adoption rates are currently low.
Table 1. 2000 Minnesota Corn-Soybean Residue Survey Results. Summary for the Lower Mississippi River Basin in Minnesota
1Fields meeting residue targets include fields with >30% residue
plus fields with >15% residue when following soybeans. Residue is measured
2 An asterisk indicates no data were collected. How different tillage systems maintain crop residue
Tillage systems can use tillage implements very effectively to leave various levels of crop residue on the soil surface. The effectiveness of seven tillage systems at protecting crop residue is described below.
Moldboard plowing followed by one or two secondary tillage operations with a field cultivator or disk before planting. This system is an aggressive tillage practice that often leaves less than 10 percent of the surface covered with crop residue after planting.
Fall chisel plowing plus spring secondary tillage with a field cultivator or disk. This tillage practice is quite aggressive and can reduce crop residue to levels that are inadequate for erosion control.
Single pass with a field cultivator in spring before planting corn after soybeans or a tandem disk before planting corn after alfalfa or soybeans after corn. This is a less aggressive tillage system that may leave adequate residue after planting.
Tillage is limited to that performed by the planter (ridge leveling) and one or two in-season cultivations (ridge building). Preformed ridges provide a drier and warmer seedbed at planting. Adequate levels of crop residue remain after planting.
Strips about 4" to 6" wide and 7" to 8" deep matched to the row-spacing of the planter are prepared in the fall with mole fertilizer knives or anhydrous knives mounted on a tool bar. Fertilizer P and K can be injected directly into the strip at the time of strip tillage. Corn is planted into the tilled "residue free" strip without any secondary spring tillage.
Combining deep slit tillage using a zone builder or other narrow subsoil shank and a planter equipped with multiple (2 or 3) fluted coulters cutting residue and preparing a seed bed gives excellent seed-soil contact directly above a deep slit.
No-Till (the planter performs all seedbed preparation):
Starter fertilizer placement and cleaning residue from the rows usually are done with the planter, but may be performed separately, sometimes in combination with fertilizer injected into a strip or band.
Soil types vary greatly on the landscape in southeastern Minnesota. It's important to understand these soil differences when deciding which tillage system to use in a farming operation. Failure to match the tillage systems to soil and landscape properties can result in reduced yield, poor erosion control and, eventually, loss of soil productivity.
Well-drained soils warm up sooner and usually require less tillage than poorly drained soils. Warmer soils generally allow earlier planting and let emerging plants withstand greater amounts of crop residues left on the surface. Poorly drained soils often require more tillage and tile drainage systems in order to achieve timely planting. A high level of crop residue on slow-to-drain soils keeps them from warming and drying quickly. Wheel traffic on these wet soils can more easily cause surface compaction and could affect the subsoil as well.
The land's aspect, or exposure to the sun, also affects the warming and drying of soil. The sun's angle of incidence on south-facing slopes in spring is intense, and soils warm up and become drier sooner than soils on north slopes.
Different crops leave different amounts of residue on the soil surface after harvest. Corn harvested for grain leaves a large amount of crop residue that does not readily break down. Much less crop residue is left on fields when corn is used for silage or corn stalks are baled for feed. When alfalfa follows corn in the rotation, tillage may be needed to incorporate fertilizer and/or agricultural lime and to develop a seedbed for the small seeds of forage crops. It is extremely important that small-seed legumes have close seed-to-soil contact for optimum germination.
The residue left following a soybean crop is minimal and does not provide adequate protective residue cover on the steep, loess soils in southeastern Minnesota. Soybean stubble should not be tilled in the fall in order to reduce the potential for soil erosion from fall rains and spring runoff of snowmelt. This leaves an opportunity for limited spring tillage before planting corn. The potential for erosion is greatly reduced when soybean residue is managed by using very limited tillage combined with contour farming and conservation structures. A straw chopper on the combine distributes soybean residue evenly, which helps in managing residue successfully the following spring. Some farmers have reported that special straw chopper knives and other combine attachments may be needed to thoroughly chop and evenly spread residue from some new soybean varieties, the stems of which tend to remain green for a longer time.
When a farmer begins using conservation tillage, starting with a relatively weed-free field greatly improves the likelihood of successful long-term weed management. Using reduced tillage on a weed-infested field requires greater weed management and can become very costly. In fields where tillage has been reduced and crop residue levels have increased, weed pressure may increase and shifts in weed species may occur. In some fields, weed species that were controlled previously with tillage, pre-plant and pre-emerge herbicides begin to sprout and grow, giving the appearance of a "new" weed in the fields. Pre-plant incorporated herbicides are no longer used in fields where little tillage is used; pre- and post-emergence herbicide applications become the primary weed control program. A wide range of effective post-emergence herbicide treatments is available today for crops grown with reduced tillage. As a result of equipment improvements, mechanical weed control can also be combined with herbicides in fields with high crop residue.
Many of the newer generations of planters and drills have options available to handle higher crop residue levels. Most planter tool bars sold today can be modified with row cleaners and/or coulters to ensure optimum seed-to-soil contact. Modern no-till drills are designed to clear residue while dropping and covering seeds as small as alfalfa at precision depths for exceptional contact with the soil, which results in good plant emergence. In recent years, narrow row spacing has become more prevalent and can be a benefit for farmers using conservation tillage. It is more difficult to do row cultivation in narrow rows, particularly on contour strips. However, rotary hoeing, combined with earlier closure of the crop canopy, helps to control weed growth. Farmers with older planters can modify their equipment with row cleaners and/or fluted coulters at a relatively low cost. Also, equipping the planter with starter fertilizer attachments increases the potential for success in high residue situations.
It is critical to establish high soil fertility before starting a conservation tillage system. Fertilizer and manure nutrients, along with agricultural lime, must be managed for optimum crop production. With conservation tillage, moderate-to-poor soil fertility in fields will intensify yield loss and diminish the probability of long-term success. Also, it can be difficult and costly to make great increases in soil pH with lime after very reduced tillage systems have been established.
Nitrogen - Nitrogen (N) is the most important nutrient for corn production. Nitrogen needs to be managed for efficient use by the crop and also to reduce environmental risk of runoff and leaching. The Minnesota Nitrogen Fertilizer Task Force recommends that no nitrogen fertilizer be fall-applied on the soils of the Karst geologic area. Surface-applied N fertilizer—such as granular urea and UAN (urea-ammonium nitrate) solution—should be incorporated mechanically or by rainfall within three days of application to eliminate loss of N to volatilization. Any significant loss of nitrogen to the atmosphere or by runoff or leaching will reduce yield and profitability. Anhydrous ammonia is injected into the soil with knives that disturb the soil surface. The injection equipment also provides a minimum tillage pass without destroying the conservation tillage practice. For additional information see University of Minnesota Extension Service publications: Fertilizing Corn in Minnesota and Fertilizer Management for Corn Planted in Ridge-Till or No-Till Systems as listed in the Resource Section at the end of this publication.
Phosphorus (P), potassium (K) and soil pH - Soil tests should be used to maintain fertility at optimum levels. For optimum crop production with all tillage systems, soil pH needs to be 6.5 or greater when alfalfa is in the rotation or at least 6.0 when soybean is grown; P should be 16-20 ppm with the Bray test and 12-15 ppm when the Olsen extractant is used; and K needs to be 121-160 ppm. Soil testing should be done every two to three years to maintain soil fertility levels in reduced-tillage fields. If additional nutrients are needed, their placement in relation to the crop is important. Placing the fertilizer close to the corn seed gives the most efficient P and K uptake. Broadcast applications of fertilizer and ag lime are best following corn where some tillage can be used to incorporate the materials and maintain adequate crop residue levels for erosion control. For a soybean-corn crop sequence, two years of requirements for phosphate and/or potash fertilizer may be broadcast and incorporated in the year following corn. Broadcast fertilizer left on the soil surface is recommended for topdressing an existing stand of alfalfa and usually is not a hazard to surface waters when runoff control measures are used. However, soluble phos-phorus could enter surface waters if fertilizer is spread on grass waterways.
Continuous corn production provides the best opportunity to apply livestock manure on a consistent basis in conservation tillage systems. Corn residue levels are adequate for erosion control following injection of manure by knife or disk application equipment. Broadcast manure should be incorporated as soon as possible after application to reduce volatilization of N, potential runoff and odors. Farmers who need to apply manure throughout the winter should observe Minnesota Pollution Control Agency rules for land application of manure.
In reduced tillage systems with a corn-alfalfa rotation, using livestock manure gives some flexibility in application timing. Corn is grown for one or two years then is rotated to alfalfa. First year corn following a good stand of alfalfa (4-5 plants per sq. ft.) in most cases requires no additional N, so manure use would be limited. However, manure applied prior to seeding alfalfa—whether in the spring or late summer (by August 15th)—gives farmers flexibility in using manure to provide the nutrients required. Regardless of the manure application method, tillage may be needed to prepare the seedbed for alfalfa and to incorporate the manure. If soybeans are in the rotation, manure can also be used effectively when applied at agronomic rates before planting. However, the small amount of soybean crop residue remaining after harvest has a minimal effect on stabilizing soil and controlling erosion, particularly on silt loam soils with steep slopes. Farmers that have substantial amounts of manure for use might consider a rotation of two or more years of corn in order to maintain adequate crop residue cover for erosion control and make the most efficient use of manure nutrients.
Many farmers are reluctant to farm with greater amounts of crop residues on their fields. They fear yield loss and don't like the appearance of a crop growing in heavy residue. Their perception is influenced by several factors: 1) upsetting the landlord, business partner or a family member; 2) ridicule from neighbors; 3) lack of crop man-agement skills to adopt conservation tillage; 4) recent purchase of equipment for aggressive tillage; and 5) on-farm research results. Overcoming the aesthetics of crop emergence in a field covered with residue takes patience, time and an understanding of the system. Farmers who have used conservation tillage learn to appreciate the "look" of a crop growing in higher levels of residue and the lower cost of production.
When evaluating potential yield loss that may result from adopting a very high-residue system such as no-till, it is important that farmers compare differences in production costs as well as expected differences in yield to arrive at a sound business management decision. The following example is intended only for illustration. Each farm will have its own production costs and risks based on equipment, management skills, crop rotation and other farm-specific factors.
The risk associated with reduced tillage is most pronounced for no-till production
of continuous corn. In the Karst region, over 40 site-years of field research
with continuous corn show that the yield difference is only 2 to 6 percent lower
for no-till compared to chisel plowing. Following is a brief comparison of average
tillage costs for no-till and chisel-plow systems, which can be used as a guide
to determine how much of the potential yield penalty from no-till may be offset
by cost savings from reduced tillage trips.
All costs above include labor, fuel, maintenance and depreciation. Estimated costs based on William Lazarus, Minnesota Farm Machinery Economic Cost Estimates for 2000, University of Minnesota Extension Service, St. Paul.
The tillage cost difference in the example is $13.56/A in favor of no-till. Weed control costs and the remaining costs through harvest would be similar for both methods. Assuming a 170 bu/A corn yield at $2.00/bu, the farmer would gross $340.00/A. If yields were 4 percent lower for the no-till system, there would be no difference in dollars returned for the two systems; however, the no-till farmer would need less labor, have a reduced line of equipment to maintain and cause less soil erosion and compaction. This example demonstrates that unless the average yield penalty associated with no-till exceeds about seven bushels of corn per acre, no-till production may be more profitable than a chisel-plow system.
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