Professor, LSU AgCenter
Crop rotations are as important, or, may be more important, in conservation till systems than in conventional till systems. Crop rotations are especially important for cropping systems with soybeans, wheat and sweet potatoes - crops that quickly lose yield and quality potential with continuous cropping practices, because of disease, insect and weed problems. In addition to the yield benefits, other benefits are derived from rotation in conservation systems are described below.
Farmers’ experience and many years of agronomic and economic research in the LSU AgCenter have convincingly demonstrated that crop rotations increase yields of the included crops. Even without the presence of definable and identifiable causes of yield limitations, such as diseases and nematodes, yield increases from rotations usually are in the range of 20 percent. When specific problems that adversely affect crop health can be identified, the yield increases from rotations will be larger.
For example, in a 25-year rotation study at the LSU AgCenter's Northeast Research Station, continuous cotton produced 1,051 pounds of lint per acre, and cotton rotated in two-year cycles with corn or grain sorghum produced 1,241 pounds per acre, an 18 percent increase. Continuous soybeans, however, that were affected by disease (charcoal rot) and nematodes produced only 32 bushels per acre, whereas rotated soybeans produced 48 bushels per acre, a 50 percent increase. Crops such as sweet potatoes cannot be grown successfully without regular use of crop rotation to control insects, diseases and nematodes. Rotational crops for sweet potatoes must be nonhost crops for nematodes.
Likewise, crop rotations provide greater options for increased income and decreased production risk. In a seven-year study at the LSU AgCenter's Macon Ridge Research Station, monocrop cotton averaged annual net returns of $124 per acre, monocrop corn averaged $251 per acre and a cotton-corn-wheat cropping system averaged net returns of $313 per acre. Clearly, crop rotations have large yield and income advantages over continuous monocropping that make rotational cropping systems advantages for most farmers.
All of the rotational cropping systems provide opportunities for using conservation tillage, and virtually all crops can be no-tilled. An example of an excellent rotation is the above mentioned three-crop, two-year system of corn followed by wheat followed by double-crop cotton or soybeans in year two, which provides maximum conservation benefits and an opportunity for maximum profitability. But most of the beneficial row-crop rotations in Louisiana are likely to be two-year rotations that involve corn/cotton, corn/soybeans, cotton/soybeans, sweet potatoes/grain sorghum, or soybeans/rice Although all these rotations are beneficial, rotations that include corn, grain sorghum and rice are preferred for conservation efforts because those are high residue crops and are needed for rotation with cotton and soybeans, which are low residue crops. Using grain crops in rotations provides additional benefits for soil quality and disease, nematode and weed control (as described in the following sections).
Most agronomic benefits of no-tillage systems are the result of crop or cover crop residue on or near the soil surface. Properly used, residue from cash crops and from cover/green manure crops will minimize soil and nutrient loss from cropland and maintain soil organic matter. Residue management is therefore the key to improving soil quality in conservation systems and also for protecting surface water quality.
If a soil is severely degraded because of many years of intensive tillage, it may take a few years to accrue noticeable benefits from rotation and conservation tillage systems. Intensively tilled soils have greatly reduced organic matter and microbial populations, and these will be restored by conservation practices, but it will take time.
In conservation systems, soil cover should be 50 percent or more at all times. Some crops, however, leave too little residue, leading to insufficient ground cover. Both soybeans and cotton are low-residue crops. Corn, grain sorghum, winter grains and rice are examples of crops that produce large amounts of residue for long-lasting ground cover. Rotating high residue and low residue crops helps to maintain sufficient cover because residue from the high-residue crop will carry over into the low-residue crop. After a low-residue-producing crop, it is beneficial to plant a winter grain or cover crop to help maintain residue for ground cover. Cover crops are discussed in Chapter 2.
Residue management is a crucial issue to deal with where winter grain crops immediately precede summer crops. The goal of residue management in these situations is to achieve uniform ground cover following harvest of the grain crop but also to, as much as possible, keep the residue from interfering with cropping practices for the following crops.
Nonuniform residue distribution results in uneven stands. The coulters or openers on the planter or drill will cut through normal levels of residue but not cut through piles of residue, the bottom of which will usually be moist. This results in hair-pinning of crop residue and poor seed-to-soil contact. Depth placement will also be affected. Depending on the thickness of the residue and planter settings, seeding depth can vary from more than 2 inches where there is little residue to less than 1 inch where the residue is 2 inches or more. This will result in uneven emergence, partial stands and variable early growth.
Residue that is unevenly distributed can also lead to weed control problems because herbicides do not reach the soil or intended targets where residue is piled up.
The best way to minimize crop residue interference is to maximize the combine cutting height, which minimizes the amount of residue going through the combine. Vertically standing crop residue that is attached to the soil is much easier to plant into than crop residue lying horizontally on the ground. Spreaders and choppers are available for all combines to help with residue distribution. Chaff spreaders also can help to spread fine materials that otherwise would be distributed directly behind the combine. When growing conditions favor high biomass production, planter attachments are available that will manage the residue and ensure good stands (which will be discussed further in Chapter 8: Equipment).
Residue cover provides all the environmental benefits of conservation tillage, such as erosion control, infiltration improvement, evaporation reduction, and enhanced soil biological activity. Properly used, residue from cash crops and from cover/green manure crops minimizes soil and nutrient loss from cropland. This is one of the most important components of conservation tillage. On the other hand, too much or badly managed residue can create significant problems during crop establishment that will have a significant effect on productivity and profitability.
A high level of diverse soil biological activity is indicative of good soil quality. Soil quality improves when biological activity increases. A diversity of soil organisms – bacteria, fungi, earthworms, insects and plant roots - contributes to soil biological activity.
Soil cover from crop residue and the absence of soil disturbance are beneficial for most of the beneficial soil-inhabiting organisms. Crop residue and rooting patterns play a primary role in determining the types and quantity of biological activity. It is the primary source of nutrients for soil organisms, and different crops benefit specific organisms. Greater diversity in the crop mix produces greater diversity in soil organisms. A more diverse and active soil microbial community will reduce pest and disease incidence because of increased competition for substrate as well as predation of pests and diseases by other organisms.
Rotating crops with a high carbon to nitrogen ratio, such as corn, cotton, small grains and rice, with low carbon/nitrogen ratio crops, such as soybeans and winter legumes, is highly beneficial for diversity of soil organisms. Crop mixes with different rooting patterns that explore the soil to different depths also are useful for soil improvement. Shallow root systems of grain crops improve soil tilth and increase biological activity to the extent of rooting depth. Deep taproots of crops such as cotton open avenues for deeper penetration of soil organisms and improved soil quality at deeper depths.
Rotating crops in systems that include year-round plantings increases the efficiency of fertilizers and mineralized nutrients. All types of fertilizer nutrients are more efficiently used when rotations are employed.
Nitrate losses are of particular concern because it is a highly mobile nutrient that can pollute both surface and groundwater if it is not used in a timely manner by crops. Although a portion of applied nitrogen can be carried over for one year or even two, residual nitrogen is more likely to be lost than carried over through a Louisiana winter. Winter grain crops are extremely efficient at finding and using residual fertilizers that are left over from corn or cotton fertilization or from a legume soybean crop.
Crop rotation is an important component of disease, weed, nematode and insect control and is often the primary control mechanism for nematodes and diseases. Disease-causing organisms and insects survive on crop residue and in the soil on root systems. Crop rotation for insect and disease management is therefore very important in conservation tillage.
Rotations help to control many of the common root and stem diseases that affect row crops. Control of reniform and root-knot nematodes especially requires crop rotations that facilitate the use of non ost crops and resistant varieties.
In no-till situations, perennial weeds can become a problem, but selection of rotational sequences can minimize establishment of these species. The different herbicide programs used for different crops helps to control development of herbicide-resistant weeds, too. Seeds of many weeds buried in the soil, if they require light for germination, will not germinate in no-till fields. Winter grain crops and cover crops are strong competitors with weeds and reduce weed infestations in late winter and early spring. Having a winter crop also provides the opportunity for selective herbicidal control of winter weeds while maintaining abundant ground cover.