Crop rotation systems are superior to the monoculture production systems that dominated the Southeast during the “cotton boom” from the mid-1800s to the 1920s. Monoculture systems grow the same crop in the same field year after year. Often, these systems dominate when one crop has greater profit potential than others that thrive in the same soils and climate. However, overtime, monoculture systems reduce yields and profit by aggravating existing problems with insects, weeds and disease, and by mining fertility.
Fungal diseases and nematodes are major causes of yield reduction in monoculture systems . For instance, gray leaf spot (Cercosport zeae-maydis), a fungal disease, causes yield reductions in no-till corn because infested corn residue retains infection from year to year . Similarly, insect-pest pressure increases because the same host plant is present each year. Weed resistance to pesticides increases because of the limited number of pesticides available for one crop .
Crop rotation has been practiced successfully for centuries. Rotations documented in ancient Roman literature included combinations of cereal crops, legumes and olive trees . The Romans concluded that with proper crop rotations, a piece of land could be farmed productively year after year without ever going fallow.
In the late 1800s, long-term studies of crop rotation were started for research and education in the United States. An example is the “Old Rotation” located in Auburn, Ala. and established by the Alabama Agricultural and Mechanical College, now Auburn University. The Old Rotation studied and compared various crops and production practices common in the Southeast. The project generated more than a century of valuable research and experience that clearly demonstrates the benefits of rotations for a variety of crop sequences . For example, soil quality and crop yield improvements were documented for alternating corn and cotton in a rotation and for adding a legume winter cover crop.
The benefits of crop rotation systems as compared to monoculture systems fall into four broad categories [14, 3]:
- Insects and disease pathogens do not multiply because the host crop is not present each year in the crop rotation. Although the pathogen or insect might still be present, its reproduction is decreased or ceases when the host plant is not present.
- Weed control is improved. The herbicides recommended for weed control vary based on the crop, so crop rotation results in more herbicide options that reduce the chance weeds will become resistant to a pesticide. Choosing pesticides with different modes of action counters weed resistance. See Chapter 11 for more information on herbicide groups and weed resistance. As of August 2016 in the United States, 80 weed species have developed tolerance to at least one herbicide group. One common chemical group, acetolactate synthase (ALS) inhibitors, has 49 resistant weeds . Many resistant weed species are prevalent in the Southeast, including Palmer amaranth (Amaranthus palmeri), Italian ryegrass (Lolium multiflorumi) and Common cocklebur (Xanthium strumarium).
- The need for fertilizer is reduced or fertilizer-application timing changes. Within a rotation, one crop provides nutrients for other crops, reducing the total amount of fertilizer needed. For instance, when a legume is followed by a grass, the legume provides nitrogen for the grass. Likewise, a legume will not need phosphorus fertilizer if the phosphorus is added to the preceding grass crop and is readily available to the legume as the grass decomposes.
- Soil organic carbon increases over time. Research from the Old Rotation compared monoculture cotton (Gossypium hirsutum) with no winter cover except cotton stubble to a crop rotation of cotton and a winter-annual-legume cover crop. The soil’s organic carbon concentrations were doubled with the cotton>legume rotation .
The Old Rotation also compared a monoculture cotton system with a two-year cotton>winter-legume cover crop>corn rotation. Soil organic carbon was similar to the monoculture cotton system with cover crops: 1.0 percent versus 0.9 percent, respectively. However, when a third crop, soybeans, was added to develop a three-year rotation of cotton>winter-legume cover crop>corn>rye cover crop (Secale cereal)>soybeans, the highest concentration of organic carbon was measured: 1.2 percent. Of the rotations studied, this three-year rotation with legume and grass cover crops gave the most benefit for building soil organic matter. Soil organic matter provides the basis for improving soil structure and overall soil tilth.
Table 7.2 shows common cash crop rotations in the Southeast with cover crops incorporated over a four-year rotation. Traditional low-residue rotations leave fields fallow in the winter with only cash crop residue on the soil surface. Cover crops can be added into otherwise fallow areas of the rotation to offer additional biomass for sequestering carbon and protecting soil from erosion.
The first line of Table 7.2 shows one of the most common four-year rotations in the Southeast: corn>winter wheat>double-crop soybeans>cover crop>corn>winter wheat>double-crop soybeans>cover crop. When wheat is listed in the table as a winter crop, it refers to wheat harvested for grain. Winter wheat provides excellent winter cover and adds valuable plant biomass for soil building if straw is left in place rather than harvested or burned. The cover crop selected will be influenced by the needs of other crops in the rotation, the farm’s long-term goals for improving soil characteristics and reducing erosion, and the time available for cover crop growth. Table 5.3 includes information concerning the seeding rate, seeding depth, dry matter production and more for several cover crops common in the Southeast.