Cropping systems and rotations

All crops are grown as part of a cropping system

A cropping system is the sum total of all crops and the practices used to grow those crops on a field or farm.

• Simple example = one variety grown each year in the same field with nutrients provided as fertilizer to replace nutrients sold off the farm with the crop
• Complex example = system where fruits, vegetables, tree crops, grain crops, forage grasses and legumes, and livestock are all grown on a farm during the course of a year with multiple harvest times and managed recycling of nutrients within the system

Even a simple cropping system is quite complex in terms of interactions of plants with soil, soil organisms and crop pests.

Monoculture: the production of a single crop in a field. This practice is considered ecologically unstable.

Rotation: a cropping sequence on a field that includes more than one crop over many years.

• Benefits have been know for many years, though types and size of benefits have changed over time as technology developed.
• Need to find balance between economic return and productivity or sustainability.

History of crop rotations in Wisconsin

Prior to 1940, four year rotation for livestock-based system. Limited or no availability of pesticides, limited power and equipment, and labor intensive.

1. oats under seeded with alfalfa or clover
2. forage legume
3. forage legume
4. corn

After 1940, rapid mechanization, use of corn hybrids and nitrogen fertilizer, and better power and equipment changed the cropping system. No longer livestock based and dependent upon complex rotations. Developed into a grain cropping system.

1. Corn-soybean 
2. Continuous corn

The Corn-Soybean Rotation

• Compared to most cropping sequences used in the world, the corn-soybean rotation is relatively young.
• During most of the 20th century corn was planted following a legume like alfalfa or clover. Only recently has the legume become soybean.

Yield response in corn-soybean rotations

• Response is usually observed.
• Reasons for response are unclear.
  • N effect
  • Tillage
  • Residue alleopathy
  • Most likely rotation effect is complex with small and interacting effects on soil microorganisms (both harmful and beneficial), nutrient supply, insects, soil structure, and other chemical, physical, and biological factors.

Good rotations:

1. include both cereals and legumes
2. improve soil structure and soil fertility
3. control soil-borne insects and some diseases
4. cut chemical costs, fertilizer costs, and fuel costs by 15-30%

Rotating hybrids and varieties

• MN: same hybrid planted in sequence tended to yield less than when a different hybrid was used the second year.
• Suspect mychorrhizae effect on corn roots.

Erosion in rotations

1. Soil is more erosive after soybeans than after corn
2. During the winter after soybeans are grown, considerable soil loss can occur via WIND EROSION. However, most soil erosion after soybeans occurs during the first or second month after the next crop is planted (if the following crop is a row crop)

Conservation Security Program (CSP)

Farmers interested in switching away from 50:50 corn:soybean acres

• Other more challenging acres remain 50:50 C:S
• Take best corn ground and grow 2yr corn: 1yr SB

Recent developments indicating that the corn-soybean rotation may not be stable

1. Diseases
   • Corn: Gray leaf spot, Mycotoxins, Anthracnose
   • Soybean: Brown stem rot, White mold, Sudden death syndrome, Rust
2. Weeds: Development of Roundup resistant weeds
3. Insects:
   • Corn: Western and Northern corn rootworm
   • Soybean: Soybean aphid, Bean leaf beetle
4. Natural gas supply in winter of 2001, 2004
5. Corn rootworm
6. Soybean cyst nematode
7. Carbon sequestration
8. Selection of pathogen races i.e. Brown stem rot
9. Gulf of Mexico hypoxia

Rotations are the "black box" of agronomic sciences. Response is almost always measured, but it is unknown how and why the response occurs.