Recall in module 5, we examined how soils, climate, and markets play major roles in determining which crops farmers cultivate. In many cases, farmers cultivate multiple crops of more than one life-cycle because the diversity provides multiple benefits, such as soil conservation, interruption of pest lifecycles, diverse nutritional household requirements, and reduced market risk. In this module, we examine some ways that farmers cultivate crops in sequence and define some of the terms for this crop sequencing.
A sole crop refers to planting one crop in a field at a time. Recall from Module 5, the seasonal crop types (Figure 7.1.1) and note that different seasonal crops could be planted in succession. A monoculture refers to planting the same crop year after year in sequence (See Figure 7.1.2). By contrast in a crop rotation, different crops are planted in sequence within a year or over a number of years, such as shown in Figures 7.1.3a and 7.1.3b. When two crops are planted and harvested in one season or slightly more than one season, the system is referred to as double cropping, as illustrated in Figure 7.1.4. Where growing seasons are long and/or crop life cycles are short (ex. leafy greens), three crops may be planted in sequence within a season, as a triple-crop.
Figure 7.1.1.: Crop Term: Seasonal Types and Example Crops. Credit: Heather Karsten
Figure 7.1.2.: Monoculture. Credit: Heather Karsten
Figure 7.1.3a.: Simple Summer Annual Crop Rotation. Credit: Heather Karsten
Figure 7.1.3b.: Dairy Perennial - Annual Crop Rotation. Credit: Heather Karsten
Figure 7.1.4.: Double-cropped annual crops. Credit: Heather Karsten
Crop rotations and double cropping can provide many soil conservation and soil health benefits that are discussed in the reading assignment at the end of this page, and in Module 7.2. Crop rotations can provide additional pest control benefits particularly when crops from different plant families are rotated, as different families typically are not hosts of the same insect pest species and crop pathogens. Integrating crops of different seasonal types and life cycles in a crop rotation also interrupts weed life cycles by alternating the time when crops are germinating and vulnerable to weed competition. Rotating annual crops with perennial forage crops that are harvested a couple of times in a growing season also interrupts annual weed life cycles, because most annual weeds don't survive the frequent forage crop harvests.
When all or most of a crop is grazed or harvested for feed for ruminant livestock, such as dairy and beef cattle or sheep, the crop is referred to as a forage crop. Examples of forage crops include hay and pasture crops, as well as silage that can be produced from perennial crops and most grain crops. For instance, silage from alfalfa, perennial grass species, corn, oat, and rye is made when most of the aboveground plant material (leaves, stems and grain in the case of grain crops) is harvested and fermented in a storage structure called a silo or airtight structure. To preserve the silage, air is precluded from the storage structure and microbes on the plant material initially feed on the crop tissues, deplete oxygen in the storage structure, and produce acidic byproducts that decrease the pH of the forage. This acidic environment without oxygen prevents additional micro-organisms from growing, effectively "pickling", and preserving the forage.
Figure 7.1.5.: Airtight upright silo. Credit: Heather Karsten
Figure 7.1.6.: Bunker silos are packed tightly with heavy equipment and covered with plastic to keep out air and moisture. The bunker silo on the right is uncovered because the silage is being removed to feed to dairy cattle. Credit: Heather Karsten