9.1.2: Intercrops and Cover Crops

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Intercrops are two or more crops that are planted together in a field at the same time or to be planted close in time and overlap for some or all of their life cycle. Intercrops may provide a range of benefits including: i. improving soil fertility, ii. increasing crop diversity and iii. reducing pest pressure. The mixtures also often produce higher yield and crop quality. There are multiple types of intercrops that vary in their spatial arrangement.

Strip intercrops are wide strips with multiple rows of one crop, that are alternated on the field with strips of one or more different crop(s). Strip intercrops are typically planted on the field contour with crops of different life cycles that protect soil from erosion throughout the year. For instance, strips of corn may be alternated with strips of perennial forage grasses that can reduce soil erosion across the field when the corn isn't growing. Or, as in the photo below, winter wheat provided live plant coverage on portions of the field in spring, prior to corn and soybean were planted. In mid-summer, corn and soybean provide live coverage after wheat is harvested; and in fall, winter wheat will be growing on some strips after corn and soybean are harvested. Having strips of different crop species can also reduce the spread of insect pests and crop pathogens compared to cultivating one crop on the entire field.

Figure 7.1.7.: Strip intercrop: Alternating strips of corn, soybean and winter wheat planted on the contour.. Credit: Heather Karsten

Row intercrops alternate rows of different crop species, usually every other row or every two rows.

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Figure 7.1.8.: Alternating rows of onion and hairy vetch. Hairy vetch is a winter annual legume that is mowed frequently to reduce competition with the onions. In this system, hairy vetch is planted to provide soil protection, suppress weeds, and add nitrogen to the soil. Credit: Heather Karsten

Mixture intercrops tend to be combined randomly when planted; such as grass and legume forage mixtures. Intercrops of different crop species (ex. native tuber mixtures) or different varieties of a crop species (ex. rice) are sometimes planted to reduce pathogen and insect pest infestations. Crop rotation and intercropping increase agrobiodiversity across an agricultural landscape, providing multiple potential agroecosystem benefits, such as i. reducing the risk of crop loss to pests and climatic stresses (ex. frosts, floods, and drought), ii. providing habitat for beneficial organisms such as pollinators and pest predators, and iii. enhancing the diversity of nutritional crops for farmers and markets. Further, integrating crops from the grass family tends to promote soil structure, while legumes enhance soil nitrogen, and integrating perennial crops protects the soil from erosion and builds soil organic matter and soil biological activity because perennials allocate a high proportion of their growth to storage organs. For instance, the photos below illustrate how both intercropping and crop rotation enhance agrobiodiversity in the high Andes of Peru.

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Figure 7.1.9.: Pasture intercrop of four perennial forage crops: tall fescue, orchardgrass, Kentucky bluegrass, and white clover. Credit: Heather Karsten

Figure 7.1.10.: Four major native tuber crops: Maca, Oca, Ulluco, and Mashua at the CIP International Potato Center in Lima, Peru. Credit: Heather Karsten

Figure 7.1.11.: Example High Altitude Andean Crop Rotation from Peru. Credit: Heather Karsten

Figure 7.1.12.: Sheep grazing perennial pastures that are typically rotated to annual crops: potato, native tuber crops, legumes, and small grains before rotating back to perennial pasture. Credit: Heather Karsten

Figure 7.1.13.: Agrobiodiversity is high across the high altitude Andean landscape due to crop rotation and genotypic diversity within fields. Credit: Heather Karsten

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Figure 7.1.14.: Diversity of potato and native tuber crops in a grocery store in Lima, Peru. Credit: Heather Karsten

Cover Crop: A cover crop is planted after a crop that is harvested and is terminated before the subsequent crop is planted. Cover crops tend to be annual crops that they can quickly establish after a harvested crop to protect the soil from erosion and provide other benefits including i. to add organic matter to the soil; ii. to scavenge nutrients and prevent nutrients from leaching out of the topsoil (also called a catch crop); iii. to support soil organisms in the root zone, iv. to suppress weeds, and v. to provide habitat for aboveground beneficial organisms, such as insects that predate on crop pests or weed seeds. Leguminous cover crops also add nitrogen to the soil when they are terminated and returned to the soil and are therefore often referred to as green manure crops. Cover crops are also sometimes referred to as "catch crops" because they can take up and retain nitrogen and other nutrients that might otherwise leach out of the rooting zone and be lost to deeper soil profiles, and potentially to groundwater.

Cover Crop Intercrops

Because cover crop species have different plant traits that provide different cropping system benefits, often two or more species of cover crops are planted together as a cover crop intercrop or cover crop mixture. For instance, small grains that scavenge nitrogen well and have fibrous roots that bind soil particles and promote soil structure are often mixed with tap-rooted legumes that fix nitrogen. Some cover crop mixtures combine plant species that establish quickly in the late summer or early fall but don't typically survive the winter, such as oats or deep-rooted radish species. Non-winter hardy species are sometimes combined with winter-hardy species such as hairy vetch, cereal rye or annual ryegrass that survive the winter and provide cover in early spring.

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Figure 7.1.15.: Annual crimson clover and winter wheat cover crop intercrop photographed in spring. Credit: Heather Karsten

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Figure 7.1.16.: Cover crop intercrop of annual cereal rye and hairy vetch (a legume) photographed in spring. Credit: Heather Karsten

Readings

Download the book Building Soils for Better Crops. Edition 3. Sustainable Agriculture Network, USDA. Beltsville, MD or read it online, Building Soils for Better Crops. Edition 3.

For this module, you will be assigned to read multiple sections. So, we recommended that you download the book. Then, read more about the benefits of cover crops in Chapter 10: Cover Crops and Chapter 11: Crop Rotations.