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Chapter 3: Classifying and naming crops.

Key Concepts

* Crops can be classified by agronomic use, nutritional use, life cycle, and climate adaptation.

* Agronomic crops include cereals, grains, pulses, forages, fiber crops, cover crops, companion crops, green manure crops, and recreational turf crops.

* Bioenergy production from starch and cellulose is an emerging technology.

* Nutritional use classification includes sugar crops, oil crops, protein crops, starchy seed crops, and root and tuber crops.

* Plants are scientifically described using two-part scientific names: the genus and species.

* Taxonomy is a system of organizing plants into species, families, orders, classes, and divisions.

Key Terms




cereal crop

companion crop

cover crop


fiber crop


grain crop


green manure crop



oil crop

protein crop


root crop

scientific name


small grain



tuber crop


Agronomy is the production of field crops while sustaining the soil resource and the environment. The word agronomy comes from the Greek words agros (field) and nomos (to manage). Most field crops are ultimately used for food production, but they have other important uses. To facilitate their description, we place crops in categories based on their agronomic, industrial, and nutritional uses. Categorization involves the use of professional and technical terminology. An aspect of categorizing crops is naming them. All crops have a common name and a two-word scientific name. Profiles of individual crops are provided in Chapters 17, 18, and 19.

Agronomic Crop Categories

This category includes cereals, grains, pulses, forages, fiber crops, root and tuber crops, cover crops, companion crops, green manure crops, and recreational turf crops.

Cereal Crops

Cereal crops are grasses that produce edible grain. The word cereal is derived from the name of the Roman goddess of grain, Ceres. Wheat, rice, corn, barley, and sorghum are the most important cereal crops grown in the world today. Cereals store energy in their seeds as starch. The starch can be extracted from the grain and used as an energy source for livestock and humans. Cereal grains contain about 75% starch, 12% protein, 2% fat, and smaller quantities of vitamins and minerals.

A popular American definition of a cereal is that cereals are processed breakfast foods to which milk is added (Figure 3-1). Today, manufacturers make numerous processed breakfast cereals from corn, wheat, rice, and oats, and these cereals are marketed to children and adults. Around 1900, William and Harvey Kellogg developed the original breakfast cereals in Battle Creek, Michigan. They developed their first products, corn flakes and shredded wheat, as low fat and fibrous health foods. Today's breakfast cereals vary greatly in nutritional content and potential health benefits.


Grain Crops

Grain is the fruit or seed of a crop plant. Similar in definition to cereal crops, the term grain crop is applied to the harvested portion of a number of grass crops such as corn. But the term also includes legumes such as soybean and kidney beans, and other plants such as flax and buckwheat. Small grain is a term applied to wheat, oat, barley, and rye because relative to corn, their seed is small.


Legumes grown for their edible, high protein seeds are known as pulses. Pulses contain up to a third of their dry weight in protein. They store energy as carbohydrates or oil. Pulse seeds are borne in pods that burst into halves when mature and dried. Pulse crops include soybean, edible field beans, field peas, lentils, peanuts, chickpeas, and cow peas. Most pulses should be roasted or boiled before human consumption to destroy chemicals which reduce the availability of nutrients. Many pulses such as field beans, chickpeas, field peas, and lupines are important components of human diets. Soybean is the most widely grown pulse (Figure 3-2). Humans consume the soybean itself in the form of soy paste (miso) and soy curd tofu, and we eat it as a germinated seed (bean sprouts). Its oil (soybean has an oil content around 13-25%) is extracted and used for cooking, and farmers use the protein rich meal for livestock feed.


Forages are any crops whose vegetative parts--including stems, leaves, and sometimes attached seed or grain--are used for livestock feed. Animals can take forages directly from pastures by grazing, or forages can be fed following storage. Grazing is the most traditional form of forage use, and "free ranging" of livestock was a common practice during the European settlement of North America. It is important today in permanent pastures, rangelands, and cropland pastures. In many regions, pasturing is restricted for many months of the year due to cold weather or drought, and stored forages are critical to meet animals' needs. Farmers store forages as hay or silage. Hay is forage that is dried to about 20% moisture and stored in the air, whereas silage is harvested at 50-80% moisture and sealed in a structure called a silo. Forages are also important in crop rotations and are often grown on erodible lands unsuitable for tillage.


Forages are natural feeds for ruminant animals and wildlife. Ruminants such as sheep, goats, cattle, and deer are capable of digesting fibrous portions of plants and can use many plant materials that humans cannot digest. Ruminants are a unique complementary meat and protein source for humans who depend on grains to supply energy nutrition. In modern meat production systems, many livestock are fed a combination of forages and grains to achieve high levels of meat or milk production. Producers grow a diversity of perennial and annual plants for forage. Species used depend on the climatic and soil conditions and the intended use.

* In the northern United States and Canada, temperate cool-season legumes such as alfalfa and clovers, and grasses such as smooth bromegrass and timothy are used for pasture, hay, and silage.

* In the southern United States, annual clovers, bermuda grass, and tall fescue are grazed and used for hay.

* In the dry regions of the western Great Plains, shrubs and native grasses such as little bluestem and buffalo grass are grazed.

* Throughout the United States, corn and small grains are harvested for silage when they are immature.

Fiber Crops

We use fiber from plants for clothing, rope, paper, and baskets. This fiber is a collection of individual fiber cells as described in Chapter 7. Cotton, flax, and hemp are fiber crops that each produce fiber of different characteristics.

Cotton fiber is mostly pure cellulose and is white. It is called a surface fiber and surrounds the seed. Cotton fibers vary in length and are twisted. Cotton is the most widely-used natural fiber crop.

Flax fiber is called bast fiber, which is the soft fiber extracted from the stem. The cloth produced after processing is called linen. Flax is the oldest crop used for fiber and cloth production. Flax fibers are longer than those of cotton and average about 2 feet in length. They consist of about 70% cellulose with some waxes mixed in.

Hemp fiber is also extracted from the stem. It is very coarse and brown. It is an extremely strong fiber used for the production of canvas, rope, and bags. Hemp is illegal to grow in the United States, but the fiber can be imported.

Root and Tuber Crops

White or Irish potatoes, sweet potatoes, and cassava are examples of root crops, or tuber crops. They are harvested for their underground storage organs that are rich in starch. Potatoes are enlarged underground stems called tubers. Though sweet potatoes and cassava seem similar to the Irish potato, they have enlarged roots, not tubers. These crops are among the most important in the world as a source of energy for human nutrition. Another tuber-producing crop is Jerusalem artichoke. In contrast to Irish potatoes, Jerusalem artichokes store energy as inulin, another type of complex carbohydrate.

Cover Crops

Farmers use cover crops to prevent soil erosion due to water or wind during periods when the soil is not protected by growing plant cover. For example, in the Midwest, summer annuals such as corn and soybean are normally in the field from May through October. The soil remains uncovered, except for crop residue, from November until the following spring. A cover crop such as winter rye is often planted in late fall to provide soil cover over the winter.

Companion Crops

Companion crops such as oat and wheat have traditionally been seeded with a small-seeded legume such as alfalfa to provide erosion control, suppress weeds, and provide some income from the land during the establishment period if either crop fails. Companion crops are harvested at maturity for grain and straw or when immature for silage. Following the harvest of the companion crop, the legume will regrow.

Green Manure Crops

Green manure crops are legumes or grasses that are incorporated into the soil while vegetative with the purpose of adding nutrients to the soil or enhancing soil quality. Sweet clover is an example of a traditional green manure crop that farmers grow expressively for the purpose of plowing down and adding nitrogen and organic matter to the soil. Other examples include alfalfa (Figure 3-3) and winter rye.



Recreational Turf Crops

People use many grasses such as Kentucky bluegrass, perennial ryegrass, and bentgrass for recreational purposes. They use Kentucky bluegrass and ryegrass extensively for lawns, soccer fields, and football fields in the northern regions of the United States. Further south, bermuda grass is often used in recreational areas such as golf courses (Figure 3-4). Varieties of these species have been selected with tolerance to wear from human traffic.

Bioenergy Crops

Bioenergy is a general term that describes production of energy from biological systems. We also use the terms biomass and biofuel to describe bioenergy. Biomass crops are those used for the production of energy. Biofuel crops are crops grown specifically for use as a dedicated fuel source for production of liquid fuels. Bioenergy crops are appealing because they are a renewable energy source. They use carbon energy captured through photosynthesis and, in contrast to fossil fuels, their combustion does not result in a net increase in the atmospheric carbon dioxide concentration. Worldwide, only about 15% of the energy used by humans comes from biomass crops. Most of this energy is used through direct combustion (burning) of forest products and crop residues.

About 85% of all the energy consumed in the United States is from nonrenewable fossil fuel resources (Table 3-1). Only 6% is from renewable sources such as plant biomass, hydroelectricity, wind, and the sun. Biomass and hydroelectricity each contributes about 45% of the renewable energy used in the United States. Most of the biomass used for energy in the United States is provided by the forest products industry, urban tree trimming, and fuel wood. Also, a small quantity of switchgrass is being co-fired with coal.

Energy Policy

Historically, people have taken for granted the availability of low-cost energy in the United States. Taxpayers have indirectly and directly subsidized the fossil fuel energy industry. In the early 2000s, public awareness about energy issues grew because of several developments.

* High prices for gasoline at the pump caused a significant change in consumer spending.

* Dependence on imported foreign oil from Venezuela and the Middle East caused national security and balance of payment concerns.

* Excessive greenhouse gas production from burning fossil fuels such as petroleum and coal was associated with global warming.

In response to these concerns, President George W. Bush identified some specific energy goals in his 2007 State of the Union Address. These include production of 35 billion gallons (133 billion liters) of biofuels from ethanol and biodiesel by 2017. This would replace a small portion of the 230 billion gallon (871 billion liter) annual consumption of gasoline. With this proposal, scientists project that about 15 billion gallons (57 billion liters) of biofuel ethanol would be produced from corn starch, and 20 billion gallons (76 billion liters) would be produced from cellulosic sources.

Starch-Based Ethanol

Ethanol is produced when yeast ferments glucose. In the United States, corn is the leading crop used for ethanol production. In 2007, almost 5 billion gallons (19 billion liters) of ethanol were produced from corn. About 2.8 gallons (11 liters) of ethanol are produced per bushel (25.4 kilograms) of corn. Therefore, an acre (0.4 hectare) of corn with a yield of 150 bushels (3800 kilograms) of corn grain produces 420 gallons (1600 liters) of ethanol. Ethanol is blended with gasoline to produce E85 fuel, made from 85% ethyl alcohol and 15% gasoline. For every barrel of ethanol produced, 1.2 barrels of petroleum is displaced. Corn conversion to alcohol occurs in several steps:

1. Milling or grinding the grain into a fine powder or corn meal.

2. Liquefying the corn meal with water and heat.

3. Using enzymes to convert the starch to glucose.

4. Using yeast to ferment the glucose to produce ethyl alcohol

[C.sub.6] [H.sub.12] [O.sub.6] [right arrow] 2 [C.sub.2] [H.sub.5] OH + 2 C[O.sub.2]

5. Separating and concentrating the ethanol from water and other impurities by using distillation and other separation technologies.

Ethanol can also be produced from sugar extracted from sugarcane and sorghum. In Brazil, ethanol made from sugarcane is a significant source of energy. Brazil is the world's leading producer of ethanol.

Corn Ethanol Challenges

The conversion of starch from corn to ethanol is appealing as an approach to grow our own fuels. Corn-based ethanol also creates a demand for corn and maintains prices for farmers. More than 100 ethanol plants in the United States have created numerous jobs in rural areas. Many of these are located in the Midwest (Figure 3-5).

However, the extensive use of corn for ethanol production is controversial. Increased production of ethanol from corn to achieve a capacity goal of 15 billion gallons (57 billion liters) will require use of nearly all the corn produced in the United States. This will divert corn that is normally used for livestock and human food. Prices of meat, eggs, and corn products will increase. Also, some people are concerned about the energy and water use efficiency of the corn to ethanol process. Finally, excessive corn production on agricultural landscapes will likely lead to high rates of soil erosion and increased nitrogen pollution of waterways.

Cellulosic Ethanol

Production of ethanol from cellulose is appealing because of the abundance of cellulose produced in the United States. Perlack et al. (2005) estimated that more than a billion dry tons (91 million metric tons) per year of low-cost fuels could be produced. Potential cellulosic ethanol sources fall into three categories.

1. Crop residues left after grain harvest, for example, corn stover, rice straw, and wheat straw.

2. Forest residues left after wood harvest and processing.


3. Dedicated energy crops such as switchgrass, alfalfa, hybrid poplar, and willow.

4. Surplus or damaged hay crops of alfalfa or smooth bromegrass.

The cellulose content of crop residue and dedicated energy crops averages about 40%, although the concentration of cellulose can be affected by crop, management, and environment. These crops also contain a significant amount of hemicellulose (~30%) and lignin (15%).

Cellulosic crops yield 60-80 gallons (227-303 liters) of ethanol per ton (0.91 metric tons) of crop biomass. Unfortunately, currently there is no large-scale production of ethanol from cellulosic feedstock in the United States. Technology is in development to greatly increase the efficiency of the conversion process. Though the starch in corn grain can be readily converted to glucose and then fermented to ethanol, the cellulose in plants is not easily converted. Cellulose is less available because it is bound to lignin and hemicellulose. Also, the cellulose structure is more difficult to degrade by enzymes than is starch. Cellulose chemistry is explained in more detail in Chapter 5.

The current process of ethanol production from cellulosic plant material has the following steps:

1. Grinding the plant material.

2. Treating with acid, pressure, and heat to break linkages between lignin, cellulose, and hemicellulose. This step also degrades hemicellulose into a mixture of simple five-carbon and six-carbon sugars (xylose).

3. Using cellulase to convert the liberated cellulose to glucose.

4. Fermenting to produce ethanol. Glucose from cellulose is fermented by yeast to produce ethanol and xylose from hemicellulose is fermented by bacteria to produce ethanol.

5. Distilling and concentrating the ethanol.


Biodiesel is a renewable substitute for petroleum-based diesel. It is produced from vegetable oils such as soybean and canola. Oil extracted from these crops is reacted with alcohol in the presence of a catalyst. About 100 billion gallons (379 billion liters) of biodiesel are produced annually in the United States. People often use it as a blend with petroleum diesel.

Nutritional Use Categories

Crops can be classified by the way we use them nutritionally. The basic components of human and animal diets are fats, proteins, and carbohydrates such as sugars and starches (see Chapter 4).

Sugar Crops

Sugar beets, sugarcane, and corn are some of the crops from which sugar is obtained. The sugar beet is a root crop grown for sucrose. Sugarcane stores sugar in a tall growing stem that is crushed during sugar extraction. Corn is the source of high fructose corn syrup, which is the leading sweetener used in drinks and candies. It is produced by conversion of corn starch from the kernel to a fructose- glucose mixture.

Oil Crops

Many crop species produce oil-rich seeds that can be processed for their oil. Oil crops produce oils that contain fatty acids that vary in saturation. Oil crops include soybean, flax, sunflowers, canola, peanuts, and cotton. Soybean accounts for half of edible oil production.

Protein Crops

Legumes such as soybean, peas, field beans, and peanuts produce seeds that are naturally high in protein. These protein crops are an important source of protein in many parts of the world. Nonlegumes can also be used for seed protein production and include quinoa, cotton, and sunflowers.

Starchy Seed Crops

Seed from the cereal grasses is naturally high in starch. These seed crops include corn, wheat, oat, barley, rye, millet, and sorghum. Nongrasses such as amaranth and buckwheat also produce starchy seed.

Starchy Root and Tuber Crops

Roots and tuber crops store their energy primarily as starch. Humans can convert this starch to energy.

Naming Plants

Every culture has given plants common names that often describe a morphological feature or a use. For example, white clover is a plant with white flowers, whereas red clover has red flowers. The challenge with common names for plants is that they vary regionally and nationally. In Europe, for instance, alfalfa is also known as lucerne, corn is known as maize, and wheat is known as corn. Orchardgrass, a forage grass, is called cocksfoot in Europe.

Scientific Names

Scientific names allow for the universal identification of plants. In the eighteenth century, the Swedish botanist Carolus Linnaeus first developed the binomial nomenclature system of plant identification. In the binomial system, plants are identified using a two-part Latin name: the genus and species. Scientific names are always italicized, with the genus capitalized. Each unique plant is called a species, and similar species form a genus. A complete scientific name also includes the name(s) or abbreviation of the name of the individual(s) who first described the species. For example, the complete scientific name for soybean is Glycine max L. with L. being an abbreviation for Linnaeus, who named this species. Species that have been subject to plant breeding and selection can be identified as a variety, cultivar (short for cultivated variety), or hybrid number. See Appendix 1 for a listing of common and scientific names.


Taxonomy is a system of organizing all life based on similar genetic or morphological characteristics. Species are the lowest taxonomic unit. Similar species are grouped into families, families into orders, orders into classes, classes into divisions, and divisions into kingdoms. The kingdom category separates plants from animals. See Table 3-2 for an example of the complete classification for corn (a grass) and soybean (a legume). For comparison, the taxonomy of a cow is provided. Two particularly relevant categories to plants are the classes and families.

Dicots and Monocots

Crop plants can be classified as dicots or monocots. Dicots are plants such as soybeans and sunflowers that have two seed leaves (cotyledons) within the seed. Monocots are plants such as corn and bluegrass that have a single cotyledon within the seed. Dicots and monocots also differ in root, stem, leaf, and flower characteristics (see Chapter 6).

Plant Families

Plant genera are grouped into families based on floral, fruit, or genetic characteristics. The families of some important crops and crop weeds are shown in Table 3-3. Two of the most important crop families--with a large number of species of economic significance--are the Poaceae (grass) and Fabaceae (legume) families. Note that family names end in the suffix-aceae.

Review Questions

1. Define and give examples of these agronomic crop categories: forages, pulses, root crops.

2. Provide two separate definitions of the term cereal. One should be an agronomic definition, the other a common use definition.

3. Define and give an example of the following nutritional crop use categories: sugar crops, oil crops, protein crops, and starchy seed crops.

4. What is the most commonly used bioenergy crop grown today? Explain the process of its conversion into fuel.

5. Give an example of a cellulosic biofuel crop. Why is conversion of a cellulosic crop to ethanol more difficult than for a grain crop?

6. What is plant taxonomy?

7. Give the scientific name of a commonly grown crop plant.

Critical Thinking

1. Should the United States convert corn grain into ethanol for use in transportation? Is this the best use of the grain?

2. Review the common and scientific names of a crop plant that you choose. Much of our English language is based on Latin. Can you interpret the Latin name?


Barnes, R. F., Nelson, C. J., Moore, K. J., & Collins, M. (2007). Forages: The science of grassland agriculture. (6th ed.). Ames, IA: Blackwell Publishing.

Capon, B. (1990). Botany for gardeners. Portland, OR: Timber Press, Inc.

Energy Information Administration, U.S. Department of Energy (2007). Official energy statistics from the U.S. government. DC: USDOE. <> Accessed on 18 October 2007.

Graham, L. E., Graham, J. M., & Wilcox, L. W. (2006). Plant biology (2nd ed.). Prentice Hall: Upper Saddle River, NJ.

Levetin, E., & McMahon, K. (1996). Plants and society. Dubuque, IA: William C. Brown Publishers.

Martin, J. H., & Leonard, W. H. (1967). Principles of field crop production. New York: The Macmillan Company.

Perlack, R. D., Wright, L. L., Turhollow, A. F., Graham, R. L., Sotcks, B. J., & Erbach. D. C. (2005). Biomass as feedstock for a bioenergy and bioproducts industry. The technical feasibility of a billion-ton annual supply. DOE/GO-102005-2135. DC: United States Department of Energy and USDA.

Simpson, B. B., & Ogorzaly, M. C. (1995). Economic botany: Plants in our world (2nd ed.). New York: McGraw-Hill, Inc.

United States Department of Agriculture--Natural Resources Conservation Service (2007). Plants Database. DC: USDA-NRCS. <> Accessed 26 October 2007.
Table 3-1
Sources of energy consumption in the United States, 2003. Energy
Information Administration, Perlack et al. (2004).

Energy source   Percent of total energy

Petroleum                 39%
Natural gas               24%
Coal                      23%
Nuclear                    8%
Renewable                  6%

Table 3-2
Complete classification of corn, soybean, and cow. The division
Magnoliophyta includes the flowering plants. The class Liliopsida
represents monocot plants, and Magnoliopsida represents dicot plants.
The family Poaceae is the grass family, and Fabaceae is the legume
family. USDA-NRCS (2007).

           Corn            Soybean         Cow

Kingdom    Plant           Plant           Animal
Division   Magnoliophyta   Magnoliophyta   Chordata
Class      Liliopsida      Magnoliopsida   Mammalia
Order      Cyperales       Fabales         Artiodactyla
Family     Poaceae         Fabaceae        Bovidae
Genus      Zea             Glycine         Bos
Species    mays            max             tarus

Table 3-3
Important families of crop and weed plants, and their characteristics.

Monocot families           Crops          Weeds

Poaceae                    Barley         Foxtail spp.
  (Grass family)           Corn, Oat      Fall panicum
                           Wheat, Rice    Quackgrass

Cyperaceae                 Water          Nutsedge spp.
  (Sedge family)           chestnut

Monocot families           Characteristics

Poaceae                    Includes about 600 genera and 9000 spp.
  (Grass family)           Most widely distributed
                           Highly specialized leaves, stems, and
                           Mostly hollow stems (culms)

Cyperaceae                 Solid, triangular stems
  (Sedge family)           Leaves three-ranked

Dicot families             Crops          Weeds

Fabaceae                   Soybean        Black medic
  (Legume family)          Peanut         Kudzu
                           Pea, Vetches   Wild licorice

Solanaceae                 Potato         Nightshade
  (Nightshade family)      Tomato         Jimsonweed

Convolvulaceae             Sweet potato   Field bindweed
  (Morning-glory family)                  Dodder

Asteraceae                 Sunflower      Dandelion
  (Sunflower family)       Artichoke      Thistle, Ragweed

Chenopodiaceae             Sugar beet     Lambsquarters
  (Goosefoot family)       Quinoa         Kochia

Amaranthaceae              Amaranth       Pigweed
  (Amaranth family)                       Waterhemp

Polygonaceae               Buckwheat      Smartweed
  (Buckwheat family)       Rhubarb        Knotweed

Linaceae                   Flax
  (Flax family)

Brassicaceae               Canola         Garlic mustard
  (Mustard family)         Mustard

Cucurbitaceae              Squash
  (Squash family)          Cantaloupe

Malvaceae                  Cotton         Velvetleaf
  (Mallow family)          Okra           Mallow

Dicot families             Characteristics

Fabaceae                   Includes 630 genera and 18,000 spp.
  (Legume family)          Seeds borne in pods
                           Conduct biological [N.sub.2] fixation
                           Seeds and vegetation rich in protein

Solanaceae                 Includes 85 genera and 2800 spp.
  (Nightshade family)      Contains alkaloids

Convolvulaceae             Viny herbage
  (Morning-glory family)   Leaves frequently heart or arrow-shaped

Asteraceae                 Largest family with 1100 genera and
  (Sunflower family)         20,000 spp.
                           Head inflorescences

Chenopodiaceae             Taprooted succulent annuals
  (Goosefoot family)       Leaves of some species shaped like
                           Flowers small and greenish

Amaranthaceae              Taprooted herb
  (Amaranth family)        Very small flowers

Polygonaceae               Pungent acid juice in stem
  (Buckwheat family)

Linaceae                   Annual with fibrous stem
  (Flax family)

Brassicaceae               Many winter annuals and biennials
  (Mustard family)         Herbage and roots often produce pungent
                           Petals often form cross

Cucurbitaceae              Includes 90 genera and 700 annual spp.
  (Squash family)          Viny crops with large fleshy fruits
                           Large bright flowers often unisexual

Malvaceae                  Includes 204 genera and 2300 spp.
  (Mallow family)          Have mucilaginous sap
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Publication:Introduction to Agronomy, Food, Crops, and Environment
Geographic Code:1USA
Date:Jan 1, 2009
Previous Article:Chapter 2: Agriculture today.
Next Article:Chapter 4: Food from plants.

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