The cultivated peanut or groundnut (Arachis hypogaea L.), originated in South America (Bolivia and adjoining countries) and is now grown throughout the tropical an warm temperate regions of the, world. This crop was grown widely by native peoples of the New World at the time of European expansion in the sixteenth century and was subsequently taken to Europe, Africa, Asia, and the Pacific Islands. Peanut was introduced to the present southeastern United States during colonial times. Peanut was grown primarily as a garden crop in the United States until 1870. As a field crop, peanut was used commonly for hog pasture until about 1930.
Peanut, an important oil and food crop, is currently grown on approximately 42 million acres worldwide. It is the third major oilseed of the world next to soybean and cotton (FAO Food Outlook, 1990). India, China, and the United States have been the leading producers for over 25 years and grow about 70% of the world crop. Peanut was ranked ninth in acreage among major row crops in the United States during 1982 and second in dollar value per acre. Production of peanut in the U.S.A. during 1989-1990 was estimated at 1.8 million tons. or about 8% of the world production of 23.2 million tons (FAO Food Outlook, 1990). In 1983, Georgia, Texas, Alabama, and North Carolina grew 80% of the 1,375,000 acres of peanut in the United States. Virginia, Oklahoma, Florida, South Carolina, and New Mexico were the other states with more than 10,000 acres of peanut.
The peanut crop in the U.S.A. is composed of four market types from two subspecies. A. hypogaea hypogaea includes the Virginia and Runner market types. The second subspecies, A. hypogaea fastigiata, includes two botanical varieties of economic importance: vulgaris, the Spanish market type, and fastigiata, the Valencia market type. Virginia peanuts have the largest pods and elongated seeds, while Runner peanuts are medium-size varieties of the Virginia type. Spanish types have smaller round seeds and Valencia is intermediate in size and shape. Valencia is grown primarily in New Mexico, Spanish in Oklahoma and Texas, and other types in the Southeast and Texas. The Runner type includes 70% of the edible trade in the U.S.A. with Virginia and Spanish accounting for 20 and 10%, respectively. Valencia peanuts generally constitute less than 1% of the U.S. market (Knauft and Gorbet, 1989).
Peanut has only occasionally been grown in northern states due to its warm temperature requirement. Use of poorly adapted varieties and improper production practices usually resulted in low yields and poor quality nuts. However, peanut has good potential as a food crop in Minnesota and Wisconsin and could be an alternative cash crop to soybean, corn, potato, or fieldbean (Robinson, 1984, and Pendleton, 1977).
All parts of the peanut plant can be used. The peanut, grown primarily for human consumption, has several uses as whole seeds or is processed to make peanut butter, oil, and other products. The seed contains 25 to 32% protein (average of 25% digestible protein) and 42 to 52% oil. A pound of peanuts is high in food energy and provides approximately the same energy value as 2 pounds of beef, 1.5 pounds of Cheddar cheese, 9 pints of milk, or 36 medium-size eggs (Woodroof, 1983).
Peanuts are consumed chiefly as roasted seeds or peanut butter in the United States compared to use as oil elsewhere in the world. Americans eat about 4 million pounds (unshelled weight) of peanuts each day. Approximately two-thirds of all U.S. peanuts are used for food products of which most are made into peanut butter. Salted and shelled peanuts, candy, and roasted-in-shell peanuts are the next most common uses for peanuts produced in this country. The remaining one-third of annual production is used for seed, feed, production of oil, or exported as food or oil. The large nuts sold as in- and out-of-shell are supplied by Virginia (confectionery or cocktail) and Runner (“beer nuts”) types. Spanish varieties supply small shelled nuts, “redskins”, and the Valencia type is used for medium-size nuts in the shell. Runner and Spanish are made into peanut butter while all types are used for peanut products that do not require a specific seed size.
Nonfood products such as soaps, medicines, cosmetics, and lubricants can be made from peanuts. The vines with leaves are an excellent high protein hay for horses and ruminant livestock. The pods or shells serve as high fiber roughage in livestock feed, fuel (fireplace “logs”), mulch, and are used in manufacturing particle board or fertilizer.
III. Growth Habits:
Peanut is a self-pollinating, indeterminate, annual, herbaceous legume. Natural cross pollination occurs at rates of less than 1% to greater than 6% due to atypical flowers or action of bees (Coffelt, 1989). The fruit is a pod with one to five seeds that develops underground within a needlelike structure called a peg, an elongated ovarian structure.
Peanut emergence is intermediate between the epigeal (hypocotyl elongates and cotyledons emerge above ground as in soybean) and hypogeal (cotyledons remain below ground as in fieldpea) types. The hypocotyl elongates but usually stops before cotyledons emerge. Leaves are alternate and pinnate with four leaflets (two pairs of leaflets per leaf). The peanut plant can be erect or prostrate (6 to 24 in. tall or more) with a well developed taproot and many lateral roots and nodules. Plants develop three major stems, i.e., two stems from the cotyledonary axillary buds equal in size to the central stem during early growth.
Bright yellow flowers with both male and female parts are located on inflorescences resembling spikes in the axils of leaves. One to several flowers may be present at each node and are usually more abundant at lower nodes. The first flowers appear at 4 to 6 weeks after planting and maximum flower production occurs 6 to 10 weeks after planting.
Eight to 14 days after pollination aerial pegs will grow 2 to 3 in. into the soil and then turn to a horizontal orientation to mature into a peanut pod. Pods reach maximum size after 2 to 3 weeks in the soil, maximum oil content in 6 to 7 weeks, and maximum protein content after 5 to 8 weeks. The peanut crop matures after 7 to 9 weeks in the soil, which is indicated by maximum levels of protein, oil, dry matter, and presence of darkened veining and brown splotching inside the pod. Peanuts usually require a minimum of 100 to 150 days from planting to maturity depending on the variety planted.
Flowering continues over a long period and pods are in all stages of development at harvest. Pegs will eventually rot in the soil (25% after 12 weeks in the soil) and the resulting loose pods are lost during the harvest. Since the pod wall is needed to protect the seed, as it is moved through the various markets from producer to processor or consumer, yields and farm prices are based on a pod rather than seed basis.
IV. Environment Requirements:
Temperature is the major limiting factor for peanut yield in northern states since a minimum of 3,000 growing degree-days (with a base of 50°F) is required for proper growth and development (Robinson, 1984). A peanut crop will not reach optimum maturity for a marketable yield to justify commercial production in areas with fewer heat units during the growing season. This eliminates some of Minnesota and most of Wisconsin as practical production areas. Little if any growth and development occur at temperatures below 56°F (Emery et al., 1969) and 86°F is reported to be optimal (Ketring, 1984).
Rainfall distribution varies greatly from western Minnesota to south eastern Wisconsin and irrigation may be a yield-stabilizing factor. University of Minnesota studies over a six-year period indicated that irrigation of sandy soil increased average yield by 1,000 to 1,450 lb/acre. However, in some years it did not increase yields appreciably, and irrigation expenses and lower land values may give an economic advantage to dryland production over use of irrigation. Research in Ontario, Canada indicated that the most critical time to apply water was during the flowering period.
Soil for peanut production should be a light-colored, light textured with good drainage, and moderately low amounts of organic matter. Such soil is preferred since it is usually loose and friable, permitting easier penetration of roots and pegs, better percolation of rainfall, and easier harvesting. Light-colored soils reduce staining of pods which ensures greater eye appeal when the crop is used for unshelled nuts. Well-drained soils provide proper aeration for the roots and nitrifying bacteria that are necessary for proper mineral nutrition of the plant. Medium to heavy soils or those with a high clay content should also be avoided due to excessive loss of pods when harvesting peanuts.
Organic matter should be maintained at a level of 1 to 2% to improve water-holding capacity of the soil and supply plant nutrients. Peanut grows best in slightly acidic soils with a pH of 6.0 to 6.5, but a range of 5.5 to 7.0 is acceptable. Saline soils are not suitable since peanut has a very low salt tolerance (Weiss, 1983).
C. Seed Preparation and Germination:
Poor stand is perhaps the most common cause of low yields. To obtain a full stand, use undamaged seed with intact seed coats and treat shelled seed with an approved seed protectant prior to planting. Planting seeds rather than pods allows for easier machine planting and more uniform stands. Robinson (1984) reported higher yields when seed was used because planting pods delayed emergence due to slower absorption of moisture into the shells.
V. Cultural Practices:
A. Seedbed Preparation:
Peanut should not be grown in the same fields for successive years, but should be produced in a crop rotation plan. Soil samples should be taken before pre-plant field preparation to determine nutrient needs. Fertilizer, if needed, may be broadcast prior to plowing. Plow 8 to 9 in. deep to completely cover plant residues, which reduces losses from stem- and peg-root diseases (Sclerotium rolfsii) and weeds. The operations necessary to produce a seedbed for corn or soybean are suitable for peanut.