The peanut (Arachis hypogaea L.) is a herbaceous, annual type of plant that grows to a height of 20-60cm. Depending on the species, the plants may grow upright and sideways with their sideward shoots to a breadth of 30-80cm. The main stem usually remains upright. The taproots penetrate to a depth of 90-120cm, creating branches within the upper soil levels that are populated by rhizobia and mycorhiza.
Peanuts grow best in a weakly acidic pH value (6.0-6.5); a pH value of 5.5-7.0 is still acceptable, and local can adapt themselves to pH values up to 7.8. Peanuts are sensitive to a high salt content in the soil (max. 4mS/cm).
Cultivation systems and diversification possibilities
Peanuts should only be planted in a three year crop rotation, at least. Otherwise, soil-borne diseases can accumulate, and humus is lost due to excessive soil loosening during the harvest.
Nevertheless, peanuts possess good soil enrichment potential for a non-legume, and act as an excellent crop prior to planting grain. The previous crop should leave little weeds behind, and be harvested early; to allow plenty of time to work over the soil to permit early sowing to take place. The previous crop should be allowed to largely go to seed. Suitable crop partners include grain, sorghum, pearl barley, maize, rice, and also: sesame, saffron, cotton, sweet potatoes and grain legumes such as mung beans (Vigna mungo) or cowpea (Vigna unguiculata).
Planting peanuts in mixed crop systems is very widely spread and is more the rule than the exception on small farm units. Some advantages are:
* Greater total production per area (despite the losses caused by overshadowing of the peanuts as opposed to monoculture); * Reduction of transpiration especially in alley-cropping; * Greater diversity of diet; * Regulation of erosion and weeds; * Reduced susceptibility to pests.
Due to their tolerance of shading, peanuts are especially suited to mixed cultivation together with tall-growing crops such as pearl barley, sorgo, maize, cotton, hibiscus (Hibiscus sabdariffa), manioc and sunflowers; and as an under-sown crop together with such other crops as bananas, pigeon peas (Cajanus cajan), Gliricidia sepium, castor beans (Ricinus communis), sugar cane or permanent crops such as coconut palms, oil palms, rubber and cocoa.
When their vegetation periods are similar in length, other crops cultivated together with peanuts will benefit by a gain in yield (in contrast to monoculture sites, peanut and pearl barley). Even greater increases in yield have been observed for other crops such as cotton –which has a vegetation period which is up to 3 months longer. The success of cultivating in a mixed system, or in agro-forestry systems, is dependent largely upon choosing site appropriate crops, the way the combination gels and from the correct placement of the peanut in the chronological order of the various species. For example, cultivation in an agro-forestry system must always be at the beginning of a cycle (planting within the first 3 years may be possible – according to which other crops are grown), because the plant’s natural attributes preclude it from being planted.
The most important ways of avoiding diseases are:
- Crop rotation;
- Choosing the right variety;
- Sufficient supply of nutrients;
- Uprooting infesting plants to stop the disease spreading;
- Destruction of any infested plant parts after the harvest.
Harvest and post-harvest handling
Time of harvesting
Because the leaves are still green when the pods are already ripe, occasional uprooting must be performed to ascertain the optimum harvesting date. Bush varieties mature in 110-130 days after sowing, branching varieties in 130-150 days. The individual seeds are ripe when:
- The structure of the pods is easily recognizable;
- These have been largely filled by the seeds within;
- The inner walls of the pods have taken on a darker color (brown). The testae have then attained the typical color for their variety.
As soon as 60-70% of the pods are ripe, any further delays in harvesting will result in losses. The optimum period for harvesting is very short. If this is not kept to, and harvesting is begun either 5-10 days before this time or after it then up to 25-50% of the production may be lost. Harvesting too late especially in hard and dry soils, results in pegs breaking off as they will already have become too brittle.
The pods from freshly harvested plants still have a moisture content of around 35-50%, and need to be rapidly dried to a moisture content of around 20-25% so they may be easily separated from the plants. The best method is to pre-dry them in windrows for 2-3 days. After the tap roots have been cut away, the plants are stacked on their leaves with the pods facing downwards.
The advantages are:
- Rapid drying;
- Avoidance of contact with the soil;
- A reduction of attacks by insects and the risk of infestation by Aspergillus spp.
The quicker the pods are dried after being uprooted, the less aflatoxin is created. Nevertheless, care must be taken not to dry them too quickly, as this could result in a weakening of the testa, which protects the seeds from decay.
Manual harvesting is still widely practiced in many countries where it is more profitable, because fewer pods are left in the soil, and they are also less likely to be damaged. Planting on ridges, in hard soils, can make the harvest easier. The plants can be extracted by hand with a hoe; this can be made easier still by cutting below all of the rows with special uprooting blades (pulled either by tractor or animals). In light soils, harvesting machines, similar to those used for potato crops can be used. For heavy soils, use special peanut uprooters. Purely mechanical harvesting is also often carried out in two stages, initially drying the peanut plants in windrows to reduce the amount of drying necessary in the artificial drying processes. The first machine uproots the plants, shakes off the soil and lays them on the ground upside-down, similarly to manual harvesting. In the second stage, a threshing machine picks up the pre-dried windrows.
Post harvest treatment
Foliage from peanut plants provides excellent, protein-rich fodder, with similar nutritional values as lucerne (alfalfa), and is therefore also harvested. It can be cut down just before the uprooting takes place and dried into hay. One method of carefully drying after the whole plant has been harvested is to dry the foliage on hay racks, or upright poles, after it has been left to wilt for a short while on a windrow. Otherwise, the valuable leaf parts can get lost.
After being dried out in the field, the pods are separated from the plants. The best results are gained when the moisture content is between 20-25%, because then the pods can be separated easily and completely. If the content is lower, then the pods and seeds will be more susceptible to damage. The best method is to separate them by hand. Sometimes, the pods are carefully beaten off with a stick, or stationary threshing and pick-up threshing machines are used.
Directly after being threshed, the pods are dried, either artificially or in the sun, until they attain a moisture content of 6-7%. Delays must be avoided at all costs to reduce the risk of infestation by Aspergillus flavus, which rapidly increases. Under 9%, the creation of aflatoxin is retarded, but protection against storage pests, whose activities create aflatoxins, is only attained at levels under 7%. For this reason, a moisture content of 6-7% is necessary. Problems can often occur when the harvest is carried out during wet weather, and the produce isn’t sufficiently dried afterwards. Only at less than 6% will the seeds become damaged (they break during shelling).
In cases of extreme sunshine and heat, it may be advisable to carry out the drying process under a roof. The weight loss during drying is compensated for by an increase in quality and a reduction of the risks involved. In regions where insufficient sunshine is available to dry out the seeds after harvesting, conditions need to be created to artificially dry the produce, to reduce any post-harvest losses and the risk of toxicity (mobile dryers).
Because usually only a few seeds are infected with aflatoxin, sorting is an effective and important preventative measure after the harvest. Strongly infected pods and seeds are either highly discolored, or will have shrunk. They can either be sorted manually or mechanically; electronic colour sorting has made it possible to sort out practically all of the aflatoxin infected and bad seeds.
The main factors which need to be observed in storing are; a low moisture content of the seeds (see: drying) and low ambient temperatures. High seed and/or room moisture content, coupled with high temperatures, are the main reasons for the creation of aflatoxins. Prevention is achieved by:
- Sufficient air circulation;
- Regulation of the relative air humidity;
- Suitable cooling;
- Sorting out the damaged and discolored pods before they are stored.
Unshelled peanuts are far easier to store than shelled, because the protective testa remains intact. If they are not to be sold in the pods, they should be shelled only immediately before being sold.
Storage pests: Most storage pests reach the seeds through a broken pod or testa, which means that careful sorting provides good protection. One exception to this rule is Attagenus fasciatus, one of the few storage pests that bore into the pods. Some varieties have formed a special resistance against storage pests due to protective substances in their pods and testa. Adding clay powder helps against Corcyra cephalonica. Most storage pests cease their activities when moisture content of the seeds is below 7% and 20% air humidity.
The aflatoxin problem
Peanuts are extremely susceptible to infection by the fungi Aspergillus flavus. The poison aflatoxin is created by fungi of the species Aspergillus flavus and Aspergillus parasiticus, which are widely disseminated in tropical and subtropical soils. All cultivation measures should be well-planned, to take this aspect into consideration. Aflatoxin in food can affect the health of both man and animal. Importing countries have set maximum tolerance values for the presence of aflatoxin in foodstuffs; to protect consumers.
For consumers in the producing countries, the risks due to the poison are more difficult to ascertain, because the larger part of the peanut harvest is consumed or sold on local markets. Thereby, there are also no “dilution” effects caused by large quantities, meaning that both humans and animals can be subjected to high doses of aflatoxin. Additionally, malnutrition leads to immune systems being weakened. By heeding the preventative measures outlined, economic or health problems regarding an aflatoxin infection should not even arise.
Infection before the harvest
The fungus penetrates the pods during the growth period whilst still in the soil and it takes place in two ways:
A) Infection through invisible damage to the pods or seeds
Mechanically damaged or bitten pods will quickly be infected by the fungus, which feeds primarily on dead and dying tissue. Hot, dry soil conditions abet attacks by termites, which are vectors for the fungi’s spores. Alternating phases of rain and drought causes the pods to split open, and producing high aflatoxin values in the seeds.
B) Invisible infections of the pods
Many pods are infected after the pegs have been pushed down into the soil. Yet when the plant enjoys good growing conditions, the fungi may remain inactive and no significant amounts of aflatoxin are produced; this is because peanut plants have a natural protection mechanism: The growing plant produces immune substances (phytoalexin), which have an anti-microbe and fungus-suppressing effect (arachidin). All cultivation measures that encourage healthy, natural growth in effect support this protection mechanism.
The production of phytoalexins decreases towards maturity, as a result of water deficiency, and ceases altogether if a drought continues. In contrast, the fungi A. flavus is still able to proliferate and create aflatoxin at much drier conditions, before it also finally ceases all activity. It is encouraged at average temperatures of 26-30°C in the upper 5cm of the soil. In dry periods, the peanut plant folds its leaves together, meaning that the ground receives even less shade, resulting in sudden increases of soil temperatures. In hot, dry conditions, A. flavus grows very rapidly, possibly due to its enemies dying off, for they breed best in hot, humid conditions, when they can keep A. flavus under control.
Sufficient irrigation is an effective way prevent the production of aflatoxin (especially during the final 4-6 weeks of the vegetative period), even when the soil temperatures are perfect for A. flavus, and 50% of the pods are infested.
Production of aflatoxin after harvesting
When the fungus has actually penetrated, it can create aflatoxin during the drying, transport and storage processes. Humidity and temperature are the two main factors. Even manufactured goods, such as peanut flour, are at risk. Although detoxification methods exist, the best approach is always prevention.
These quality requirements for peanuts, with their minimum and maximum values, are generally issued by the authorities or importers. Yet agreements may be reached between individual manufacturers and importers upon different values, providing they still conform to official requirements. Quality requirements Minimum and Maximum values Appearance Specific according to Quality grade Taste and smell Variety-specific, fresh, not moldy Purity Free of foreign substances (sand, stones, shell rests, insects etc.) Water Max. 5.0% Peroxide units Max. 1.0 milli-equivalent peroxide per kg fat Free fatty acids Max. 0.5% Residues Pesticide Not measurable Bromide and ethylene oxide Not measurable Heavy metals Lead (Pb) Max. 0.50mg/kg Cadmium (Cd) Max. 0.10mg/kg Mercury (Hg) Max. 0.03mg/kg Micro-organisms Total organisms Max. 10,000/g Yeasts and mold fungi Max. 500/g Enterobacteriaceae Max. 10/g Coliform Max. 10/g Escherichia coli Not measurable Staphylococcus aureus Max. 100/g Salmonella Not measurable in 25g Mycotoxins Aflatoxin B1 Max. 2 Zg/kg Aflatoxins B1, B2, G1, G2 Max. 4 Zg/kg