The peanut plant (Arachis hypogaea L.) is grown mainly for its seeds, which are used either as food (snacks, peanut butter, etc.) or for their edible oil. After peanuts are harvested, aerial parts of the plant become available in large quantities and are used fresh or dried as a nutritious livestock feed in all peanut-producing countries.
As with other legume crop residues, the terminology used to name peanut crop residues are numerous: they are called haulms, vines, tops, stover, hay or straw, depending on the country or local tradition. While these names may not always be exact synonyms (e.g. peanut hay can be made out of vegetative plants rather than from the mature plants remaining after the harvest), they are used quite interchangeably in the literature. In American English the term peanut hay often describes what is a typical crop residue, though it can also describe an actual hay.
Morphology and cultivation
Peanut is an annual herbaceous plant growing to a length of 30 to 50 cm. The plant can be erect or prostrate with a well developed taproot and many lateral roots and nodules. The leaves are opposite and pinnate with four leaflets; each leaflet is 1 to 7 cm long and 1 to 3 cm across. The flowers are 1.0 to 1.5 cm across, bright yellow or yellowish orange with reddish veining. They are borne in axillary clusters on the stems above ground and last for just one day. 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. 8 to 14 days after pollination, a short stalk at the base of the ovary elongates to form a thread-like structure known as a "peg". This pushes the ovary down 5 to 8 cm into the soil, where it develops into a pod. Pods are 3 to 7 cm long, normally containing one to four seeds. 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. Flowering continues over a long period, and at harvest pods are in all stages of development. Pegs will eventually rot in the soil and the resulting loose pods are lost during harvest (Putnam, 1991).
Peanut crop residues
Peanut crop residues consist of leaves, stalks (vines) and remaining pods left in the field after the peanut harvest. There is a considerable variation in quality, depending on the harvest method, storage and on the proportions of plant materials included in the residue. Like other legume hays, peanut forage is subject to leaf shattering, which increases the proportion of stems and diminishes its nutritional value (Myer et al., 2010). Peanut crop residues can be fed fresh, dried or ensiled (Hill, 2002). Depending on the livestock production system, peanut crop residues can be used as a supplement or as a sole feed (Etela et al., 2011). The peanut crop yields large amounts of good quality forage and is an important, and sometimes major, provider of fodder wherever it is grown. Dual-purpose peanut varieties capable of producing appreciable quantities of both grain (peanuts) and good-quality hay are being developed and disseminated in Africa and Asia (Etela et al., 2011).
In the USA, peanut hay is produced throughout the peanut belt, and is fed as a winter feed supplement to growing stocker cattle, beef replacement heifers, and wintering cow herds. In autumn, if harvesting conditions are good, large amounts of peanut hay can be baled in a few weeks, and the production costs are cheaper than for grass hays. In drought years, peanut often becomes the primary hay source for cattle farmers. However, it is only made on 25 to 40% of the total peanut acreage, as many peanut producers, particularly those who do not have to feed cattle, forget hay production and incorporate the residual vines as organic matter (Hill, 2002).
In West Africa, peanut haulms are extensively fed to ruminants, especially in the dry season. Smallholder crop-livestock farmers consider forage and seed value with equal weightings. In the subhumid zone of West Africa, farmers prefer late-maturing cultivars to early-maturing types as the former provide more forage for livestock. Furthermore, the sale of forage is a major source of household income (Larbi et al., 1999). Crop residues such as peanut or pigeon pea haulms have been shown to provide up to 80% of livestock feed in densely populated areas of Nigeria (de Leeuw, 1997). In the Gambia, peanut hay is the traditional feed resource of choice, especially in urban areas. It is transported into peri-urban livestock facilities, where it is used in zero-grazing systems during the dry season of about nine months (Asaolu et al., 2010).
In India, peanut crop residues are widely used to feed livestock (NDDB, 2012). A study, conducted in villages with dairy enterprises in the Deccan Plateau, concluded that grain/pods and crop residues (sorghum and groundnut) almost equally contributed to the feed resource in mixed-crop livestock systems. In some villages, peanut hay comprised between 40 and 80% of the crop residues. In peanut-based crop-livestock systems, peanut hay was used for 8 months of the year in non-intensive dairy villages, and all year round in intensive dairy villages (Devi et al., 2000).
Peanut grown for forage
Peanut is now rarely specifically grown for forage, as the seed crop is much more valuable, though this was common in the first half of the 20th century in the USA (Sheely et al., 1942; Gobert et al., 1994; Myer et al., 2010). The rhizoma (or perennial) peanut (Arachis glabrata), a similar species that produces few seeds, is grown for this purpose (French et al., 2006; Hill, 2002). In the USA, there have been some promising efforts, but not yet conclusive, to again use peanut as a forage crop only (Myer et al., 2010).