Cocoyam as Animal Feed 3: Production and nutrient composition of cocoyam leaves
Published Date: 27th July 2020
Cocoyam leaves appear in clusters of long arrowhead shapes that point towards the earth and grow a few meters high on erect stems. They are attached to the long, fleshy leafy stalks or petioles that are green, red, or purple in color, and attached slightly off the center of each leaf blade. Cocoyam leaf, as a green vegetable forms an important part of the diets of rural people at many tropical locations where the crop is cultivated. It is the fourteenth most consumed vegetable worldwide. It is consumed freshly boiled or used as a vegetable to supplement starchy meals or soups in many parts of West, and Central Africa, especially Ghana, southern Nigeria, and the Cameroons. The leaves may also be harvested, and preserved by sun-drying for use during the dry season when vegetables are generally in short supply. In Ghana, cocoyam leaves are produced on a subsistence basis, and itinerate pickers who may not be farmers dominate the harvesting and marketing. Some cocoyam species such as taro (Colocasia) and tannia (Xanthosoma), also produce edible inflorescence, which is usually surrounded by stem leaves of the corm. In these species, the emergence of inflorescence usually marks the maturity of the cocoyam, and are readily harvested as a vegetable. In South America and some parts of Asia, it is used in dressing salad, while it is consumed as a delicacy in southern parts of Nigeria. Traditionally, fresh cocoyam inflorescence is used as a vegetable in soup and yam porridge or may be dried, milled, and used as spices in soup preparations to impact desirable color and flavor.
In recent times, however, there has been a steady decline in cocoyam production at its traditional farming locations in West and Central Africa, and the leaves have become underutilized probably because of the change in culinary habits that accompany urbanization. It is therefore common to see previously edible cocoyam varieties growing in the wild as weeds, which are cleared during farm preparation. At waste dump sites and moist areas, Xanthosoma species grow naturally and yield a large quantity of above-ground biomass in the form of leaves and petioles. The plants have 5 - 7 green leaves each time, and the sizes of the leaves are usually largest during the rainy season. New leaves emerge every 2 - 3 weeks, and eventually, die and fall to the ground after some weeks. These leaves could be harvested and used as alternative feed raw material for animals, especially at locations where they are abundant. Indeed, cocoyam leaves are among the numerous foliage used traditionally by smallholder farmers in many tropical countries to feed animals, due to their richness in essential nutrients. In Colombia, under intensive pig farming system, Xanthosoma cocoyam leaf has been found to be a very promising forage for its re-growth capacity, high biomass yield, and palatability.
Cocoyam forage yield
Taro (Colocasia esculenta) has been reported to yield up to 370 tons/ha/year of foliage, made up of leaves and petioles. Hang and coworkers however reported yearly C. esculenta foliage estimate of approximately 200 tons/ha, in Vietnam, with the leaves representing 50 percent of the foliage dry matter. Taro leaves are rich in protein, while the petioles are rich in soluble carbohydrates. An important study at the Ecological Farm (TOSOLY) of the UTA Foundation, Colombia reported the yield potential of tannia cocoyam (Xanthosoma sagittifolium) in experimental plots. The plant spacing adopted was 70 cm between rows and between plants in the row. Leaves and petioles were harvests at approximately 30-day intervals for about 20 months. Fresh biomass (leaves and petioles) yield was equivalent to 128 tons/ha/year, 14.5-ton dry matter, and 1.9-ton crude protein/ha/yr. On a dry matter basis, the leaves accounted for 60 percent of the biomass and 87 percent of the crude protein, while the petioles made up the rest. Research has shown that cocoyam leaves have high protein content that is readily degradable, and are good sources of minerals, and vitamins A and C. They contain many more proteins than the corm. The leaves of most cocoyam varieties however taste acrid and can cause sharp irritation and burning sensation of the lips, mouth, and throat, similar to what obtains with the corm when eaten raw. There is therefore the need to enhance their nutritive values using processing methods that readily eliminate the anti-nutrients.
Nutrients composition of cocoyam leaf meal
It has been reported that cocoyam leaves are very high in moisture content, rich in proteins, vitamins, and minerals. Fresh leaf and petiole of X. sagittifolium together have been reported to weight 450 to 650 g, with leaf component accounting for 40 percent and the petiole 60 percent. The average dry matter value of the fresh leaf is 11 percent, indicating its high moisture content. Studies at the Silpakorn University, Phetchaburi, Thailand, showed that taro (C. esculenta) leaves on analysis yielded, 9.2, 29.7, 4.3, and 15.2 percent ash, crude protein, ether extract, and hemicellulose contents respectively. Other studies have reported 12.4 percent crude fiber and a range of 16.0 – 26.0 percent crude protein as influenced by the rate of cutting, while the calorific value was 20.3-kilocalories per 100 grams. It has also been reported to contain many high levels of starch and total soluble sugar than most tropical forages, but similar in organic matter and fiber contents.
Studies conducted at the Ecological Farm (TOSOLY) of the UTA Foundation, Colombia reported the proximate composition of X. sagittifolium to be 24.8, 13.3, and 14.2 percent crude protein, ash, and crude fiber respectively. The values of calcium, phosphorus, potassium, and magnesium have also been reported as 1.8, 0.2, 3.2, and 0.2 percent dry matter respectively, indicating the need to provide additional sources of phosphorus when the leaves are to be fed to monogastric animals. The protein in X. sagittifolium has also been reported to be rich in lysine (46 g/kg crude protein), methionine (27.2 g/kg crude protein), cysteine (12.2 g/kg crude protein), methionine+cysteine (26.9 g/kg crude protein), and threonine (49.5 g/kg crude protein). It, therefore, contains higher threonine value than soybean, while the methionine+cysteine value was up to the ideal protein value. A major limiting factor in the X. sagittifolium leaves is however the low digestibility of the protein.
Researchers at the Federal University of Technology, Owerri, Nigeria, analyzed the proximate and mineral compositions of the X. mafafa leaf meal as a possible feedstuff for poultry. Crude protein, ether extract, crude fiber, total ash, and nitrogen-free extract values were reported as 28.5, 15.2, 3.5, 7.8, and 35.3 percent respectively, indicating relatively high crude protein and ether extract values. The leaf meal was also found to be exceptionally rich in calcium, and moderate in magnesium, potassium, phosphorous, and sodium concentrations. Another study at the Federal University of Technology, Owerri, Nigeria, analyzed the nutrient compositions of the mature inflorescence of X. sagittifolium and C. esculenta. On dry matter basis, X. sagittifolium inflorescence was found to contain much higher crude protein (37.9 percent) than C. esculenta (22.56 percent), although the values show that both flowers are rich in protein. The ash and crude fat contents were however similar at values of 6.23 - 6.61 and 13.13 - 14.05 percent respectively. C. esculenta on the other hand recorded much higher crude fiber (15.32 percent) and total carbohydrate (25.11 percent) contents than X. sagittifolium (12.92 and 13.75 percent respectively), indicating that cocoyam inflorescence is high in protein but low in carbohydrate. This study particularly highlighted the fact that X. sagittifolium inflorescence contains much higher crude protein than any cocoyam leaf meal.
Anti-nutrients in cocoyam leaf meal
The realization of the nutritional potential of cocoyam leaf is usually adversely affected by its anti-nutrient content, which limits its use in the raw form. Similar to the situation in the roots, the presence of calcium oxalate crystals in cocoyam foliage, is chiefly responsible for its acridity. Holloway and coworkers report the total and insoluble oxalate content of the leaves of nine different cultivars of taro grown in Fiji to ranged from 278 to 574 mg/100 g wet matter, with the edible leaves generally containing lower levels of total oxalates than the inedible ones. Researchers at Osun State Polytechnic, Iree, Nigeria, reported that raw C. esculenta leaves contain anti-nutrients such as oxalic acid (55.2 mg/kg), tannins (34.5 mg/kg), and phytates (27.0 mg/kg), indicating that oxalates are the major anti-nutrients in raw cocoyam leaves. Total oxalate content of C. esculenta foliage grown in Vietnam ranged from 115 to 362 mg/100 g wet matter, indicating significant differences in cultivars growing in different tropical regions. Research at the Federal University of Technology, Owerri, Nigeria, has also reported that raw X. maffafa leaves contain high levels of tannin, trypsin inhibitor, flavonoids, and phenols but low levels of cyanide, alkaloids, and phytate.
Again, the X. sagittifolium and C. esculenta inflorescence studies at the Federal University of Technology, Owerri, Nigeria, showed that fresh cocoyam inflorescence is also associated with irritating sensation in the mouth and throat due to the presence of oxalate salts, and is the limiting factor to their consumption. Other phytochemicals found in the inflorescence were alkaloids, flavonoids, glycosides, phenols, saponins, steroids, and tannins, with C. esculenta containing more alkaloids than X. sagittifolium (9.80 and 6.22 percent respectively), while it was the opposite for saponins (6.61 and 5.50 percent respectively). Analysis of petiole sap from X. sagittifolium revealed that the saponins content was 4.71 percent, flavonoids 0.51 percent, alkaloid 4.31 percent, tannin 1.25 percent, and glycoside 527.5 ppm, while C. esculenta petiole contains 4.20 percent saponins, 1.80 percent flavonoids, 5.50 percent, 1.16 percent tannins, and 865.8 ppm glycosides. These results indicate that there are more anti-nutrients in the inflorescence than the petioles.
The effects of these anti-nutrients usually include reduced feed intake, digestibility, nutrient utilization, and weight gain, while the acidity is caused by needle-like oxalate crystals. Indeed, a significant amount of the total calcium in cocoyam foliage may be locked up as insoluble calcium oxalate, leaving very insignificant amounts of free calcium in the leaf tissue. The chief limitation of the use of cocoyam leaves or inflorescence as a vegetable for humans or feedstuff for livestock, therefore, is the presence of oxalates, which can form non-absorbable salts with Ca, Fe and Mg, rendering these minerals unavailable.
Processing to improve the nutrient value of cocoyam leaf meal
Different processing methods such as drying, boiling, and fermentation have been used to reduce the effects of the anti-nutrients and improve the nutritional value of cocoyam leaf. As human food, cocoyam leaves are generally, boiled, blanched, steamed, stewed, fried or pressure cooked to improve their digestibility, nutrient availability, and minimize its poisonous effects. Simple heat treatment methods such as boiling and blanching have been used to effectively process cocoyam leaves for animal feeding. Blanching is the cooking process in which a food substance usually vegetables or fruits are scalded in boiling water and removed after a brief time interval. Materials are blanched to soften them, partly or fully cook them, to remove or reduce the presence of toxins and improve their palatability, digestion, and utilization as food or feed.
For example, the blanching of C. esculenta leaf at 80OC for 5 minutes was reported to result in a substantial reduction of its oxalate, phytate, and tannin contents and improvements in carbohydrate and crude protein levels. It has also been reported that there is not much difference in the total soluble solids in steam blanched and hot water blanched cocoyam leaf samples. Research at the University of Fort Hare, Alice, South Africa, however, showed that boiling for 5 minutes significantly reduced the phosphorus, potassium, and zinc contents of leaves of different C. esculenta accessions, while calcium, magnesium, sodium, and copper levels were not significantly affected. The iron levels in all the samples were particularly increased by the boiling process. Again, boiling for 5 minutes resulted in a 16 – 78 percent reduction in oxalate content, 28 – 61 percent in tannin, and 17 – 41 percent in phytate contents. Boiled C. esculenta leaves may, therefore, serve as good iron sources in the diets of a weaned pig to aid optimal blood formation and growth.
Blanching with an electrolyte solution is also practiced, and usually helps to minimize the period of heat exposure and nutrients losses from the material. An interesting study was carried out at the Federal University of Technology Owerri, Nigeria, to determine the effects of blanching with different concentrations of palm fruit bunch ash derived electrolyte solution on the proximate, mineral and phytochemical contents of X. mafafa leaves. Ash solutions of 0.25, 0.5, and 0.75 percent were used to blanch the leaves at 100OC for 3 minutes. The blanched leaves were allowed to cool in the hot solution before decanting the solution and drying the leaves. Ether extract and crude fiber contents decreased with increasing ash electrolyte concentration, while the total ash and nitrogen-free extract increased. There was a very high increase in the potassium levels, while calcium, magnesium, and sodium increased marginally. Phosphorus levels, however, decreased with increasing concentration of the electrolyte solution. The treatment also resulted in a significant decrease in tannin, phenol, cyanide, trypsin inhibitor, and alkaloids contents up to 0.5 percent electrolyte concentration, while carotenoids, flavonoids, and phytate concentrations increased. These results showed that blanching with ash electrolyte solutions could be used to improve the nutritional value of cocoyam leaves.
Researchers at the Ecological Farm (TOSOLY) of the UTA Foundation, Colombia have developed a simple method of enhancing the feeding value of cocoyam leaves. Whole leaves including petioles of X. sagittifolium were harvested and subjected to size reduction by mechanical maceration into very small particles. The macerated leaves were thoroughly mixed and ensiled in air-tight rigid plastic containers for 7 days at tropical ambient temperature. The silage produced was of good quality as judged by smell and color. Information on the nutrient composition of the ensiled leaves was however not provided. Researcher at Makerere University, Kampala, Uganda, also studied four processing methods (boiling, ensiling, soaking, and wilting) of reducing oxalates levels in X. sagittifolium leaves. The leaves were macerated, mixed with sugarcane molasses at a level of 4 percent (fresh weight basis), and ensiled in airtight plastic bags at average room temperature (26oC) for 21 days. The chopped leaves were wilted under the sun (44oC) for 3 hours, while soaking was done in tap water at room temperature for 24 hours. Boiling was achieved by placing the macerated leaves in boiling water for 1 hour. All the treated samples were subsequently oven-dried at 60oC. Wilting reduced the total oxalate content of the leaves by 21 percent after 3 hours, while boiling reduced the level by 52 percent, ensiling by 44 percent and soaking by 42 percent, indicating that boiling, ensiling and soaking were better methods of reducing the total oxalate content in cocoyam leaves.
Enzyme treatment has also been used to improve the nutrient content of taro leaves. To achieve this, researchers at Silpakorn University Phetchaburi, Thailand, dried macerated taro leaves in a hot air oven at 60°C for 24 hours, and subsequently treated them with the commercial enzyme (Hemicell®) at 1 percent (w/v) inclusion level before incubation at 55°C for 6 hours. The enzyme-treated taro leaves recorded lower fat and hemicelluloses contents (2.96 and 14.55 percent respectively) than the fresh leaves (4.31 and 15.18 percent respectively). Enzyme treatment however resulted in significantly higher values of reducing sugar (29.78 and 6.23 mg/g, respectively), while crude protein and cellulose contents were not affected by the treatment. The study, therefore, shows that enzyme treatment effectively breaks down the fiber content of taro leaves resulting in a reduction in fiber content and an increase in reducing sugar content.
Large quantities of cocoyam leaves are produced annually in many tropical regions. These leaves have traditionally been fed to animals in many tropical countries, due to their high nutrient values, although extensive use is limited by the presence of anti-nutrients in the leaves. Boiling, soaking, and fermentation have been shown to be the best methods of reducing the effects of the anti-nutrients and improving the nutritional value of cocoyam leaf meals. Feeding trials with processed cocoyam leaf meal in monogastric and ruminant animals will be discussed in the next article.
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