La culture de l'amarante, légume-feuilles tropical : avec référence spéciale au sud-Dahomey

1. INTRODUCTIONAmaranth is the most cultivated leaf vegetable of the humid tropics and as such comparable to spinach in the temperate regions. Efforts to improve the crop are hampered by a shortage of knowledge. The object of this study is to fill up a part of this gap. The experimental data were gathered at a horticultural extension centre in Dahomey (Centre de Formation Horticole et Nutritionnelle, Porto-Novo) and at the Agricultural University of Wageningen.2. THE CULTIVATION OF AMARANTH IN SOUTH-DAHOMEYThe densely populated southern part of Dahomey has a humid tropical climat. The dry season lasts from December to March. On the compounds and in the fields, amaranth is found almost exclusively in the rainy season (April-November) but the marketgardeners around the big cities cultivate this crop under irrigation throughout the year. In the main rainy season (May-July) harvests are low as a result of an excess of diseases and pests and probably also by leaching of the soil and by a lower light intensity.The following cultivation methods can be distinguished:a. transplanting with narrow spacing (< 10 x 10 cm) and harvesting after 3 to 4 weeks by uprooting,b. transplanting with wide spacing (>20 x 20 cm) and harvesting by repeated cuttings, during 2 to 3 months.Direct sowing sometimes is practised as mixed cropping with food crops and in compounds, but not by the market-gardeners. Objections against this method are the longer occupation of the plot, weed growth, and a product with a heterogeneous quality.While growing amaranth, the gardeners have to contend mainly with the following problems:1. to obtain good quality seeds2. to choose the most suitable cultivation method (spacing, havest time, frequency and height of cutting)3. to improve and maintain the soil fertility4. to control diseases, pests and weeds. See table 1-4 and figure 1.3. CONSUMPTION AND FOOD VALUEThe consumption of leaf vegetables in the humid tropics amounts to about 25 g per person per day net weight or 40 g of rough product. Extreme amounts of 250 g occur. Nutritionists advise a daily intake of 100 g for balancing the usually one- sided menu in the tropics. In Dahomey and in many other regions, about half of the consumption of leaf vegetables consists of amaranth.Leaf vegetables are rich in food-stuffs, especially provitamin A, vitamin C, iron and calcium, while also the protein content is quite important, especially for children who do not receive enough animal protein. When calculated on dry matter base, the food value of different species of dark green leaves does not show much variation. The dry matter content in itself is important, as it may vary from 15 % (for instance Amaranthus cruentus) to 6 % (for instance Basella alba). Loss of food- stuffs (like vitamin Q can be kept low, if the vegetables are cooked with very little water and during a short time and if no potash but salt is used. Throwing away the cooking liquid is often unavoidable.If the consumption does not exceed 100 g per person per day, oxalic acid and free nitrates in amaranths are most unlikely to be harmful.See table 5-9 and figure 2.4. BOTANY AND GEOGRAPHYThe collective name 'amaranth' for this group of leaf vegetables comprises all species and cultivars of the family of Amaranthaceae, principally Amaranthus cruentus L., A. dubius Mart. ex Thell., A. tricolor L. and Celosia argentea L. The taxonomic distinction between species is difficult and many synonyms exist. The grain-amaranth, still cultivated in tropical mountanous regions of America and Asia (A. caudatus L., A. hypochondriacus L. and A. cruentus L.) are also used as leaf vegetables.See figure 3 - 8.5. GROWTH ANALYSESAmaranthus cruentus and Celosia argentea both appear to be quickly growing plants. The former differs from the latter by a somewhat faster initial development, a higher dry matter content, a shorter generative phase and a higher seed production.Harvesting by uprooting yields the highest leaf production about six weeks from transplanting, when the plants become generative. Gardeners commonly harvest some weeks earlier because then quality is better (less and more tender stems). In a field trial Amaranthus produced 2.7 t of leaves (dry matter) per ha in six weeks, or 6.4 g.m-2.day -1and Celosia 2.3 t or 5.5 g.m -2.day -1. This leaf production might be increased to maximally 3.5 t in four weeks by various cultivation measures. When the leaves reach a closed cover with a leaf area (LAI) of about 7 times the surface of the soil, the bottom leaves will drop and no increase in total leaf weight can be obtained.Harvesting by regular cuttings causes that the plant tries to restore the lost leaf area after the cutting. To obtain a high total production the choice of a suitable spacing and harvest method (see ch. 7) and the prevention of early flowering are important (ch. 6).See table 10-13 and figure 9-16.6. ECOLOGICAL INFLUENCESThe photosynthesis of leaves of Amaranthus cruentus and of Celosia argentea was measured. The optimum temperature for Amaranthus was not reached at 40 °C whereas Celosia showed no increase of the photosynthesis rate above 35 °C. At 0.45 cal.cm -2.mn -1, the highest measured light intensity, and at 40°C, Amaranthus was not yet light-saturated, as opposed to Celosia. The photopressed in&nbsp;μg of CO 2 per cm 2of leaf area. When expressed in&nbsp;μg of CO 2 per mg of leaf dry matter, the photosynthetic rate of Amaranthus at low light intensities showed somewhat lower values than Celosia and at high light intensities higher values than Celosia. The transpiration coefficient of Celosia proved to be much higher than that of Amaranthus. Celosia has a lower diffusion resistance for water-vapour but a higher resistance for CO 2 . The conclusion that Amaranthus cruentus is a C4-cycle plant and Celosia argentea a C3-cycle plant, was confirmed by a study of the leaf anatomy.A number of cultivars, tested for day length reaction, were indifferent or behaved like qualitative or quantitative short-day plants. The early flowering of Amaranthus in Dahomey probably is not mainly a photoperiodical symptom, but more a result of waterstress, the use of too old seedlings for transplanting and cutting at a too high level during the first cutting.In relation to the frequent presence of palm trees in the fields of Dahomean gardeners, the influence of shade on leaf production was studied. Shade appeared to be disadvantageous. The decrease of production by shade is possibly worse for Amaranthus than for Celosia.See tabel 14-26 and figure 17-22.7. PLANT SPACING AND CULTIVATION METHODIf harvest is done by uprooting after 3 to 4 weeks from transplanting, a high planting density of about 100 plants per m2 will be desirable. Often the gardeners plant at denser spacing because of the frequent thinning out by a fungus disease Choanephora. If repeated cuttings are wanted, a number of 20 to 40 plants per m2 will be adequate. The first cutting should not be too low, for this impedes regrowth. A height of 20 cm appears to be suitable. If cutting at a higher level is practised early flowering may occur. A frequency of cutting every three weeks appears to be better than every two weeks.Both cultivation methods have advantages and disadvantages. For harvest by uprooting the labour intensive transplanting is a handicap, but weeding will not be necessary. If harvested by cuttings Choanephora may cause a lot of damage.See table 27-30 and figure 23-27.8. SOIL AND FERTILIZINGAmaranth is a crop which requires a high level of soil fertility and which is exhausting for the soil. A number of field trials have shown that amaranth has a good response to NPK-fertilizing with a high potassium content. Fresh town refuse appeared to be an excellent type of organic manure. Although the composition is very variable, this fresh town refuse generally has a high fertilizing value and a low C/N quotient. Composting appeared to be intensive in labour requirement and led to an important loss of minerals, especially of N, and a loss of C. On poor and moderately fertile soils, the application of 400 kg/ha of NPK (10-10-20) and 25 t/ha of town refuse is recommended. Without organic manure the duration of fertilizer influence will be shorter and leaching of part of it is risked.Split application of fertilizer may show good results, especially in the rainy season, on poor soils, and during a long cultivation period. Half of it may be applied during preparation of the plot, the other half during the growth period. It is applied in granular form or dissolved in the irrigation water.In Dahomey, the manuring of successive crops of amaranth and other vegetables during many years, with high applications of about 50 t/ha per cultivation has converted poor sandy soils into good soils for gardening. See table 31-51 and figure 28-29.9. DISEASES AND PESTSIn Dahomey, Amaranthus cruentus L. has been cultivated for years on the same plots without any rotation. This is possible because the plant is not affected by the common soil diseases which affect many other vegetables. The worst diseases and pests are:a. damping-off, caused by Pythium aphanidermatum (Eds.) Fitzp. Control by cultivation measures: sowing not too densely (2 g/m 2), no shade above the seedbed, good manuring with organic matter, good drainage. Celosia is less affected than Amaranthus.b. wetrot caused by Choanephora cucurbitarum (Berk. et Rav.) Thaxter, a fungus which may cause a lot of damage under wet conditions, especially after transplantation and after a cutting. The infection is less concentrated in spots than that of Pythium. Control measures: dense planting (open places of dead plants will then be filled up by neighbouring plants), no shade, good manuring, no scattering of woodash on plants, no ammonium sulfate in the sprinkling water. Weekly treatments with 3 kg/ha of maneb with carbatene may prevent damage. Celosia is less sensitive.c. caterpillar damage caused by Hymenia recurvalis F.; control with bromophos, carbaryl or lindane was satisfactory.d. stemborer Lixus truncatulus F. is the most noxious if it attacks young plants, less than four weeks old. Repeated treatments with the insecticides enumerated under c. appeared to be effective. It is recommended to use seedlings not older than three weeks for transplanting. Celosia is not affected.e. Meloidogyne incognita Chitwood is a serious pest of Celosia. Most Amaranthus cultivars appeared to be bad hostplants, therefore this crop is suitable for use in rotation schemes with sensitive vegetables. After soil fumigation with DD and a crop of Celosia the Meloidogyne population showed a quick recovery.See table 52-56.10. WEED CONTROLMost weeds can easily be controlled by weeding. An exception is Cyperus rotundus L., which has spoiled many plots of gardeners. In a control trial executed during the rainy season, weekly weeding appeared insufficient. A double spraying with glyphosate destroyed 95 % of the Cyperus tubers.See table 57-58.11. BREEDINGAll the tested Amaranthus species are monoecious wind pollinated plants. The hermaphrodite flowers of Celosia are pollinated by insects. In the field the percentage of self fertilization might be rather high. The chromosome number in the genus Amaranthus is 2 n = 32 (A. cruentus, A. caudatus), 2 n = 34 (A. tricolor) and 2 n = 64 (A. dubius). The chromosome number in the genus Celosia is 2 n = 36 or 72. Hybrids between several species of Amaranthus have been reported. Some breeding work has been done, for instance in India.In Dahomey, local and foreign cultivars were tested. The selection criteria are connected with growth habit, productivity, disease resistance and leaf quality (colour, taste). The following cultivars were selected: Amaranthus cruentus cv. Fotètè, A. dubius cv. Klaroen and Celosia argentea cv. Avounvô Rouge. A cultivar lacking anthocyan was selected from the last mentioned.In an hothouse in Wageningen, some thirty cultivars of six different species were observed. Marked phenotypical differences as to growth habit, production, daylength reaction, protein-N and oxalic acid content became evident. See table 59-60 and figure 30.12. MULTIPLICATIONIn Dahomey 2000 kg/ha of seeds could be harvested from the cv. Fotètè (spacing 40 x 70 cm) in three months. For the cv. Klaroen this amount was 600 kg in four months and for cv. Avounvô Vert 700 kg in five months. The 1000 grains weight of Amaranthus seeds is about 350 mg, that of Celosia seeds about 1 g. Fresh amaranth seed becomes germinative after drying. The light/dark reaction of the phytochrome is capable of inducing or inhibiting germinative power. Generally, seeds germinate better in the dark. Therefore in practice either the seed has to be lightly covered with soil or the seed bed should be covered.Various methods for preservation of seeds were tried out. Small quantities of dry seeds (humidity 12 %) in a well-closed pot, if so required with the addition of a dehydrator, can be kept viable for months. Large quantities stocked in cotton bags in an airconditioned room, keep their viability for at least one year.In Dahomey seeds are distributed in plastic bags. The quantity is based on 2 g of Amaranthus or 3 g of Celosia per m 2of seedbed, sufficient for 1000 plants. See table 61 - 62 and figure 3 1.