RESEARCH NOTE
M.N. Versteeg Agronomist, IITA Benin
V. Koudokpon Agronomist, DRA, RAMR project, Benin.
A high incidence of imperata grass (Imperata cylindrica) causes farmers to abandon already scarce land for farming in the densely populated (150- 500 inhabitants/km2) Adja plateau of Mono province in Southwest Benin. Imperata is among the most persistent and difficult weeds for tropical smallholders (Akobundu, 1987). It has been shown to cause very serious growth inhibition of maize and sorghum through the allelopathic action of its roots (Eussen et al. 1978).
So far, the method most practiced to recover fields heavily infested with imperata is to dig up the rhizomes for drying out and burning, generally carried out by hired labor. Very few farmers are able, however, to pay for or execute such an operation. A leguminous cover crop, mucuna (Mucuna pruriens var. Utilis), has now been shown to effectively help control the spread of this weed.
Since 1986, a team of scientists from the Beninese Directorate of Agricultural Research, the Royal Tropical Institute (KIT) of Amsterdam, and IITA's Benin station is carrying out on- farm research (OFR). In this collaborative research project, IITA contributes agronomic expertise and materials, whereas KIT helps with socioeconomics and, since 1989, with animal husbandry knowhow.
In the beginning years, school demonstration fields were installed to show farmers new practices, including alley cropping and live mulch. The region has a bimodal rainfall pattern, and mucuna was planted during the first season to show its potential as a fast-growing leguminous cover that restores soil fertility. In addition to providing nitrogen-rich organic matter, mucuna also improves soil physical properties (Hulugalle et al. 1986) and controls the population of several plant parasitic nematodes (Caveness 1988).
The farmers, however, were more impressed by the way this aggressively growing plant smothered imperata patches. So the next year, having observed that mucuna had brought down imperata density to less than 10 % of the original stand, 20 farmers asked for mucuna seeds, to try it out on their own imperata-infested fields. The OFR team brought them the seeds and explained clearly how to establish the mucuna, but it left the whole execution and management of the fields to the farmers themselves. This limited intervention by researchers showed clearly that the technique is simple enough for an easy farmer's adoption.
Description of the Technology to Control Imperata by Mucuna
The technology explained to the farmers was to slash the imperata field and immediately plant the mucuna in the same way as maize is traditionally planted, but with only one mucuna seed per pocket. This gives a plant spacing of about 0.80 m x 0.80 m (15000-16000 plants/ha), for which about 15 kg/ha of seeds is needed. As the mucuna germinates, the imperata immediately reproduces new shoots from the rhizomes. These shoots were slashed for a second (and last) time after about 3 weeks, when the mucuna has produced 3-4 leaves. Although imperata continued to form shoots, the young mucuna plants used them as a support to twine and rapidly developed a dense canopy, which eliminated light and smothered the imperata. As a consequence, the remaining underground rhizomes of imperata were exhausted through the maintenance respiration of the imperata biomass. To obtain an effective smothering effect, farmers should plant early in the first season, preferably not later than the beginning of May. Although later plantings may still obtain a dense cover, the period of smothering becomes too short for sufficient exhaustion of the rhizomes, as all the mucuna dies during the harmattan period (December- January).
In the succeeding year, preferably maize (or another fast-growing tall crop) should be planted directly into the mulch and as early as possible, without burning the mucuna cover . By doing so, the elimination of both the remnant regrowth of imperata and the volunteer mucuna from shattered seeds is greatly facilitated. Additional hand- weeding is an important aspect of this integrated imperata control method, which, however, did not create problems for most farmers, as it coincided with normal field preparations and first weedings of maize. Often the amount of work was even less than in conventional fields, because of the absence of other weeds and of better competition from the more vigorously growing maize due to the fertility contribution of the mucuna/imperata organic matter.
Where fields with imperata still need to be cropped by farmers, an optional method for establishing mucuna is to relay mucuna into an early-planted maize crop, after the first weeding of the maize. An extra weeding must still be given to the maize/mucuna association. After the maize harvest, the mucuna cover will usually develop very rapidly and the control of imperata may be as effective as with sole-planted mucuna.
Course of the Farmer-Managed Tests to Control Imperata by Mucuna
Of the 20 farmers who received seeds in 1988, about 14 succeeded in producing a dense mucuna cover. Failures were due to omitting the second imperata slashing, and to excessive spacing. During the harmattan period, however, one third of the well smothered fields caught fire from burning farmers' fields adjacent. The drying mucuna cover thus appeared extremely sensitive to fire. If such burning occurs rather early, the imperata is freed from mucuna's smother cover and is able to resprout quickly and to replenish its rhizomes with new assimilates.
Even so, the imperata control in most of the other fields was impressive. In fields where farmers did a good complementary weeding during the 1989 first season maize, there was virtually no more imperata in the rotation crop during the second season. Farmers were also enthusiastic about the extra yield (500 kg/ha for local maize and 800 kg/ha for the improved variety, SEKOU- TZSRW), attributable to the enhanced soil fertility associated with the mucuna/imperata organic matter. This fertility effect was better than the one obtained after a leguminous cover of pigeonpea and persisted also during the second growing season (Fig. 1). Even more impressive was the fact that in the next year 1989 we observed 103 farmers who planted mucuna in their fields. About 30 of them had got the seeds and knowhow from their colleagues; they planted the mucuna without the direct intervention of research or extension staff. The real number is probably even higher, because of still unreported fields. Such a spontaneous spread is the best proof of farmers' adoption of an introduced technology.
For next year (1990), the provincial agency (CARDER-Mono) will try to enhance the spread of the technology to many other parts of Mono, while also in other provinces and even in other countries, tests on this methodology have been started. With similar success, it is expected that in comparable ecologies in Africa, mucuna will find its place in many farming systems and imperata will rapidly lose its damaging impact.
References
Akobundu I.O. 1987. Weed science in the tropics: principles and practices. John Wiley & Sons, Chichester, UK. 522 pp.
Caveness, E. 1988. Observations on soil nematode population development under a Mucuna utilis fallow. Internal report 10 - 1987/1988, Root, Tuber and Plantain Improvement Program, IITA, Ibadan, Nigeria. 6 pp.
Eussen, J.H.H, S. Slamet, & D. Soeroto 1976. Competition between alang-alang (Imperata cylindrica (L.) Beauv.) and some crop plants. Biotrop Bull. 10. 24 pp.
Hulugalle, N.R., R. Lal & C.H.H. ter Kuile 1986. Amelioration of soil physical properties by mucuna after mechanized land clearing of a tropical rain forest. Soil Science 141: 219-224.
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