Document(s) 25 of 36

Résumé

Les études sur les légumineuses pouvant être introduites dans les systèmes de culture dans les vallées au nord du Ghana ont commencé au milieu des années 1980. Bien que les légumineuses à graines comme le niébé, l'arachide et le soja produisent une biomasse et une quantité de N plus élevées que celles des plantes de couverture telles que Calopogonium, les paysans ne sont pas disposés à les utiliser comme engrais verts au détriment de la production des graines. Cependant, les variétés de 60 jours de niébé intéressent les paysans parce qu'elles servent à la fois d'engrais verts et de production de graines. Les légumineuses utilisées comme cultures de relais avec le riz n'intéressent pas les paysans parce qu'un semis précoce de la légumineuse dans le riz empêche la croissance de ce dernier tandis que, semée tardivement, la légumineuse souffre et ne s'établit pas. L'utilisation des légumineuses comme jachère antérieure à la culture de riz donne de meilleurs résultats. Une culture de niébé précédant la culture de riz, par exemple, accroît le nombre de talles productives et le rendement grainier.

Introduction

Northern Ghana, comprising the Upper East, Upper West, and Northern administrative divisions, lies between lat. 8° and 11°05´N and between long. 0°30´E and 3°W and covers 9.97 x 10⁶ ha. Rainfall in the area has a unimodal pattern, from April to October, with the peak in September, and totals 1 000 – 1 100 mm. In the first 2 or 3 months, the rainfall is too erratic for farmers to cultivate a staple food crop.

Subsistence agriculture is the predominant occupation, and the crops are sorghum, millet, rice, maize, legumes (groundnuts, cowpea, and soybean), and cotton. The lack of appropriate land-management and -conservation practices has resulted in severe degradation of the environment: a changing vegetation cover, further decreases in soil fertility, increased soil compaction and erosion, disturbed hydrological regimes, and high weed infestation. The consequence of this in the past few decades has been a falling agricultural-productivity index (kilograms crop yield per capita).

Materials and methods

Relay cropping of rice with Calopogonium

In this trial, Calopogonium was interseeded into rice 1 and 2 months after the rice was planted and 2 weeks before the rice was harvested, with a no-interseeding control. The experiment had a randomized complete-block design, replicated four times. Yield data and growth habits of both crops were used to assess treatments.

Calopogonium as an improved fallow legume

In the improved-fallow study, treatments were 1, 2, and 3 years of Calopogonium fallow and weedy fallow, with different levels of fertilizer, in a split-plot design. Type of fallow was the main plot, and levels of fertilization were the subplots. Continuous rice cropping was the control treatment. Weed infestation and grain yield were recorded, and an economic analysis of the different systems was undertaken. This work, located at an on-farm site at Yepeligu and an on-station site at Nyankpala, is ongoing.

Results

Relay cropping of rice with Calopogonium

When Calopogonium was seeded 1 month after the rice, it competed with the rice; when it was seeded 2 months after the rice, it germinated poorly, probably as a result of shading, high soil-water levels, and low soil temperature. A good crop of Calopogonium could not be established when planted 2 weeks before the rice harvest.

Improved-fallow study

Annual dry-matter yield of Calopogonium measured in November–December ranged from 5 to 8 t ha^-1. This biomass dried up during the dry season and formed a carpet. At the beginning of the rains, in April, Calopogonium seeds that dropped on the ground in the fallow plots germinated to form a regenerated fallow, in contrast to the bush-fallow and continuous-rice fields. Biomass incorporated from this regenerated fallow in June ranged from 1 to 3 t ha^-1 and contained about 2% N. A 3-year Calopogonium fallow significantly increased soil organic matter and cation exchange capacity. Fertilizer application had no effect on these properties. However, differences were not significant after 1 year of fallow.

Rice fields preceded by 3 years of Calopogonium had a significant reduction in weed biomass. This effect was highly significant with the application of fertilizer (Figure 1). Both 1 and 3 years of Calopogonium fallow resulted in significantly higher rice-grain yields per hectare than bush fallow did. A partial budget analysis of the systems at both sites showed that Calopogonium fallow was the most profitable system (Table 1).

Figure 1. Effect of fallow and fertilizer on weed biomass at 28 d after planting, Yepeligu. Note: BF, bush fallow; CF, Calopogonium fallow; CR, continuous rice; R, rice.

Table 1. Partial budget analysis of rice produced in different fallow systems, Yepeligu, 1995.

Treatment a (years)	Fertilizer (kg ha-1)	Grain yield (kg ha-1)	Incremental benefit (GHC ha-1)	Incremental cost (GHC ha-1)	Profit (GHC ha-1)
2 CR after 2 CF	0	1 093	-75 000	1 976	-76 976
	30	1 602	124 345	56 563	67 782
R after 3 BF	0	1 200	20 379	4 940	25 319
	30	1 659	138 957	1 976	84 370
2 CR after 2 CF	0	1 268	38 095	1 976	36 119
	30	2 330	286 680	56 563	230 117
R after 3 CF	0	1 580	117 791	4 940	112 551
	30	2 664	373 000	54 587	318 421
4 CR	30	2 246	290 896	59 527	231 369
Note: In 1998, 2 292 Ghanaian cedis (GHC) = 1 United States dollar (USD). a Continous rice without fertilizer was the control. BF, bush fallow; CF, continuous fallow; CR, continuous rice; R, rice.

Document(s) 25 of 36