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Résumé

Les couvertures de sol aident à diminuer les risques d'érosion du sol dans les cultures arbustives, particulièrement avant la fermeture du couvert arbustif. Parmi les différentes espèces de couverture de sol testées avec le cacao au Ghana, celles qui ont eu le plus de succès sont Centrosema pubescens, Pueraria phaseoloïdes et Flemingia congesta. Ces couvertures de sol ont été aussi efficaces pour le désherbage et l'amélioration de la fertilité des sols épuisés par des cultures de plantations. L'arrivée du mouton de la race djallonké dans les plantations a évité l'encombrement des cultures arbustives en fournissant de surcroît l'occasion de produire de la viande. Après 20 ans de pâturage, Centrosema et Pueraria ont pratiquement été remplacés par une grande variété d'espèces spontanées.

Introduction

The cultivation of plantation tree crops — such as oil palm, citrus, cashew, coconut, rubber, papaw, and cola — carries a high risk of soil erosion until the tree-crop canopy closes. Several cover crops have therefore been tested with plantation tree crops in Ghana. The most successful cover crops include Centrosema pubescens, Pueraria phaseoloïdes, and Flemingia congesta.

Cocoa itself covers the soil properly, so its requirement for a cover crop is less crucial. Nevertheless, some cover-crop work has been done with cocoa. In experiment 1, a number of shrubby cover plants were tested for their ability to recondition forest soils exhausted by cropping; cocoa was used as a test crop, and natural bush fallow was used as a control (given a cocoa yield index of 100). In experiment 2, creeping covers were tested in cocoa, and in experiment 3, mixed creeping and erect cover plants were tried in cocoa. Results of the three experiments are shown in Table 1.

Table 1. Performance of various covers in cocoa.
Experiment 1 (rejuvenation of exhaustively cropped soils)		Experiment 2 (creeping cover in cocoa)		Experiment 3 (mixed creeping and erect cover in cocoa)
Cover	Score (%)	Cover	Score (%)	Cover	Score (%)
Natural regrowth	100	Natural regrowth	100	Natural regrowth	100
Flemingia congesta	588	Calopogonium mucunoïdes	207	Indigofera sumatosa	241
Pennisetum purpureum	341	Pueraria phaseoloïdes	183	Tephrosia spp. (mixed)	218
Leucaea ieucocephala	170	Mimosa invisa	100	Desmodium asperum	195
Cajanus cajan	154	Centrosema pubescens	158	Indigofera spicata	183
Tithonia diversifolia	42			Cassia tora	158
				M. invisa	156
				Crotolaria longithyrsa	144
				T. diversifolia	18
Source: Adapted from annual reports of the Kade Agricultural Research Station, University of Ghana, Legon, Ghana.

Most of the cover crops were hard seeded and required treatment before planting. The seed was steeped in concentrated H₂SO₄ for 10 min, rinsed in water, and planted immediately. Alternatively, it was soaked in warm water (75°C) for 15 min. Broadcasting untreated seed immediately after burning the bush has also worked well on some oil-palm estates.

Livestock integration with tree crops

Cover crops are invariably so vigorous that they have to be slashed to prevent them from choking the tree crops. However, many cover crops are nutritious (Table 2) and palatable to animals. Sheep were therefore introduced to graze them and to convert their threat into an opportunity to produce meat. Under research-station conditions, forest-type sheep (Djallonke) reached 25–30 kg in 2 months, compared with 15–25 kg in the savanna zone. Lamb mortality was 5–6% on station and 24% on farm, whereas the national average for scavenging flocks is 40%.

Table 2. Chemical composition of cover plants grazed by sheep on a tree-crop plantation.

Genus	% of DM					ppm of DM
	N	P	K	Ca	Mg	Zn	Cu	Mn
Pueraria	3.5	0.21	2.0	0.51	0.23	26.0	11.3	360
Centrosema	3.2	0.20	2.3	0.45	0.15	29.3	11.3	320
Panicum	2.4	0.19	2.8	0.28	0.16	20.0	2.0	187
Note: DM, dry matter.

After 20 years of being grazed, the Calopogonium has disappeared and Centrosema and Pueraria have been reduced drastically in the plantations, having been displaced by a wide range of volunteer species. Some of these invaders, which were introduced in screening trials for effective cover crops, have become cosmopolitan on the station. Among these are Mimosa pudica, Axonopus compressus, Brachiaria lata, Oplismemus burmanii, Desmodium sooparius, Panicum maximum, Synedrella modiflora, Byrsocarpus coccineus, Lantana camara, and Oxalis corniculata. Chromolaena odorata, which entered Ghana in about 1960, has adapted very well to the climate and is widespread on the plantations.

Documento(s) 27 de 36