12. The Speargrass (<i>Imperata cylindrica</i> (L) Beauv.) menace in Ghana: Incidence, farmer perceptions and control practices in the forest and forest-Savanna transition Agro-ecological Zones of Ghana

Abstract

Surveys to determine farmers’ practices, perceptions and the incidence of speargrass were conducted in the forest and forest-savanna transition zones of Ghana in 1996 and 2000. Mean farm size was 1.2 ha, fallow and cropping length was 4.7 and 4.5 years, respectively, with a mean cropping intensity factor (CIF) of 49%. Speargrass had been present in the area for over 40 years and was perceived as the most noxious weed. Eighty- six percent of fields that relied on slash-burn method of land preparation had severe speargrass infestation. Infestations > 50% cover, mean density of 33 plants m^-2 and shoot height range of 15–300 cm were observed on 60% of the fields. Fifty-one percent of farmers reported of inadequate level of control with current control practices. Glyphosate was applied on fields with > 50% speargrass cover. Speargrass becomes a problem after 3 years continuous cropping from fallow and, under severe infestation, most farmers abandon fields to natural fallow. A dense regrowth of Chromolaena odorata is indicative of a speargrass-suppressed field. Follow-up weed control could be 3-6 times/season depending on initial land preparation, type of crop and/or level of infestation. Cost of weed control was 20–60% higher on speargrass-infested field ($71/weeding/ha) than on other fields, and weeding may take 20–25 mandays/ha. Farmers perceived average yield losses of 30–80% ha^–1 due to speargrass interference, implying a national average crop loss ha^-1 of $31–$84, $155–$414 and $272–$727 for maize, cassava and yam systems, respectively. Reductions in food quality due to the piercing nature of the rhizomes was also paramount.