Abstract:
Field and screen house studies were conducted to: 1) investigate farmers’ perception
of speargrass (Imperata cylindrica (L.) Beauv.) and existing management practices, 2)
recommend a legume cover to control speargrass and improve soil fertility, 3)
determine the effect of integration of land preparation, chemical application and cover
crop on growth and survival of speargrass and 4) evaluate the growth and yield of
maize and yam on speargrass managed fallow. The studies involved a 3 x 3 factorial
arrangement of land preparation and cover crop laid as a split plot design and a 23
factorial arrangement in a split-split -plot design of land preparation, chemical
application and cover crop at Mampong and Wenchi in the forest and forest-savanna
transition zones. The mean fallow or crop length was 4.7 and 4.5 years respectively.
Speargrass was ranked the most noxious weed because of its deleterious effect on
crops through competition and piercing nature of emerging shoots from rhizomes on
crops. It becomes a problem after three years cropping and was present on 94% of the
fields surveyed with high incidence (86%) on those burnt at time of land preparation.
Farmers perceived yield losses due to speargrass was <30 to >80 % ($326 - $869/ha).
In the field studies ploughing delayed the regrowth of speargrass up to 8 weeks while
Mucuna suppressed growth up to 32 weeks. Stylosanthes reduced speargrass dry
weight by 77 % relative to natural fallow and 55 % relative to Mucuna while Mucuna
reduced weight by 48 % relative to the natural fallow at 44 weeks. The legume fallow
reduced speargrass rhizome dry weight by 69-77 % compared to the natural fallow and
suppressed shoot regrowth > 50 % on following crop. Stylosanthes or Mucuna fallow
increased maize grain yield by 62 and 98 % respectively over the natural fallow. The marginal rate of return (MRR) of changing from natural fallow to Stylosanthes or
Mucuna fallow was 113 and 153 % respectively. Soil analysis showed an increase in
organic C and available K and a decrease in available P, Ca and Mg. Total N was
medium for all cover (0.10 - 0.17 %) at Mampong but low at Wenchi (0.04 - 0.13 %).
Glyphosate application after land preparation increased effectiveness of control by 61
% up to 3 months, while planting Mucuna after glyphosate effected speargrass control
beyond 6 months. Glyphosate-Mucuna combination reduced the incidence of speargrass
by 50 % and rhizome dry weight by 81 % while glyphosate or Mucuna reduced weight
by 64 and 60 % respectively over the natural fallow. Speargrass rhizomes pierced 1%
of harvested yam tubers where glyphosate was applied, 7 % from non-glyphosate plots,
15% from burned plots and 9 % from ploughed plots. The fungus Colletotrichum gloeosporiodes, which causes anthracnose, was isolated from the leaves of yam and Mucuna. In the screen house, 5 % of the rhizomes from legume fallow sprouted
against 18 % from the natural fallow. Speargrass management through integration of
land preparation method, chemical or legume cover is economically viable.
