Typha control efficiency of a weed-cutting boat in the Lac de Guiers in Senegal: a preliminary study on mowing speed and re-growth capacity

Prolific growth of Typha australis in the lower part of the Senegal River a nd the Lac de Guiers resulted from changed ecological conditions following the construction of two high dams in the Senegal River. Fluctuation of the water level has decreased markedly and the water has changed from brackish to fresh as the inflow of salt water from the ocean is prevented. The effic iency of a hydraulic weed cutting boat (model Conver 480 H) has been tested in various plots in the Lac de Guiers. This lake, connected with the Seneg al River, is economically very important as it is the city of Dakar's major drinking water supply and also provides irrigation water for vast agricult ural areas. Cutting, performed at 20 cm and 50 cm below the water surface, was very effective in removing Typha stands. However, due to the excessivel y thick stems of the Typha plants, cutting took much more time compared to similar operations in Europe. It was estimated that approximately 35 h were required to clear one ha of dense Typha stands. Directly after cutting re- growth was observed during a relatively brief period (approximately three m onths), however, subsequently all newly formed growth tips in the plots had suddenly disappeared. It was concluded that this sudden collapse was linke d to a destruction of the root zone due to anoxic conditions. Eventual re-g rowth was relatively slow as after one year the plots were still largely fr ee from Typha plants. Efficiency of mowing was not related significantly to depth of cutting.