A Case of Cyperus spp. and Imperata cylindrica Occurrences on Acrisol of the Dahomey Gap in South Benin as Affected by Soil Characteristics: A Strategy for Soil and Weed Management
Because of the limiting efficacy of common weed control methods on Cyperus spp. and Imperata cylindrica their occurrences in tropical agroecologies and the effect of soil properties in suppressing these species were investigated in south Benin (Cotonou), a typical ecology of the Dahomey gap. Weeds and soil samples were collected twice early and later in the rainy season in 2009 at four topographic positions (summit, upper slope, middle slope, and foot slope). Sampling was done according to Braun-Blanquet abundance indices (3 and 5) and the absence (0) of Cyperus and Imperata in a quadrat, respectively. The relationship between their respective abundances and soil parameters (texture, C, N, P, K, Na, Ca, Mg, and Fe) was explored. Weed occurrence was less related to soil texture, and Imperata growth was more influenced by soil nutrients (K, Ca, and Fe) than Cyperus spp. Soil cation ratios of K？:？Mg and Ca？:？Mg were the main factors that could be changed by applying K and/or Mg fertilizers to reduce Cyperus and/or Imperata occurrence. Maintaining high Fe concentration in soil at hillside positions can also reduce Imperata abundance, especially in the Dahomey gap. 1. Introduction Weeds are notorious yield reducers that are, in many situations, economically more important than insects, fungi, or other pest organisms [1, 2]. The yield loss due to weeds is almost always caused by an assemblage of different weed species, and these can differ substantially in competitive ability [3]. In rice growing agro-ecologies of West Africa, weed species assemblages include nutsedges and speargrass that are perennial and serious threats [4]. Imperata cylindrica (L.) Raeuschel (speargrass) is a common and persistent weed in upland ecology. It reproduces through seeds and rhizomes. This weed is particularly difficult to control, as it is tolerant to fires and shallow cultivation due to its extensive underground network of rhizomes. The weed tends to be abundant where fields are regularly cultivated and burnt, as it recovers rapidly from disturbance, and burning induces flowering. It exerts great competition on crops [5, 6]. In moist to hydromorphic upland areas, some of the most intractable weed problems in rice are due to the perennial sedges Cyperus rotundus L. (purple nutsedge) and Cyperus esculentus L. (yellow nutsedge). Their tubers and seeds can remain dormant to survive periodic flooding or dry seasons. These species are able to multiply rapidly through tubers which can be greatly accelerated by soil tillage [7]. Because of these characters, I. cylindrica and Cyperus
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