Weed species composition and diversity in flood recession farming in the Okavango Delta, Botswana

Abstract

Globally, flood recession farming is practiced in wetlands such as lakes, floodplains, swamps and rivers. Major wetlands where flood recession is practiced include the Dianchi and Kunming lakes in China, the Yunnan River in Malaysia, the Pantanal in Brazil, the Sudd Swamps of the Sudan, the Omo Valley in Ethiopia, Tana River in Kenya and Rufiji River in Tanzania. In Botswana it is practiced along the Chobe River and the Okavango Delta. It is locally known as molapo farming. It is generally perceived as giving higher yields than dryland because it is practised in an environment with high soil moisture and fertile soils. Agroecosystems in floodplains can modify plant communities by creating micro-climatic conditions that favour some and disfavour other plant species. Previous studies on flood recession farming focused more on yields and less on weeds. Most phytosociological work on floodplains in the Okavango Delta has been focussed on natural vegetation communities and their response to hydrology, while little attention has been given to the use of seasonal floodplains for agricultural production. Therefore this study seeks to determine the weed species composition and diversity in flood recession farming in the Okavango Delta. The study was conducted in Shorobe, Makalamabedi and Lake Ngami in north western Botswana. Vegetation sampling was conducted in 36 molapo fields from March to April 2016. The fields were categorised according to the number of years they were cultivated continuously. The categories were: cultivated for 5 years; 10 years; 15 years and uncropped sites. Full species counts were done in 1m2 quadrats placed at 10m intervals along a line transect. Soil samples and GPS coordinates were taken from each field. Hierarchical cluster analysis was used to determine weed species communities using indicator values derived from Indicator Species Analysis. A multi-response permutation procedure was used to test the hypothesis of no difference between the groups of species communities. Kruskal-Wallis was used to test for statistically significant difference in species diversity between species communities. Diversity indices (Shannon‟s index and Evenness) were used to determine ix species diversity for each cultivation frequency within and across the study sites. Kruskal Wallis was used to test for significant difference in species diversity between the cultivation frequencies. Rank of abundance was used to determine the association of weeds with the cultivation frequencies. To evaluate whether the mean biomass of crops and field status (weed free and weedy) were statistically different from one another, Independent-t test was used. Results showed no statistically significant difference (p ˃ 0.05) in species diversity (Shannon‟s index) between weed species communities in flood recession farming but species evenness (Equitability index) was significant at p ˂ 0.001. There was a significant difference (p = 0.042) in diversity indexes across the 4 cultivation frequencies. The 5 year frequency recorded the highest median score (Md = 1.352) while the 15 year frequency recorded the lowest median score (Md = 1.035). Corchorus tridens L. was the most abundant weed species at the 5 and 15 year frequencies. Cynodon dactylon (L) Pers. was most abundant in the 10 year frequency while Cyperus esculentus L. was most abundant in uncropped fields. The canonical correspondence analysis results indicated that variations in weed species composition in flood recession farming were explained more by available phosphorus and flood frequency. There was a statistically significant difference t(14) = 7.553, p≤ 0.001 in the mean maize biomass scores between weed free and weedy fields. Cultivation frequency influenced weed species composition and diversity in flood recession farming areas of the Okavango Delta. Uncropped sites were composed of wetland species that tolerate some dry periods or seasonal flood plains. Flood recession farming fields were dominated by common weed species which are also troublesome in dryland arable farming. Flooding frequency and available phosphorus were the main drivers of composition and diversity of weed species in molapo farming. For instance, species like Cyperus longus, x Zornia glochidiata and Alternathera sessilis were found mainly in molapo fields with high flooding frequency. This study has provided an inventory of the most abundant and trouble-some weed species in flood recession farming in the Okavango Delta. The study has also revealed that high cultivation frequency tends to suppress weeds hence reduction in their diversity. And at a moderate cultivation frequency weeds species were activated and newly created micro climate favoured their germination and growth. The study have assisted in understanding the status of weeds in molapo farming as such provided valuable information which might be useful when designing a sound and successful weed management program in flood recession farming areas in the Okavango Delta.