Thesis (BS Biology) -- University of the Philippines Mindanao, 2010

Weed management design should balance the need to reduce weed population, on one hand, and the needs to preserve weed diversity and avoid build up of resistance to herbicides, on the other. To determine the effect of manual and chemical (paraquat and glyphosate) weed management on the abundance and diversity of broadleaf weed species in a banana plantation, counts, dry weights, seed bank and diversity indices of broadleaf weed species were compared in to identical experiments in 15% and 25% slopes. In addition, evolution of paraquat resistance in Ageratum conyzoides was investigated to determine the effects of sustained paraquat application. From the seed banks, a total of 10 broadleaf species were found in both experimental fields: Ageratum conyzoides, Bidens pilosa, Cleome rutidosperma, Calapogonium muconoides, mimosa invesa, Hedyotis corymbosa, and Younggia japonica. Seedling emergence and diversity indices showed an increasing trend in seed banks in both 15% and 25% slope over 3 years but there was no significant difference between the two treatments. Field counts and biomasses of broadleaves were significantly lower in the chemical plots to the manual plots. The sustained dominance of A. conyzoides was also observed in the field but chemical control was better than manual controo in reducing the population of this weed. Species C. rutisdosperma and B. pilosa, which were dominant in the first year of sampling, continuously declined from the second year sampling. Along with the decline was the increase in D. cordata and A. indica in the 15% and 255 slopes respectively. The shifts in weed composition have ecological significance discussed in this report. Experiments on Ageratum conyzoides show the paraquat resistance is present in this species and the frequency and fitness of resistant plant may increase with sustained paraquat application. In conclusion, chemical weed control is advantageous not only because it is more effective in reducing the weed population, but it also favored a shift in weed population to more ecologically favorable composition. The risk of developing herbicide resistance is real, but this can be managed by diversifying herbicides