Burning in an Imperata fallow/upland rice farming system

This is a companion paper to an earlier one (Menz & Grist (1996) No. 6) in which the agroforestry model SCUAF and a companion constructed economic model were used to model the consequences of reducing fallow length and available land area under shifting cultivation for a typical model Philippines upland low fertility site. In that paper the removal of Imperata cylindrica biomass was considered to be undertaken manually, although it is frequently removed by burning, which process has further negative implications for soil fertility. In this paper the previous modelling exercise is (1) extended by explicitly analysing the impact of burning Imperata on soil fertility and erosion, crop yield and profitability, and (2) targeted to circumstances thought to be currently prevailing in many upland areas in which land area is held fixed at 2 ha, there is a relatively short fallow length of 4 yr, and Imperata is the fallow species. Burning is shown to be the the most profitable method used currently for clearing Imperata before cropping, despite its effects on site degradation; the impacts on rice yields and profitability were small compared with the effects of shortening the fallow period to less than 4 yr (results obtained from this option are presented in an appendix). The use of herbicides to remove Imperata was also considered; differences between the profitability of this option and burning were not large, and where burning was more profitable, herbicide cost reductions of 25% would reverse this trend.