Asiatic cotton can generate similar economic benefits to Bt cotton under rainfed conditions

Transcription

1 Asiatic cotton can generate similar economic benefits to Bt cotton under rainfed conditions Supplementary note The minimum effect size (d) that our test is able to detect considering our sample size (n=17 rainfed Bt G. hirsutum; n=19 rainfed G.arboreum; n=15 irrigated Bt G. hirsutum) and a statistical power of 80% is d=0.96 if rainfed Bt G. hirsutum is compared to rainfed G.arboreum, and d=1.02 if rainfed Bt G. hirsutum is compared to irrigated Bt G. hirsutum. According to Cohen (1992) 32, these are large effect sizes. This means that in our study, we would fail to detect (if present) small differences between the performances of the cotton practices being compared. However, with the large variability in crop performance due to external factors including climate and soil quality, we believe that large differences (and not small differences) are a necessary requirement to conclude that one cotton variety is performing better or worse than another variety. In fact, our sample size allowed us to detect significantly large differences in pesticide and fertilizer expenditure between the cotton practices being compared and in revenue and yield between rainfed Bt G. hirsutum and irrigated Bt G. hirsutum (Table 1). This confirms that, if present, we would successfully detect large differences on net revenue between cotton practices. Ideally, we would have compared the power of our study with the effect sizes found in previous studies, but to our knowledge, there are no studies that compared yield and net revenue (delivering Mean ± SD data) between Bt G. hirsutum and G. arboreum farms, or between rainfed Bt G. hirsutum and irrigated Bt G. hirsutum farms. 32. J. Cohen, A power primer. Psychological Bulletin 112, (1992). NATURE PLANTS 1

2 Supplementary Figures and Tables Supplementary Fig. 1. Relationship between the number of times Bt G. hirsutum was irrigated (irrigation frequency), and the yield produced (Kg/acre). This relationship is significant and non-linear (F(1,13) = 6.69; P = 0.023). 2 NATURE PLANTS

3 SUPPLEMENTARY INFORMATION Supplementary Fig. 2. Farmer s wealth index according to each cotton practice. There were significant differences in farmers' wealth between cotton practices (F (2,48) = 4.57; P < 0.05). Bonferroni tests revealed that these significant differences were due to Bt G. hirsutum farmers with irrigation being wealthier than Bt G. hirsutum farmers under rainfed conditions. No significant differences on wealth were found between G. arboreum farmers and Bt G. hirsutum farmers, either irrigated or rainfed. NATURE PLANTS 3

4 Rainfed Bt G.hirsutum vs. Rainfed G. arboreum Rainfed Bt G.hirsutum vs. Irrigated Rainfed G. arboreum vs. Irrigated Bt G.hirsutum Wealth index B estimate P -value B estimate P -value B estimate P -value B estimate P -value Pesticides 950 < < < Fertilizers 1269 < < < Manure* <0.05 * Note that for manure the analyses used were quasipoisson GLM Supplementary Table 1. B estimates and P-values of the multiple regressions used to assess the relationship between farmers wealth and pesticide, fertilizer and manure expenditure, including cotton practice as a dummy variable. 4 NATURE PLANTS

5 SUPPLEMENTARY INFORMATION Household Schooling Vehicles TVs Non-farm income Food insec. Landholding Irrigated landholding Household size Years schooling Num vehicles Num TVs Non-farm income Months food insec Landholding area Irrigated landholding Supplementary Table 2. Matrix of P-values corresponding to the matrix of Pearson s correlation coefficients for the eight socio-economic variables related to farmers wealth. NATURE PLANTS 5