Bt Brinjal The scope and adequacy of the GEAC environmental risk assessment

50 Bt Brinjal: The GEAC environmental risk assessmentA yield gap is the difference between the average yield obtained by farmers and the “potential” yield for a given agriculturaltechnology, and can be estimated in several ways. One of the most common ways to measure potential yield of an agriculturaltechnology is through experimental yield trials, such as those conducted in the MST and LST for Bt brinjal. Typically there isa substantial yield gap between the experimentally obtained yields and those that the average farmer obtains (Fig. 3). This iscalculated as the yield gap, YG E= Y E– Y F, where Y Eis the yield estimated in the experiment and Y Fis the average farmer yield.The proportional yield gap for experiments (PYG E) can be calculated as PYG E= Y F/Y E. This can be used to adjust data obtainedfrom controlled experimental conditions to average farmer conditions for a given agricultural technology, such as Bt brinjal.The data in the MST and LST were obtained from experiments conducted under controlled conditions at experimentstations and agricultural colleges. These data will overestimate yields that would be obtained by farmers according to anestimated proportional yield gap, PYG E. This implies that the yield benefit from Bt brinjal will have been overestimated in theMST and LST (Box 12). If PYG Ecan be estimated, it is possible to use the MST and LST data to estimate the true expectedbenefits to the average Indian brinjal farmer (presumably a large-scale producer), subject to the limitations mentioned in theprevious section.Yield gaps for brinjal have not been summarised in the literature, so there is no ready value that can be used for PYGE inthis case. However, all reported yield gaps are positive (Lobell et al. 2009), meaning that the yield estimated in an experimentaltrial of an agricultural technology is always larger than the average farmer yield with the same technology. Moreover, given thedifficulty growing vegetable crops compared to grains, one might expect that the yield gap for a vegetable crop would be as largeas or larger than the yield gap for staple grains. Armed with these two assumptions, it is possible to use published data on yieldgaps in grain crops (Box 12). These considerations give a yield gap of 54%, meaning that the estimated benefit of Bt brinjalprovided by the MST and LST must be multiplied by 0.54 to give an estimate of the yield benefit to the average large-scale,commercial producer.Finding 25. The expected maximum potential yield benefit from hybrid Bt brinjal is probably ≤43.7 q/ha for largescalecommercial farmers and ≤7.2 q/ha for small-scale resource-poor farmers; about 16 % of the time hybrid Bt brinjalis not expected to out-yield non-Bt brinjal.A maximum potential yield benefit from Bt brinjal for the average large-scale commercial farmer can be calculated from theMST and LST data provided in the Dossier (volume 6) and the Supplemental Materials. This is a maximum potential yieldbenefit because it is estimated under well-controlled experimental conditions, and the yield gap is expected to reduce the actualbenefit that could be obtained by the average large-scale commercial farmer.The MST data are aggregated across all three years of the studies. At each site, an experiment to compare Bt brinjal withnon-Bt brinjal was replicated four times in small plots of 27m 2 . This is smaller than the average field size even for small-scaleresource-poor farmers, but is larger than some fields. Experiments were conducted under controlled conditions using standardcommercial brinjal production practices. During the first two years of MST, no insecticides for the control of BFSB wereapplied to the control treatment, non-Bt brinjal (Fig. 4, No Icide Control). During the final year of MST, insecticides wereapplied to the non-Bt brinjal to control BFSB as needed (Fig. 4, IcideControl). It should be expected that the yield benefit of Bt brinjal Figure 4. Yield benefit of Bt brinjal calculated frommulti-site trials (Dossier, volume 6).should be higher when compared to a no-insecticide for BFSB treatmentthan when compared to a treatment where BFSB is controlled withinsecticides. However, the MST data does not meet this expectation.The average yield benefit with a no-insecticide control treatment is 139q/ha, while it is 192 q/ha with an insecticide control treatment. Thereare many possible reasons for such a difference, but no explanation canbe independently verified. Therefore, all of the MST data are pooled.The results from each of the 19 MST experiments are shown in Fig.4. One trial had a very large yield advantage for Bt brinjal of 420 q/ha,while three trials (16% of total trials) had almost no yield benefit. On