Changes in net returns might have an indirect environmental benefit if low incomes led to agricultural land being abandoned or if, as is sometimes claimed, high incomes allowed farmers to devote money to environmental improvements. Net returns affect sustainability broadly defined, so as to include social sustainability, by their effect on farming livelihoods.
Econometric analysis of the ERS/NASS 1997 data, using the relevant state average prices for inputs and outputs, showed a significant increase in the net returns from herbicide-tolerant cotton for the surveyed states as a whole and from Bt cotton in the southeast (Fernandez-Cornejo & McBride 2000, Table 9). There was no significant overall increase in net profits for herbicide-tolerant soybean. Results for the other GM crops were not mentioned.
These results help to explain the rapid uptake of GM cotton by US farmers, but do not explain why herbicide-tolerant soybean has been rapidly adopted too. Comparison of herbicide-tolerant and conventional soybean in 1997 showed that net profits varied from region to region. Net profits from GM soybean were significantly higher than those from conventional soybean in the Heartland land resource region, which had 70% of the total US soybean area in 1997 (Fernandez-Cornejo & McBride 2000, Tables 10 and 6). An alternative explanation for the rapid uptake of GM soybeans is that they became available just at the time when soybeans became eligible for support payments in the US (Directorate-General for Agriculture 2000).
To summarise, evidence of benefits from GM crops grown commercially is so far based on the analysis of two or three years' survey data at most. Results vary by crop-trait combination, by region and by season. Overall, results so far suggest that adoption of Bt cotton in the US can lead to reductions in the insecticides used against the target pests, although in some regions the use of other insecticides may be increased to cope with pests not controlled by the Bt toxin. The net environmental impact is unclear. For Bt corn, there may be some savings in insecticide use, although less than one-tenth of conventional corn in the US is usually sprayed to control the target pest: the European corn borer. For herbicide-tolerant soybean, the results suggest an overall decrease in the herbicides used, despite an increased use of glyphosate. This represents a decrease in environmental impact in terms of the persistence and toxicity of the changed mix of herbicides, as well as in terms of the overall quantity used. For herbicide-tolerant cotton and corn, there is little evidence so far of a significant overall reduction in the use of herbicides.
There are many factors that complicate the analysis of survey results. For example, it seems that adopters of GM crops are likely to have larger farms, be better educated and use more inputs than non-adopters. Even when more evidence is gathered so that the analyses can be more detailed, there remain many unanswered questions about what the resulting changes in farm management, including changes in pesticide use, may mean in terms of their environmental impact.
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