Herbicide resistance: Does prevention pay?
People tend to glibly state that, whenever it's an option, prevention of a problem is better than curing it or living with it. When you look closely at a particular problem, the old saying is sometimes true, sometimes not. That is exactly the case with herbicide resistance. Past economic analysis has found that prevention usually doesn't pay, but we've recently found that it can do in the important case of glyphosate resistance.
Previous economic analyses of the preemptive use of strategies to delay the onset of herbicide resistance have concluded that they are often not economically attractive for selective herbicides, largely because the only effective delaying strategy is abstinence from use (e.g. Pannell and Zilberman 2001). There is certainly a benefit in preserving a herbicide for use later, but if it comes at the cost of not being able to use the herbicide now, so what? In most cases, there doesn't seem to be a strategy which can generate additional applications of a herbicide prior to the onset of resistance.
However, there does seem to be an exception in the case of the important non-selective herbicide glyphosate. The frequency of glyphosate resistance genes is low enough for their local extinction to be possible. This means that use of multiple control practices may be able to eliminate any weeds that survive glyphosate application. A particular version of this strategy, known as "double knockdown", involves a follow up application of paraquat after glyphosate. Biological modelling of this option has been encouraging.
Some colleagues and I set out to analyse the economics of the resistance problem for glyphosate, to see if preservation of the herbicide could be economically attractive (Weersink et al., 2005). There are several complexities behind this question, including the need to spend more in the short run to reduce costs in the long run. This means that discounting plays an important role.
Because every farmer has a different history of herbicide use, our approach was to calculate the break-even period before resistance onset. If glyphosate resistance is expected to occur before that break-even period, a farmer would benefit from adoption of the resistance-avoiding strategy, even though it is more expensive in the short-term.
The calculated break-even periods varied from three years to 26 years, depending on factors such the extra expense that would occur once the herbicide became resistant, and the discount rate. For realistic assumptions, the break-even period was often around 10 years.
This raises the question of how many years farmers expect to be able to use glyphosate before resistance kicks in. A national survey of 380 Australian grain growers in 2003 found that, on average, grain growers expect that they will get glyphosate resistance in at least one field in 12 years, with 8 percent of growers expecting it in less than five years and 31% in less than 10 years.
So it seems that there are farmers who would probably benefit economically from adopting the double-knockdown strategy: those farmers with expectations for more rapid onset of resistance. Those with average expectations about this would only benefit if they have a relatively low discount rate, a relatively long planning horizon, and/or a relatively high expected cost of weed control after resistance onset.
For most selective herbicides it seems that abstinence is usually the only effective way to prevent resistance development, leading to an unhappy choice between strategies that might be characterised as "just say no" and "accept the inevitable". For glyphosate resistance it appears that a third option is sometimes optimal: "short-term pain for long-term gain".
David Pannell, The University of Western Australia, David.Pannell@uwa.edu.au
Weersink, A., Pannell, D.J., and Llewellyn, R.S. (2005). Economics of pre-emptive management to avoid weed resistance to glyphosate in Australia. Crop Protection 24: 659-664. full paper (77K)
Pannell, D.J. and Zilberman, D. (2001). Economic and sociological factors affecting growers’ decision making on herbicide resistance. In: D.L. Shaner and S.B. Powles (eds.) Herbicide Resistance and World Grains, CRC Press, Boca Raton, pp. 251-277. full paper (110K)
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