Agriculture, Economics

57 – Herbicide resistance: Does prevention pay?

People often say that preventing 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.

There have been a number of past economic analyses looking at whether it is worth using strategies to delay the onset of herbicide resistance. This studies have tended to conclude that it is not economically attractive to use these strategies pre-emptively (prior to the onset of resistance) largely because the only effective strategy for delaying resistance is not using the herbicide (e.g. Pannell and Zilberman 2001). There is certainly a benefit in preserving a herbicide for use later, but it often comes at the cost of not being able to use the herbicide now, and that cost tends to outweigh the benefit. For most herbicides, there doesn’t seem to be a strategy that will allow more addition applications of a herbicide in future than you have to give up in the short term.

However, at least in Australia, there seems to be an exception in the case of the important non-selective herbicide glyphosate. The frequency of glyphosate resistance genes is low enough in most Australian crop fields for it to be possible to drive those genes to extinction. This could happen if farmers use additional control practices to eliminate any weeds that survive glyphosate application. A particular version of this strategy, known as “double knockdown”, involves a follow-up application of the herbicide 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.

Because every farmer has a different history of herbicide use, we had to think carefully about how to do the study in a way that would be relevant to all farmers. Our approach was to calculate the “break-even period before resistance onset”. If the farmer expects glyphosate resistance to occur quite soon (within say a few years), it is more likely to be worth trying to prevent it. If resistance is not expected for a long time, it is less worthwhile taking preventative action now. The break-even period is the number of years at which the result switches from prevention being worthwhile to not being worthwhile. If resistance is expected to occur within the 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 break-even periods we calculated in our economic model varied from three years to 26 years, depending on factors such the extra expense that would occur once the herbicide became resistant, and the interest 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 quite a few farmers who expect glyphosate resistance to occur before the break-even period, meaning that they would benefit economically from adopting the double-knockdown strategy. Those farmers with expectations for more rapid onset of resistance are the ones who should adopt it.

On the other hand, it is not worthwhile for all farmers. For farmers with average expectations about the time until resistance onset, it would only be worth investing in prevention in a particular set of circumstances. The farmer would need to have a relatively long planning horizon, a relatively high expected cost of weed control after resistance onset and a relatively low interest rate.

For most 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 in Australia it appears that a third option is sometimes optimal: “short-term pain for long-term gain”. For this third option to be worthwhile in other places as well, there needs to be a weed control option that is not too expensive and is highly effective at killing the weeds that survive glyphosate application.

David Pannell, The University of Western Australia

Further Reading

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)