Part 4. The Cost of Herbicide Resistance

Herein, an outstanding example of estimating the cost of herbicide resistance in a grass weed:

Contents

1. Calculating the Cost of Herbicide Resistance

2. Example: Complete Budgeting Required

Farmers, like all businessmen, take decisions in the context of their own enterprise. Hence, if they cannot be convinced that herbicide resistance in weeds will have a significant impact on their profits, they will not adopt measures to prevent its development. The basis of calculating the cost to the individual farmer of herbicide resistance is relatively straightforward. The problem associated with such an exercise is obtaining the information required to come to a robust conclusion. This is because the factors have to be identified that affect the development of resistance and, in some cases, data have to be acquired in order to calculate the economic impact of alternative farming practices or systems.

1. Calculating the Cost of Herbicide Resistance

Define three approaches to cropping systems/practices and herbicide strategies that produce:

. The highest profits regardless of the possible development of resistance (A in Figure 1).

. The highest profits when the enhanced metabolism form of herbicide resistance has fully developed and/or where target site resistance restricts cropping systems/practices or herbicide choice (B in Figure 1).

. The highest profits whilst preventing the development of resistance (C in Figure 1).

. The difference in the profitability of Approaches A and C is the cost of preventing the development of resistance. It is a matter of opinion whether the cost of resistance is the difference between the profitability of Approaches A and B or C and B.

. Complete budgeting is required where more than one weed species is involved and/or where major changes in cropping and/or husbandry are involved. Partial budgeting will provide a robust answer where only changes in herbicide practice have to be adopted in order to prevent the development of resistance and to control resistant weeds.

2. Example: Complete Budgeting Required

The annual cost to the farmer of enhanced metabolism of herbicides by black-grass (Alopecurus myosuroides) in winter wheat (Triticum aestivum) in England.

Black-grass is an annual grass weed of Northern Europe that shares the same life cycle as autumn sown crops. Whilst individual plants are not very competitive, they can rapidly build up to infestations that dramatically reduce yields in subsequent autumn sown crops (Moss, 1980).

Continuous autumn sowing, non-plough tillage and September sowing, favour black-grass and the development of resistance to herbicides (Approach A in Figure 2; Moss and Cussans, 1985). On the two farms studied, located in Peldon in Essex, there is cross-resistance to many effective selective herbicides used for the control of black-grass in cereals and other crops (Moss and Clarke, 1995).

The mechanism is enhanced metabolism and it is assumed to be fully developed because the level of resistance has not changed since it was first identified in 1984 (Moss and Cussans, 1985). However, herbicides still give some control of the weed. Complete budgeting demonstrated that it remains more profitable to retain continuous winter wheat rather than change the pattern of cropping, even though some major changes in cultivations and crop husbandry have been necessary to contain black-grass.

These changes involve the adoption of ploughing and late sowing (after mid-October). There is also the continuing necessity of a high usage of herbicides. Ploughing increases costs and delayed drilling reduces yields and this current approach (B in Figure 2) is now less profitable than an approach that, again with the benefit of hindsight, may not have developed resistance had it been adopted originally. This third scenario involves sowing continuous winter wheat in early October after plough tillage, where one herbicide application is usually sufficient to give adequate control of black-grass in Figure 2).

The calculations based on the physical data provided by the two farmers at Peldon and which are fully described by Orson and Harris (1997), demonstrate the high cost, in terms of lost yield and increased costs, of adopting a policy to prevent resistance. However, the cost of resistance is significantly higher (Figure 2).

Figure 3 demonstrates the impact that the full expression of enhanced metabolism form of resistance has on farm profits when approach B is compared to approach C, based on the physical data from the two farms in question.

This shows that the additional cost of controlling herbicide resistant weeds has a far larger impact on form profits when the value of the crop output is low. The price of wheat in the UK at the time of writing is around £75/tonne. At his price, resistance can turn profit into loss or magnify losses significantly.

3. Example –Partial Budgeting

The annual cost to the farmer of target site resistance of water plantain (Alisma plantago-aquatica) and rice club-rush (Scirpus mucronatus) to sulfonylurea herbicides in paddy rice (Oryza sativa) in Italy.

In this particular instance, the cost of resistance is restricted to higher herbicide costs and also the costs involved with the additional number of times herbicides have to be applied. There may also be further costs resulting from reduced yields because of the poorer weed control sometimes experienced with the herbicide programs adopted to control resistant weeds. Table 1 details the additional annual herbicide and application costs of Approaches B and C (Figure 1) in the absence of red rice and Heteranthera spp (e.g. H. zosterfolia). that require very complex herbicide policies. For the sake of simplicity, it has been assumed that there has been no loss of yield due to poorer weed control in Approach B.

The assumption made in this example is that the additional pre-emergence application of oxadiazon one year in three will prevent the resistance of all target weeds in addition to water plantain and rice club-rush.

In this example the cost of preventing resistance is significantly less that the cost of resistance.

Table 1. Additional annual cost (compared to Approach A in lira/ha) of target site resistance of water plantain and rice club-rush to sulfonylurea herbicides in paddy rice in Italy (Approach B) and the annual cost of its prevention (Approach C).

Farming system (see Figure 1)	Herbicide strategy	Additional herbicide and application costs (lira/ha)

Approach A	Sulfonylurea + molinate -- post-emergence	-
Approach B	Oxadiazon -- pre-emergence Propanil + MCPA -- post-emergence 1 Propanil – post-emergence 2	131,200
Approach C	Oxadiazon -- pre-em. (one year in three) Sulfonylurea + molinate -- post-emergence	28,700

4. Conclusions

The examples show that calculating the cost of herbicide resistance to the farmer is a necessary step in order to:

. persuade farmers to adopt anti-resistance strategies where the cost is small compared to the cost of resistance. The examples suggest that this may apply particularly where target site resistance may occur and where there are alternative herbicides available.

. persuade farmers, where the cost of the occurrence and the adoption of a strategy to prevent the occurrence of enhanced metabolism, which reduces the efficacy of many of the available herbicides, are both high, to at least adopt the cheaper components of an anti-resistance strategy and to monitor carefully any development in resistance.

. locus research and advisory efforts. For instance, the exercise on black-grass resistance has highlighted the importance of monitoring methods to measure the development of enhanced metabolism.

Source

Jim H. Orson

Morley Research Centre, Morley St. Botolph,