SEA Working Paper 01/13
Farmer perceptions of the herbicide resistance ‘wall’
Rick S. Llewellyna,b, Robert K. Lindnera, David J. Pannella & Stephen B. Powlesb
a
School of Agricultural and Resource EconomicsAbstract
The prospect of a serious herbicide resistance problem has often been referred to by agronomists as ‘hitting the wall’; a point from which the established selective herbicide dependent cropping system can continue no further. In this paper, we examine herbicide resistance in Western Australia and farmers’ perceptions of the ‘resistance wall’. Are we there yet, is it avoidable, is it moveable, and is it costly?
Introduction
The modern Australian cropping systems that have led to the introduction of erosion-preventing reduced tillage practices and increased crop diversity are threatened by the development of herbicide resistance. On large areas of the wheatbelt, important and ubiquitous weeds in Australian cropping regions can no longer be controlled by selective herbicides on which farmers once relied . This has led to the availability of effective selective herbicides to control such weeds being considered a potentially exhaustible resource , with farmers’ management requiring consideration of both short and longer-term costs of prevention and management .
This approach to considering resistance management has largely been developed through economic studies of insect pest management problems where rapid resistance development to a very limited range of pesticides has led to the complete exhaustion of pesticide efficacy in some situations. Essentially, farmers face an optimal resource use problem balancing exploitation of the pesticide or an investment in its conservation, usually through the use of more costly alternative practices.
For Australian grain growers to consider investment in preventing herbicide resistance to be profitable there are several factors that are likely to be important in the resistance management decision. These include:
In this paper, perceptions of Western Australian grain growers relating to these and other factors are examined, with the objective being to identify potentially influential factors in the adoption of preventative herbicide resistance management strategies. Where possible inconsistencies between grower perceptions and research and field experience have been identified. Opportunities for extension to improve farmer decision-making are discussed.
Data
The data presented in this study is from a survey of 132 randomly selected grain growers from within the Dalwallinu (DAL) shire (64 growers) and Katanning-Woodanilling (KAT) shires (68 growers) of Western Australia. Properties managed by growers in the DAL region were larger on average (3864 ha), had a greater proportion of land cropped (70 per cent), and received a lower average annual rainfall (approx. 325mm) compared to properties in the KAT region (1812ha, 55 per cent, 450mm). Farm visits were conducted in February-March 2000 and interviews conducted with the primary cropping decision-maker(s) on each farm based on a fully-specified questionnaire. Most questions on herbicide resistance and weed management, focused on the most important cropping weed, annual ryegrass (Lolium rigidum) , and resistance to herbicides in the Group A (ACCase–inhibitors) and B (ALS-inhibitors) herbicide groups. These represent the most common forms of herbicide resistance in Western Australia . As the resistance status results indicate, the two regions represent an area of the Western Australian wheatbelt where more intensive cropping and herbicide resistance is well-established (DAL) and an area where cropping has only relatively recently become more intensive and weed populations with serious levels of resistance are not yet widespread (KAT).
Herbicide resistance status
A question of primary importance is to what extent do growers have ryegrass susceptible to at least some selective herbicides remaining on their cropping land? That is, is the conservation of selective herbicide susceptibility still relevant? In this section, data on the resistance status of growers’ cropping land as a whole, their paddock identified as being typical of cropping land on their farm, and the paddock identified as having the most serious resistance problem (if applicable), are used to examine the extent and intensity of resistance development on growers’ properties.
It is recognised that only a small proportion of cropping land has recently been tested for resistance status. Of the growers surveyed in this study, 37 per cent had used herbicide resistance testing, with only 6 per cent of all cropping land having ever been tested. Therefore, it should be assumed that measures of resistance status are based on growers’ own observations and subjective perceptions rather than relatively objective test results. The survey identified resistance development at two levels. Where a grower described a herbicide as ‘no longer worth using’, the weed population is described here as being ‘resistant’. Where the grower described the herbicide as ‘less effective but still worth using’, the weed population is described here as ‘developing resistance’.
Of the 132 growers interviewed, most growers still had a Group A herbicide option remaining for ryegrass control, even on their paddock identified as having the most serious resistance problem on their farm. Only 21 per cent of growers reported having forms of resistance other than ryegrass resistance to Group A and B herbicides. On paddocks nominated as most serious and typical, just six per cent and two per cent had other forms of resistance. Clearly, ryegrass resistance to Group A and B herbicides is the most common form of resistance, justifying the focus on these forms of resistance in this study. Other forms of resistance comprised mainly resistant wild radish and wild oats.
Given the large difference in the level of resistance between the two regions, results are presented separately (Table 1). In DAL, 86 per cent, and in KAT 60 percent, of growers indicated that they had some level of herbicide resistance. The most notable differences are in the resistance levels of typical cropping levels (i.e. comparing modal resistance levels for the properties). In DAL, the proportion of typical cropping paddocks reported to contain a ryegrass population with resistance to the important Group A ‘fop’ class of herbicides was found to be four times higher than in KAT (Table 1).
Table 1. Percentage of growers in Katanning-Woodanilling (K) and Dalwallinu (D) regions with herbicide-resistant ryegrass populations
| Form of resistanceb |
Any |
Any |
B |
B |
Fop |
Fop |
Dim |
Dim |
Clet |
Clet |
|
K |
D |
K |
D |
K |
D |
K |
D |
K |
D |
|
| In Most Serious resistance paddocka | ||||||||||
| With resistance or developing resistance |
60 |
86 |
44 |
83 |
46 |
66 |
13 |
48 |
7 |
17 |
| With resistance |
38 |
61 |
27 |
52 |
27 |
38 |
7 |
20 |
3 |
3 |
| In Typical cropping paddocka | ||||||||||
| With resistance or developing resistance |
26 |
61 |
18 |
59 |
12 |
47 |
3 |
27 |
0 |
8 |
| With resistance |
13 |
31 |
9 |
25 |
4 |
17 |
2 |
8 |
0 |
2 |
a
Refers to resistance status of paddock as a whole e.g. ‘with resistance’ is defined as ‘herbicide not worth using on that paddock’.b
Data is shown for ryegrass resistant to Group B herbicides (B), Group A ‘fop’ herbicides (Fop), Group A ‘dim’ herbicides (Dim), the Group A ‘dim’ herbicide clethodim (Clet), and resistance in any weed to any herbicide (Any).
For growers with Resistance, the proportion of cropping land affected also differed greatly between regions (not shown in a table). Of the growers in KAT with a resistant population, the average proportion of land where a herbicide is no longer worth using to control a weed population (Resistant) was 18 per cent. For those with Group B resistance, 16 per cent of land contained a ryegrass population on which a Group B herbicide was not worth using and for Group A fops, the figure was 17 per cent. In DAL, the proportions were much higher, being 48 per cent of land for any herbicide, 39 per cent for Group B resistance and 47 per cent for Group A resistance.
The extensive development of herbicide resistance in Western Australian cropping systems means that the majority of growers involved in this study have some experience with resistance development on their property. This is predominantly through ryegrass resistance to the Group A fop and Group B herbicides. Although the majority of growers in both regions have a herbicide resistant weed population on some area of their farm, Group A fop herbicides are still considered to be worth applying for ryegrass control on the majority of cropping land in both regions. At least one Group A dim herbicide (i.e. clethodim) remains effective on almost all typical cropping paddocks. No grower involved in this study has completely exhausted the stock of selective herbicides for ryegrass control on typical cropping paddocks. All growers still have the opportunity to conserve the effectiveness of at least some herbicides. Therefore, the question of conserving or exploiting the remaining herbicide resource is one that is relevant to all growers.
Resistance development
For growers to perceive herbicide susceptibility to be an exhaustible resource, an understanding of resistance evolution is required. To investigate growers’ understanding of the relationship between herbicide use and resistance development, a hypothetical scenario was used. The scenarios focused on the genetic selection for resistance development by eliminating most farm-specific and other agronomic factors that may have influenced the responses detailed in the above section. Growers were asked how many applications they believed it would take for resistance to develop if only diclofop (a Group A fop herbicide) or glyphosate was used as the only means of weed control in consecutive years, assuming no prior resistance selection pressure.
The distributions for diclofop and glyphosate, based on growers’ median responses, are shown in Figure 1. The median grower expected that approximately five applications was the most likely number of diclofop applications that could be applied before it was no longer worth using for ryegrass control. Growers in DAL, a region where diclofop resistance is more common, are less certain than growers in KAT that it will require 5 applications (i.e. DAL growers have higher CV’s than KAT growers, t-test P=0.002). There was no significant difference in the mean CV between regions for glyphosate. The expected distributions for diclofop are consistent with the figures derived from modelling such scenarios . The very few cases of glyphosate resistance development mean that the parameters determining the development of glyphosate resistance are largely unknown.
Figure 1. Subjective probability distribution of years before resistance develops to diclofop and glyphosate (if the herbicide is used consecutively as the only means of weed control) for growers in KAT and DAL. Distributions use median responses for lowest, highest and most likely number of applications.
The expected values for each grower’s distribution for the perceived number of diclofop applications were calculated. The means are presented in Table 2. Overall, the mean expected number of diclofop applications before resistance develops is 6.0. The proportion of growers with an expected number of diclofop applications less than eight was 90 percent in DAL, and 78 percent of growers in KAT. Research results suggest that resistance is highly likely to develop under the scenario presented here in less than eight applications.
Table 2 The mean number of effective diclofop applicationsa (EV) expected by farmers to be possible before resistance develops on a previously untreated population.
|
Mean |
Median |
Std. Dev |
Those expecting diclofop resistance after less than 8 shots |
|
|
KATb |
6.3 |
5.7 |
2.4 |
77.6 % |
|
DAL |
5.7 |
5.3 |
1.6 |
90.2 % |
|
All |
6.0 |
5.7 |
2.1 |
83.6 % |
a
if used consecutively as the sole form of weed controlb
Difference between regions significant (P=0.006) based on F-test
There were no significant differences between regions for the expected number of glyphosate applications before resistance. The mean number of glyphosate applications expected was 15.9 (Std. Dev. 11.2). The relatively high standard deviation is not surprising given that at the time of the survey there was no example of glyphosate resistance in the Western Australia. However, a larger proportion of KAT growers (18.1 per cent) have an expected number of glyphosate applications below eight, compared to growers in DAL (8.5 per cent). That is, growers in KAT were more likely to attribute a glyphosate resistance development rate similar to that of diclofop. The greater use of glyphosate by growers in DAL, without yet having produced a resistant population, may explain this difference.
In general, the results suggest that average perceptions of diclofop resistance development amongst growers in both regions are consistent with research opinion and resistance modelling results. For both diclofop and glyphosate, the average grower does not appear to have exceedingly high expectations of the number of effective applications able to applied before resistance will develop. However, in the case of diclofop, a small but notable proportion of growers expect to be able to apply more applications than research results would suggest is probable.
On-farm prediction of herbicide resistance development using application history
Herbicide management decisions that require consideration of the cost of resistance not yet developed require some level of predictive ability of herbicide resistance development in the field. A major question is how well growers can use knowledge of the number of herbicide applications to predict how many more effective herbicide applications can be applied to a paddock?
Results presented here are for Group A fop resistance development (Figure 2). It should be noted that, unlike some previous studies of the relationship between resistance status and herbicide applications , the data is entirely based on grower knowledge of herbicide history and resistance status. The analyses therefore do not necessarily represent the biological relationship between selection pressure and resistance, but rather growers’ ability to observe the relationship. Most growers do not have written records that extend back to the year that the first of the Group A herbicides were released. When asked to rate how certain they were of the number of applications received using a scale of 1 (extremely uncertain) to 7 (extremely certain), the median response was 6 (Mean 5.5, Std. Dev. 1.5), suggesting a high level of certainty despite the general lack of full written records. No grower had herbicide resistance test results from the previous year, so resistance status responses were also dependent on grower estimates and observations.
Figure 2 The relationship between the number of Group A fop applications received by paddocks and resistance status of ryegrass as reported by growers (None, n=93; Developing, n=24; Resistant, n=14).
The relationship between the number of remaining Group A fop applications reported by growers and resistance status is shown in Figure 2. Clearly, there is a positive but loose relationship. For example, resistant paddocks had received as few as five or as many as 15 applications. Biological explanations for this include different starting gene frequencies and varying levels of seasonal selection pressure. While the high possibility of recall error needs to be acknowledged, the results clearly demonstrate the high level of uncertainty associated with the use of herbicide application knowledge (paddock herbicide history) as a predictor of resistance development.
Future availability of new herbicides
The availability of new herbicide products able to control weeds resistant to existing herbicides replenishes the stock of herbicide susceptibility and diminishes the relative value of any investment in the conservation of older herbicides. Growers were asked to give the least, modal and greatest number of years before they expected such a new product for selective ryegrass control to become available. Of all growers, 21 per cent believed that the most likely time of availability (modal value) is in 3 years or less.
Expected values were calculated and showed no significant difference between regions. The vast majority of growers (86 per cent) expected a new ryegrass selective herbicide to be available within 10 years, with 52 per cent of growers expecting such a product to be available in less than 6 years.
The results indicate a high level of confidence that a new mode of action herbicide will be developed for selective ryegrass control in the near future. When these results are compared to the expected number of Group A applications remaining, it becomes clear that a large proportion of growers do not expect to exhaust Group A susceptibility on their typical paddocks before a substitute herbicide option becomes available. Given that there are no post-emergent selective ryegrass herbicides with novel modes of action known to be in the later stages of development, it is considered unlikely that such a product will become available to growers within 5 years. A lack of awareness of the time required for commercial herbicide development, particularly for the Australian market, may explain some growers’ optimism regarding the arrival of a new herbicide within the next few years.
Expected herbicide use in 10 years time
To gain a longer term perspective, growers were asked how they expected the total number of all herbicide applications to be used on their cropping land in 10 years time to differ from the number currently used. Despite expectations of more herbicide resistance, growers generally expect no decline in herbicide use. Of the growers in both KAT and DAL, 87 per cent expect to be using the same or more herbicide applications in 10 years time. Of all growers, 32 per cent expect to use more herbicide applications in 10 years time, 13 per cent less, with 55 per cent expecting to use the same as they do currently. This indicates that the vast majority of growers expect herbicides to remain the primary weed control method and foresee no reduction in the number of herbicide applications used on their property.
Reversion of herbicide resistance
In some forms of herbicide resistance, the resistant plants suffer a fitness penalty relative to susceptible plants. If the herbicide is no longer used there may be a gradual return of the population to susceptibility . Pausing herbicide use for a period and allowing time for this reversion of resistance to take place can be used as a management tool as it can act to replenish the stock of susceptibility. However, neither a fitness penalty nor reversion has been reported for Group A and B resistance in ryegrass .
To elicit grower perceptions regarding the reversion, or longevity, of herbicide resistance, a hypothetical scenario was used where a paddock with 50 per cent of the ryegrass plants resistant to diclofop is left uncropped and untreated by any herbicides for five years. Growers were asked to give their expected probability that after the five years, the percentage of resistant plants will have fallen from 50 per cent to 10 per cent or less (i.e. back to a level where diclofop can offer reasonable control).
The mean and median responses suggest that most growers are aware that regression is not likely (Table 3). However, a very high level of dispersion in the response is evident. Growers in KAT believed that reversion was more likely than growers in DAL. This is possibly a result of more growers in DAL having observed resistant populations for several years. That up to 46 per cent of growers (in KAT) perceive regression to be likely, and almost 14 per cent of all growers perceive regression to be very likely, suggests that improved extension of ryegrass fitness and reversion studies could be valuable in improving growers’ understanding.
Table 3. Grower perceptions of the probability (chance out of 10) that the percentage of diclofop resistant ryegrass plants in a paddock will regress from 50 per cent to 10 per cent or less over 5 years, in the absence of cropping and herbicide applications.
|
Mean |
Median |
Std. Dev. |
Those rating reversion likely (≥5/10) |
Those rating reversion very likely (≥8/10) |
|
|
KATa |
3.9 |
3 |
3.1 |
46.3 % |
14.9 % |
|
DAL |
2.7 |
1.5 |
3.0 |
25.4 % |
12.7 % |
|
All |
3.3 |
2.5 |
3.1 |
36.2 % |
13.8 % |
a
Two-tailed t-test for differences in KAT and DAL means, P = 0.03
Eradication of ryegrass
The focus of this study is on the long-term management of a weed. The possibility that the most important weed, in this case, ryegrass, can be eradicated has potentially major implications. It is possible that some growers perceive that if control costs rise as a result of resistance, investing large sums to attempt to eradicate the resistant weed species may become an economical option. This has the potential to reduce the perceived cost of resistance management in the long term and hence reduce the incentive for conserving susceptibility. For this reason, growers’ perceptions of the ability to eradicate ryegrass were measured.
Growers were asked if they believed that completely eradicating ryegrass from one of their typical paddocks was possible. Responses to this question are likely to have involved consideration of extreme, possibly impractical, measures. Only 27 per cent of growers in KAT and 33 per cent of growers in DAL stated that eradication was possible. Therefore most growers see no option other than ongoing ryegrass management.
Pre-empting this response, growers were also asked about reducing ryegrass density to the point of near-eradication (i.e. a density so low that the grower could walk across the paddock and not see any ryegrass). They were asked how many years of completely preventing seed-set would be required to achieve this, from a starting point of 1000 plants/m2. Only two growers believed this was not possible and responded with ‘never’. Of the remaining growers, the median number of years was five for both regions (mean years in KAT: 7.9, std. dev. 7.2; mean in DAL: 6.6, std. dev. 3.8). Only 2 per cent of growers in KAT and 6 per cent of growers in DAL believed it would take less than 3 years to achieve.
Most growers believe that eradication of ryegrass is practically impossible. Given the difficulty and cost of preventing all ryegrass seed-set for a prolonged period, the results also suggest that only a few growers perceive a rapid reduction in ryegrass density to very low levels to be a practical option. In general, ryegrass management is expected to remain a long-term management issue in cropping systems.
Externalities
Compared to studies of insecticide use and insect resistance, externalities have not received much consideration in weed science and herbicide resistance studies. This difference can be attributed to the relatively mobile nature of many prominent insect pests and the higher toxicity of many major insecticides. The survey included two questions designed to assess perceptions of mobility and health. These two variables have the potential to explain influences on herbicide use behaviour exogenous to paddock-level weed management.
Mobility of resistance: resistance as private property
If growers were to perceive herbicide-resistant weeds (or genes) to be highly mobile then they may expect to gain a resistance problem regardless of any attempt to prevent resistance development on a particular paddock. This would be expected to reduce the incentive for adopting herbicide susceptibility conservation practices. Growers were asked what the probability was (chance out of 10) that a paddock with no resistance development would gain a resistance problem within 10 years through means other than herbicide use. Pollen flow, resistant seed blown or brought in were given as examples.
Of all growers, 54 per cent perceived importation of resistance to be likely (probability ≥ 0.5). The proportion of growers believing it is highly likely to occur (probability ≥ 0.8) was 21 percent. The average perceived probability was 0.46. This suggests that mobility has the potential to be a factor in growers’ resistance management decisions. However, it should be noted that the question did refer to a relatively long 10-year time frame, which may be beyond the time scale for resistance development through herbicide use. There was also no specification of a form of resistance, so it is likely that responses were given with the most common forms of resistance in mind.
The results demonstrate that mobility is perceived to be possible by most growers, and likely by many growers. Although there are no known studies to confirm the validity of this response, there appears to be no reason to assume the response represents a misperception of the likelihood of importation in a non-specific, long-term scenario. As the question did not specify whether the resistance was to come from a neighbour’s property or the grower’s own adjacent paddock it is not possible to judge to what extent the result indicates that resistance is a common property resource. The results suggest that many growers believe that herbicide resistance genes have mobility at either the inter-paddock or inter-farm level. Therefore, herbicide resistance may not be considered to be an entirely private property problem by all growers and factors relating to common property resource management may be relevant in understanding grower behaviour.
Health
There is a possibility that some growers use reduced levels of herbicide based on factors other than weed management economics. One of these reasons could be concern about the effect of pesticide use on health . Growers were asked if they would change the amount of herbicide they used if they became absolutely convinced that there were no health risks associated with herbicide. Overall, 14.4 per cent said they would change the amount of herbicide they use (16.2 per cent in KAT, 12.5 per cent in DAL). Only 2.2 per cent of growers (3 growers) stated that the increase would be greater than ‘slightly more’, with no growers stating that ‘much more’ herbicide use would be the result. The results suggest that, in general, health concerns are not resulting in conservative use of herbicides.
The perceived value of herbicides and the cost of herbicide resistance
A decision to invest in the conservation of herbicide susceptibility implies that the ability to use effective herbicides has an economic value to the grower. When resistance develops it cannot be assumed that the total cost of herbicide resistance is only the cost of substituting alternative weed management practices to compensate for the loss of herbicide efficacy in cropping. As there are numerous crop rotation options and possible non-crop options, such as livestock grazing, that may be affected, the overall cost of resistance to a farm business can be difficult to specify. The questions included here aim to provide a more complete measure of the perceived cost of resistance, with the cost of implementing IWM practices being just one of the components integrated into the response.
Growers were asked two main questions relating to the cost, or expected cost, of resistance for their property. The simplest related to perceived changes in the average gross margin per hectare if no herbicides could be used. A more complex question examined growers’ willingness to pay for land with various levels of resistance. Both questions examine the extent to which growers value the herbicide resource.
Herbicide resistance and annual returns
Using an agreed benchmark figure for ‘gross margins’ over the past 4 years (in almost all cases $70/ha per year, based on ABARE figures of annual returns per ha.), growers were asked what this figure would be if no herbicides could be used on their property. Growers in DAL expected annual returns to halve if no herbicides could be used (Table 4). Growers in KAT expected a smaller reduction than growers in DAL. The difference between regions is expected as the KAT region offers higher returns from livestock enterprises which do not require the same level of herbicide input.
Table 4. Perceptions of growers in DAL and KAT of the annual return per hectare of arable land if no herbicides could be used on their property (compared to $70/ha as current return benchmark).
|
Mean ($/ha) |
Std. Dev. |
Reduction in mean return (%) |
|
|
KAT |
44.2 |
18.7 |
37 |
|
DAL |
34.8 |
22.3 |
50 |
a
KAT and DAL significantly different (P<0.01) using two-tailed t-test (Stata:ttest)
On average, DAL and KAT growers placed an annual value of $25.80/ha and $35.20/ha respectively on the ability to use herbicides. Only 13 per cent of all growers expected no reduction in gross margin if no herbicides could be used. The hypothetical inability to use any herbicides is an extreme scenario, particularly in terms of herbicide resistance development. The results do, however, indicate the high value placed on the herbicide resource by grain growers.
Willingness to pay for cropping land with resistance
A more comprehensive measure of the perceived cost of resistance is required if the measure is to be relevant to herbicide management decisions. Willingness to pay (WTP) for cropping land was used to measure growers’ perceptions of the cost of herbicide resistance. The potential future returns of agricultural land are capitalised into land prices, along with other variables . Agricultural land that offers lower expected returns from agricultural production, due to resistance for example, can be expected to have a lower price if all other factors remain the same. Theoretically, the difference in the WTP for land with and without resistance should represent the growers’ perceptions of the cost of resistance in net present value terms.
In this study a baseline land value was obtained by asking growers to assume that they wanted to buy more cropping land and the only land available was neighbouring land, identical in all respects to their typical cropping land. Their WTP for this typical land was then compared to their WTP for land identical in all respects except for resistance status. To account for the resistance status of typical paddocks, a hypothetical paddock that had no herbicide resistance and had no herbicide application history was also valued. Unfortunately there has been no study of the determinants of Western Australian cropping land prices that allows for comparison with actual land valuation. In any case, all resistance scenarios included here are uncommon or non-existent as Group A fop herbicides can still be used on most typical cropping land (Table 1).
The mean WTP for typical cropping land in KAT was $1085/ha (Std. Dev. $279) and in DAL, $587 (Std. Dev. $206). The effect of resistance status on WTP relative to growers’ typical land is shown in Figure 3. This shows that when the differences in typical land value are taken into account, growers in KAT and DAL devalue land with ryegrass resistant to Group A and B herbicides at a similar rate. Growers in both regions would pay approximately 24 per cent less for land where no Group A and B herbicides could control the ryegrass.
Figure 3. Growers willingness to pay for cropping land with different levels of herbicide resistance relative to the willingness to pay for typical cropping land. Note: Resistance status is cumulative (e.g. ‘& Gly.’ represents resistance to fops, Group B’s, dims and glyphosate).
For the more serious levels of resistance (resistance to glyphosate or resistance to all herbicides), growers in DAL proportionately devalue land to a greater extent than growers in KAT (Figure 3). As was the case for expected gross margins, this can be explained by the higher returns that can be expected from livestock enterprises in KAT, which results in a higher land value despite the inability to use herbicides. Growers in DAL are, on average, prepared to pay 13 per cent more for land with no herbicide history or resistance relative to cropping land like their own typical cropping land. In KAT the premium is 7 per cent. This most likely reflects the greater level of resistance in the typical paddocks that were used for the baseline value in DAL.
The majority of growers indicated that they would be willing to pay less for land with resistant ryegrass. Approximately 73 per cent of all growers stated a devalued WTP for land with ryegrass resistant to all Group A fop herbicides, with 82 per cent of growers devaluing land with ryegrass resistant to all Group A and B herbicides. Land also with ryegrass resistant to glyphosate was devalued by 95 per cent of growers. The proportion of growers devaluing land to some extent did not significantly differ between regions.
Herbicide resistance is perceived to reduce farm returns. This implies that the alternatives to cropping with the use of effective ryegrass herbicides are costly. The inability to use post-emergence selective herbicides and glyphosate to control ryegrass in a major cropping region (DAL) is expected to reduce cropping gross margins and land value by approximately 50 per cent. The common forms of resistance (Group A and B) are expected to have a smaller cost, although most growers indicate a reduced willingness to pay for land with any common form of resistant ryegrass.
Summary
Most growers have some herbicide resistant weed populations on their property, with ryegrass resistant to Group A and B herbicides being by far the most common form. This is reflected in the generally high level of awareness of factors associated with the common forms of resistance development. While most growers have some resistance on their properties, most growers still have a post-emergence selective herbicide option for ryegrass control on the most of their cropping land. Since it is rare for growers to have ‘hit the resistance wall’ on all of their cropping land, the decision problem of exploitation or conservation of the remaining herbicide resource remains relevant to almost all growers. The results indicate that growers perceive that hitting the resistance wall has a high cost should it be reached.
Growers are generally aware of herbicide resistance development and the likley number of herbicide applications able to be applied effectively. This includes awareness of the large degree of uncertainty associated with predicting resistance development. Although, on average, growers hold perceptions of resistance development consistent with research knowledge and observation, some growers appear to hold misperceptions about some aspects. For some variables the opportunities for information to facilitate improved decision-making are more obvious as a greater proportion of the population appears to hold misperceptions. These variables include the probability of resistance reversion and the probability of a new mode of action ryegrass herbicide becoming available within the next three seasons, both of which appear to be over-estimated by a notable proportion of growers. For most perceptions, however, information is likely to have a greater effect on reducing the variance amongst growers than on the overall average perception. This is due to the fact that the typical grower was found to have a generally high level of knowledge regarding factors relating to herbicide resistance.
This study includes several factors that may be important in attempting to understand why farmers may or may not decide to invest in action to conserve herbicide susceptibility. The substantial proportion of growers who expect replacement herbicides to become available in the short-term suggests that many growers may not perceive that herbicide availability will be exhausted. In other words, the development of new herbicide technology may see the resistance wall be moved further into the future. The high level of growers who perceive resistance reversion to be likely suggests that many growers believe (probably incorrectly) that it may be possible, with time, to ‘reverse from the wall’ and then resume herbicide use. There are also a substantial number of growers who expect that cropping land will gain a resistance problem regardless of how that land is managed, for example, via external sources such as neighbouring land. The importance of this in reducing the incentive for growers to invest in preventing herbicide resistance development remains to be determined. However, the result does suggest a higher than expected degree of resistance mobility that could influence resistance management decisions.
Several of the perceptions discussed in this paper form part of further analyses examining factors associated with the growers’ adoption of alternative weed management practices. The potential for targeted extension to influence these and other perceptions is also the subject of further study. As suggested by the results presented here, a range of short and longer-term factors need to be considered when attempting to understand the farmer’s herbicide management problem. The promotion of a more conservative approach to the use of the herbicide resource will be assisted if such factors are recognised.
Acknowledgements
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Citation: Llewellyn, R.S., Lindner, R.K., Pannell, D.J. and Powles, S.B. (2001). Farmer perceptions of the herbicide resistance ‘wall’, SEA Working Paper 01/13, School of Agricultural and Resource Economics, University of Western Australia. http://www.general.uwa.edu.au/u/dpannell/dpap0113.htm
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