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High Levels of Herbicide Resistance in Rigid Ryegrass (Lolium rigidum) in the Wheat Belt ofWestern AustraliaAuthor(s): Rick S. Llewellyn and Stephen B. PowlesSource: Weed Technology, Vol. 15, No. 2 (Apr. - Jun., 2001), pp. 242-248Published by: Weed Science Society of America and Allen PressStable URL: http://www.jstor.org/stable/3988799 .Accessed: 16/04/2013 23:17

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Weed Technology. 2001. Volume 15:242-248

High Levels of Herbicide Resistance in Rigid Ryegrass (Lolium rigidum) in the Wheat Belt of Western Australia'

RICK S. LLEWELLYN and STEPHEN B. POWLES2

Abstract: A random survey of 264 cropping fields in the Western Australian wheat belt was conducted to determine the extent of rigid ryegrass resistance to commonly used acetolactate synthase- and acetyl-CoA carboxylase-inhibiting herbicides. Rigid ryegrass infestation density was assessed and seed samples collected and subsequently tested for resistance to diclofop-methyl, clethodim, chlorsulfuron, and sulfometuron. Of these randomly collected populations, 46% exhibited resistance to diclofop-methyl and 64% to chlorsulfuron, with 37% exhibiting multiple resistance to both herbicides. Only 28% of tested populations were classified as susceptible to both diclofop-methyl and chlorsulfuron, although all but one population were susceptible to clethodim. Large differences in the proportion of fields containing resistant populations were found between agronomic areas, re- flecting different cropping and, therefore, herbicide use history. There was no significant association between resistance status and the density at which rigid ryegrass was present. Herbicide-resistant rigid ryegrass populations are now more common than susceptible populations across much of the Western Australian wheat belt. Nomenclature: Chlorsulfuron; clethodim; diclofop-methyl; sulfometuron; rigid ryegrass, Lolium rigidum Gaud. #3 LOLRI. Additional index word: Weed density. Abbreviations: ACCase, acetyl-CoA carboxylase; ALS, acetolactate synthase.

INTRODUCTION

Western Australia is Australia's largest grain-producing state, with recent production in the wheat belt region averaging 11 million ton of grain from 7 million ha of crop sown (Anonymous 1999a). The cropping system is based around wheat (Triticum aestivum L.) production (4.5 million ha), with lupin (Lupinus angustifolius L.) (1 million ha) as the most extensively grown noncereal crop. The Western Australian wheat belt is widely per- ceived to have one of the world's most significant herbicide resistance problems. This is attributed to wide- spread multiple herbicide resistance in rigid ryegrass, the most important weed species of Australian cropping. However, despite a relatively long history of resistance development, the extent of the problem has not previously been quantified.

I Received for publication August 1, 2000, and in revised form January 8, 2001.

2 Ph.D. student, Western Australian Herbicide Resistance Initiative and Agricultural and Resource Economics Group, Faculty of Agriculture, Univer- sity of Western Australia, Nedlands, WA 6907; Professor, Western Australian Herbicide Resistance Initiative, Faculty of Agriculture, University of Western Australia, Nedlands, WA 6907. Corresponding author's E-mail: rllewell @ agric.uwa.edu.au.

I Letters following this symbol are a WSSA-approved computer code from Composite List of Weeds, Revised 1989. Available only on computer disk from WSSA, 810 East 10th Street, Lawrence, KS 66044-8897.

Rigid ryegrass demonstrates an ability to rapidly de- velop resistance to the acetyl-CoA carboxylase- (AC- Case) (Heap and Knight 1990; Tardif et al. 1993) and acetolactate synthase-(ALS) inhibiting herbicides (Chris- topher et al. 1992), the first of which were introduced to the Australian cropping market in the early 1980s. These herbicide groups include most of the herbicides regis- tered for crop-selective control of rigid ryegrass in West- ern Australian cropping systems. Diclofop-methyl, the first ACCase-inhibiting herbicide available in Western Australia, was rapidly adopted by growers, primarily for selective control of rigid ryegrass in wheat. Other AC- Case-inhibiting herbicides became popular for control of grass weeds in dicot crops. Chlorsulfuron, the first of the ALS-inhibiting herbicides available, was also rapidly adopted by growers in the 1980s, largely because of its ability to selectively control rigid ryegrass and a broad spectrum of dicot weeds in wheat (Powles and Bowran 2000).

A study of Western Australian rigid ryegrass populations by Gill (1995) found that the development of resistance to the aryloxyphenoxypropanoate ACCase-inhibiting herbicides, such as diclofop-methyl, and the sul- fonylurea ALS-inhibiting herbicides, such as chlorsulfuron, was most common. Although multiple resistance

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WEED TECHNOLOGY

and cross-resistance in rigid ryegrass has been well doc- umented (Hall et al. 1994; Tardif et al. 1997), it is widely experienced that many aryloxyphenoxypropanoate-resistant populations remain susceptible to some of the cyclohexanedione ACCase-inhibiting herbicides.

It is important to quantify the extent and severity of herbicide resistance. Improved understanding of the extent and distribution of herbicide resistance should aid the targeting of herbicide resistance-related research and extension to specific cropping areas. Growers currently faced with managing highly resistant weed populations are likely to have different information requirements and face different costs compared with those still in a posi- tion to delay or prevent herbicide resistance (Orson 1999). A measure of the extent of resistance also pro- vides a useful benchmark for future monitoring of resistance development. Some surveys have measured the extent of resistant weed populations (e.g., Beckie et al. 1999; Bourgeois et al. 1997; Nietschke 1997; Pratley et al. 1993). No such study has previously been conducted in Western Australia.

The surveying of a large number of randomly chosen fields also presents the opportunity to measure the density of weeds present and relate this to herbicide resistance status. In studies examining economic aspects of herbicide-resistant rigid ryegrass management (e.g., Gorddard et al. 1995), the costs of resistance can be placed in two general categories. One is the cost of re- placing the herbicide to which resistance has developed with alternative weed control methods or herbicides (Be- ckie et al. 1999), and the other is the cost of yield re- ductions that may result from higher weed levels. By examining the relation between herbicide resistance status and weed density, it is possible to gain some insight into the relative importance of the latter.

In this paper we present data from a random survey of a large number of cropping fields within a range of agronomic areas in the Western Australian wheat belt. The study focuses on the most important weed, rigid ryegrass, and ACCase- and ALS-inhibiting herbicides, the herbicide classes to which rigid ryegrass is most likely to have developed resistance.

MATERIALS AND METHODS

Survey. In southern Australian cropping agroecosys- tems, rigid ryegrass infesting crops generally flowers in September and produces mature seeds by October. Seed maturation occurs before grain harvest, which usually takes place in November to December. In this survey, mature rigid ryegrass seed from plants infesting cropped

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Figure 1. Map of southwestern Western Australia showing agronomic areas from which rigid ryegrass populations were collected for herbicide resistance testing. Latitudes and average annual rainfall isohyets are shown.

fields was collected before harvest in 1998 (October to November). Two hundred sixty-four in-crop fields were randomly visited across eight agronomic areas of the central Western Australian wheat belt (Figure 1). Defined by three latitude and rainfall zones, this large area in- cludes a range of historical cropping intensities and selective herbicide use. Within each of the eight agronomic areas, 33 fields were randomly selected by traveling a predetermined distance (5 km) along minor and major roads and then surveying the nearest in-crop field. A hand-held global positioning system unit was used to record latitude and longitude for each site. The vast majority of surveyed fields (168) were wheat, with the re- mainder comprising lupin (27), barley (Hordeum vulgare L.) (26), oat (Avena sativa L.) (23), canola (Brassica napus L.) (17), chickpea (Cicer arietinum L.) (2), and field pea (Pisum sativum L.) (1).

An area of approximately 5,000 m2 was surveyed in each field by two people each following an inverted V- shaped path, beginning no less than 20 m from the edge of the field. Mature rigid ryegrass spikes were collected from plants along the sampling path. If more than 10 seed-producing rigid ryegrass plants were found within the sampling area, the seed sample was retained. If fewer than 10 seed-producing plants were found, no sample was retained, as it was considered that too few seedlings for testing may result or the seed sample may not sat- isfactorily represent the plant population in the sample area. Rigid ryegrass density was visually assessed on the basis of the following categories: none (no plants visible in survey area), very low (< 10 plants found in the sam-

Volume 15, Issue 2 (April-June) 2001 243



LLEWELLYN AND POWLES: HIGH LEVELS OF HERBICIDE RESISTANCE IN RIGID RYEGRASS IN AUSTRALIA

pling area), low (< 1 plant/M2), medium (1-10 plants/ mi2), high (> 10 plants/M2), and very high (> 10 plants/ mi2, rigid ryegrass dominating crop).

Resistance Testing. Seed samples representing 191 rigid ryegrass populations were collected and stored under normal summer temperatures before being germinated and grown for herbicide resistance testing from May to August 1999 at the University of Western Australia cam- pus. This is the normal growing period for this species in southern Australia. Plants were grown outdoors in 30 cm X 40 cm X 10 cm trays (potting mix of 50% com- posted pine bark, 25% peat, and 25% river sand, regu- larly watered) containing approximately 25 plants from each of three populations. A small number of populations did not achieve satisfactory germination or estab- lishment, resulting in fewer than 191 populations being tested.

ACCase-Inhibiting Herbicides. When the majority of rigid ryegrass plants were at the three-leaf stage (all had reached the two-leaf stage), diclofop-methyl was applied at 375 g/ha in two passes with an adjuvant4 (0.25% v/v) using a hand-held sprayer delivering 118 L/ha at 200 kPa. Mortality was recorded 14 d after treatment. Pop- ulations were classified as resistant if more than 20% of rigid ryegrass plants survived the herbicide treatment. Populations in which 1 to 20% of plants survived were classified as developing resistance. Where all plants were killed by the herbicide treatment, the population was classified as susceptible. All surviving individuals in the diclofop-methyl-resistant populations were subsequently treated with the cyclohexanedione herbicide, clethodim (48 g/ha with 1% v/v spray oil5). This herbicide was chosen to determine the extent to which the populations were resistant to both aryloxyphenoxypropanoate (diclofop-methyl) and cyclohexanedione (clethodim) herbicides. It is widely experienced that diclofop-methyl-resistant populations can initially be controlled with clethodim.

ALS-Inhibiting Herbicides. Initial testing for resistance to ALS inhibitors was conducted on 183 populations using chlorsulfuron (30 g/ha) applied to plants at the one- to two-leaf stage using the same procedure and adjuvant4 (0.1% v/v) as described for the diclofop-methyl testing. Three weeks after treatment, visual assessments were conducted and populations classified as resistant, devel-

4BS 1000 (1,000 g/L alcohol alkoxylate) Crop Care Australasia Pty. Ltd., Australia.

5 Hasten (704 g/L esterified [ethyl-based] canola oil and nonionic surfac- tants) Victorian Chemicals, Australia.

oping resistance, or susceptible as described above. Vi- sual assessment was used because of the nature of the response of rigid ryegrass to chlorsulfuron treatment in the absence of crop competition where distinct mortality does not always occur. Populations classified as resistant were trimmed to a height of 2 cm and allowed to regrow under glasshouse conditions before being treated with sulfometuron (30 g/ha) and adjuvant (0.1% v/v). It has been reported that rigid ryegrass resistance to sulfometuron indicates an ALS target-site-based resistance mechanism (Burnet et al. 1994).

Statistical Analyses. Because of the noninterval variables, analyses involving density were largely restricted to chi-square and rank order measures of association. To allow expected values to be of a sufficient size, rigid ryegrass density classifications of < 1 plant per m2, 1 to 10 plants per m2, and > 10 plants per m2 were used in contingency table tests. However, an ANOVA was also performed to test for significant differences in rigid ryegrass density between agronomic areas using the six rigid ryegrass density scores (log-transformed). For the populations tested for diclofop-methyl and chlorsulfuron resistance, a 3 X 3 chi-square contingency table test was used to determine associations between rigid ryegrass density and resistance to each herbicide. To test for any relation between resistance status to both diclofop-methyl and chlorsulfuron and rigid ryegrass density, populations were given a score of 2 for each diclofop-methyl and chlorsulfuron resistant classification and 1 for each developing resistance classification. This score was used in a 4 X 3 chi-square contingency table test to determine any association between combined resistance status and density. Proportional reduction in error measures of association for ordinal variables, gamma and Kendall's tau- b, are included to show the strength and direction of association (-1 to 1) for tests with chi-square P < 0.2. As a method to incorporate any effect of field location and crop type on density, least-squares regressions were also conducted using a log-transformation of the rigid ryegrass density score as the dependent variable. Instead of agronomic area, latitude and rainfall zone (< 325 mm = 1, 325 to 450 mm = 2, and > 450 mm = 3) were used. A dummy variable was used for crop type (wheat = 1, crop other than wheat = 0) and resistance status was scored as above.

RESULTS AND DISCUSSION

Extent of Resistance to ACCase-Inhibiting Herbi- cides. Of the 185 randomly collected rigid ryegrass pop-

244 Volume 15, Issue 2 (April-June) 2001



WEED TECHNOLOGY

Table 1. Proportion of plants from randomly selected Western Australian rigid ryegrass populations resistant to the aryloxyphenoxypropanoate ACCase-inhibiting herbicide diclofop-methyl.

Resistance Number of % of classificationa Resistant plants populations populations

% survivors

Resistant 81+ 13 7 61-80 6 3 41-60 10 5 21-40 14 8

Subtotal 43 23

Developing resistance 1-20 43 23 Susceptible 0 99 54

Total 185

aResistant (> 20% survivors), developing resistance (1-20% survivors), and susceptible (zero survivors).

ulations tested for resistance to diclofop-methyl, approximately 46% exhibited resistance (Table 1). As expected, there was variation in the percentage of resistant individuals within each rigid ryegrass population. Ten per- cent of all populations contained greater than 60% diclofop-methyl-resistant individuals. The relatively high number of populations with a low to moderate proportion of resistant plants (e.g., 1 to 40%) suggests that many growers may stop selecting for further diclofop- methyl resistance before the percentage of resistant plants reaches very high levels. This is understandable given that anything less than 90% mortality by herbicide application can be regarded as commercial herbicide fail- ure. Overall, across the vast area surveyed, 23% of all fields tested contained a rigid ryegrass population classified as resistant (> 20% survival) to diclofop-methyl.

The rigid ryegrass populations were collected from agronomic areas with different intensities of cropping history and therefore different levels of herbicide selection. As expected, given the varying history of herbicide

use in the different agronomic areas, the proportion of resistant populations varied greatly among areas (Table 2). In an agronomic area known for a recent history of continuous cropping and selective herbicide use (area 2), 96% of the tested populations were resistant or developing resistance. In this area, only one population was susceptible. This contrasts with areas where there has been less cropping, and consequently less selective herbicide use. In area 6, for example, all but one population were found to be susceptible. In this area, with higher rainfall and longer growing seasons, livestock grazing has continued to be more extensively practiced than cropping, although recent trends are toward more cropping. On the basis of Australian Bureau of Agricultural Economics figures (Anonymous 1999b), much of area 6 has approximately 30% of arable land cropped in any one season, with approximately 15% of the crop area being dicot crops. In area 2, both the proportion of land cropped and the proportion of dicot crops are approximately double these levels. Dicot crops such as lupins are often recognized as having a greater requirement for the postemergent selective grass control that ACCase- inhibiting herbicides provide. In this survey, areas 2 and 6 represent extremes in recent cropping intensity and, consequently, selective herbicide use. It is therefore expected that these two areas also represent the extremes of herbicide resistance development.

Treatment with clethodim resulted in 100% mortality for all but one of the diclofop-methyl-resistant populations. This result indicates that resistance to diclofop- methyl is unlikely to result in cross-resistance to clethodim. Susceptibility to at least this one ACCase-inhibiting herbicide generally remains even in areas with high diclofop-methyl resistance. The low level of clethodim resistance also indicates a relatively low use of this par-

Table 2. Rigid ryegrass populations from eight Western Australian agronomic areas classified according to resistance to the ACCase-inhibiting herbicide diclofop- methyl (Dicl.) and the ALS-inhibiting herbicide chlorsulfuron (Chlor.).

Resistance classificationa

Resistant Developing Susceptible Number of populations tested

Area Dicl. Chlor. Dicl. Chlor. Dicl. Chlor. Dicl. Chlor.

% of populations tested in area

1 12 9 12 27 76 64 25 22 2 73 62 23 27 4 12 26 26 3 40 67 40 11 20 22 20 18 4 24 18 38 45 38 36 21 22 5 15 50 15 30 70 20 20 20 6 0 0 4 21 96 79 23 24 7 13 58 26 17 61 25 23 24 8 7 44 30 26 63 30 27 27 All 23 38 23 26 54 36 185 183

a Resistant (> 20% survivors), developing resistance (1-20% survivors), and susceptible (zero survivors). Chlorsulfuron classification based on visual assessment.





ticular herbicide to date. Clethodim resistance is likely to become more common in future years, as use increases. However, large increases in the extent of clethodim resistance are unlikely to be a result of selection with diclofop-methyl. This cannot be said for other ACCase- inhibiting herbicides, including some cyclohexanedi- ones, to which diclofop-methyl resistance may be more likely to confer cross-resistance.

Extent of Resistance to ALS-Inhibiting Herbicides. Of the 183 randomly collected populations tested with chlorsulfuron, 64% were resistant or developing resistance (Table 2). As for ACCase-inhibiting herbicides, area 2 had the fewest susceptible populations and area 6 had the most. Overall, chlorsulfuron resistance was more common than diclofop-methyl resistance, with the greatest differences between chlorsulfuron and diclofop-methyl resistance levels found in the southeastern areas (areas 5, 7, and 8). In these areas, a range of agronomic and economic factors has resulted in a greater use of ALS- inhibiting herbicides relative to ACCase-inhibiting herbicides for rigid ryegrass control. In four of the eight agronomic areas, the proportion of resistant populations was 50% or greater. Overall, 38% of fields tested contained a chlorsulfuron-resistant population. These results indicate extensive resistance to chlorsulfuron and explain chlorsulfuron's greatly diminished value as a rigid ryegrass herbicide in most areas.

Of the 68 chlorsulfuron-resistant populations that were tested with sulfometuron, 93% (63 populations) were classified as resistant to sulfometuron (> 20% of plants surviving) (data not shown). This result suggests that target-site (ALS) resistance mechanisms were responsible for the vast majority of populations classified as chlorsulfuron-resistant. The result also supports the classification of the populations as chlorsulfuron-resistant.

Extent of Multiple Resistance to Both ACCase- and ALS-Inhibiting Herbicides. A total of 177 populations was tested for resistance to both diclofop-methyl and chlorsulfuron as representative ACCase- and ALS-inhibiting herbicides. Of these, 72% (128 populations) had some level of resistance to one or both of the herbicides. A significant association between diclofop-methyl and chlorsulfuron resistance was found (Table 3). As indi- cated by the measures of association commonly used for ordinal data, gamma and tau-b, the association is, as expected, positive. Only 8% of diclofop-methyl-resistant populations were susceptible to chlorsulfuron, whereas 40%o of chlorsulfuron-resistant populations were suscep-

Table 3. Multiple herbicide resistance to both the ACCase-inhibiting herbicide diclofop-methyl and the ALS-inhibiting herbicide chlorsulfuron in randomly collected rigid ryegrass populations.a

Chlorsulfuron Diclofop-methyl classificationc

classificationb Resistant Developing Susceptible Total

Number of populations

Resistant 23 17 27 67 Developing 13 13 21 47 Susceptible 3 11 49 63 Total 39 41 97 177

a Chi-square statistic = 24.8, 4 d.f., P < 0.001, Gamma = 0.46, Kendall's Tau-b = 0.31.

b Chlorsulfuron classification based on visual assessment. c Resistant (> 20% survivors), developing resistance (1-20% survivors),

and susceptible (zero survivors).

tible to diclofop-methyl. This suggests that fields with chlorsulfuron resistance are more likely to have diclofop- methyl remaining as an effective rigid ryegrass herbicide than vice versa. Without information on herbicide use histories and resistance mechanisms, further inferences are difficult. However, the results are consistent with an earlier nonrandom survey of rigid ryegrass populations in Western Australia (Gill 1995). Gill reported that some populations that had received mainly ACCase-inhibiting herbicides exhibited cross-resistance to ALS-inhibiting herbicides, although no populations that had received mainly ALS-inhibiting herbicides exhibited cross-resistance to ACCase-inhibiting herbicides. Given the high frequency of ALS target-site resistance found in this survey, it is clear that rigid ryegrass with multiple herbicide resistance mechanisms is common in the Western Aus- tralian wheat belt.

Extent and Density of Rigid Ryegrass Infestations. Rigid ryegrass was found in 230 of the 264 sampling areas surveyed (87%). The overall median rigid ryegrass density was low, with no significant difference between agronomic areas (ANOVA, log-transformation, P = 0.2, 263 d.f.; chi-square test, P = 0.23, 14 d.f.). Only 16% of fields contained rigid ryegrass at a high or very high density. These results suggest that although the presence of rigid ryegrass was consistent throughout all of the surveyed areas, it was, in general, at relatively low densities at the time of surveying. It is emphasized that the survey was conducted late in the growing season after the period in which most rigid ryegrass control practices are conducted by growers.

Resistance Status and Density. Resistance status was found to be independent of rigid ryegrass density for both diclofop-methyl (chi-square test, P = 0.45, 4 d.f.), chlorsulfuron (chi-square test, P = 0.33, 4 d.f.), and the




WEED TECHNOLOGY

Table 4. Populations classified according to rigid ryegrass density and a combined resistance score based on diclofop-methyl and chlorsulfuron resistance status. Populations sampled from wheat crops.

Resistance Rigid ryegrass density

scoreb 10/M2 Total

0 8 12 3 23 1 8 6 5 19 2 22 10 8 40 3 6 11 6 23 4 8 2 4 14 Total 52 41 26 119

aChi-square statistic = 11.4, 8 d.f., P = 0.18, Gamma 0.02, Kendall's Tau-b = 0.01.

b Score based on the sum of diclofop-methyl and chlorsulfuron resistance status where 2 is awarded for each resistant classification (>20% survivors), 1 for each developing resistance classification (1-20% survivors), and 0 for each susceptible classification, i.e., populations with a score of 4 are resistant to both herbicides (chlorsulfuron resistance status visually assessed).

combined resistance score (chi-square test, P = 0.058, 8 d.f., gamma = -0.07, Kendall's tau-b = -0.05). To allow for any interaction with the crop type from which the population was sampled, the analyses were also conducted using just populations sampled from wheat crops. Again, no significant association between density and resistance status to diclofop-methyl (chi-square test, P = 0.39, 4 d.f.), chlorsulfuron (chi-square test, P = 0.12, 4 d.f., gamma = -0.01, Kendall's tau-b = -0.01), or the combined resistance score was found (Table 4).

Using least-squares linear regression, chlorsulfuron resistance (P = 0.48), diclofop-methyl resistance (P = 0.66), and the combined resistance score (P = 0.85) again showed no significant association with density. As suggested by the homogeneity of rigid ryegrass density across agronomic areas, there was no significant association between latitude or rainfall and density in any model, with no model explaining more than 1% of variation (adjusted R2).

Although statistical interpretation is limited because of the use of ordinal visual assessment data, these analyses show that diclofop-methyl and chlorsulfuron resistance status was not significantly related to the density of rigid ryegrass found at the time of sampling. This suggests that rigid ryegrass can be maintained at low densities despite resistance to diclofop-methyl and chlorsulfuron. The current availability of cost-effective alternative rigid ryegrass control options such as some cyclohexanedione, preseeding, and precrop emergence herbicide treatments may partly explain this result. How- ever, without knowledge of each field's management, further interpretation is difficult.

In summary, rigid ryegrass resistance to the major herbicides diclofop-methyl and chlorsulfuron has

reached very high levels across much of the central Western Australian wheat belt. A large proportion of populations exhibit multiple resistance to the ACCase- and ALS-inhibiting herbicides. However, there are some areas remaining where these herbicides are still effective across most of the cropping land. The study also reveals that although a large proportion of cropping land contains rigid ryegrass resistant to diclofop-methyl or chlorsulfuron, the cyclohexanedione herbicide clethodim remains effective as a selective rigid ryegrass control op- tion across all areas. Thus, although the opportunity for growers to act to conserve the effectiveness of diclofop- methyl and chlorsulfuron generally remains in only a few areas, the opportunity to conserve the effectiveness of at least one rigid ryegrass-selective herbicide (clethodim) is available to the vast majority of growers across all areas. As selection for aryloxyphenoxypropanoate resistance appears to rarely result in resistance to all cy- clohexanediones, growers have had the opportunity to exploit some ACCase-inhibiting herbicides and yet still retain the effectiveness of clethodim. Extension strate- gies may benefit from incorporating a greater level of acceptance of the herbicide use paradigm that, to now, has generally involved growers exploiting susceptibility to some herbicides until resistance develops. A focus on the economic benefits of conserving susceptibility to just some or one of these selective herbicides would be more consistent with growers' past herbicide use and current herbicide status, and, as such, relevant to the greatest number of growers.

ACKNOWLEDGMENTS

This project was funded by the Australian Grains Re- search and Development Corporation. R.S.L. thanks the Co-operative Research Centre for Weed Management Systems for additional support. We also thank the mem- bers of the Western Australian Herbicide Resistance Ini- tiative research team who assisted in the field survey.

LITERATURE CITED

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Anonymous. 1999b. Australian Farm Surveys Report. Australian Bureau of Agricultural and Resource Economics, Canberra.

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Bourgeois, L., I. N. Morrison, and D. J. Kelner. 1997. Field and producer survey of ACCase resistant wild oat in Manitoba. Can. J. Plant Sci. 77: 709-715.

Burnet, M.W.M., Q. Hart, J.A.M. Holtum, and S. B. Powles. 1994. Resistance





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Tardif, F J., J.A.M. Holton, and S. B. Powles. 1993. Occurrence of a herbicide resistant acetyl-coenzyme A carboxylase mutant in annual ryegrass (Lol- ium rigidum) selected by sethoxydim. Planta 190:176-181.

Reviews of Weed Science-Volume 6, 1994

This volume contains an array of reviews that should be of great interest to practical weed scientists, weed biologists, ecologists, plant anatomists, plant physiologists, soil scientists, soil microbiologists and pesticide scientists.

This Review may be ordered for $20.00 per copy from the Weed Science Society of America, P.O. Box 7050, 810 East 10th St., Lawrence, KS 66044-8897. (Shipping charge: $3.50 for first copy; $0.75 for each additional copy.) Ph: (800) 627-0629 (U.S. and Canada), (785) 843-1235; Fax: (785) 843-1274; E-mail: wssa@ allenpress.com. Remittance to accompany order.

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Article Contentsp. 242p. 243p. 244p. 245p. 246p. 247p. 248

Issue Table of ContentsWeed Technology, Vol. 15, No. 2 (Apr. - Jun., 2001), pp. 199-398Front MatterResearchHigh Frequency of Chlorsulfuron-Resistant Wild Radish (Raphanus raphanistrum) Populations across the Western Australian Wheatbelt [pp. 199-203]Sweet Corn (Zea mays) Cultivar Sensitivity to CGA 152005 Postemergence [pp. 204-207]Imazamethabenz Persistence in a Wheat (Triticum aestivum): Potato (Solanum tuberosum) Rotation [pp. 208-215]The Relationship of Goosegrass (Eleusine indica) Stage of Growth to Quinclorac Tolerance [pp. 216-219]Adjuvant Efficacy with Quinclorac in Canola (Brassica napus) and Turfgrass [pp. 220-223]Susceptibility of Dry Edible Bean (Phaseolus vulgaris, Cranberry Bean) to the Rotary Hoe [pp. 224-228]Effect of Tillage, Row Spacing, and Herbicide on the Emergence and Control of Burcucumber (Sicyos angulatus) in Soybean (Glycine max) [pp. 229-235]Effectiveness of Ammonium Thiosulfate to Enhance Weed Control and Reduce Cotton (Gossypium hirsutum) Injury [pp. 236-241]High Levels of Herbicide Resistance in Rigid Ryegrass (Lolium rigidum) in the Wheat Belt of Western Australia [pp. 242-248]Weed Control in Field Corn (Zea mays) with RPA 201772 Combinations with Atrazine and S-Metolachlor [pp. 249-256]Syngenta Quick-Test: A Rapid Whole-Plant Test for Herbicide Resistance [pp. 257-263]Effects of Herbicides on Italian Ryegrass (Lolium multiflorum), Forage Production, and Economic Returns from Dual-Purpose Winter Wheat (Triticum aestivum) [pp. 264-270]Responses of Potato (Solanum tuberosum), Tomato (Lycopersicon esculentum), and Several Weeds to ASC-67040 Herbicide [pp. 271-276]Timing Weed Removal in Field Pea (Pisum sativum) [pp. 277-283]Effect of Seeding Rate of Drilled Glyphosate-Resistant Soybean (Glycine max) on Seed Yield and Gross Profit Margin [pp. 284-292]Cross-Resistance in and Chemical Control of Auxinic Herbicide-Resistant Yellow Starthistle (Centaurea solstitialis) [pp. 293-299]Response of Double-Crop Glyphosate-Resistant Soybean (Glycine max) to Broadleaf Herbicides [pp. 300-305]Broomrape (Orobanche cumana) Control in Sunflower (Helianthus annuus) with Imazapic [pp. 306-309]Kyllinga squamulata Control in Bermudagrass Turf [pp. 310-314]Flood Depth, Application Timing, and Imazethapyr Activity in Imidazolinone-Tolerant Rice (Oryza sativa) [pp. 315-319]Fluazifop-P Inhibits Terbacil Metabolism in Strawberry (Fragaria × ananassa) [pp. 320-326]Influence of Norlurazon Placement on Yellow Nutsedge (Cyperus esculentus) [pp. 327-331]Sweet Corn (Zea mays) Cultivar Sensitivity to RPA 201772 [pp. 332-336]Purple Loosestrife (Lythrum salicaria) Seed Germination [pp. 337-342]Resistance to BAY MKH 6562 in Wild Oat (Avena fatua) [pp. 343-347]Effect of Row Spacing and Herbicides on Burcucumber (Sicyos angulatus) Control in Herbicide-Resistant Corn (Zea mays) [pp. 348-354]Effect of Soil Moisture on Efficacy of Imazethapyr in Greenhouse [pp. 355-359]Economic Evaluation of Diclosulam and Flumioxazin Systems in Peanut (Arachis hypogaea) [pp. 360-364]Response of Rotational Crops to BAY MKH 6561 [pp. 365-374]Biological Control of Woollyleaf Bursage (Ambrosia grayi) with Pseudomonas syringae pv. tagetis [pp. 375-381]Cheat (Bromus secalinus) Control with Herbicides Applied to Mature Seeds [pp. 382-386]

NoteA Modified Power Tiller for Metham Application on Cucurbit Crops Transplanted to Polyethylene-Covered Seedbeds [pp. 387-395]

Technology Notes [pp. 396-397]Back Matter [pp. 398-398]

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