Impact of generic herbicides on current and future weed problems and crop management

Transcription

1 Impact of generic herbicides on current and future weed problems and crop management Jim Orson Morley Research Centre, Morley, Wymondham, Norfolk, NR18 9DB, UK Miles Thomas Pesticide Usage Surveys Group, Central Science Laboratories, Sand Hutton, York, YO4 1LZ, UK Per Kudsk, Danish Institute of Agricultural Science, Flakkebjerg, Denmark ABSTRACT Some generic herbicides remain relevant to current and future cropping systems despite re-registration in the European Union reducing the number on the market, particularly for minor crops and uses. They will continue to play a major role in minor crop production and in broad-leaved weed control in broad-acres crops. Indeed, the cost and efficacy of some generic herbicides and/or the possibility of the introduction of herbicide tolerant crops in Europe is reducing the incentive to discover and develop new active substances. However, increased registration standards and/or the lack of flexibility in usage to meet the demands of modern cropping systems have already resulted or may result in the reduction in usage of other generic herbicides in major crops. On the other hand, current changes in weed infestations may favour the use of some generic herbicides, at least in the short term. Herbicide resistance has resulted in the increased usage of some generic herbicides and the decreased usage of others. BACKGROUND Generic herbicides are defined for the purposes of this session as those that are off-patent and available from a source other than, or in addition to, the original manufacturer. In terms of agriculture within the European Union (EU) they are listed in either Table 1 or Table 3. Many current generic pesticides were instrumental in the rapid development of today's farming systems and the associated increase in yields in the 1970s and 1980s. Effective herbicides allowed farmers in Northern Europe for the first time to only grow crops on land most suited for their production. No longer did crops have to be grown solely for their contribution to weed control through the rotation. Higher yields and supported prices during the 1970s resulted in winter wheat becoming economically attractive and there was a rapid intensification of winter wheat, particularly on the heavier soil types in Northern Europe. These soils can sustain high yields of autumnsown crops but spring sown cropping is unreliable due to poor seedbeds and possible late sowing resulting from wet soil. Unfortunately, black-grass (Alopecurus myosuroides) shares the same growth cycle as autumn sown crops and is associated with heavy land farming.

2 Although individual plants of this small seeded grass weed are not very competitive, numbers can build up rapidly in successive autumn-sown crops if high levels of chemical control are not achieved. The introduction of chlorotoluron and isoproturon in the early 1970s provided the very high levels of control required of this weed in winter wheat and also some control of wild-oats (Avena fatua and Avena ludoviciana spp. ludoviciana) and a range of broad-leaved weeds. Thus winter wheat could be grown intensively or even continuously and could also be sown earlier in the autumn. This not only reduced the risk of wet soils preventing reliable establishment but also spread the work peak associated with a rotation that comprises solely of autumn established crops. During the 1970s there was also a rapid increase in the area of production of winter oilseed rape in Northern Europe. The European Community heavily supported this crop in order to develop vegetable oil production in Europe. Again, like cereals, autumn sowing results in higher and more reliable yields than spring sowing. Furthermore, this broad-leaved crop could be grown very successfully on heavy soils, so further encouraging rotations that comprised solely of autumn-sown crops on this soil type. It was the introduction of the aryloxyphenoxypropionates, commonly known as the 'fops', in the early 1980s that provided the reliable control of annual grass weeds, including black-grass and volunteer cereals, in annual broad-leaved crops. The autumn sowing of winter wheat also provided higher yields on some lighter soils that were traditionally less prone to black-grass infestation, such as the limestone brash soils. The late 1970s also saw the start of a significant trend to autumn-sown rather than springsown barley, particularly on the medium to light soils in Northern Europe. This was also encouraged by increased yield potential, improved cultivars and the introduction of effective fungicides. Hence, on these soil types, where a higher proportion of cropping was sown in the autumn, black-grass infestations also developed and these were again controlled by isoproturon and chlorotoluron. Throughout Northern Europe the increased intensification of autumn-sown crops also resulted in a change in the spectrum of annual broad-leaved weeds occurring in rotations. Cleavers (Galium aparine), the speedwells (Veronica spp.) and field pansy (Viola arvensis) became more prevalent, particularly as they were not controlled by isoproturon or chlorotoluron. In addition, with the earlier sowing of winter cereals, common poppy (Papaver rhoeas) plants grew too large to control in the spring with herbicides such as MCPA. These and other broad-leaved weeds in winter cereals were controlled at that time with bromoxynil plus ioxynil mixed with mecoprop. Since then the introduction diflufenican, which is now a generic herbicide, and fluroxypyr and metsulfuron-methyl, which are both usually used in mixture with generic herbicides, have improved the reliability of control. However, the control of broad-leaved weeds is still not reliable in autumn-sown broad-leaved crops, including oilseed rape. This all suggests that the European farmer, having developed current systems based on generic herbicides, can continue to rely on them in the future. However, there are significant issues that will influence the future usage of generic herbicides. These include:

3 Relevance to current and future cropping systems Environmental impact European registration reviews Market size Herbicide resistance Herbicide tolerance FACTORS AFFECTING THE CURRENT AND FUTURE USAGE OF GENERIC HERBICIDES Relevance to current and future cropping systems Most generic herbicides have witnessed a decline in usage as more effective and appropriate herbicides are introduced (Table 1). It is unlikely that cropping systems will return to those that enabled the effective exploitation of herbicides such as 2,4-D and dicamba. Current systems encourage weeds that are not controlled by these herbicides and, with earlier drilling of winter cereals, weeds are too advanced by the time these herbicides can be applied. In addition, the desire to reduce labour and machinery costs results in farmers requiring a wider application window than these herbicides offer. Furthermore, reduction is usage may also be due to lack of ability to tank-mix other pesticides. This is particularly true of bifenox. Naturally, where a generic herbicide is relatively cheap for the farmer to use, fits in with the current usage of labour and machinery, is very effective and currently used on a large scale, such as diflufenican in winter cereals, its continued large scale use is relatively assured in the medium term. The cost and efficiency of such herbicides reduces the incentive to discover and develop alternatives. This assumes that resistance to such generic herbicides does not develop and environmental problems are not identified. Increases in the area infested with a specific weed may also influence the usage of specific generic herbicides. The usage of chlorotoluron has increased in cereals over the last two years in the UK because of increased incidence of Italian rye-grass (Sissons, Makhteshim, personal communication). Some generic herbicides have unique uses. For instance, asulam is the only herbicide approved in the UK for bracken (Pteridium aquilinum) control in grassland, which is an important but minor use. Hence, its usage will continue at current levels unless there is a change in the current status of the weed or use is restricted by registration requirements. Environmental impact When the generic herbicides were first introduced there were less stringent registration standards, particularly on environmental impact. There has been a steady increase in the usage of isoproturon over the last thirty years or so due to its low cost and wide spectrum of control of annual grass weeds (Table 1). Survey data suggest that, whilst the number of sprayed hectares has increased, the average dose used has decreased. This may be for two reasons. Firstly, the maximum dose is now less commonly used alone to control black-grass. Secondly, unlike more recent introductions, it has broad-spectrum control of a range of annual grass weeds in cereals, such as annual

4 meadow-grass (Poa annua). These are controlled at lower doses and may be becoming more common as a result of the now long-term adoption of a higher proportion of autumn-sown crops on all soil types. Isoproturon now commonly occurs in surface waters at levels above those stipulated in EU directives. There are calls by water companies to reduce the usage and/or maximum dosage of this herbicide in order to reduce their costs of ensuring that water supplies meet modern standards. Currently the UK registration authorities are reviewing the usage of this herbicide in order to reduce its movement to watercourses. It is for the same reasons that the generic herbicide mecoprop was replaced by the active optical isomer mecoprop-p, effective at half the dose of the racemic mixture. In addition, the non-crop usages of atrazine were withdrawn because of contamination of watercourses although its use in maize (Zea mais) continues (Table 1). European registration reviews/market size Active substances registered in or before 1993 are being reviewed by the European Commission (EC). A significant number will not be supported by crop protection companies because the costs of generating data required cannot be justified in terms of sales within the EU. Those active substances that are unsupported will be withdrawn in July Selective herbicides are affected most by this process because fungicides and insecticides tend to have uses across a greater range of crops. This will have a significant impact on minor uses, mainly in horticulture but also in larger area crops and forestry. For example, fomesafen, terbutryn and terbuthylazine will not be supported and this will mean that there will be no pre-emergence herbicides available for use in peas produced in the UK for quick-freezing and canning. Fomesafen is not a generic herbicide and is widely used for post-emergence weed control in dwarf bean (Phaseolus vulgaris) production in France. Manufacture of some unsupported active substances may continue for markets outside the EU. Derogation may be granted by the EC for essential uses of some of these particular herbicides for a short period to allow time for alternative weed control measures to be developed. Tables 1 and 3 indicate those generic herbicides that are likely to be withdrawn from the EU in 2003 unless derogation is granted. Herbicide resistance Herbicide resistance in black-grass was first recorded in the UK in Since then the enhanced metabolism mechanism of resistance has become almost endemic in areas of intensive winter wheat production. Whilst the generic herbicides isoproturon and chlorotoluron gave, on average, around 99% control in the early 1970s, these herbicides often now give below 50% control. This is insufficient to contain populations and in many cases farmers have now switched partially or wholly away from these herbicides for the control of black-grass to alternative and more effective herbicides, such as clodinafop-propargyl and very recently to flupyrsulfuron and pendimethalin mixtures. On the other hand, herbicide resistance in black-grass has resulted in the increased usage of the generic herbicides trifluralin and tri-allate. Whilst these herbicides offer only partial control of black-grass, their efficacy is or is only partially reduced by the enhanced metabolism mechanism of

5 herbicide resistance in this weed and they are now commonly used to supplement the efficacy of other herbicides (Table 1). The use of aryloxyphenoxypropionate herbicides has been limited by the need to manage herbicide resistance. The only generic herbicide in this group is fluazifop-p-butyl (Table 1). The efficacy of more recently introduced herbicides from the cyclohexanedione group, such as cycloxydim and tepraloxydim, is not affected by the enhanced metabolism form of resistance in black-grass and also may not select for this trait. However, herbicides from both these groups generally do not control black-grass with the rapidly increasing recorded instances of the target site mechanism of resistance. Nonetheless, tepraloxydim has some level of control of target site resistant black-grass. Hence, there is a need to temper the use of herbicides from both these groups. There is now increasing concern regarding herbicide resistance in wild-oats and Italian ryegrass (Lolium multiflorum). The same resistance mechanisms found in black-grass have been recorded but, of the generic herbicides, there has been no resistance recorded in wild-oats in cereals to isoproturon and flamprop-p-isopropyl. There is the wider issue that generic herbicides offer alternative modes of action and hence may play an important role in the prevention of the development of herbicide resistance. A few herbicide modes of action dominate weed control and there is particular concern over the increased usage of the sulfonylureas across a range of crops. Target site resistance to this mode of action is now common in common poppy in northern Mediterranean countries and generic herbicides such as bromoxynil and ioxynil with mecoprop or an alternative herbicide have an important role in the control of these resistant stocks. Herbicide tolerant crops Those generic herbicides that provide effective and cheap broad-leaved weed control in the non-major crops, such as oilseed rape and sugar beet, may continue in the medium term to be used on a wide scale. This is partly because in these crops, some of which are relative small uses in world markets, herbicide-tolerance to non-selective herbicides may be introduced, reducing further the incentive of companies to develop selective alternatives. Hence, in oilseed rape and sugar beet, herbicides such as chloridazon, lenacil and metazachlor will continue to be used at current levels until the introduction of herbicide tolerance to glyphosate or glufosinate. However, the lack of incentive to develop selective herbicides for broad-leaved crops will reflect on future options in minor broad-leaved crops that may not be subject to genetic modification. REFERENCES Chapman, P. J. (2001). The European Commission Review of Plant Protection Products: Essential Uses. Proceedings of the BCPC Conference Weeds 2001, 3, 9C - 2. Thomas, M. R. (1997). Pesticide Usage Survey Report 100: Review of usage of pesticides in agriculture and horticulture throughout Great Britain MAFF Reference Book PB MAFF Publications. Wise, C. J. C. (2001). Loss of herbicides for minor crops Impact on European Growers. Proceedings of the BCPC Conference Weeds 2001, 3, 9C - 4.

6 Table 1. Sprayed area** (hectares) of all the generic herbicides* used in Great Britain in 1998 and of some alternative herbicides used in grassland, arable crops and forage maize in Great Britain ,4-D* Cereals 24,929 3,141 5,302 2,274 3, Grassland 387,788 39,788 39,788 66,172 66,172 84,611 All broad-acre crops 412,717 42,929 45,091 68,447 69,758 84,683 Asulam* Grassland 501,623 23,174 23,174 34,003 34,003 44,876 All broad-acre crops 501,623 23,212 23,594 34,003 34,003 44,876 Atrazine* All broad-acre crops 3, ,364 6, ,219 Fodder crops 14,766 24,120 24,120 71,019 71, ,579 Bentazone* Cereals 15,283 11,914 13,275 7,776 5,826 3,272 Potatoes 203 6,316 4,456 4,561 6,416 8,159 Pulses 19,928 39,366 32,952 59,619 57,754 57,764 All broad-acre crops 49,037 87, ,359 97,560 86,711 87,733 Bifenox* Cereals 83,235 74,945 56,472 17,647 2, All broad-acre crops 83,235 74,945 56,472 17,647 2, Bromoxynil* Cereals 1,128, , , , , ,355 All broad-acre crops 1,201, , , , , ,537 Carbetamide* Oilseeds 31,107 23,080 7,495 8,232 9,985 7,670 All broad-acre crops 34,879 27,108 12,190 10,180 14,383 12,943 Chloridazon* Sugar beet 107, , , , , ,564 All broad-acre crops 107, , , , , ,787 Chlorotoluron* Cereals 293, , ,418 55,884 95,665 34,939 All broad-acre crops 293, , ,418 55,884 95,665 34,939 Chlorpropham* Sugar beet 20,210 21,331 21,278 13,817 10,010 11,335 All broad-acre crops 20,210 21,331 21,278 29,941 26,134 11,606 Clodinafop-propargyl Cereals , ,160 All broad-acre crops , ,160

7 Table 1. (contd.) Sprayed area** (hectares) of all the generic herbicides* used in Great Britain in 1998 and of some alternative herbicides used in grassland, arable crops and forage maize in Great Britain Cyanazine*# Cereals 73,453 63,890 41,084 10,352 38,891 21,151 Oilseeds 9,322 22,884 19,313 14,763 20,574 20,413 Pulses 36,078 36,447 36,941 49,085 60,287 82,404 All broad-acre crops 125, , ,657 75, , ,836 Desmedipham* Sugar beet , , ,204 All broad-acre crops , , ,204 Dicamba* Cereals 113,443 50,438 42,739 54,373 68, ,706 Grassland 591,434 89,758 89, , ,047 98,932 All broad-acre crops 704, , , , , ,638 Dichlorprop*# Cereals 114,693 37,624 20,608 10,408 2,899 2,580 Grassland 24,257 1,135 1, All broad-acre crops 138,950 38,759 21,743 10,463 2,953 2,580 Diclofop-methyl* Cereals 146, ,150 57,834 72,993 90,485 81,950 All broad-acre crops 158, ,851 60,780 73,191 90,485 81,950 Diflufenican* Cereals 269, , , ,943 1,194,105 1,423,315 All broad-acre crops 269, , , ,943 1,194,316 1,424,368 Ethofumesate* Sugar beet 44, , , , , ,304 All broad-acre crops 81, , , , , ,757 Fenoxaprop-P-ethyl Cereals , , ,298 All broad-acre crops , , ,298 Fenoxaprop-ethyl# Cereals 0 185, , ,567 98,376 28,997 All broad-acre crops 0 185, , ,567 98,376 28,997 Flamprop-M-isopropyl* Cereals 199, ,914 75,129 38,215 20,672 26,900 All broad-acre crops 199, ,134 75,349 38,487 20,944 26,917 Fluazifop-P-butyl* Oilseeds 119, , ,488 81,560 89, ,618 Sugar beet 30,315 35,310 19,211 36,066 18,345 17,999 All broad-acre crops 155, , , , , ,570

8 Table 1. (contd.) Sprayed area** (hectares) of all the generic herbicides* used in Great Britain in 1998 and of some alternative herbicides used in grassland, arable crops and forage maize in Great Britain Flupyrsulfuron-methyl Cereals ,351 All broad-acre crops ,351 Glyphosate* Cereals 283, , , , , ,563 Oilseeds 34,293 44,669 80,016 89,096 60, ,690 All broad-acre crops 476, , , , , ,608 Ioxynil* Cereals 1,256, , , , , ,244 Grassland 59,910 11,846 11,846 39,303 39,303 31,143 All broad-acre crops 1,316, , , , , ,387 Isoproturon* Cereals 1,521,205 2,183,976 2,180,436 1,991,091 3,046,165 3,492,811 All broad-acre crops 1,521,665 2,183,976 2,181,248 1,991,091 3,046,376 3,493,785 Lenacil* Sugar beet 4,042 50,741 98, , , ,395 All broad-acre crops 4,992 50,741 98, , , ,395 MCPA* Cereals 568, , , , , ,148 Grassland 1,107, , , , , ,084 All broad-acre crops 1,691, , , , , ,113 Mecoprop* Cereals 1,771,370 1,030, , , , ,938 Grassland 666, , , , , ,472 All broad-acre crops 2,438,121 1,154, , , , ,412 Mecoprop-P Cereals 58, , , , , ,825 Grassland 0 4,377 4,377 61,653 61,653 88,654 All broad-acre crops 58, , , , , ,478 Metamitron* Sugar beet 276, , , , , ,053 All broad-acre crops 276, , , , , ,053 Metazachlor* Oilseeds 63,507 76, , , , ,882 All broad-acre crops 63,507 76, , , , ,177 Methabenzthiazuron* Cereals 62,449 68,894 53,064 27,769 14,399 2,141 All broad-acre crops 62,449 68,894 53,064 32,264 18,870 2,141

9 Table 1. (contd.) Sprayed area** (hectares) of all the generic herbicides* used in Great Britain in 1998 and of some alternative herbicides used in grassland, arable crops and forage maize in Great Britain Metoxuron*# Potatoes 5,851 10,017 1,486 1,639 1, All broad-acre crops 6,120 10,292 1,789 1,639 1, Paraquat* Cereals 59,205 61,549 26,853 16,352 24,529 19,508 Potatoes 99, , , , , ,340 Sugar beet 4,042 9,624 20,682 7,382 4,566 12,570 All broad-acre crops 268, , , , , ,666 Pendimethalin* Cereals 191, , , , , ,482 Pulses 14,945 4,378 18,161 16,519 12,471 27,332 All broad-acre crops 209, , , , , ,157 Quizalofop-P-ethyl* Oilseeds ,742 Sugar beet All broad-acre crops ,896 Quizalofop-ethyl*# Oilseeds 28,586 95,663 72,446 34,331 35,268 25,671 Sugar beet 6,063 2,630 4,917 2,064 3,073 1,072 All broad-acre crops 34,649 98,293 78,375 38,688 38,341 26,743 Simazine* Cereals ,444 61, , ,820 Pulses 99, , ,613 92,242 83,458 90,378 All broad-acre crops 99, , , , , ,197 Terbuthylazine* Cereals 0 0 2,710 3,999 36,410 33,191 Pulses 71,367 45,787 30,835 38,292 35,028 26,315 All broad-acre crops 83,105 60,640 48,128 53,198 91,142 74,423 Terbutryn*# Cereals 45,350 33,704 14,708 9,692 23,103 36,077 Potatoes 11,778 24,545 26,869 21,230 31,084 24,501 Pulses 86,294 58,447 42,944 47,514 49,344 35,577 All broad-acre crops 143, ,696 84,520 78, ,530 96,168 Tralkoxydim Cereals , , ,415 All broad-acre crops , , ,415 Tri-allate* Cereals 73, , ,697 87, , ,877 All broad-acre crops 81, , , , , ,458

10 Table 1. (contd.) Sprayed area** (hectares) of all the generic herbicides* used in Great Britain in 1998 and of some alternative herbicides used in grassland, arable crops and forage maize in Great Britain Trifluralin* Cereals 329, , , , , ,725 Oilseeds 9,955 12,779 28,909 46,642 32,984 98,657 All broad-acre crops 341, , , , , ,373 * = generic herbicide ** sprayed area is active substance area treated (Thomas, 1997) # = likely to be withdrawn for usage in 2003 unless designated as an essential use Table 2. Great Britain crop areas (ha), 1998 (source: DEFRA, 2000) Cereals - wheat, barley, oats, rye and triticale Oilseeds - oilseed rape and linseed (excluding industrial crops on set-aside) Potatoes - all ware and seed potatoes Pulses - field beans (Vicia faba) and pease for harvesting dry Sugar beet Grassland - including sole-right rough grazing All broad-acre crops (excludes maize for forage, includes all grass including sole-right rough grazing) Maize for forage 3,366,760 ha 619,615 ha 158,382 ha 212,319 ha 188,355 ha 10,132,751 ha 14,866,537 ha 102,600 ha Table 3. Usage 1 in Great Britain of other generic herbicides not featured in Table Acetochlor * Amitrole 7, ,923 1,923 0 Benfluralin * Bromacil# * Butralin * Chlorsulfuron 68,132 6,949 1,061 1, Chlorthal-dimethyl * Dichlobenil * Diuron ,923 1,923 0 Ethalfluralin * Fluometuron * Mefluidide * Napropamide 4, Oxyfluorfen * Picloram * Propachlor , Propanil * 1 = active substance area treated (Thomas, 1997) on crops listed in Table 2 * = No recorded uses on the crops listed in Table 2 over this period # = likely to be withdrawn for usage in 2003 unless designated as an essential use