1. Introduction
Food
production is perhaps the most important occupation of most of the population
of the globe. However, in the developed world, the population
engaged in primary agriculture is declining rapidly. For example, in the UK, the population working
on the land has declined by over 77% since 1945 (Robinson & Sutherland, 2002) and now involves much less than 1% of the population.
This reflects profound changes in agriculture over the past three hundred
years, dating from the agricultural revolution of the late 1700s.
The commercialisation of artificial fertilisers in the 1850s has been followed
by progress in mechanisation, plant breeding and the development of pesticides
over the past fifty years (Stoate et
al., 2002). The most recent
advances have been made using molecular biology to genetically modify crops.
The first generation of genetically modified (GM) crops is now grown on
over 50 million ha round the world (James,
2001), the majority of which are in the developed world, but a fifth in the developing
world. Two approaches have been taken
initially, first to introduce pest and disease resistance into crops and secondly
to introduce herbicide tolerance genes. Pest and disease resistance can result in reduced pesticide inputs,
whilst herbicide tolerance may allow some simplification of weed control.
Herbicides continue to be the largest part of pesticide sales across the
globe, reflecting the major constraint on crop yield caused by weed species. So-called genetically modified herbicide-tolerant (GMHT) crops,
whilst extensively grown elsewhere in the world, are subject to restrictions within
Europe, not having achieved approval for general use. There are concerns in regard to the movement
of transgenes (e.g. Desplanque et al.,
2002) and on possible impacts on biodiversity (Firbank et al., 2003a). The technology
remains contentious, with the need for thorough evaluation of the risks and benefits
under different conditions (Marshall, 2003; Pretty, 2001; Tester, 2001). The reduction
in pesticide use where crops have been engineered for resistance can be contrasted
with herbicide-tolerant crops that require agrochemicals (Marshall, 2001; 2003).
The first large-scale experimental field evaluation of such GMHT crops
has been the subject of recent research in the UK (Firbank et al., 2003a; Perry et al., 2003). The Field Scale
Evaluation of GM crops has compared the biodiversity associated with four GMHT
crops with conventionally grown cultivars, using split field treatments.
As such, the comparison is between two approaches to crop management, rather
than a test per se of the genetic component of the contrasted cultivars. Using GMHT cultivars, a broad spectrum herbicide is used to control
most weed species, often at a later stage in the growth of the crop compared with
conventional herbicides. The latter products
often control only a limited number of weed species with sufficient safety to
the crop. The development of GM crops
is only the most recent example of technologies that intensify land management
and the selection pressures on components of agroecosystems.
In
the UK, agriculture is a key component of land use, with farming and forestry
together amounting to over 75% of the land surface (Dalton
& Brand-Hardy, 2003). Nevertheless,
agriculture already has significant impacts on the environment, as noted by the
UK Environment Agency:
Collectively,
farming has the following impacts:
·
Farmers have addressed the most acute problems associated with slurry
and silage through investment in better infrastructure with government support.
This has reduced the number of serious pollution incidents (Category 1) between
1989 and 2000 from 522 to 21. However, 27 per cent of Category 1 and 2 water pollution
incidents still arise from agriculture.
- In 2000, more than 30 of the 50
pesticides found most frequently in surface waters, above the EU drinking water
limit (0.1 micrograms per litre) were plant protection products predominantly
used in agriculture.
- Livestock farming contributes
about 85 per cent of the UK’s ammonia emissions. This makes up about half of the
nitrogen deposition, and leads to increased acidification of sensitive soils and
waters.
- Phosphate leaching, and loss of
soil containing phosphate, significantly contributes to the eutrophication of
rivers and lakes. Excessive algal growth results in up to 200 freshwaters annually.
- There is increasing evidence that
surface water run-off rates are increasing -as a result of the loss of woodland,
pastureland, and changes in soil management. This increases the risk of flooding.
- Similarly, sedimentation of salmon
and trout rivers damages spawning gravels and fish egg survival.
- Changes in land management linked to
more intensive agriculture have been associated with the loss of habitat and biodiversity
and notably the loss of certain key species of farmland birds.
Taken from: http://www.environment-agency.gov.uk/business/444304/444312/306330/306360/?lang=_e
Pesticides
residue levels are analysed regularly by the UK Environment Agency as an indicator
of water quality. A number of pesticides
are found in water samples above the target levels (Fig. 1). The data indicate a marked decline in atrazine
residues after 1993; this probably reflects the restricted use of the product
after that date. Prior to 1993, atrazine
was widely used on railways, rights of way and industrial areas, often where hard
surface runoff contributed to residues in surface waters. An increase in atrazine incidents in the mid
to late 1990s may be associated with increased maize production.
Fig
1. Percentages of water samples found
with eight herbicides at concentrations above the 100 ng/l level between 1993
and 2001.
Forage
maize (Zea mays L.) is grown widely across southern Britain.
It is harvested for whole-crop silage and is now an important part of livestock
farming, particularly for the intensive dairy sector in the UK. A minor horticultural crop, sweetcorn, is also
grown, particularly in the south-east of Britain, where summer temperatures are
generally higher than elsewhere in the UK. The area of maize has grown fourfold since
1989 (Garthwaite & Thomas, 1999) and in 1997, six years ago, the acreage was estimated
at 109,413 ha (Garthwaite et al., 1997). The maize acreage
in 2002 was estimated as 121,000 ha (Nix,
2003), indicating that the rapid increase in area during the
1990s has now stabilised.
In
the 1997 Pesticide Usage Survey, maize was the major fodder crop grown, amounting
to 52% of the total area of fodder crops in Great Britain.
The amounts of the crop grown in different regions of Great Britain are
shown in Table 1. The south west of Britain
grows the largest amount of maize, reflecting the predominance of dairy and beef
enterprises in the region. The Midlands
and south east also grow significant amounts of fodder maize.
Table
1. Acreages (ha) of maize and all
fodder and grassland crops in Great Britain in 1997.
Data from (Garthwaite et al., 1997).
| |
Northern |
Midlands
& Western |
Eastern |
South
Eastern |
South
Western |
Wales |
Scotland |
Great
Britain |
|
Maize | 1873 | 23382 | 8768 | 21120 | 48818 | 5452 | - | 109413 |
|
All
fodder and grassland | 1299492 | 1104254 | 324762 | 469031 | 1226138 | 1353541 | 4355173 | 10132392 |
Maize
crops have been of concern to those responsible for the environment, as the crop
has been implicated in contributing to diffuse pollution.
These concerns focus on soil erosion and increased sedimentation in rivers,
the use of cattle slurry as a fertiliser on the crop (maize is capable of high
nitrogen uptake) and the use of triazine herbicides for weed control.
The impacts of farming systems on soil erosion and sedimentation in rivers
and streams has become one focus of the UK Environment Agency.
The wider growth of maize crops in southern and western Britain is implicated
in increased sedimentation in some catchments with trout and salmon rivers, as
a result of soil erosion.
Although
it accounts for only 1.1% of the crop area of England and Wales, maize can cause
problems in some areas. Pollution from maize production is due to the use of high
levels of fertilisers and the use of atrazine as a pesticide. Erosion is often
associated with this crop because soils are left bare in winter. From www.environment-agency.gov.uk.
Atrazine
has been declared a “dangerous substance” by the North Sea Conferences, which
consider the environmental health, management and exploitation of this marine
area.
This
report examines 1) the current management of forage maize and the closely related
sweetcorn in the UK, 2) evaluates changes in management that may result from the
deregistration of the herbicide atrazine, and 3) assesses the likely patterns
of weed control that might arise from the introduction of genetically-modified
herbicide-tolerant (GMHT) cultivars, especially Liberty Link glufosinate-ammonium
(hereafter glufosinate)-tolerant maize, in the light of experiences in the USA
(Owen, 2003).
2.
Current management of fodder maize in the UK
As
most maize cultivars benefit from high summer temperatures and reasonable moisture,
crops are mainly grown in southern Britain. A
significant proportion of maize is grown continuously in the same field; Knott
(2002) estimated this at around 50% of the crop. This aspect of maize production enhances particular
problems, particularly in regard to crop protection, as continued cropping encourages
the build-up of resistant weeds, pests and diseases. The continued use of particular crop protection
pesticide programmes also selects for resistance; a number of triazine herbicide-resistant
weed biotypes occur in maize fields.
Crops
are drilled in April and May, significantly later than other spring-sown arable
crops. The crop is sown on wide rows, typically 0.75m
apart, at a drilling rate of 100000 seeds/ha. A pre-cultivation herbicide, such as glyphosate is sometimes used.
All seed is dressed, commonly with thiram to prevent damping off diseases
that kill seedlings. The majority of maize crops were treated with
the herbicide atrazine in 1997. The herbicide
is soil-acting, killing seedlings as they emerge through treated soil, and is
therefore usually applied at the early post-emergence stage, once maize has established,
but sometimes pre-emergence. The herbicide is adsorbed onto soil particles,
giving residual weed control, in comparison to foliar-acting herbicides, such
as glyphosate and glufosinate-ammonium, which are absorbed only via leaves.
The current estimated cost of atrazine application in maize is between
£5 and £6.50 per ha (Nix, 2003), which is a relatively small part of the variable costs
of the crop (Table 2).
The
crop is usually harvested for whole-crop silage from late-September into October,
with yields averaging 40 tonnes/ha at 30% dry matter.
Chopped material is placed in silage clamps and fed to cattle after fermentation.
Table
2. The yield, variable costs and contractor
costs (£/ha) of growing silage maize (Nix, 2003).
| |
£/ha |
|
Yield | 40 tonnes/ha |
|
Seed | £120 |
|
Fertilizer | £55 |
|
Sprays | £35 |
|
Total variable costs | £210 |
|
| |
|
Contract drilling | £40 |
|
Harvesting | £75
or £125 with carting
& clamping |
|
Area payment (CAP) | £69.53 |
|
Average sale (£25/tonne @ 30% dry matter) | £1000 (sold
standing at £425-525/ha) |
3.
Sweetcorn, a minor crop
A
small area of sweetcorn (1600 ha) is drilled or transplanted in the UK, in late
April/May on wide rows 0.75m apart if grown for hand-picking (Knott,
2002). Sweetcorn is
initially slow-growing when temperatures are low and it seldom forms a complete
canopy before the end of July. Spring
emerging annual weeds can smother sweetcorn at early growth stages and perennial
species, particularly common couch, can cause crop suppression. Even when mature, the canopy allows light penetration
and weeds grow beneath it. Sweetcorn is
often grown continuously on the same sheltered field and repeated use of atrazine
leads to a build-up of black nightshade (Solanum nigrum), a species that
is poorly controlled by the herbicide. There are a number of weeds that have evolved resistance to triazine
herbicides under such conditions, such as groundsel (Senecio vulgaris).
4.
The weed flora of maize crops
Maize
crops are grown either as part of a rotation in arable or grassland or as a continuous
crop. In arable crop rotations, the weed flora reflects
the soil type and the field cropping history, and is commonly a mixture of annual
dicotyledonous and grass weeds. In grass rotations, the weed flora is more often a mixture of selected
grassland annual weeds, notably chickweed (Stellaria media) and annual
meadow grass (Poa annua), with other components of the previous swards.
It
is well understood that the timing of final cultivations has a major impact on
the weed flora of arable crops. Winter-sown
crops support an autumn-germinating flora, while spring-sown crops are dominated
by spring germinating species (Chancellor,
1985; Hald, 1999). Thus, maize,
being late-spring sown, supports a particular weed flora of those species that
either germinate in the late spring and early summer or all year round. One species of note is typical of maize fields,
black nightshade (Solanum nigrum). This species germinates late in the year (Fig. 2), together with
fat hen (Chenopodium album), the mayweeds (Matricaria and Tripleurospermum
species) and greater plantain (Plantago major). Other spring-germinating weeds of the Polygonaceae
are often also well-represented.
In
continuous maize, where the crop is grown in the same field year after year, the
selection pressure is such that the weed flora tends to become dominated by the
later germinating weed species, especially black nightshade, which is poorly controlled
by the standard herbicide programme applied.
In addition, perennial weeds, particularly couch grass (Elytrigia repens)
can become problematic in continuous maize fields. Some grass weeds, such as meadow grasses (Poa spp.), can
also be difficult to control in this grass crop. A particular concern in continuous maize cropping is the encouragement
of resistant weeds, such a black nightshade. Also, the evolution of herbicide resistance
is promoted where the same herbicides are used year after year (Caseley et al., 1991).
A
number of the spring germinating weeds are important components of the diet of
farmland birds, especially in stubbles over winter (Vickery
et al., 2002). It is possible
that maize could be an important crop for farmland birds, as stubbles are often
present over winter before the crop is sown, and spring germinating weeds are
encouraged. However, maize is particularly
susceptible to competition from weeds, especially early in the life of the crop,
so the evidence is that following herbicide applications the crop is rather poor
from a biodiversity perspective (Vickery et al., 2002). The Farm Scale
Evaluation results also indicate that conventionally managed maize crops tend
to have fewer weeds and associated invertebrates, compared with GMHT cultivars
under the herbicide regime imposed (Heard
et al., 2003a; Heard et al., 2003b).
Fig. 2. Germination
periodicity of common arable and horticultural weeds. From Grundy
& Jones (2002) after Hance
& Holly (1990).
5.
Weed competition in maize and the critical weed-free period
Maize
is highly susceptible to the effects of weed competition early in the life of
the crop. As the crop is planted on wide rows, the canopy
rarely closes. The plant, whilst it is
generally robust, grows upright in its early stages. At this point, competition for light and nutrients
from weeds causes highly significant yield losses. Without weed control, maize yield is significantly
reduced (Table 3).
Table
3. Comparison of maize yield data from
plots treated with atrazine (Conventional) or left untreated.
Experimental data from Somerset (Vickery
et al., 2002).
| | | Conventional | No
herbicide | s.e.d. |
| Maize |
t/ha fresh weight |
40.85 |
25.75 |
2.458 |
|
t/ha dry weight |
12.54 |
8.02 |
1.057 |
| plant density
No/m2 | 104.44 | 109.91 | ns |
ns = not significant
The
need to control weeds during the early stages of the crop is known to be critical,
e.g. (Evans et al., 2003b). Weed control
once maize plants have grown to 0.5m height does not radically affect yield.
Thus maize growers pay particular attention to herbicide applications at
drilling or shortly afterwards.
Maize
is grown extensively in North America, where it is known as corn, and herbicide-tolerant
cultivars have been grown there for more than seven years.
There is a considerable literature on weed control in GMHT maize. The need for efficient weed control early in
the life of GM crops requires residual herbicide use, especially atrazine (Hamill et al., 2000). Interactions
between weed control and nitrogen are also documented. Increased nitrogen supply to the crop can result
in the need for extended weed control (Evans
et al., 2003a; Evans et al., 2003b).
A
field study was conducted in 1998 and 1999 at over 30 locations throughout the
North Central region of the USA to determine the effect of time of weed removal
on weed control and yield of glyphosate-tolerant corn (Gower
et al., 2002). Herbicide treatments
were applied when the grass weed giant foxtail (Setaria faberi Herrm.)
was 5, 10, 15, 23, or 30 cm tall. Average
corn yield was 101, 99, 93, 93, and 80 % of the weed-free control, respectively.
Average weed control for these treatments was greater than 90 % for grass
and broadleaf weeds, but was occasionally as low as 70 % due to late emergence
of annual grasses and Amaranthus species or difficulty in controlling Ipomoea
species. Most effective and consistent
annual grass control occurred when glyphosate was applied to grasses at least
15 cm in height. The problem of weed re-infestation
after single applications of foliar-acting herbicides was significant.
6.
Current weed control in UK maize
Typically,
maize in the UK receives two herbicide applications per year, each a different
product. On average, 100% of the crop receives a seed
treatment, 98.1% receive herbicide sprays, 6.9% insecticide and 0.8% receive molluscicide
or repellents. In terms of the herbicides
used, the Pesticide Usage Survey indicates the treated area in GB in 1997 (ha)
is approximately twice that planted, i.e. there are two applications on average
per crop (Table 4).
Table
4. Pesticides used on maize (area
treated with different products) in the UK in 1997.
| Chemical group |
Area (ha) treated out
of 109413 ha planted in 1997 |
|
Fungicides | - |
|
Growth regulators | - |
|
Herbicides | 208687 |
|
Insecticides | 10307 |
|
Molluscicides & repellents | 1172 |
|
All seed treatments | 224427 |
|
All pesticides | 444593 |
To
quote the 1997 Pesticide Usage Survey: “Seed treatments accounted for 50% of all
applications to maize and the most extensively used seed treatments were of thiram
(42%) and methiocarb (35%). Atrazine was
applied to 53% of the herbicide treated area of maize and was used mainly for
general weed control. Bromoxynil, applied
to 23% of the same area, was used for the same reason but also to control nightshade
(Solanum nigrum, black nightshade). Gamma-HCH
was used on 85% of the insecticide treated area, the main reasons given for its
use being the control of wireworm or a combination of wireworm and leatherjackets
in maize crops grown after grass. There
was no recorded use of fungicides or growth regulators on maize.” (Garthwaite et al., 1997).
An analysis of the herbicides
used in 1997 on maize indicates the commonest treatment was atrazine (Table 5).
This triazine herbicide is very persistent in soils, lasting up to one
year depending on applied dose. This can restrict the crops that can be grown
following applications. Because of its
persistence, there is a high risk of movement via surface and through-flow to
both surface and ground waters. Residues have been recorded frequently in water samples across the
globe where the product is used for agricultural, horticultural and industrial
weed control. Restrictions in the use
of the herbicide have been imposed during the period the chemical has been in
use.
The second most frequently
used herbicide in maize is bromoxynil. This herbicide controls broad leaved (dicotyledonous) weeds. Where late-germinating weeds or species that
are resistant to atrazine are present, an application of this herbicide will control
most of the non-grass weeds present.
Table 5. Usage of herbicides on maize
and sweetcorn crops grown in Great Britain (spray hectares) taken from the CSL
Pesticide Usage reports. Herbicides
used on 10% of the crop or more in red; herbicides not supported in the EC Review
in blue; herbicides no longer manufactured or withdrawn in green.
| Herbicides | Maize (1997) | Sweetcorn (1999) |
Crop
area | 109413 | 1,690 |
| Herbicides Total weeds | | |
| Diquat/paraquat | | 60 |
| Glyphosate | 18815 | 404 |
| Paraquat | 1325 | 14 |
| Herbicides Broad-leaved &
grass weeds | | |
| 2,4-D | 214 | |
| Atrazine | 109579 | 2,085 |
| Benazolin/2,4-DB/MCPA | 138 | |
| Bromoxynil | 47117 | |
| Clopyralid | 2032 | 15 |
| Cyanazine/pendimethalin | 4839 | |
| Fluroxypyr | 3514 | 0 |
| MCPA | 83 | |
| Mecoprop | 264 |
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