Herbicide drift into hedgerows: Extent, effects and mitigation
Marianne Bruus1, Beate Strandberg1, Christian Kjær1, Per Løfstrøm2, Helle Vibeke
Andersen2
1National
Environmental Research Institute (NERI), University of Aarhus, Vejlsøvej 25, DK-8600 Silkeborg,
Denmark
2 NERI, University of Aarhus, Frederiksborgvej 399, DK-4000 Roskilde, Denmark
Email contact: mbp@dmu.dk
1. Background and aim
In heavily populated countries like Denmark hedgerows serve not only as protection against wind erosion,
but also as ecological refuges with a recreational value. Previous studies have shown considerably more
plant species in hedgerow bottoms at organic farms than at conventional farms, indicating that herbicide drift
from field spraying may affect plant species composition. Furthermore, some hedgerow trees (e.g. hawthorn
and bird cherry) have proved sensitive to very low dosages of herbicides. The aim of the presented studies
was to quantify herbicide drift into hedgerows under realistic spraying conditions, compare spray drift values
with the sensitivity of hedgerow species to selected herbicides and estimate the effect of various mitigation
methods on both hedgerow bottom vegetation and fruit yield of hedgerow trees.
2. Methods
Herbicide drift into hedgerows was measured on five occasions. Each time the herbicide metsulfuron methyl
and/or the dye marker sodium fluorescein were applied by tractor in five tracks parallel to the hedgerow.
Spray deposit was measured at different height both on masts in the field and in the hedgerow. A drift model
was established to calculate the vertical distribution of spray drift in hedgerows under varying spraying
conditions such as time of spraying, choice of nozzles and nozzle pressure and introduction of unsprayed
buffer zones.
Herbicide effects on hedgerow bottom flora were studied by sowing a mixture of 33 selected species in 2001
and subsequently treating the field with low glyphosate dosages every year. 10 replicates of each treatment
were established. Glyphosate effects were measured as number of species, species composition and cover
and biomass of single species.
Our knowledge of the sensitivity of common hedgerow trees to herbicides was extended by exposing
Swedish whitebeam and elder to low dosages of metsulfuron methyl and measuring the effect on fruit yield.
3. Results and discussion
1.1.
Herbicide drift and mitigation methods
The drift measurements showed large variations, but nevertheless a very clear pattern. Spraying close to the
hedgerow resulted in herbicide drift mainly to the lower parts of the hedgerow, whereas herbicide applied far
from the hedgerow was more evenly distributed in the hedgerow (Figure 1). As a consequence unsprayed
buffer zones will mainly reduce deposition in the lower part of the hedgerow (Figure 1). In contrast, chosing
drift reducing nozzles (e.g AI 11004) or nozzle pressure (1 atm vs. 3 atm) will reduce spray deposition at all
heights of the hedgerow, as will spraying at times when wind speed is low and air humidity high, i.e. early in
the morning (Figure 1).
Buffer zone
0m
3m
6m
24 m
6
4
2
0
B
8
Nozzle type and pressure
XR 11002, 3 atm
XR 11002, 1 atm
AI 11004, 3 atm
6
4
2
2
4
6
8
10
Metsulfuron deposition, ng/cm
12
2
8
Time of spraying
6 am
11 am
4 pm
6
4
2
0
0
0
C
Height in hedgerow, m
8
Height in hedgerow, m
Height in hedgerow, m
A
0
2
4
6
8
10
12
Metsulfuron deposition, ng/cm
14
2
0
2
4
6
8
10
12
Metsulfuron deposition, ng/cm
14
2
Figure 1. Estimated effects of unsprayed buffer zones (A), nozzle type and pressure (B) and time of spraying (C) on
herbicide deposition in hedgerow. The graphs show how deposition varies with height.
It is also worth noting that the spray drift measured in the present study significantly exceeds the spray drift
values commonly used in relation to risk assessment.
1.2.
Effects of herbicide drift on hedgerow bottom vegetation
Drift-relevant dosages of glyphosate had very different effects on the biomass of single species. Grasses
were generally less sensitive than dicot species, and sheep’s fescue even increased in biomass because it
was less sensitive to glyphosate than normally more competitive grass species like couch grass and
common bent grass (Figure 2). Consequently, glyphosate affected the species composition and the number
of species present.
C
2
100
Number of species/0.5m
B
50
2
Biomass (g d.w./0.5m )
A
80
30
60
Cover (%)
40
20
20
10
0
40
0
14,4
72
0
360
Glyphosate application (g a.i./ha)
0
14,4
72
8
7
6
5
4
3
360
21-03-2005
2005
Glyphosate application (g a.i./ha)
2006
21-03-2007
2007
glyphosate 0 g a.i./ha
glyphosate 14,4 g a.i./ha
glyphosate 72 g a.i./ha
glyphosate 360 g a.i./ha
common bentgrass (Agrostis capillaris)
sheep's fescue (Festuc ovina)
couch grass (Elytrigia repens)
other species
bare soil
couch grass (Elytrigia repens)
sheep's fescue (Festuca ovina)
common bentgrass (Agrostis capillaris)
21-03-2006
Figure 2. Effects of low dosages of glyphosate on biomass of single species (A), species composition (B) and number of
species (C) within experimentally established field plots. The experiment was established in 2001 and had been treated with
glyphosate (0, 14.4, 72 and 360 g a.i./ha) and nitrogen fertilizer (100 kg N/ha) every year in May. Figure A and B show data
from 2007, i.e. after six treatments, and figure C shows data from the years 2005-2007. The arrows at figure C show the time
of glyphosate application.
1.3.
Effects of herbicide drift on fruit yield of hedgerow trees
Elder was almost as sensitive as hawthorn to metsulfuron methyl, whereas Swedish whitebeam was
somewhat less sensitive. Due to the diffences between species in sensitivity and the in vertical distribution of
fruits, elder and hawthorn benefit more from the described spray drift reducing measures than does Swedish
whitebeam (Figure 3).
0,8
0,6
0,4
0,2
0,0
0
6
Unsprayed buffer zone, m
24
C
1,0
0,8
Relative fruit yield
B
1,0
Relative fruit yield
Relative fruit yield
A
0,6
0,4
0,2
0,0
XR 11002, 3 atm XR11002, 1 atm AI 11004, 3 atm
Nozzle type and pressure
1,0
0,8
0,6
0,4
0,2
0,0
6 am
11 am
4 pm
Time of spraying
Hawthorn
Swedish whitebeam
Elder
Figure 3. Effects of unsprayed buffer zones (A), nozzle type and pressure (B) and time of spraying (C) on fruit yield of
hawthorn, Swedish whitebeam and elder relative to fruit yield of unsprayed trees.
4. Conclusions
Both hedge bottom flora and hedgerow trees may be affected by herbicide spray drift from field spraying.
Effects may be reduced by various measures. Unsprayed buffer zones will reduce effects mainly on
hedgerow bottom flora, whereas drift reducing nozzles and pressure as well as early morning-spraying will
benefit both hedgerow bottom flora and fruit yield of hedgerow trees.
Acknowledgement – The research described was supported by the Danish National Environmental
Protection Agency.