216 R.C. Robinson et al. Agriculture, Ecosystems and Environment 79 2000 215–231
and enhance the efficacy of asulam applied aerially to bracken Horsnail and Robinson, 1984; Robinson,
1984. The pertinent work by Marrs and Frost, how- ever, had evaluated drift for helicopters which were not
set-up exclusively with RD nozzles. When this mea- sure is adopted under commercial conditions, bracken,
itself sensitive to asulam, rarely shows a response beyond a few metres downwind of the boom ends.
Missed-strips sometimes arise which reflect a sharp edge to the sprayed swath and an absence of high lev-
els of drift which would otherwise assist swath match- ing. It was therefore suspected that the 160 m buffer
zone was too restrictive of helicopter spraying in many areas where invasive bracken required control, espe-
cially in upland localities intersected by numerous water-courses.
Recognising these factors, a protocol was approved by the Environment Agency England and Wales to
demonstrate, through further monitoring, that a re- duced buffer-zone was feasible when helicopters were
fitted exclusively with RD Raindrop nozzles. The protocol called for monitoring of commercial-scale
spraying of typical upland bracken, again using a R. acetosa bioassay to compare with the work of Marrs
and Frost. Drift of asulam itself was to be mea- sured using high performance liquid chromatography
HPLC methods to avoid debate over use of tracers. Drift fallout on sampling cards was to be measured
to correlate bioassay data with dosage. Airborne drift was also to be monitored on vertical ‘strings’ to
permit comparison with other, definitive drift studies.
Such studies Wilson, 1995 have indicated that drift-reduction nozzles used on helicopters can de-
crease drift to levels comparable with conventional tractor-operated boom spraying. Where such compa-
rability may be demonstrated, interpretation of limited assay data and the choice of buffer zones required can
be resolved by the widely-accepted agricultural prac- tice pertaining to tractor-drawn sprayers. Part of this
study therefore undertook such a comparison based on the data of Lloyd and Bell 1983.
2. Materials and methods
Monitoring of helicopters applying asulam to bracken on a commercial scale was carried out
in 1997 at two upland moorland sites in the North York Moors National Park. At Site 1 Crossley Side,
Little Fryup Dale; Grid Ref. NZ 710 055 spray was applied to a 7.4 ha block within an extended area of
mostly uniform, low-density bracken approximately 50 cm tall across a 10–30
◦
± concave slope. Heather
Calluna vulgaris, including Erica cinereum and E. tetralix, bilberry Vaccinium myrtillus and crow-
berry Empetrum nigrum ground-cover was present under the bracken. Drift was monitored within the
same area of vegetation on slightly rising ground. At Site 2 Goathland Common; Grid Ref. SE 816
995 spray was applied to a 4.3 ha block of vigorous, uniform bracken 75–100 cm tall across a 10
◦
slope. Drift monitoring was conducted on an adjacent area,
mostly free of bracken sprayed in 1996, although vigorous heather and bilberry was present.
2.1. Site layout for spray-deposit and drift monitoring Within each site, a rectangular block was selected
for treatment with a downwind edge perpendicular to the wind direction. This base-line was flagged with
balloon markers to guide the pilot to obtain an accurate first-pass of the helicopter no other markers were em-
ployed. All the remainder of the spraying took place successively upwind of, and parallel to, this line, in 22
passes made cross-wind in opposite directions, each pass being approximately 300 or 200 m long at Site 1
and 2, respectively.
Spray deposit assessment and drift monitoring were carried out at each site along duplicate transects A and
B, approximately 50 m apart, lying ± parallel to the wind and extending to 150 m Site 1 or 200 m Site
2 downwind of the base-line. The different sampling distances reflected the space available on-site over rea-
sonably smooth terrain given the wind direction on that day. The transects also extended 30 m upwind of
the base-line to measure spray deposits achieved di- rectly under the initial helicopter passes.
Sampling-tables were used as supports for all papers, cards and pots see further, located at inter-
vals along each transect. The tables comprised a flat 77 mm×127 mm alloy plate located on a peg which,
inserted firmly in the ground, held the plate approx- imately 30 cm above ground-level the height of the
heather and bilberry ground-cover at both sites. Where more than one table was required at each
point, they were set-up not more than 30 cm apart
R.C. Robinson et al. Agriculture, Ecosystems and Environment 79 2000 215–231 217
from each other and across the wind direction. Where necessary, any bracken fronds in the vicinity were cut
back to create an open avenue, approximately 2 m wide, along the transects.
Spray deposition and levels of drift were estimated using four different means:
i Ciba Geigy, 52 mm×76 mm, water-sensitive pa- pers were used to visualise spray deposits under the
helicopter and the drift fallout downwind of the ap- plications. Papers were laid out on tables attached by
a rubber-band at 1 m intervals, beginning at the zero line, to 50 m downwind; thence at intervals of 5 m
to 100 m downwind. Significant visible fallout was not anticipated beyond this distance. Papers were in-
spected for deposit levels and droplet quality on-site immediately after spraying. Xerox copies of each pa-
per were also made as permanent records.
ii Fyne Papers acrylic-coated, 290 gsm, 75 mm× 106 mm ‘Astralux’ cards were used to sample the spray
applied under the helicopter as well as the levels of drift fallout downwind. Each card was attached to
the sampling tables in a horizontal position by a sin- gle rubber band. The cards were laid out on tables
at 2 m intervals, beginning at the zero line, to 50 m downwind; thence at intervals of 5 m to 100 m down-
wind; and beyond that, at intervals of 10 m to 150 m downwind Site 1 or to 200 m downwind Site 2.
To monitor the dosage applied under the helicopter, cards were also laid out within the transects upwind
of the base-line every 2 m, to a distance of 30 m. Im- mediately after spraying, each card was placed within
a labelled polybag and returned to the laboratory for estimation of asulam by HPLC.
To check the possibility of cross-contamination from handling sampling-tables already used on Site
1, control cards on Site 2 were handled on-site in the same manner as the ‘live’ cards, but not later exposed
to spray drift. In subsequent analysis of these control cards for asulam, no cross-contamination was found.
iii Vertical drift-sampling ‘strings’ on 10 m masts were used to sample levels of airborne drift. Three
masts were erected 50, 100 and 150 m downwind of the base-line at each site, adjacent to Transect B.
Each mast supplied by Silsoe College and described by Gilbert and Bell, 1988 carried duplicate, vertical,
10 m ‘strings’ of 2 mm OD polythene tubing small diameter for sampling fine, airborne spray droplets.
Each string comprised five sections, each 2 m long, linked by crimp-connectors. Immediately after spray-
ing, each 2 m string section was placed in a labelled polybag and returned to the laboratory for estimation
of asulam by HPLC.
iv Potted seedlings of R. acetosa Common Sorrel were used for bioassay purposes, to monitor
the spray applied under the helicopter and the lev- els of drift downwind. R. acetosa was chosen as a
bioassay because it grows rapidly and uniformly from seed and is sensitive to asulam recommended for
the control of Rumex species as Docks in pasture. The Rumex seedlings were grown in the greenhouse,
sown in 80 mm square plastic pots in John Innes No. 2 compost and thinned to 10 plants per pot before use
at the 1–2 leaf stage, approximately 20–30 mm tall. This growth stage proved more manageable than the
larger 10–40 leaf plants used by Marrs and Frost. A stackable tray system minimised damage to seedlings
during handling, transport and work on-site.
Single pots of Rumex seedlings were placed on sampling-tables in the same locations as the ‘Astralux’
cards. After spraying, no overhead watering was used and precautions were taken to protect the plants from
rainfall. As soon as possible after treatment, all pots were returned to the greenhouse for growing-on.
Visual assessments of the foliage of exposed seedlings were made 14, 22, 30, 36 and 48 days after
treatment DAT for Site 1 or 12, 20, 28, 34 and 46 DAT for Site 2. Percentage reduction of healthy leaf
area was estimated by comparison with control plants, effectively integrating necrosis, chlorosis and bulk
reduction effects. Control pots were not exposed to the helicopter treatments but were otherwise handled
identically. Two independent sets of controls were maintained for each of the two sites. The assay had
not reached maturity maximum levels of recovery or damage by the last assessment at 4846 DAT 7
weeks.
2.2. Analysis of cards and ‘strings’ Each polybag containing cards or ‘strings’ was
shaken with 20 ml of a mix of 85 parts NaH
2
PO
4
buffered to pH3 with O-phosphoric acid and 15 parts acetonitrile. This brought the asulam into the mo-
bile phase and also tended to separate the asulam from potential contaminants. Each bag was allowed
to stand for 30 min before a 20 ml sample was taken
218 R.C. Robinson et al. Agriculture, Ecosystems and Environment 79 2000 215–231
for estimation at 270 nm by reverse-phase HPLC with 1 ml min
− 1
flow rate against an external stan- dard using a peak integration method. The limit of
detection was approximately 10 ppb asulam in the 20 ml extracts. This was equivalent to a dose rate
of approximately 0.3 g ha
− 1
of asulam. By means of spiked samples, mean recovery of asulam from the
cards was estimated at 87.6 including a correction for ‘shading’ by the rubber-bands. In the absence of
a suitable determination, recovery from the ‘strings’ was assumed to be 100.
2.3. Application Each helicopter was set-up exclusively with Dela-
van RD Raindrop nozzles in order to minimise drift. These disccore hollow-cone nozzles employ a sec-
ondary swirl chamber which significantly reduces fines compared with conventional nozzles of the same
output Stewart and Gratkowsky, 1976; Grumbles et al., 1980. Typically, the droplet spectrum from an
RD nozzle comprises 1 of droplets with diame- ters 100 mm, with VMD ranging from 400 to over
1500 mm, depending on output Tate, 1977.
At Site 1, a Bell 47-G3B1 helicopter was fitted with 36 RD3 D525 nozzles at an angle of 90
◦
to the di- rection of flight, giving an operating boom width of
10.5 m. Working spray pressure was quoted as 30 psi and flying speed as 78–83 km h
− 1
at a height of 2–3 m. At Site 2, a Robinson R22B helicopter was fitted with
16 RD7 D845 nozzles at an angle of 130
◦
to the direction of flight, giving an operational boom width
of 8.5 m. Working spray pressure was quoted as 20 psi and flying speed as 64–74 km h
− 1
. This helicopter em- ployed a greater flying-height of 3–5 m to compensate
for its narrower swath width. One full helicopter load of asulam spray-mix was
applied at each site for drift monitoring purposes. A small quantity of non-ionic wetter alkyl phenol ethy-
lene oxide condensate, as Techsol ‘Enhance’ Site 1 or ‘Enhance Low Foam’Site 2, was added to each
tank mix as recommended for spraying bracken with asulam. At each site, the duration of spraying with
turns lasted for just over 8 min. No other spraying was carried out on-site until all monitoring work was
complete.
A portable Met-250 weather station recorded wind speed and direction at 2 m height above ground at
1 s intervals during the spraying. Site 1 was sprayed in gusty conditions with a mean wind speed of
16.7 km h
− 1
close to the CAA maximum limit of 18.5 km h
− 1
or 10 knots at 14.5–16
◦
C with 80–85 RH. Site 2 was sprayed at a mean wind speed of
11.1 km h
− 1
at 19–20
◦
C with 78–91 RH. 2.4. Comparative data for tractor-operated spraying
Lloyd and Bell 1983 measured drift to 200 m downwind of multiple passes of a 12 m tractor-drawn
boom-sprayer running cross-wind along a single track. Data for airborne drift retained on 11 m verti-
cal 2 mm ‘strings’ and fallout retained on horizontal 2 mm ‘strings’ were measured at mean wind speeds
of 14.6–17.9 km h
− 1
. To compare these data with those measured in the current study, it was necessary
to make corrections for the much larger total area sprayed by the helicopters upwind of the monitoring
positions.
Equations were fitted to the drift characteristics for the single-track tractor spraying Table 1. By sub-
stitution at successive 12 m intervals, these equations predicted the increments of fallout expected at the
same fixed monitoring positions for 12 m bouts of the tractor sprayer made successively upwind of the
original track. The 22 helicopter passes in the current work equated to upwind distances of 231 m and 187 m
at Sites 1 and 2, respectively. These distances were equivalent to approximately 19 and 15 cross-wind
passes of the 12 m tractor sprayer respectively, giv- ing the number of incremental substitutions required.
Assessed at 8 m downwind, this procedure generated values which compared favourably with those cal-
culated by Lloyd and Bell by their own graphical methods.
Computed fallout values were converted to a per- centage of the applied volume rates per ha according
the nozzle sizes on the tractor sprayer. Estimated fall- out from the helicopter at the same monitoring posi-
tions were calculated as means of the values shown in Fig. 2 and corrected to a percentage of the estimated
quantities of asulam applied per ha.
Corrected for ‘string’ length, airborne drift values were converted to a percentage of the spray volumes
applied to a notional 1m wide strip upwind of a 1 m wide ×10 m tall window. The helicopter data were cor-
rected to the same window dimensions and converted
R.C. Robinson et al. Agriculture, Ecosystems and Environment 79 2000 215–231 219
Table 1 Mean values for drift from a single pass of a tractor-drawn 12 m boom sprayer
a
A Data for airborne drift ml per 1 m wide×11 m tall window Nozzles,
Volume rate Mean wind
Airborne drift at distances Equation for drift
R
2
and pressure l ha
− 1
speed km h
− 1
downwind m characteristic
b
8 20
50 100
8005 fan 2.5 bar 270.0
17.9 1.42
0.75 0.30
0.13 y=11.2570 x
− 0.9465
0.9869 11003 fan 2.5 bar
161.7 16.2
1.61 1.20
0.47 0.27
y=8.6143 x
− 0.7371
0.9598 11001 fan 2.5 bar
54.2 14.6
1.01 0.63
0.32 0.17
y=4.7264 x
− 0.7045
0.9859 D313 cone 5.0 bar
65.8 17.3
1.98 1.33
0.86 0.40
y=7.6788 x
− 0.6072
0.9484 B Data for drift fallout ml m
− 2
Nozzles, Volume rate
Mean wind Drift fallout at distances
Equation for drift R
2
and pressure l ha
− 1
speed km h
− 1
downwind m characteristic
b
8 20
50 100
8005 fan 2.5 bar 270.0
17.9 0.096
0.038 0.01
0.006 y=1.0481 x
− 1.1420
0.9886 11003 fan 2.5 bar
161.7 16.2
0.112 0.046
0.014 0.012
y=0.7455 x
− 0.9406
0.9602 11001 fan 2.5 bar
54.2 14.6
0.086 0.028
0.008 0.005
y=0.9089 x
− 1.1605
0.9878 D313 cone 5.0 bar
65.8 17.3
0.154 0.097
0.019 0.006
y=3.2832 x
− 1.3271
0.9501
a
Data from Lloyd and Bell, Tables 5 and 6 1983.
b
x = distance downwind m from the edge of the sprayed area.
to percentages of the weights of asulam applied to the same 1 m strip.
3. Results
