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
Timed video recordings at both sites confirmed that the combinations of nozzles, pressures and flow-rates
were close to nominal, tabulated values. At Site 1, these checks indicated an applied total volume rate of
30 l ha
− 1
and a mean asulam dose of 3.6 kg a.i. ha
− 1
. Although this reduced dose may be critical to control
of bracken, it did not compromise the drift study itself. At Site 2, the checks showed an applied total volume
rate of 35 l ha
− 1
and a mean asulam dose of 4.4 kg a.i. ha
− 1
recommended for bracken eradication. The total quantities of asulam applied were estimated as
26.4 and 19.1 kg at Sites 1 and 2, applied to estimated areas of approximately 7.4 and 4.3 ha, respectively.
3.1. Visual droplet fallout on papers and cards The water-sensitive cards positioned directly under-
neath the helicopter confirmed the large droplets and few fines associated with typical RD Raindrop nozzle
applications by helicopter. At Site 1, irregular deposits upwind of the base-line revealed a ‘missed-strip’ con-
firmed by the pilot. The droplet impacts at Site 1 were generally elongated and ‘tailed’ due to the wind. At
both sites, every card showed some impacts, although their size and number were extremely small furthest
from the sprayed areas, consistent with drift fallout.
3.2. Asulam deposits and drift fallout measured on cards
Fig. 1 shows the mean recovery of asulam, be- neath or close to the first helicopter passes. At both
sites, the first pass was well centred over the base-line which was thus a valid reference for drift compar-
isons. Where downwind distances are quoted with re- spect to this base-line, distances from the edge of the
sprayed area will be less by approximately 5 m ap- proximately one-half of the swath width. Fig. 1 con-
firms the missed-strip at Site 1. The mean doses of applied asulam between 30 m upwind and 4 m down-
wind of the base-line, were 2.31 and 4.37 kg a.i. ha
− 1
at Sites 1 and 2, respectively. Levels of asulam fell
220 R.C. Robinson et al. Agriculture, Ecosystems and Environment 79 2000 215–231
Fig. 1. Asulam dosage recovered from cards close to the helicopters.
rapidly downwind of the treated zones, falling be- low 2 of the recommended dose approximately 15 m
downwind of the base-line at both sites. Fig. 2 shows the asulam fallout recovered from
cards downwind of the base-line in semi-log format. The scatter of points reflects the uneven terrain and
small differences in the angles of the cards to the hor- izontal which will have affected the capture-efficiency
of drifting droplets. At Site 2, fallout levels approached the limits of the detection method approximately
R.C. Robinson et al. Agriculture, Ecosystems and Environment 79 2000 215–231 221
Fig. 2. Asulam drift fallout recovered from cards.
200 m downwind. Although similar patterns of drift were recorded at both sites, higher levels of fallout
were recovered at Site 1 beyond approximately 25 m, attributed to greater wind speeds during application.
The higher doses within 25 m at Site 2 were attributed to greater flying height of the helicopter and the
greater mean dose applied close to the base-line, both tending to increase local spray ‘displacement’ effects.
222 R.C. Robinson et al. Agriculture, Ecosystems and Environment 79 2000 215–231
Fig. 3. Linear regression of asulam fallout log–log plot.
Fig. 3 shows estimated equations for the drift fall- out characteristics. To avoid the exaggerated fallout
of heavier droplets in the spray ‘displacement’ zone immediately downwind of the base-line arguably not
true drift, the regressions were undertaken for data 10 m or more downwind of the base-line. At Site 2,
the lowest recorded values for fallout diverged from the regression line, possibly because of reduced cap-
R.C. Robinson et al. Agriculture, Ecosystems and Environment 79 2000 215–231 223
ture efficiency of smaller droplets on flat papers with increasing distance from the source.
Total asulam fallout outside the sprayed areas was estimated by integration of the regression equations in
Fig. 3, for an area downwind of, and the same width as, the sprayed blocks. Between limits of 10 m and
1 km, for block widths of 320 and 232 m at Sites 1 and 2, estimated total fallout was 0.61 and 0.40 of
the total asulam applied at each site, respectively. It is coincidence that these estimates reflect the percent of
driftable fines droplets 100 mm diameter quoted for RD Raindrop nozzles Ware et al., 1975; Anon., 1980.
With mean wind speeds of 16.6 and 11.1 km h
− 1
, re- spectively, the two total drift estimates appeared to
conform to the linear relation between drift and wind speed predicted by Hobson et al. 1993. This seems
confirmed by the ratio of 1.4 between the gradients of the two lines in Fig. 3 the indices for the power
equations shown compared with a ratio of 1.5 for the mean wind speeds. Using the two equations in Fig. 3,
the mean distance at which fallout dropped below 1 of the applied mean dose of asulam was estimated as
36.0 m and 27.4 m for Sites 1 and 2, respectively.
3.3. Airborne drift of asulam captured on vertical ‘strings’
Fig. 4 shows the quantities of asulam recovered from the duplicate 2 m ‘string’ sections on each mast
after adjustment of data for Site 1 where the masts were bowed by the wind. Significant trends towards
zero did not appear on any of the masts at any string position. This suggested that turbulent air-mixing had
occurred at both sites, resulting in driftable droplets exceeding 8–10 m height, before the air had reached
50 m downwind of the spray release point. This effect is commonly seen for tractor-operated spraying as
well as aerial application Parkin and Merritt, 1988.
Compared with Site 2, higher levels of airborne drift were captured at Site 1, attributed to greater
wind speeds during spraying, and where there was also greater variation with sampling height. At Site 1,
least drift was picked-up on the ‘strings’ nearest the ground which was attributed to the filtering effect of
the bracken covering this site. At Site 2, where bracken was largely absent downwind of the sprayed block,
this effect was not apparent. Fig. 5 shows the mean levels of airborne asulam
recovered per 10 m ‘string’ at each mast position. It is assumed that each ‘string’ sampled a 2 mm×10 m
cross-sectional air-stream or ‘window’. Table 2 shows the overall airborne drift off the block estimated to
10 m height with the lateral dimension of this win- dow extended to the width of the sprayed block. As-
suming no loss of drift over the tops of the masts, differences in the measured quantity of drift passing
through one window and the next ought to account for the fallout observed on the cards laid out between the
same two masts. Table 3 makes a comparison of such fallout estimates with measured values, showing that
the ‘string’ data tended to overestimate fallout mea- sured on the cards, although there was good agree-
ment between the two estimates at Site 2. At Site 1, however, data for the masts predicted double the fall-
out compared with measured values from cards. This discrepancy may be the result of higher wind speeds
at Site 1 causing more drift to be lost over the tops of successive masts, especially as the masts were known
to have been bowed. The greater wind speeds at Site 1 may also have enhanced droplet capture efficiency
on the ‘strings’ and reduced the impaction of droplets onto flat cards.
3.4. Bioassay results Assessed in the greenhouse after spraying, the pot-
ted Rumex seedlings showed expected, high levels of damage in pots which had been closest to the sprayed
areas but with damage decreasing rapidly with dis- tance downwind. This pattern was consistent with
measured fallout while most of the plants except those nearest the treated areas showed rapid recovery
with time.
In Transect A, Site 1, a discontinuity in the assay did not correlate with measured fallout and these scores
were treated as anomalous. The effect was attributed to exposure of one tray of seedlings 24 pots to differ-
ent temperature and light levels during long-distance transport after the spraying. It highlighted the sensi-
tivity of the assay to external factors and, to avoid effects of such variation inherent in the assay, mean
score values, not outlying values, were subsequently used to deduce appropriate buffer zones in respect of
asulam fallout.
224 R.C. Robinson et al. Agriculture, Ecosystems and Environment 79 2000 215–231
Fig. 4. Asulam recovery from drift mast strings. Y-axes refer to duplicate 2 m strings at each of 5 lvels 5=level nearest ground.
Growth-responses over the assessment period were established from the scores for each pot compared with
the asulam fallout recovered from the card adjacent to each pot. Fig. 6 shows logistic dose-response curves
function described by Pestemer and Pucelik-Günther, 1997 fitted for assessments at 3 and 7 weeks
post-spray intermediate assessments omitted. The level of correlation and the ± homogenous mix of data
between both sites sprayed 2 days apart, suggest a valid assay for predicting potential effects of asulam
fallout. The data in Fig. 6 indicate a no-effect dosage of
approximately 10 g a.i. ha
− 1
of asulam at 3 weeks post-spray. At 7 weeks post-spray, most seedlings had
completely recovered from a dose of 100 g a.i. ha
− 1
2.27 of the recommended dose. The data suggest that the assay had not reached maturity by 7 weeks
and that seedlings exposed to yet higher doses of asu-
R.C. Robinson et al. Agriculture, Ecosystems and Environment 79 2000 215–231 225
Fig. 5. Mean recovery of airborne drift of asulam.
lam would also recover. However, doses over 1000 g a.i. ha
− 1
would be mostly lethal to R. acetosa under the conditions of the assay.
Figs. 7 and 8 summarise the Rumex responses to fallout means of duplicate transects in relation to
time and distance downwind at Sites 1 and 2, respec- tively. At 2 and 3 weeks post-spray, the seedlings
showed some ephemeral responses beyond 100 m downwind at both sites. Considering the apparent
sensitivity of the assay to handling and growing
226 R.C. Robinson et al. Agriculture, Ecosystems and Environment 79 2000 215–231
Table 2 Estimated airborne drift of asulam
Distance from base-line m Airborne drift grams asulam
Airborne drift total asulam applied Site 1
a
Site 2
b
Site 1
a
Site 2
b
50 73.14
12.46 0.28
0.07 100
36.61 5.94
0.14 0.03
150 19.97
2.51 0.08
0.01
a
Site 1: data for window 320 m wide×8 m tall; 26.4 kg asulam applied.
b
Site 2: data for window 232 m wide×10 m tall; 19.1 kg asulam applied.
conditions, marginal observations at this range and timing were considered indeterminate. At 4 weeks,
no effects were evident more than 50 m downwind and, as the assay matured, all plants except those
within 10 m of the base-line continued to recover. At 7 weeks, no effects were evident beyond 20 m of the
base-line approximately 15 m from the end of the spray boom. This corresponded with the no-effect
level of approximately 100 g a.i. ha
− 1
shown in Fig. 6. All lethal effects were confined to within 10 m
of the base-line or approximately 5 m from the end of the spray boom.
3.5. Comparison of drift data for tractor-operated spraying
Fig. 9a and b compare fallout values derived for the tractor sprayer data with values from the helicopter
applications. It is seen that fallout from the helicopters exceeded that from typical tractor spraying within a
limited zone only, to approximately 15 m downwind of the treated areas taking the worst case at Site 1.
This effect is attributed to the lateral ‘displacement’ of spray resulting from the greater height of spray-release
from the helicopters and the turbulence associated with the rotors. This deposition is not representative of fall-
out of finer droplets over greater distances, beyond
Table 3 Drift fallout: predicted values masts vs measured values cards
a
Site 50–100 m zone
100–150 m zone Data for cards measured
b
Data for masts predicted
c
Data for cards measured
b
Data for masts predicted
c
1 15.6
36.5 8.0
16.6 2
6.8 6.5
2.3 3.4
a
Data for grams asulam.
b
Integrated values using regression equations from Fig. 3.
c
Values by subtraction — data from Table 2.
say 15 m of the edge of the treated area. At greater distances, the fallout values for the helicopters were
consistently below those for the tractor sprayer 11003 nozzle data BCPC ‘Medium’ spray quality.
The fitted exponential curves in Fig. 5, for the air- borne levels of asulam recovered during the helicopter
work, were used to extrapolate these data to the same downwind monitoring positions as the tractor sprayer
data of Lloyd and Bell in Table 1. Power equations offered poorer fits and were not used. Tooby 1997
also suggested that power equation extrapolations can be misleading when attempting to set appropriate safe
distances for buffer zones. Fig. 9c and d compare the derived airborne drift for multiple passes of the tractor
sprayer with data for the helicopters from Sites 1 and 2, respectively. At both sites, data from the three mast
positions 50, 100 and 150 m fell within the range of values predicted for the tractor sprayer. The greater
levels of airborne drift at Site 1 are attributed to the greater wind speeds at this site.
4. Discussion
