Determination of buffer zones to protect seedlings of non-target plants from the effects of glyphosate spray drift

Agriculture, Ecosystems and Environment, 45 ( 1993 ) 283-293

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Elsevier Science Publishers B.V., Amsterdam

Determination of buffer zones to protect seedlings of non-target plants from the effects of glyphosate spray drift R. H. Marrsa,*, A. J. Frostb, R. A. Plant b, P. L u n n i s b aNess Botanic Gardens, Universityof Liverpool Environmental and Horticultural Research Station, Ness, Neston S. Wirral, L64 4AY, UK bNERC, Institute of Terrestrial Ecology, Monks Wood,Abbots Ripton, Huntingdon, PEI7 2LS, UK (14 January 1993)

Abstract

There is an increasing need to protect semi-natural vegetation from the potential effects of herbicide drift. One way to protect sensitive sites is to surround them with a no-spray buffer zone. Earlier estimates of buffer zone size based on bioassay experiments with established perennials suggested zones needed to be 6-10 m wide. In this paper four bioassay experiments are reported, where seedlings grown in trays were exposed downwind of glyphosate applications and taken to a glasshouse for assessment. Three experiments were done with Lychnisflos-cuculi seedlings including one with different surrounding grass structures, and Experiment 4 tested the response of 15 species typical of seminatural vegetation. The mortality of Lychnis flos-cuculi varied between experiments and appeared more or less unaffected by grassland structure except immediately downwind of the sprayer. The multi-species experiment indicated a wide sensitivity to spray drift, and one species was affected between 15 and 20 m downwind. Thus, seedlings of some species were affected at greater distances than established plants, indicating either greater capture of drift or a greater sensitivity. On sites where seedling establishment is an important mechanism for community regeneration, buffer zones may need to be 20 m wide.

Introduction Although there has been concern that herbicide spray drift from agricultural land could damage adjacent semi-natural vegetation (Sheail, 1985 ), there is little quantitative evidence to support this view. A number of experimental studies have investigated the effects of herbicides at full dosage on species found in semi-natural communities (Balme, 1956; Yemm and Willis, 1962; Way and Chancellor, 1976; Parr and Way, 1984; Marts, 1984, 1985; Marshall and Birnie, 1985; Marshall, 1988; Willis, 1988 ). However, the impact of the lower doses found under drift conditions has received much less attention. As *Corresponding author.

© 1993 Elsevier Science Publishers B.V. All rights reserved 0167-8809/93/$06.00

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it is now a legal requirement in Britain that all spray operators take all reasonable steps to minimize the effects of spray drift on non-target species in adjacent vegetation (FEPA, 1989), policies to minimize damage must be developed. One way to derive a sensible working approach for the protection of seminatural communities is to surround them with an unsprayed buffer zone (Marrs et al., 1992a). The difficulty for conservationists is determining the size that these buffer zones need to be to afford adequate protection. Preliminary estimates of buffer zone size have already been determined for herbicides using bioassay studies for two situations: (a) aerial applications of asulam to control bracken (Marrs et al., 1992b), and (b) for tractor-mounted applications in normal agricultural situations (Marrs et al., 1989, 1991a,b, 1992c). The buffer zones developed for tractor-mounted sprayers have been derived entirely from experiments on established perennial species, although effects of surrounding grassland structure, age/size of plant, interspecific competition and repeat applications have been included (Marrs et al., 199 l a,b). In these bioassay experiments very little impact was found in the downwind zone, and it was suggested that a buffer zone of between 6 and 10 m should give adequate protection. However, these authors pointed out that no measurements had been made of the effects of drift at the regeneration stage, when seedlings are establishing. This paper, therefore, extends these preliminary bioassay experiments to investigate impacts on establishing seedlings exposed to glyphosate drift downwind of a tractor-mounted sprayer. Three experiments were done with L. flos-cuculi (nomenclature follows Clapham et al., 1987), a species used throughout the studies of spray drift with perennial plants (Marrs et al., 1989, 1991 a,b, 1992c). The objectives of two experiments were to determine the buffer zone width required to protect this species and the third experiment assessed the influence of two surrounding grass structures on seedling mortality. A comparative study with 15 species was also done to determine the approximate range of variation in buffer zone width in a range of species typical of semi-natural habitats. Materials and methods

Lychnis flos-cuculi studies

L. flos-cuculi seeds were sown thinly in 48 seed trays (220 m m × 175 m m × 55 m m ) filled with SAI GP compost. Approximately 2 weeks after sowing the numbers of seedlings in each tray ranged from 140 to 250 seedlings (BBCH scale 12; Lancashire et al., 1991 ) per tray. Trays were then placed at varying distances downwind of the sprayer (with appropriate untreated controis) in three separate experiments: Experiment 1. In September 1990 four replicate trays were placed at 0 m

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(i.e. receiving the full application rate from the outer 50 cm of the spray jet ) and at 0.5, l, 1.5, 2, 3, 4, 5, 6, 8 and l0 m downwind of the sprayer. Experiment 2. In September 1991 five replicate trays were placed at 0, 5, 10, 12, 14, 16, 18, 20, 25, 30, 35 and 40 m downwind of the sprayer. Experiment 3. In September 1991 trays were placed at 0, 5, 10, 15 and 20 m downwind of the sprayer in four replicated transects in each of two grassland structures: 'short' (approximately 5 cm) and 'tall' (approximately 80 cm ) grassland; height measured by ruler to tip of tallest leaf.

Multi-species study Seeds of 15 species ( Betonica officinalis, Digitalis purpurea, Galium verum, Geum urbanum, Hypericum perforatum, Lotus corniculatus, Lycopus europaeus, Pimpinella saxifraga, Plantago media, Primula elatior, Primula vulgaris, Ranunculus acris, Silene alba, Teucrium scorodonia and Verbascum thapsus) were germinated and the seedlings pricked out into small plant pots (45 mm X 45 m m × 65 m m ) containing SAI GP compost. In September 1991 between 20 and 120 seedlings depending on species (BBCH scale 14; Lancashire et al., 1991 ) were placed at 0, 5, 10, 12, 14, 16, 18, 20, 25, 30, 35 and 40 m downwind of the sprayer.

Spraying procedure Spraying was done using a tractor-mounted team sprayer with a 6 m boom, fitted with 12 Lurmark Red 03-F80 (BCPC code F80/1.20/3) flat fan nozzles, held 80 cm above the ground. The spraying was aimed at reproducing likely field conditions, and was done at a tank pressure of 2 bar and at a tractor speed of 6 km h - i. Glyphosate was applied at 2.2 kg ai h a - 1 in four sequential upwind swathes from the 0 m position giving a total treated area of 70 m × 14 m. Wind speed during all experiments was monitored with a Vector Instruments R500 recording anemometer and was always between 2 m sand 3 m s- ~at a height of 2 m during experimental sprayings.

Seedling assessment and analysis After exposure to the spray drift the trays were maintained in a glasshouse and watered carefully to prevent herbicide being washed off. Three to 4 weeks after exposure to the herbicide drift each seedling was examined and classified visually as either healthy or dead (a severely damaged category was also included in Experiment 1 with the L. flos-cuculi seedlings). Where appropriate a range of regression models were fitted to the relationship between mortality and distance downwind including logistic, exponential and polynomial equations using MLP (Ross, 1980 ). These equations were used to calculate a

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(a) Dead 100 4

80'

60' 40,

20

0

(b) Damaged 100

•

80'

% 40'

2

2

4

6

8

I0

(c) Healthy I00 80 60 40 20 0 0

2

4

6

8

I0

Distance downwind (m)

Fig. 1. Effects of glyphosate on percentage of L. flos-cuculi seedlings classified as (a) dead, (b) severely damaged, and (c) healthy after exposure downwind ofglyphosate spray in Experiment l; mean values + standard errors (n = 4 ) are presented, U = unsprayed controls.

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LDistso value for the approximate point where 50% mortality occurred. All equations gave similar results and here only the logistic equations are presented.

Results Lychnis flos-cuculi studies Experiment 1. Large numbers o f seedlings were either killed or damaged over the entire 10 m distance downwind o f the sprayer (Fig. 1a ). All seedlings were killed under the sprayer and at 0.5 m downwind. Thereafter there was a steady decline in numbers killed until 10 m where 30% mortality occurred. The numbers of surviving but damaged seedlings increased with increasing Mortality (%)

20]

IOO •

80 ¸

60

40

20

0

~ o

lO

20

30

40

50

Distance downwind (m)

Fig. 2. Effects ofglyphosate on seedlingsof L. flos-cuculi after exposure downwind of a sprayer in Experiment 2; mean values + standard errors (n = 5) are presented.

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Table 1 Mortality (%) of Lychnisflos-cuculi seedlings after exposure downwind to a glyphosate application in both 'short' ( 5 cm) and 'tall' (80 cm) grassland; mean values _+standard errors (n = 5 ), fitted logistic equations and LDist5o values are presented Distance downwind of the sprayer ( m )

Short grassland

Tall grassland

0 5 10 15 20

91+1 35+5 9+2 3+0.5 2+0.6

98+1 15+3 10+2 1 +0.3 1 +0.2

LDistso range (m)

Grass treatment

A

C

B

M

Variation accounted for by regression

(%) 0-5 0-5

Short Tall

1.04 -9.04

174.09 2276

0.64 - 17.57

-0.33 -0.17

94.1 94.2

Logistic equations fitted to data: mortality (%) = A + C / ( 1 + EXP (-B (distance ( m ) - M ) ) )

Table 2 Mortality of seedlings in the comparative species experiment: data for logistic equations relating percentage mortality after exposure downwind from a glyphosate application and LDistso ranges are presented. The logistic equation fitted was mortality (%) =A + C~ ( 1 + E X P ( - B(distance(m) - M ) ) ) LDistso range (m)

Species

A

C

B

M

Variation accounted forby regression

(%) 0-5

7-10

10-15

15-20

Geum urbanum Pimpinellasaxifraga Plantago media Primula vulgaris Digitalispurpurea Lycopuseuropaeus Primula elatior Silenealba Betonica officinalis Galium verum Lotuscorniculatus Ranunculus acris Teucriurn scorodonia Verbascum thapsus Hypericumperforatum

0 -3.74 0.40 8.70 1.74 2.01 - 1.88 -0.41 2.79 4.01 -0.28 1.79 2.42 15.48 2.16

70.0 3656 99.6 78.3 112.8 103.7 123.7 101.9 115.4 89.7 103.6 98.2 97.8 74.0 98.3

- 2.00 -0.23 - 1.97 - 1.99 -0.32 -0.49 -0.31 -0.54 -0.23 - 1.08 -0.74 - 1.84 - 1.88 -0.45 -0.74

4.10 -15.65 4.02 4.63 5.82 5.75 6.93 8.75 8.75 10.44 11.10 11.01 12.59 12.13 16.60

99 62 99 99 99 99 94 98 98 99 98 99 99 94 99

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L[3istso

120

~

L[3istso

¢ Sm

C

5-10m

100'

80,

40

30 ~

Distance downwind (m)

LOist~;)

qO--qSm

L[3istEs o

120

100 ¸

IO0

BO

80

40

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40

(m)

IS--ROm

0

20

1'0

2O OkCan<~ downwind (m)

3b

40

0

1'0

~)

30

dovmv,4nd (m)

Fig. 3. Fitted curves of the responses of seedlings in the multi-species study to glyphosate; data are ranked according to the LDistso ranges given in Table 2; parameters of the logistic curves are also presented in Table 2. Species codes: (A) Geum urbanum, (B) Pimpinella saxifraga,

(C) Plantago media, (D) Primula vulgaris, (E) Digitalis purpurea, (F) Lycopus europaeus, (G) Primula elatior, (H) Silene alba, (I) Betonica officinalis, (J) Galium verum, (K) Lotus corniculatus, (L) Ranunculus acris, (M) Teucrium scorodonia, (N) Verbascum thapsus and (0) Hypericum perforatum.

distance downwind up to 10 m (Fig. lb ). Very few individuals were healthy even at 10 m compared with more than 80% in the untreated controls (Fig.

lc). Experiment 2. Mortality declined rapidly to 10% over the first 10 m (Fig. 2 ), thereafter it remained at about 10% up to 18 m and after 20 m declined to the levels equivalent to the untreated controls (1%). The fitted logistic equation was Mortality (%) = 0.64 + 5227/1 + Exp (0.22 (Distance + 18.38 ) ) which described 98.2% of the variation and gave an LDistso value of between 0 and 5 m. Experiment 3. Similar results were obtained to Experiment 2 with a mor-

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tality of 10% found at 10 m (Table 1 ). The only points at which the grassland structure appeared to have any effect on mortality was directly under the sprayer and at 5 m downwind. The greatest difference between grassland treatments was at 5 m downwind where mortality was lower in the 'tall' than in the 'short' grass (Table 1 ).

Multi-species study There was a wide range of responses ranging from four species with an LDistso mortality 0-5 m to the other extreme with Hypericum perforatum at 15-20 m (Table 2 ). Response curves for all species show that for most species there was little damage after 20 m (Fig. 3). Where seedling mortality was detected above 20 m it was at relatively low levels: Betonica officinalis declining to 25 m, and both Primula vulgaris and Verbascum thapsus having low but detectable mortalities up to 40 m. Although Primula vulgaris had a detectable mortality up to 40 m, it was less sensitive than the related species Primula veris in the immediate downwind zone (Table 2 ).

Discussion Seedlings are clearly more sensitive to glyphosate spray drift in the area downwind of tractor-mounted sprayers than established perennial plants. Seedlings were sensitive up to 20 m downwind, and a few species showed a small effect on seedling mortality even between 20 and 40 m. In contrast, almost no significant effects on a range of performance measures have been found with established perennials at 8 m downwind (Marrs et al., 1991 a,b; Marrs et al., 1992c). These performance measures included detailed measurements of flowering, seed production and seed viability when three herbicides (glyphosate, MCPA and mecoprop) were applied annually over three or four seasons (Marrs et al., 1992c). This contrast between seedlings and established plants might have been expected to some extent as it is well known that seedlings are more sensitive to direct applications of herbicide. However, in the experiments where seedlings were exposed to herbicide drift this increased sensitivity was maintained, probably as a result of an interaction between the efficiency of drift interception and the sensitivity of seedlings to low herbicide doses. A further important result from this study was the differences found between the three experiments with L. flos-cuculi, which were all done in similar wind speeds, using standardized spraying equipment and the same methodology. In Experiment 1 severe effects were found at the m a x i m u m distance tested ( 10 m ) , but in Experiments 2 and 3 the most damaging effects were confined to the first 10 m. This between-experiment variability was not found in our previous studies with established perennials, where essentially similar

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results have been found in 17 separate experiments (Marrs et al., 1989; Marrs 1991 a,b, 1992c). The heterogeneity between experiments with establishing seedlings may reflect differential interception of the herbicide drift by the small seedlings. Drift interception may have varied between experiments because of subtle differences, either in the meteorological conditions affecting drift movement, or in the proportion of droplets intercepted via gravitational fallout and inertial impaction pathways. The different responses at 5 m downwind in the two grassland structures supports the view that differential interception may occur in the immediate downwind zone, as this can be affected by both the roughness coefficient (Zo) and eddy or frictional velocity (U*). Both of these parameters increase with increasing grassland height and foliage density, thus, increasing interception of small droplet drift (Elliott and Wilson, 1983; Williams et al., 1987 ). If inertial impaction was the main pathway for drift capture the mortality should have been greater in the 'tall' grassland structure. However, where differences between grass heights occurred the opposite response was found. This means that either gravitational fallout was the more important process in drift deposition at 5 m, or that the grassland plants intercepted most of the drift and protected the seedlings. The interception and subsequent effects of herbicide drift in different vegetation structures is extremely complex and needs a great deal of additional study (Marrs et al., 1991b). Initial estimates of minimum buffer zones size for the protection of seminatural vegetation from herbicide spray drift devised originally from experiments on established perennials were between 6 and 10 m (Marrs et al., 1989 ). These authors argued that the results were preliminary and may need modification in the light of additional studies. Clearly, where seedling regeneration is an important mechanism for the maintenance of plant communities, then buffer zones need to be wider and as a first approximation 20 m appears reasonable. It may even be possible to categorize semi-natural communities into those where seedling regeneration is relatively infrequent (e.g. productive hedgerows and field margins where the vegetation is mainly composed of aggressive dominants), and where buffer zones could be set at 6-10 m, and those where seedling regeneration is a frequent and important mechanism for the maintenance of species diversity (e.g. grazed chalk downland) and buffer zones need to be 20 m. These two groups represent the ends of a continuum and it would be necessary to plan such a strategy with care, especially in intermediate situations. Further studies are needed to quantify the importance of seedling establishment as a regeneration mechanism in a range of situations. It is worth speculating on the relevance that seedling mortality caused by spray drift has in relation to other natural causes of seedling death. Indeed it is possible that herbicide induced mortality merely replaces mortality from another environmental factor, i.e. only killing seedlings that are inherently weak. However, on sites of nature conservation interest it is important to

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minimize external damaging effects on vegetation. If buffer zone as advocated here were used to protect sensitive sites, then herbicide users can demonstrate that they have taken all reasonable steps to safeguard them. Acknowledgements This work was funded by both the Nature Conservancy Council and the Department of the Environment (PECD - 7 / 2 / 5 0 ).

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Marshall, E.J.P. and Birnie, J.E., 1985. Herbicide effects on field margin flora. Proc. 1985 Br. Crop Prof. Conf., pp. 1021-1028. Parr, T.W. and Way, M.J., 1984. The effects of management on the occurrence of agricultural weeds in roadside verges. Aspects Appl. Biol., 5:9-10. Ross, G.J.S., 1980. MLP: maximum likelihood program. Rothamsted Experimental Station, Harpenden. Sheail, J., 1985. Pesticides and Nature Conservation: The British Experience 1950-1977. Clarendon Press, London. Way, M.J. and Chancellor, R.J., 1976. Herbicides and higher plant ecology. In: L.J. Audus (Editor), Herbicides, Physiology, Biochemistry, Ecology, Vol. 2. Academic Press, London, pp. 145-172. Williams, C.T., Davis, B.N.K., Marrs, R.H. and Osborn, D., 1987. Impact of pesticide drift. NERC contract report to NCC, Institute of Terrestrial Ecology, Huntingdon. Willis, A.J., 1988. The effects of growth retardants and selective herbicide on roadside verges at Bibury, Gloucestershire over a thirty-year period. Aspects Appl. Biol., 16:19-26. Yemm, E.W. and Willis, A.J., 1962. The effects of maleic hydrazide and 2, 4-Dichlorophenoxyacetic acid on roadside vegetation. Weed Res., 2:24-40.