Summary

Omfang og effekt af herbicidafdrift til læhegn

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 that some hedgerow trees (e.g. hawthorn and bird cherry) are sensitive to very low dosages of herbicides. The aim of the present study was to quantify herbicide drift into hedgerows under realistic spraying conditions, compare spray drift values with the sensitivity of hedgerow species to the selected herbicide and estimate the effect of various mitigation methods on the fruit yield of hedgerow trees.

Spray drift into hedgerows was measured on five occasions under varying meteorological conditions. Spraying was performed with a conventional tractor-mounted sprayer, and on every occasion the dye marker sodium fluorescein was applied to the experimental field in five tracks parallel to the hedgerow. On one occasion the herbicide metsulfuron methyl was also applied. Spray deposition was measured at different heights both on masts in the field and in the hedgerow. On basis of the collected data a spray 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.

Our knowledge of the sensitivity of common hedgerow trees to herbicides was extended by exposing Swedish whitebeam and elder to a series of low metsulfuron methyl dosages and subsequently measuring the effect on fruit yield.

Herbicide drift and mitigation methods

The drift measurements showed large variations, but nevertheless a very clear pattern. Spraying close to the hedgerow resulted in spray drift mainly into the lower parts of the hedgerow, whereas herbicide applied far from the hedgerow was more evenly distributed in the hedgerow. Correspondingly, the developped model calculated that unsprayed buffer zones will mainly reduce deposition in the lower part of the hedgerow. In contrast, chosing drift reducing nozzles or nozzle pressure 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. Application of reduced herbicide dosages will result in linearily reduced spray deposition, i.e. reducing the dosage by 50 % will reduce spray drift by 50 %.

It is worth noting that the spray drift measured in the present study significantly exceeds the spray drift values commonly used administratively to assess the risk of pesticide effects on nature.

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 in the vertical distribution of fruits, elder and hawthorn benefit more from a careful choise of spraying eqpuipment and time and from reduced herbicide dosages than does Swedish whitebeam, and only elder benefits significantly from unsprayed bufferzones.

Calculations involving the above-mentioned results as well as data on the frequency of the studied hedgerow species, the number of birds feeding on hedgerow berrries during the winter season and the feeding requirements of those birds indicate that not only hedgerow trees, but also higher levels of the food web may benefit from mitigation of spray drift into hedgerows.

Furthermore, the hedgerow bottom flora is very likely to benefit from all the modelled mitigation methods, including unsprayed buffer zones, since they all reduce spray drift into the lower part of the hedgerow.