Herbicidet Starane 180S’ effekt på pollenproduktionen hos mælkebøtte og rødkløver - påvirkninger af fødeudbudet i markhegn på konventionelle og økologiske brug

Summary

Conventional farming and its use of herbicides does not only affect the weeds in cultivated fields. Herbicides also affect the hedge bottom vegetation of the adjacent hedgerows because of spray drift deposits from the fields.
Reduced pollen production and viability followed by poor seed set has been observed for many different Glyphosate-resistant crops. This may also be the case for wild flowering plants outside the fields, if they are exposed to herbicides during their flowering period. Hedgerows are, because of their shape and placement in the landscape, greatly affected by field management. An alternative to conventional farming is organic farming, where the use of pesticides is forbidden. The fact that pesticides are forbidden in organic farming, could be the reason that the diversity of the hedge bottom vegetation is richer in organic hedgerows than in conventional hedgerows. Flying insects are of great value to the hedgerows because they function as pollinators. These insects feed on pollen and nectar produced by the flowering plants in the hedgerows. Reduced pollen production will have consequences for the survival of the insects and this may have consequential knock-on effects on the whole ecological structure of the hedgerow.
The aim of this study is to examine if there is a difference in accessible pollen, as food source for the insects, between conventional and organic hedgerows. Furthermore, the objective is to investigate whether this eventual difference in the production of pollen, can explain the difference that has been observed in the diversity of plants between hedgerows near conventionally farmed fields compared to organically managed fields.
The study includes field observations of twenty organic and ten conventional hedgerows, where the flowers of the hedge bottom vegetation have been counted five times during the growing-season. The greatest importance is attached to the dose/response experiments, which have been carried out in a greenhouse in order to test the effects of the herbicide Starane 180S on flowering and pollen production on two important wild living pollen producers Taraxacum sp. and Trifolium praténse L. The herbicide doses used were 0, 5, 25 and 100 % of the recommended field dose. 5 % represents the estimated spray drift deposit in conventional hedgerows and the unsprayed plants represents the organic hedgerows.
In the hedgerows 203 flowering plant species were observed. 106 of these occurred exclusively in organic hedgerows, while only 13 plant species were found solely in conventional hedgerows. The two types of hedgerows had 84 species in common. The average number of flowering plant species per hedgerow was 53, 8 ± 6,68 for the conventional hedgerows and 89,0 ± 6,19 for the organic hedgerows. Taraxacum occurred in 85 % of the examined organic hedgerows and had a number of 11,5 ± 22,0 flowers per hedgerow. In conventional hedgerows Taraxacum was found in 50 % of the hedgerows but with only 0,3 ± 0,5 flowers per hedgerow. Trifolium praténse occurred in 40 % of the examined organic hedgerows with only 0,15 ± 0,5 flowers per hedgerow, but it was not observed in conventional hedgerows.
Reduced doses of Starane 180S did not affect the flowering of the one year old Taraxacum, only the plants treated with full dose had a significant lower production of flowers. The number of pollen per anther was 30 % higher for treatment 25 and 100 % than for the untreated plants. The treatments with 25 and 100 % of Starane 180S caused a great reduction in flowers for Trifolium praténse, but neither of the doses affected the production of pollen. Starane 180S was only fatal at full recommended dose, and only for Trifolium praténse. Not even on seedlings did reduced doses of Starane 180S have lethal effects on either Taraxacum or Trifolium praténse. However there was a significant reduction in plant biomass on the seedlings after exposure to Starane 180S in drift relevant doses, and together with the tendency that Starane 180S causes a reduction in the number of flowers, the sublethal effects of Starane 180S can help explain the difference in plant diversity that has been observed between conventional and organic hedgerow bottom vegetation.
Although Starane 180S has been proved to have some effect on Taraxacum and Trifolium praténse, the use of this herbicide in conventional farming is not believed to have as severe consequences for the insects and vegetation of the surrounding nature as other herbicides have.