Wilde ganzen en cultuurgrasland in Nederland

In the period 1965-1985 numbers of migrating geese wintering in The Netherlands increased notably: from approximately 100.000 to 600.000 birds. Not all these birds are present during the whole winterseason: a maximum of about seventy percent of the total number of geese occurring in The Netherlands is present at the same time during the month of January. Along with this spread in time, geese are distributed geograffically, mainly over the lower parts of the country. In this respect the province of Friesland plays a significant part since almost sixty percent of all wintering geese stay in this northern province for a longer period.

Since some goose species (Branta leucopsis, Br. bernicla and Anser brachyrhynchus) have a close season, and others (Anser anser, A. fabalis and A. albifrons) can be shot only during a certain period (from September 1st till January 31th) and time of day (half an hour before sunrise till 10.00 a.m.), any damage caused by goose-grazing is completely compensated for by the Dutch government, more specifically the Ministry of Agriculture and Fisheries.

A side-reflection is the recognition that all scaring techniques are effective only for a fairly short period and require not only persistence but also the presence of an area where the geese can be scared to, either a refuge or a place where they will be tolerated. This is not yet the case. Along with the increase in geesenumbers a steady rise in the number of complaints by dairy farmers became manifest every year.

Per province an annual review of all complaints concerning goose-damage is presented as well as the crops concerned. Over ninety percent of these complaints concern goose-grazing on pasture land, in more than ninety seven percent restricted to the province of Friesland, the main problem being yield losses at first cut.

Yearly amounts paid by the Game Fund to compensate for these losses increased from sixthousand Dfl. in 1974 to one and a half million Dfl. in 1984; every year more than half a the total amount is paid to Frisian farmers.

The intensification of dairy farming in The Netherlands is discussed, the main topics being changes in grassland utilisation, the application of growing quantities of mineral fertilizers, the increase in yearly reseed and the use of new mixtures. In combination with an improved drainage this resulted in a shortening of the nonproductive winter-period. An increasing number of wintering geese benefit from the fact that more grass of a better quality is available in autumn and spring.

From an inventory of methods used by the provincial Game Damage Committees to assess goose-damage, their appears to be very little conformity, mainly caused by the fact that the subject was never thoroughly studied. In order to find a uniform, standardized method for the assessment of yield losses due to goose-grazing, a four year research program was carried out by the Centre for Agrobiological Research (CABO, Wageningen). In the study area, the northern Dutch province of Friesland, the effect of goose- grazing was studied on growth and production of Lolium swards in a temperate climate.

Experiments were conducted over three winter periods (1982-1985) of which the first two may be classified as "mild" winters, the last one was "hard" and characterized by very low grazing intensity. The difference in sward height between the grazed (g) and ungrazed (u) plots at the time of last goose-grazing (V) turns out to be an essential parameter in explaining losses in herbage production at the time of the first cut; it also provides a method for calculating growth retardation (in days) due to goose-grazing.

Goose-grazing generally causes loss of herbage yield in the first cut (early bite); this effect does not occur in subsequent cuts. The extent of this loss (in kg dm) is correlated with:
- the difference in sward height (grazed - ungrazed) at time V;
- the undisturbed yield level at the time of harvest;
- the temperature sum (T-sum) at the time of harvest.

Moreover after a mild winter the T-sum at the time of the last heavy goose-grazing plays a significant role; in a hard winter this is the case with the T-sum at V.

Retardation can also be calculated from the absolute yield loss (in kg dm) and net daily production of the corresponding sward (in kg dm day ^-1). Generally speaking yield losses are bigger after a mild than after a hard winter (and so is retardation). Goose-grazing causes a reduction of the LAI-values of the grass; this results in a decline in daily dry-matter production (kg dm 100 m ^-2day ^-1) as a result of which yield losses occur in the first cut. These losses are always bigger than the calculated herbage consumption by geese in the preceding period, both expressed in kg dm 100 m ^-2.

Comparison of goose-dropping contents with the botanical composition of the grazed sward show that there is no significant food selection, the former being a true reflection of the latter. Goose-grazing may cause an improvement in early bite quality (in terms of kVEM kg dm ^-1) but the kVEM-loss per hectare is so big that the former advantage pales into insignificance in relation to the latter loss of quantity (one feed unit VEM = 1.65 kcal = 1.65 * 4.1868 J).

One and even two winters of goose-grazing did not noticeably affect the total number of plant species in the sward, nor dry weight percentages or number of tillers dm ^-2per species.

We could not demonstrate any permanent effect of the combination of goose-grazing, treading and manure on the P, K and N content of the upper soil layer; we did not find any effect on permeability, carrying capacity (solidity) and moisture level of that same layer.

The agricultural importance of goose-droppings should be considered as insignificant: in terms of nitrogen supply for example this will be less than ten percent of the farmers N-gift for the first cut. Moreover such plant nutrients as the geese provide have come directly from the soil on which the birds feed and are not additions.

A guideline is presented to the assessment of goose damage to herbage production using the difference in T-sum values at the time the sward is grazed for the last time (V) and at the time of harvest. A grassland utilization scheme is discussed, in particular for use and management of the first cut.

Small adaptations in such a scheme might help in lessening the conflict between dairy farming and goose-grazing in The Netherlands.