Seed harvesting impacts on grassland
Extract from the Lowland Grassland Management Handbook
The Lowland Grassland Management Handbook
This updated and expanded handbook offers practical advice on grassland management.
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6.14 Wild flower seed harvesting
With the increase in projects designed to re-create herb rich grasslands there has been an increasing demand for native wild flower seed. This has led to the exploitation of semi-natural hay meadows for seed and the development of two types of machines to harvest it. Suction harvesters, which are usually tractor mounted, normally consist of a series of suction heads resting on wheels pulled over the grassland sward. The heads can be set at different heights and the seeds are picked up by suction and collected into a large collecting vessel. In order to maximise the variety of seed collected, it is normal for the process to be repeated on a number of occasions during June, July and early August. A brush harvester utilises a large brush to sweep seed out of the crop. This machine was developed by Emorsgate Seeds of Kings Lynn and normally operates on two harvests in the season. This machine is probably less damaging to invertebrate populations as it removes fewer individuals.
Following use of the seed harvester the crop may still be cut for hay. Harvesting of seed could have an impact on the populations of vascular plants and invertebrates.
The points relating to the timing of cutting in section 6.10 are also relevant in this context.
The impact of seed harvesting on the botanical composition of the sward depends on the proportion of seeds removed from the range of species, the timing of seed set of individual species and the extent to which individual plant species rely on seed production as a regenerative strategy. Harvesting machines do not collect all seed but lighter ones are more efficiently collected by the suction harvester.
Initial advice (which may require modification in the light of further research) is as follows:
- Where grasslands are characterised by populations of annual species, such as yellow rattle Rhinanthus minor, which do not form persistent soil seed banks and which normally set seed prior to hay cutting, it is advisable to either not harvest every year or only harvest a proportion of the site or area in any one year.
- Where there are no significant populations of annual species, it may still be advisable to avoid harvesting every year or to harvest only a proportion of the site in any one year. This then allows early maturing perennial species to set and shed seed in some years. English Nature has stipulated, for example, on one site that one fifth of the meadow may be harvested on a five year rotation.
Large numbers of invertebrates occur in hay meadows, but often there are relatively few species and those that do occur have life cycles which fit in with regular hay management (Kirby 1992).
Large numbers of invertebrates can be collected during wild flower seed harvesting (Waring 1990). Some species are likely to be more vulnerable than others to extinction or to reduction in their populations as a result of seed harvesting and this will depend largely on the initial size of population present and the nature of specific species life histories, including mobility.
- It may be inadvisable to change grassland management from extensive grazing to hay and aftermath grazing in order to harvest a seed crop. Invertebrates associated with seed heads are particularly vulnerable.
If a meadow is known to support rare or local invertebrate species which may be vulnerable to seed harvesting (advice may be needed from an entomologist), it is advisable to prohibit its use for this purpose. If it is decided to go ahead, then the following measures may reduce the impact of seed harvesting on invertebrates:
a. Cordon off areas if a species occurs on only part of a site.
b. Only harvest a proportion of the site in any one year. For example, a site could be harvested in strips with one strip in every five being harvested for seed.
c. Do not harvest every year. This is based on the assumption that invertebrate populations reduced by harvesting may recover if left for a year or two.
d.Populations of rare and vulnerable species should be regularly monitored.
e. Spreading seed onto sheets after collection allows a high proportion of invertebrates to escape thus mitigating the impact of harvesting.