Plant community ecology

The invasion of agricultural grasslands by weed species may substantially lower their nutritional value to grazing animals. Once established, weed control may be dependent on the use of herbicides or physical removal. In Ireland, perennial ryegrass monocultures or perennial ryegrass/clover swards are typically used in intensively managed agricultural grassland systems.

However, previous studies have demonstrated that increases in forage plant diversity are associated with reduced weed invasion and greater plant community stability (Finn et al., 2013; Connolly et al., 2009; Frankow-Lindberg et al., 2009). This project aims to investigate the effects of increasing the botanical diversity of swards on unsown weed invasion.

Eight pasture mixtures were selected using a constrained simplex-centroid design with three functional groups; grasses, legumes and herbs. There was a constraint imposed on the design so that there was at least 40% grass in each mix, therefore no more than 60% legume or herb (after Kirwan et al. 2007). This was repeated at three levels of species richness, with 1, 2 or 3 species per functional group (24 mixtures). The design was replicated at four different nitrogen (N) rates (0, 45, 90, 135 kg N/hectare/year), resulting in a total of 96 plots.

Experimental plots were mown to a height of 4 cm to simulate 28-day rotational grazing. Forage sub-samples were taken from three harvests during the growing season (April, June and August). Above-ground dry matter yield (DMY) of both sown and unsown species were measured. Prior to harvest, the community composition of unsown species in each plot was assessed with a 1 m2 quadrat, placed at the centre of each plot. Weed invasion was measured as the proportion of total yield composed of unsown species biomass.

Results from this experiment will allow practical recommendations for methods to reduce weed burden on grassland farms using multi-species sward mixtures. This work will also allow us to explore theory around biological invasion along a diversity gradient.

References
Connolly, J. et al. (2009). Effects of multi-species swards on dry matter production and the incidence of unsown species at three Irish sites. Irish Journal of Agricultural and Food Research, 48, 243-260.
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Finn, J.A. et al. (2013). Ecosystem function enhanced by combining four functional types of plant species in intensively managed grassland mixtures: a 3-year continental-scale field experiment. Journal of Applied Ecology, 50, 365-375.
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Frankow-Lindberg, B.E., et al. (2009). Biodiversity effects on yield and unsown species invasion in a temperate forage ecosystem. Annals of Botany, 103, 913-921.
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Kirwan, L., et al. (2007). Evenness drives consistent diversity effects in intensive grassland systems across 28 European sites. Journal of Ecology, 95, 530-539.
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