Plant diversity in grasslands: Multi-species swards outperform even in drought conditions

Six-species swards outperformed perennial ryegrass monocultures and were considerably more resistant to drought, research has found.

The research from Teagasc, Johnstown Castle and Trinity College Dublin shows that multi-species mixtures receiving 150 kg per hectare per year of nitrogen fertilizer out-yielded perennial ryegrass monocultures receiving double that amount of fertilizer (300 kg per hectare per year).

Increases in plant diversity up to six species in intensively-managed grasslands reduced the impact of drought and produced more yield with less fertilizer.

Higher-diversity lower-input mixtures produced higher yields than low-diversity high-input monoculture

Averaged across the two years of the trial, mixtures with all six species produced the highest yields, and yielded more than the best of the six monocultures. Under rainfed conditions, the mixture with equal proportions of all six species at sowing (produced 11.8 tons/ha/year) outperformed the best-performing monoculture (produced 10.5 tons/ha/year).

Even after increasing fertilizer on perennial ryegrass in monoculture, it did not yield as much as the highest diversity mixture.

Higher-diversity mixtures produced similar yields under drought as other comparisons under rainfed conditions

As expected, total annual yields were generally reduced by the experimental drought. Nevertheless, the higher-diversity lower-input mixtures under unfavorable drought conditions achieved similar yields to those from the low-diversity, high-input comparison under favorable rainfed conditions. This indicates the potential for multi-species mixtures to mitigate the effect of more variable weather conditions on grassland yields across the whole year.

Dr. John Finn, a senior researcher at Teagasc's Environment Research Centre at Johnstown Castle, said: "The need for research on drought in Irish grasslands has become all too obvious in recent years, and that need will only increase in the coming decades. The use of multi-species is one strategy to improve the resilience of grassland production."

Dr. Caroline Brophy, Associate Professor at Trinity College Dublin, said: "It is crucial that we reduce nitrogen fertilizer inputs to improve the sustainability of Irish agricultural grasslands. Using innovative statistical approaches, we show that multi-species mixtures provide a solution to reduce fertilizer usage, without compromising on grassland production."

The most productive swards were a combination of species from the three functional groups of grasses, legumes and herbs. With legume proportion between 30 and 70%, yields were better than the best monoculture.

Teagasc Walsh Scholar Guylain Grange, who conducted the experiment, said: "Multi-species mixtures can be a practical, farm-scale solution for intensive grassland production with lower nitrogen fertilizer input, and can mitigate the risk associated with summer droughts due to climate change. Further research at Teagasc is investigating the use of multi-species mixtures under grazed conditions."