Farmland ecology: Four-species mixtures increased weed suppression in intensively managed grasslands

Diversity robustly suppresses weeds in intensive grassland systems

How can we get more from less use of natural resources in agricultural systems? The use of multi-species mixtures (plant diversity) in intensively managed systems has been proposed as one strategy to improve agricultural sustainability. We show that four-species forage mixtures greatly increased weed suppression in intensively managed grasslands.

Uncontrolled weed growth can represent a major source of inefficiency, diverting nutrients, water, light and labour to an undesirable form of biomass, while herbicides incur significant environmental and economic costs. In pastures, weeds can impair forage quantity and quality resulting in reduced animal production, and increase the need for reseeding with its consequent costs. If diversity helps in maintaining a low level of weeds in pastures (and increases yield) it can increase the sustainable production of higher quality forage compared to systems relying on monocultures.

We conducted a co-ordinated continental-scale field experiment across 31sites to investigate this. The study included 15 plant communities at each site, two grasses and two legumes each sown in monoculture and 11 four-species mixtures varying in the relative proportions of the four species. At each site, one grass and one legume species was selected as fast establishing and the other two species were selected for persistence. Mixtures were designed to reduce reliance on fertiliser nitrogen. Previously we showed strong effects of plant diversity in enhancing total biomass, biomass of sown species and N capture across the 11 four-species grass-legume mixtures (Finn et al. 2013, Suter et al. 2015). Here, we summarise results showing similar strong effects of mixtures on weed suppression in the same experiment. This work is currently being published in the Journal of Applied Ecology under the title ‘Weed suppression greatly increased by plant diversityin intensively managed grasslands: a continental-scale experiment’, DOI: 10.1111/1365-2664.12991. (Open Access)

Our findings.

Averaged across all sites: Average weed biomass in mixture over the whole experiment was 52% less (95% confidence interval 30% to 75%) than in the most suppressive monoculture (transgressive suppression). Transgressive suppression of weed biomass persisted over time, being found in each year for each mixture (which varied in sown composition between 20% to 80% grass). Weed biomass was consistently low and relatively similar across all mixtures and years. Within a site weed biomass varied much less in mixtures than in monocultures.

At each site: Across sites, the growth condition differed remarkably from Meditteranean (Span, Sardinia) to sub-Arctic (Iceland, north Norway). Yet, at each site the weed biomass for almost every mixture was lower than the average weed biomass across the four monocultures. The average proportion of weed biomass in mixture was less than in the most suppressive monoculture in two thirds of sites. Transgressive suppression of weeds was maintained across years, and was independent of site productivity.

Implications of our work: Weed invasion can be diminished through a combination of forage species selected for complementarity and persistence traits in systems designed to reduce reliance on fertilizer nitrogen. In this study, effects of diversity on weed suppression were consistently strong across mixtures that varied widely in species proportions and over time. The level of weed biomass did not vary greatly across mixtures varying widely in proportions of sown species. These findings suggest that the composition of the mixtures need not be tightly controlled either at sowing or subsequently, making the system easier to manage. We conclude that these diversity benefits of weed suppression, which occurred in parallel with increased forage yields and nitrogen capture in intensively managed grasslands, are relevant for the sustainable intensification of agriculture and, importantly, are achievable through practical farm-scale actions, which are readily applied, as certain European regions demonstrate.

References

Connolly J, Sebastià M-T, Kirwan L, et al. Weed suppression greatly increased by plant diversity in intensively managed grasslands: A continental-scale experiment. J Appl Ecol. 2017. Open Access.

Finn, J., et al. (2013) Ecosystem function enhanced by combining four functional types of plant species in intensivelymanaged grassland mixtures: a 3-year continental-scale field experiment. Journal of Applied Ecology, 50, 365-375.

Suter, M., et al. (2015) Nitrogen yield advantage from grass-legume mixtures is robust over a wide rangeof legume proportions and environmental conditions. Global Change Biology, 21, 2424-2438.