Farmers have increasingly sown a single type of grass in their fields over the past 100 years, and then added chemical fertiliser to increase their harvest. But new research suggests that there are alternatives that are cheaper and can increase the potential of these grasslands to feed livestock.
Author
- Caroline Brophy
Professor in Statistics, Trinity College Dublin
My research team and I were particularly interested in the potential of mixing up the species of plants grown in agricultural grasslands and what the benefits might be.
This meant the sowing of two grasses, two legumes (for instance, red clover and white clover) and two herbs (such as plantain and chicory) together in a field. These groups of species can play different roles in a grassland. For example, legumes can extract nitrogen from the atmosphere, and herbs can have deep roots.
We wanted to find out if mixing plants that differ in their ecological traits can provide a nature-based and sustainable solution to reducing chemical fertiliser use.
There was another potential benefit. If farmers could find a way to reduce their chemical fertiliser use, it would lower their operational costs, and also benefit the environment .
The price of chemical fertilisers around the world has risen in the past five years. Costs tripled from 2021 to 2022 , and while they have reduced since , they are still higher than pre-2021 prices. So there's a financial motivation to reduce use.
Chemical fertilisers also harm the environment, including by releasing greenhouse gases and leaching nitrate .
In our new international study published in Science , my colleagues from the LegacyNet project and I have shown that planting mixtures of different species can improve grassland yields compared to conventional practices, and crucially do so while using substantially less chemical nitrogen fertiliser.
This means farmers could save money and reduce their environmental footprint by doing this.
How was the research carried out?
The team conducted the same experiment at 26 research institutes across Europe, North America, Asia and New Zealand. At each site, we had plots sized at least three metres by five metres ranging from just one species up to six species, managed with moderate levels of chemical nitrogen fertiliser. We also had plots with a single grass species, managed with at least twice the amount of fertiliser.
In each case we measured crop yield. The research showed that planting a variety of species produced 11% higher yields than sowing a single grass, despite the single grass being treated with more than double the chemical nitrogen fertiliser.
This was in part due to the inclusion of legume species - for example, white clover and red clover, in our six-species multispecies mixtures. Legumes can extract nitrogen from the atmosphere through a natural process.
And this "free" nitrogen is released into the soil to be used by all species in the grassland. Since grass and legumes work well together, a now widespread farming practice is to sow 70% of one grass and 30% of one legume. This is often a combination of the grass perennial ryegrass and the legume white clover.
Does this mean that by growing just one grass and one legume we could get just as much yield as a multispecies mixtures? Not necessarily, because for example, herbs have deeper roots compared to grasses, which can bring benefits such as increased water uptake from lower down in the soil. Indeed, previous research has shown that multispecies mixtures can even mitigate the yield losses associated with drought conditions .
Our research found that there was an 18% increase in yield from using our multispecies mixture, compared to sowing 70% of one grass and 30% of one legume.
We also found that having two grasses, two legumes and two herbs was better than having just one grass, one legume and one herb.
From Roman times, agricultural grasslands have been used to feed grazing animals or as part of a crop rotation system , where farmers plant different crops in the same field in rotation, and also leave it fallow. We now know that we can improve grassland systems by combining two grasses, two legumes and two herbs in a field.
Adapting to climate challenges
Climate change poses a significant risk to farming livelihoods, agricultural production and food security. Our experiment spanned a range of climates, which allowed us to investigate the potential of multispecies mixtures to cope with rising temperatures. We found that as temperatures increased, the benefits of multispecies mixtures increased even further, compared to previous practices.
This suggests potential for farmers to increase the productivity of their lands using less fertiliser despite increasing temperatures.
Historically, farmers understood that increasing the species diversity of a grassland could improve productivity compared to sowing a single species .
But the prevalence of using a single crop and applying high amounts of chemical fertilisers has increased massively over the past century , a period ironically termed the "green revolution". Food shortages after the second world war prompted the use of technologies such as fertilisers, herbicides, pesticides and farm machinery as ways of protecting crops.
And yields from agricultural grasslands flourished as these technologies grew in popularity and until recently, chemical nitrogen fertilisers were relatively cheap.
However, the evidence showing how chemical nitrogen fertilisers emit the potent greenhouse gas called nitrous oxide is now much better known. Chemical fertilisers can cause other environment problems such as nitrate leaching into groundwater . Creating chemical fertilisers is an intensive industrial process that relies heavily on burning fossil fuels.
In the past two decades, scientific studies testing species diversity in agricultural grasslands began to emerge . There was evidence that mixing grass and legume species could improve the yields compared to grass species being grown on their own .
Our study further advances this knowledge to show that we can do better by planting mixtures of grasses, legumes and herbs, and with less chemical fertiliser.
This research confirming that farmers can produce more by using less fertiliser is a win for farmers' pockets and a win for the environment.
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Caroline Brophy receives funding from Research Ireland Frontiers for the Future program, grant number 19/FFP/6888; the Department of Agriculture, Food & the Marine, project ref. 21R456; and the European Union's Horizon 2021 doctoral network program under the Marie Skłodowska-Curie grant agreement No. 101072579 (LegumeLegacy).
