Attempts to develop grain crops such as wheat or maize that can be harvested year after year have repeatedly failed. What remains are cereal varieties that are neither truly annual nor perennial, but something in between – and that do not fulfil their intended purpose. Yet these failed experiments may end up solving a completely different major problem in agriculture.
Farmers around the world struggle to maintain high nutrient levels in their soil – a prerequisite for stable crop yields. The problem is especially pronounced in the periods between harvest and sowing, when fields are left bare – fallow periods – and winter rains wash nutrients out of the soil.
Research shows that these periods are a key source of nutrient loss, greenhouse-gas emissions and declining biodiversity. To counter this, farmers sow cover crops to retain nutrients when no wheat, barley or oats is growing in the fields.
But the solution is far from ideal. Farmers must invest both time and money in sowing cover crops without any direct financial return, and the effect varies widely depending on weather conditions and timing. As a result, many farmers do not use them unless required to do so, and uptake often depends heavily on policy requirements and subsidy schemes.
“In Denmark, planting cover crops to reduce nutrient leaching is mandatory – and to be honest, it is a real nuisance for farmers,” says Virginia Anne Nichols, Assistant Professor at the Department of Agroecology of Aarhus University in Denmark.
There is a long line of attempts to address this problem, from perennial crops to drone-based sowing and permanent ground cover. All have shown promise but also clear limitations. There may, however, be another way that several research groups around the world are now beginning to explore.
The idea is to use failed attempts at perennial crops directly in the field instead of sowing separate cover crops. These plants can cover the soil on their own, potentially saving farmers significant work.
The mechanism is simple: after grain harvest, the plants regrow from growth points and roots already established in the soil – a phenomenon known as post-harvest regrowth. Instead of starting from scratch with new seeds, the plants continue to take up nutrients during the period when the soil would otherwise be bare.
“Here we propose an alternative solution in which farmers simply leave the harvested plants in the field,” says Virginia Anne Nichols.
The results have been published in Agricultural Systems.
Not quite perennial, not quite annual – but something new
This type of plant is described as something entirely new: not truly perennial but not a classic annual either. The researchers call them plusquam-annuals, meaning “more than annuals” - plants that act like annuals because they produce only a single harvest but then continue growing as a kind of afterlife in the field without necessarily setting new seeds.
Most crops grown in fields around the world are annuals – and that is key to understanding the idea that Virginia Anne Nichols and colleagues are exploring.
“Wheat, barley and oats are annual plants – they die after harvest and must be sown again the following year. So for many years, farmers have dreamed of perennial crops that can simply keep producing harvests,” explains Virginia Anne Nichols.
But even the most promising perennial cereal varieties have shown marked declines in yield in trials, often already in the first year, making them unsuitable as reliable crops.
A century of breeding – and the plants that fell in between
For more than a century, plant breeders and agronomists have tried to turn some of the world’s most widely used annual crops into perennials, but their efforts have not really borne fruit.
“Existing studies show that these intermediate forms have already appeared in experiments in which annuals and perennials have been crossed. So the potential is there – we do not have to reinvent the wheel.”
Virginia Anne Nichols came across this, among other things, at a conference focusing on the possibility of developing perennial crops from annual species.
“At the meeting, researchers explained that the crops could grow, be harvested and then regrow. But the plants often died before they had time to set new seeds for the following season. For many, that meant the crops were a failure – but I saw a completely different potential,” she explains.
Virginia Anne Nichols and colleagues now want to use these failed experiments as crops that can effectively function as their own ground cover. In practice, farmers can sow the crops, harvest them, and then allow them to regrow and cover the soil after harvest – without it mattering if they die later in the season. This removes the need to sow separate cover crops while offering a more reliable solution than newly sown cover crops.
Letting the crop do the job: roots, regrowth and trade-offs
“Cover crops often struggle because they are sown late in the season, when temperature and light do not provide favourable growing conditions. As a result, they often establish poorly or not at all. But these new crops already have their roots in the ground and have captured the nutrients. That is why it is a major advantage if you can simply leave them to keep growing,” says Virginia Anne Nichols.
The study suggests that this established root system may be the key advantage: the roots are in place and can immediately take up and retain nutrients while continuously adding carbon to the soil.
The new study also highlights that the approach comes with trade-offs – including fewer opportunities to introduce new plant species into crop rotations and reduced nitrogen fixation compared with traditional cover crops. But planting a cover crop into the regrowth is also an option.
How well the idea works will also depend on local conditions. In some regions, nutrient leaching is primarily an autumn problem, whereas in others it occurs mainly in spring – placing different demands on the crops’ regrowth and productivity.
Virginia Anne Nichols is already testing a “failed” perennial rye as a self-covering crop. The idea has been shared early in the hope that others will begin to explore it as well.
“We already have the plants, and even though they cannot be used for the purpose for which they were originally developed, we may be able to use them in a different way. We might also be able to use genetic tools to make them even better suited to acting as their own cover crops. We would like to explore this further,” she says.
