With farmers under pressure to reduce input usage, one Nuffield scholar has analysed the use of cover cropping and intercropping as a means of reducing reliance on fertilisers and pesticides. Abby Kellett reports.
Kent farmer, Andrew Howard is approaching the end of his Nuffield scholarship travels, which have taken him around the world to study the benefits and challenges associated with multi-cropping.
While Mr Howard has already had five years' worth of experience using cover crops, he was keen to investigate intercropping as a means of reducing pest pressure, increasing nitrogen efficiency and to act as a ‘soil conditioner’ on his 350 hectares of arable land.
On his farm, cover crops precede all spring crops and when possible are planted post-harvest and before establishment of the following winter-sown crop.
See also: Cover crops: How are they best used?
Mr Howard says: “Everything we do here is to build organic matter and to look after our soil, so I don’t like to see the land fallow or bare at all, especially not over winter.
“Any time we have more than six weeks with no crop in the ground, we try and sow a cover crop, even in the summer.”
Mr Howard believes both cover cropping and intercropping have a place in his farming system and acknowledges they both have challenges and benefits associated with them.
“One benefit of intercropping is that you harvest a cash crop, while with cover cropping, you can only graze it. The benefit of cover cropping is that you can add more species to increase diversity and you don’t have to worry about harvesting mechanically."
Intercropping is where two or more crops are growth together in the same field. Typically crops are planted in alternating rows or are totally intermixed. Research has shown there are a number of benefits associated with the close proximity of different plant species.
Source: adapted from AHDB
Both practices have advantages associated with using multiple species. Since each species uses soil and space differently, species are able to share resources and complement each other, according to Mr Howard.
“If you look under a hedge or at the edge of a field, there is more green biomass than in the middle of the field. Species here are healthy, disease free and have low pest pressure without the use of inputs.
“This is what we are trying to mimic using intercropping and cover cropping. Both of which should allow us to reduce input costs and increase crop growth.
“Nature doesn’t like monocultures where all plants are genetically the same. They require the same resources, they get the same diseases and pests so if one gets attacked then all the plants will eventually.
“However crops with multiple species can use different soil and aerial resources because their rooting and canopy growth is different. Instead of competing against each other they share resources."
As well as reduced input costs, Mr Howard identified increased carbon deposition and improved soil health as additional benefits.
He also considers intercropping as a means of spreading risk: “By sowing more than one crop, in which one may do better than another depending on the year, you should always have a crop to harvest, as it is unlikely that they will all fail.”
Separating species at harvest is the biggest challenge, according to Mr Howard. At present, he is trialling intercropping on a small scale and plans to hire a seed cleaner this year to separate grain based on size.
“Knowing which varieties are suitable for intercropping is crucial. For example, making sure all species will ripen at a similar is important to maximise yield and crop quality.
“The best varieties for intercropping are not necessarily the same as the best monocrops. Also, there are agronomic challenges such as reduced weed control options,” says Mr Howard.
Of the trial plots set up by Mr Howard, linseed and oats have appeared the best combination in terms of growth and rooting.
He believes the difference in rooting between these species may have allowed for optimum nutrient uptake. As well as this, pest damage has been reduced where this mix was planted.
“The plot containing linseed with lucerne has been hammered by flea beetle but for whatever reason linseed and oats seem to have far less pest damage.
“It’s about trialling different combinations and seeing what works on your soil type and under individual farm pressures,” says Mr Howard.
The potential for reducing nitrogen inputs is a key driver in Mr Howard’s decision to explore intercropping and he hopes to reduce his nitrogen usage by 50 per cent in the next five years.
An ambitious goal but something he believes he can achieve, partly through the use of intercropping, without suffering a yield penalty.
He says: “Artificial nitrogen has increased yields dramatically but this has come at a cost. Firstly the reduction in soil organic matter and health.
“Secondly, excess nitrogen especially in the nitrate form is directly linked to pest, disease and weed pressure. So increased yields have led to an increase in pesticide usage and the tools we have to deal with these side effects are failing quickly.
“To remain productive in the future we need to grow healthy crops in healthy soil. To increase soil health and plant health we need to reduce artificial nitrogen. I want a soil that provides the goodness to the plants naturally and free.”
During his Nuffield travels, Mr Howard met French farmer, Hubert Charpentier, who has been using lucerne as a permanent cover crop which is known as a ‘living mulch’.
This has effectively reduced his reliance on fungicides and N in particular, while also improving soil organic matter.
Mr Charpentier first plants the lucerne with winter oilseed rape (OSR) as opposed to cereals, which would be too competitive for the lucerne.
After the OSR is harvested the light gets down to the lucerne and it grows all summer. He then drills winter wheat directly into the lucerne in October.
The lucerne has successfully reduced competition from grass-weeds which have also been supressed by the inclusion of two broad-leaved crops in the rotation and by adopting a no-till system.
He uses some broad-leaved weed herbicides to remove relevant weeds, to suppress the lucerne and to reduce competition with the cereal crop.
Mr Howard believes the lucerne’s deep roots are key to its suitability as a permanent cover crop: “The main reason lucerne is a good living mulch is that it is a very deep rooting plant and so does not compete with the wheat for water. Unlike white clover which has shallower roots."
Mr Charpentier had found that the optimum amount of nitrogen to apply in this system is 100kg/ha, which typically gives a yield of 8t/ha, a normal yield for the area but with nearly half the N fertiliser.
Where this mulch is used, Mr Charpentier is able to halve or avoid fungicide use depending on the season. This method, which reduces reliance on pesticides, has inspired Mr Howard to trial this practice on his farm in the future.
Please note: Updated from print version first published June 10