Good rooting is critical for high yields in wheat and oilseed rape crops and research suggests today’s crops may be failing to extract sufficient water in drier periods.

Roots have long been neglected with little research in recent years, because of the difficulty in studying them in fields. However, a renewed interest in soil health has seen more focus on rooting.

See also: Wet weather raises rooting worries for winter cereals

A key measure of rooting is the root length density and researchers believe there is a critical threshold of 1cm of root/cu cm of soil.

“The thinking is if the density is above 1.0, rooting is at a level that will abstract the available amount of water and water-soluble nutrients,” explains Adas crop physiology research scientist Charlotte White.

Therefore, being below 1.0 means there are insufficient roots to extract the water and nutrients that are available.

Adas has pooled data from control OSR plots in on-farm trials carried out from 2013 to 2014 and Dr White discovered root length density below 40cm was often lower than the critical 1cm/cu cm.

“This suggests that below 40cm, today’s crops are not able to extract sufficient amounts of water and nutrients. And we know water is strongly linked with yield.”

Rooting depth decrease over time

One question is whether this has changed over time? Dr White explains there is limited historical data on rooting at depth, but looking at topsoil (top 20-30cm), it is clear there was a lot more rooting in older trials (1970s and 1980s), which may suggest there has been a decrease over time.

This may be due to changes in soil fertility, drainage, increased compaction or a change in cultivation system. Breeding is another possibility, possibly inadvertently breeding for crops with less rooting in the topsoil. However, while the reason is still not clear, she suspects it is a combination of these.

Lack of rooting

So what does the lack of rooting mean? Dr White says reduced density deeper in the soil profile may mean yields are being constrained in drier years.

“Improving rooting could offer higher yields in drier years,” she suggests.

Syngenta technical manager Jason Tatnell believes limited rooting could be one of several potential reasons why growers are not seeing wheat crops reach 20t/ha and may also be to blame for the plateauing of yields.

Dr White’s advice to farmers who suspect a rooting problem is to get a shovel and look at what is going on below the surface.

“Look at drainage and soil health. Poor drainage does not do roots any good. Any compaction also needs to be dealt with, as it can inhibit root growth.”

However, mole draining and subsoiling do not go as deep, so the use of growth regulators and phosphites may be the way to encourage rooting at depth.

Growth regulators divert growth from the canopy to roots by reducing the height of the growing point. Research is now focusing on this rooting effect.

German rooting trials

Mr Tatnell points to two separate German glasshouse trials where the use of Moddus (trinexapac) + chlormaquat led to a 11% and 12% increase in root mass, when applied at similar rates to UK. Another glasshouse trial looking at P uptake at different soil moisture levels saw a 12% and 17% increase with the use of a regulator.

Field trials looking at the effect on K uptake of regulator use coupled with applying more fertiliser saw a greater response with a PGR than just applying more K, suggesting rooting was limiting uptake.

“We are seeing a more efficient plant,” says Mr Tatnell.

Glasshouse trials with wheat in drier conditions found improved water use efficiency because of a smaller canopy, falling from 480ml/g to 540ml/g when applying Moddus + chlormaquat instead of chlormaquat alone. Grain yields were also 22% higher, he says.

Helping crops cope with dry conditions

“PGRs are setting plants up to be better able to cope with extremes, for example a more resilient crop if you get a really dry spring.

“That [increased resilience] is why in our lodging trials, you might not see any lodging, but there is still a 0.2-0.3t/ha benefit. This could be down to better rooting,” he says.

Adas work has confirmed the rooting benefits in oilseed rape across six trials applying metconazole in spring. It increased root length density at 40-100cm from 0.478 to 0.601 and this was estimated to equate to a yield increase of 0.2-0.3t/ha in a dry season.

In-field work is ongoing in cereals, where rooting increases have been seen with PGR use, especially on lighter soils. But Dr White says key questions such as why there has been a decline in rooting at depth and how farmers can reliably increase rooting at depth to support high yields remain unanswere.