As autumn-drilled crops wake up this spring, encouraging roots and tillering will be a priority on many soils. CPM looks at the effects of the winter and how to minimize the damage.

The ability for plants to produce new roots is limited.

By Lucy de la Pasture

It’s not just growers who have had a pretty miserable time this winter. Many of the autumn crops that did go in the ground were drilled late and have spent most of the time sitting in cold, waterlogged soils with precious little potential for growth.

ADAS crop physiologist, Dr Sarah Clarke, says that although growers could get onto light land, few autumn cereals were planted on heavy land around Gleadthorpe in Nottinghamshire.

“Where crops have spent a prolonged period in soggy ground conditions, the likelihood is that yield potential will already be reduced.” One study found that wheat yields were reduced by 20-24%, she says, when exposed to around seven weeks of waterlogging.

“When plants are waterlogged for an extended period, there’s a decrease in root proliferation and overall plant growth,” she explains.

Soon after waterlogging, root metabolism shifts from aerobic respiration to the less efficient glycolysis to produce energy, leading to a reduction in adenosine triphosphate (ATP) production. The energy deficit at root level means the root hydraulic conductivity is severely reduced.

Even though wheat and barley have the capacity to grow adventitious roots with aerenchyma (air-filled spaces), which enables cells to continue respiration and water and nutrient uptake, their growth is slowed under waterlogged conditions.

Even when soils drain, the damage to the yield potential of the crop has already been done, highlights Sarah. “Depending on the duration of waterlogging, the ability for plants to produce new roots is limited. Crops will probably require less total nitrogen where yield potential has been adversely affected, although account should be taken of the likely increased N leaching through excessive rainfall.”

But all is not lost, plant root growth can still be manipulated by the use of PGRs early in the season and this is likely to be an important tactic to reduce the risk of lodging later.

Plants get anchorage from their coronal roots and different varieties can have very different root architecture. Coronal roots are much more rigid that the thinner absorption roots responsible for nutrient and water uptake. They’re formed early in the plants life and become fully developed by the time crops reach GS39.

This means there’s a crucial phase in the spring when the coronal roots are still developing, and these are likely to have been adversely affected particularly after the wet winter, points out Dr Paul Fogg, crop production technical lead at Frontier Agriculture.

Paul explains that plants produce higher levels of gibberellic acid as day length increases and suppressing its production with PGRs will help claw back some yield potential. He prefers the use of gibberellic acid-type inhibitors early in the season – such as chlormequat, Moddus (trinexapac-ethyl) and Canopy (prohexadione-calcium+ mepiquat chloride) – and Terpal (mepiquat chloride+ 2-chloroethylphosphonic acid) later in the season, if required. Moddus and Canopy are known to increase the thickness of the cell walls in the stem so are useful tools for weaker-strawed varieties, he points out.

“The lack of both below and above-ground biomass this spring will mean PGRs will play an important role in preventing further loss of yield potential. Applications to promote rooting and tillering have to go on early as once the crop goes into stem extension, the ability to influence plant roots and tiller numbers is reduced,” he says.

As well as an early chlormequat application, Paul says early nitrogen, potash and phosphate will also be needed by plants. He also believes phosphites have a role in maximizing the underground architecture of crops.

“Phosphites are genetic signaling compounds and trigger the plant roots to look for phosphate, which has a key role in root growth. If you’re using a phosphite then it needs to be before GS30 and it’s important that the crop has access to adequate fresh phosphate to get the best results,” he comments.

Georgina Wood, technical manager at Syngenta, advocates splitting Moddus application to include the T0 timing, as this has shown to have the greatest effects on root enhancement.

“Stronger rooting assures improved anchorage and moisture and nutrient uptake, but also enhances the crops tolerance of take-all infection. A second dose at T1 has shown to give the biggest responses in terms of height management, stem strength and tolerance of eyespot,” she says.

This year, independent variety profiling trials at Harper Adams University have given a crucial insight into more precise tailoring of agronomy decisions for PGR timing and application rates. Georgina highlights that tall varieties, with extended internodal interval, are typically weaker and benefit from greater PGR control and promotion of thicker stem walls.

“The work has shown that for Evolution and Graham, for example, the T0 Moddus treatment is a priority to enhance anchorage strength. While growers will obviously need to take into account local conditions and field situations, it does give a useful guide.”

Paul agrees that variety has a big impact on how a PGR programme is put together. In particular, he highlights the importance of the speed of variety development in the spring to GS30, which is affected by the time of drilling.

The Harper Adams research has also revealed that even in some varieties, for instance Gleam and Grafton, where there’s no difference in AHDB Recommended list ratings, with or without a PGR, Moddus can  help with stem anchorage or basal strength.

Georgina is advising a tank-mix for wheat PGRs this spring, using Moddus at 0.1 to 0.2 l/ha and chlormequat (750 g/l) at 1.0 to 1.25 l/ha at both the T0 and T1 application, with the rates adjusted to the specific situation at each timing.

“One of the key advantages of Moddus at the earlier timing is faster uptake if conditions are cool. And the research has shown that where it’s used in tank-mix, the faster uptake appears to carry the chlormequat into the plant quicker too, which benefits both tank-mix partners.”

Moddus has been shown to be taken up at up to three times faster than chlormequat at a temperature of 7°C.  With the average April temperature of 7.9°C for the past three years, Moddus could have a two or three- week advantage in achieving activity in the plant, compared to chlormequat, believes Georgina.

Paul adds chlormequat needs a minimum temperature of 8-9⁰C for maximum uptake and agrees the alternatives, Canopy and Moddus, are much less temperature dependent. He adds that adjuvants can help get PGR into the plant but points out that it still has to be able to metabolise it.

“In a season where every effort is going to be required to get crops rooting strongly and growing well, that could be hugely beneficial,” he adds.

But it’s later drilled winter barley crops which may have the most to gain from spring PGR treatments this season. Last season’s trials at the Syngenta Innovation Centre near Newark highlighted the increased risks of lodging from later drilling dates, for both hybrid and conventional winter barley varieties.

October drilled plots of Bazooka, drilled at 200 seeds/m², and KWS Amistar, at 325 seeds/m², were largely unaffected even by the wet conditions and heavy rains through June, with just 1.6 and 2.9% lodging respectively. But plots drilled in November, at slightly higher seed rates of 250 and 400 seeds/m², suffered significant lodging effects in both varieties.

“These trials have confirmed the higher risk of lodging for later drilled barley crops this season, primarily through reduced root mass and the need to develop strong stems to hold up heavy seed heads,” explains Georgina.

“Treatment rates will depend on specific varieties and situations, but since we know hybrid barley tends to be taller, the PGR programme should be more robust throughout,” she suggests.

“A later application of Terpal at GS 37-39 helps to reduce the incidence of brackling and necking in heavier yielding hybrids, but it would only likely be justified in high risk situations with conventional varieties.”

Paul’s approach is that the emphasis should be on getting the early part of the PGR programme right and decrease the risk of root lodging.

“That means to strengthen the crop’s foundation by encouraging stronger crown roots and shortening the internodes at the base of the stem. Lodging risk can then be managed later in the season.

Straw is a commodity that’s likely to be in high demand and valuable this season, given the acreage of winter cereals in the ground. Growers will want to maximise their gross margins for the crop and not overly reduce the amount of straw, which will also be part of the balancing act when it comes to PGR decisions, reasons Paul.

“Xemium fungicides – such as Serpent (fluxapyroxad+ pyraclostrobin) or Adexar (fluxapyroxad+ epoxiconazole) – have proven themselves to really help manage straw quality, which is something to bear in mind,” he adds.