Tough decisions lie ahead for flood-hit crops

Growers are advised to assess soil conditions before deciding whether waterlogged crops are still viable – or fields should be left to recover for next season.

Tough questions may need to be asked when it comes to choosing between managing struggling crops already in the ground, redrilling fields altogether or putting in a cover crop to improve soil structure, say agronomists.

Exceptionally wet autumn and winter weather continues to pose serious difficulties, says Neil Watson, technical manager for Hutchinsons. While some early-sown winter crops are looking good, others have been hit hard, he says.

Crucially, when soils become waterlogged, this starves roots of oxygen, says Mr Watson. This causes them to stop functioning, preventing the movement of water or nutrients from the roots up to the top growth.

Knock-on effects

“Ironically, the stomata close, almost like in drought conditions, which affects photosynthesis, with knock-on effects for nutrient uptake, tillering and biomass accumulation.

“In a normal situation, tiller production might be around 1,200/m2, producing 500-600 ears/m2, but in extremely wet conditions, particularly when waterlogging occurs close to emergence, tillering may be nearer half that.”

In severe situations, crops can also “cannibalise” nutrients from older leaves to support newer growth, causing early leaf senescence, says Mr Watson.

Roots often adapt to wet conditions by growing closer to the surface to facilitate gas exchange in anaerobic soils. Shallow rooting presents issues as the season progresses though, especially when conditions turn dry and crops need more scavenging ability for water and nutrients, he says.

“Without the foundations, you haven’t got the critical structure to build growth throughout the season.”

Different options

The first step is to assess the soil and crop condition in individual fields, identifying issues and crop requirements. Building early nutrition to boost root development and build biomass is crucial, and early nitrogen is one of the most important considerations.

“The first N-Min results for this year show there is less residual nitrogen in the soil than we’d normally see, primarily as a consequence of nitrate leaching, says Hutchinsons fertiliser manager Tim Kerr.

But it’s a quadruple whammy; there’s less nitrogen in the soil, less in the crop, poorer rooting, and reduced mineralisation due to the impact of waterlogging on soil microbes. We’re facing a very different scenario to previous years.”

Applying nitrogen in ammonium nitrate form is generally preferred as this is more readily available to plants than urea-based fertiliser, which must go through two chemical changes – both reliant on microbial activity – for it to become plant-available.

“In normal conditions, plants are slower to respond to urea fertiliser, but if soils are in a less than ideal state, that process could be even slower. Inhibited urea is even slower to work, so be careful what you use.”

Soil nutrients

Early sulphur will also be important on almost every soil this year, as sulphate, like nitrate, is easily leached, Mr Kerr says. “Ideally, apply sulphur with the first nitrogen, or alongside the potash. Rates should generally be in proportion to the amount of nitrogen applied.”

Boron is another easily leachable nutrient, and testing of samples in recent years shows over 70% to be low in boron, especially on lighter land.

“Deficiency symptoms are rarely seen, but boron plays a vital role in root and shoot development. Trials in 2020 showed a positive response from foliar boron applied at GS 30 and GS 39.”

Potassium and magnesium are other nutrients to consider. They can be easily lost from lighter soils with a low cation exchange capacity, while phosphate availability may also be lower this year, not because of leaching, but due to losses from soil runoff in heavy rain.

“In cold, wet soils, the microbes that convert exchangeable P to available P slow down, so crops can quickly exhaust the soil solution, resulting in deficiency symptoms, even where soil indices are high,” adds Mr Kerr.

“Foliar products are an option if there is sufficient leaf area, however phosphites may be a more effective option to develop rooting and improve future scavenging ability.”

Manipulating rooting

Timely application of plant growth regulators could play an important role in manipulating crops to offset some of the damage done by waterlogging or flooding, says Alice Cannon, regional technical support manager for Hutchinsons.

“The power of PGRs is not just about straw shortening; the chemistry can manipulate crops in many other ways, depending on the product choice, rates and timing.”

Tramline and small plot trials last year, for example, showed early PGR applications at T0 (GS 30-31) and T1 (GS 32) can significantly improve stem diameter and stem width, boosting the plant’s lodging resilience – something that will be particularly important in shallow-rooted crops that may be more prone to lodging as biomass develops.

The same trial also found a benefit to tiller survival from early PGRs, says Ms Cannon.

“Tillers will abort from stem extension, and that’s simply not an option this year given we’re already facing a reduction in tillering due to the compromised foundation period. The potential for fertile tillers is determined by the number of leaves present at stem extension, so keeping green area index going through March is crucial to building yield.”

Care is needed with product choice and rates on backward crops, she cautions, while more forward early-sown cereals might benefit from slightly higher rates.

“Growth stage 30 will be an extremely important timing this season. It’s traditionally a disease control timing, but it’s just as important to manipulate crops at this stage to maximise their potential.”

Disease control

While it is hard to predict disease pressure during the main growing period, certain diseases are likely to be at greater risk due to prolonged wet weather and lack of significant cold temperatures, so close monitoring and early action will be vital.

Mild, wet, conditions have favoured Septoria development in more advanced, early-sown wheats, while eyespot could be more problematic given the wet autumn and generally low varietal resistance, says David Howard, Hutchinsons head of integrated crop management.

“Wet soils may increase take-all spread, and mildew is likely to be a greater risk in stressed crops, perhaps more so in barley, where risk from the rain splash diseases net blotch and rynchosporium are also likely to be greater.

While more backward, later-sown wheats may face lower Septoria pressure, later drilling does usually increase rust risk, and with a lack of hard frosts to kill inoculum so far this winter, rust could be a challenge in some situations, says Mr Howard.

“Ironically, we might see some benefit where flooding has taken away the lower leaves that can carry rust or Septoria spores over winter, but that will be balanced against the damage flooding’s done to crop stress and development.”

Early action at T0 and T1 will be crucial, with fungicide choices tailored to the specific needs of individual fields, Mr Howard says. He also notes previous trials that have shown good results from using elicitors around GS 30 to increase energy uptake in stressed crops, thereby supporting natural disease protection.

Products that mimic natural disease defence mechanisms are also worth considering for Septoria and mildew protection, but more so as a preventative measure. Perhaps the biggest challenge for disease control will be around spray timings in patchy crops.

Mr Howard advises growers to time sprays based on the parts of the field with the highest proportion of higher yield potential. “Don’t wait for poor areas to catch up and risk letting disease into the good parts.”

He also recognises the need to get on with early nitrogen to give backward crops a boost, but says care is needed, as large doses can cause excess nitrogen in the leaf, potentially increasing disease pressure.

How waterlogging affects crops

Alters physical soil properties – e.g. reduced soil porosity and gas diffusion, leading to soil oxygen deficiency, limiting root growth and function

Consolidation of soil structure, making it harder for roots to penetrate and access nutrients

Impacts gas transfer from soil to roots

Compromises nutrient uptake

Denitrification (in saturated, anaerobic soils) – reduces nitrate to nitrite which is toxic to plants and releases gases

Nutrient leaching, especially on lighter soils (particularly nitrogen, sulphur and boron)

Anaerobic conditions inhibit activity of soil microbes involved in many processes, including organic matter decomposition, mineralisation, N fixation

Shallower rooting = lodging risk and reduced nutrient uptake

Delayed or reduced tiller production

Anaerobic conditions cause mineralisation of organic sulphur to sulphides rather than sulphate

Key actions

Assess fields to identify issues and decide on the most appropriate management

Do test digs to asses the impact of waterlogging on soils and root development

Check plant populations to determine a realistic yield potential

Tailor inputs to crop need and yield potential, considering all available options

Consider re-drilling or sowing spring cover crops on worst areas with no viable crop