Canopy management critical

Field Applied Research Australia managing director Nick Poole said growing a wheat or barley crop pivots around anthesis, or flowering.

"Targeting optimum flowering windows by matching cultivar with sowing dates will not only help to avoid heat stress and minimise frost risk, it will also help maximise growth and align the most important development period for setting yield potential (stem elongation to head emergence) with environment," he said.

"Grain quality and yield are often the inherited consequences of pre-anthesis management and growth.

"Pre and post-anthesis water use is driven by the canopy and needs to be balanced carefully with the likely yield potential of the region."

Excessive canopy growth will transpire more of the available water before anthesis. This can cause a post-anthesis shortage that could lead to lower grain weights, reduced grain quality and increased screenings.

Excessive canopy growth, particularly from earlier sowing, high seeding rates and high levels of soil-available nitrogen produce more tillers and their competition for resources will result in thinner stems and a high risk of lodging, particularly in higher rainfall zones or irrigated scenarios.

Growers should aim to manage their crop so it reaches maximum light interception during the critical stem elongation period.

Excessive growth prior to that does not contribute greatly to improved yields. Thick, closed canopies also increase the fungal disease pressure.

"Growers have a lot of agronomic levers at their disposal for managing canopy growth," Mr Poole said.

"The most effective are pre-sowing decisions such as variety selection and plant density, which intersect with time of sowing, soil fertility, available moisture and local disease risk factors.

"Once those opportunities have passed, N, grazing and plant growth regulators are the main tools for managing canopy growth problems."

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Cereals use N most intensely during stem elongation.

From growth stage 30 through to ear emergence, the plant builds the canopy structure it needs to intercept sunlight for producing the carbohydrate sugars that will fill the grain.

The first of those carbohydrate sugars are stored in the stem before flowering since there are no grains to fill and the activity of the leaves directly fills the grain after anthesis. This means the amount and timing of N inputs can be used to manage canopy growth once seasonal variables become clear.

Mr Poole said if a crop was underperforming during its vegetative phase, N could be applied between tillering and early stem elongation to promote growth and tillering.

"As long as there is a sufficient canopy to capture light during the late stem elongation period, poor growth during the vegetative period does not have to translate into reduced yields," he said.

"If the canopy growth is excessive, delaying N until later in stem elongation and even up to flag leaf development can help limit stem height and reduce the risk of lodging, particularly in high-rainfall regions."

Poor canopy development may be an indicator of a more significant plant growth problem, such as root damage or soil constraints.

The effectiveness of using N to increase canopy growth will depend on the nature of these underlying factors.

Simply increasing the number of tillers may not lead to a corresponding yield increase. Along with delaying N inputs, chemical PGRs offer a second intervention.

"During early stem elongation, PGRs reduce crop height by inhibiting the effects of the plant growth hormone gibberellin," Mr Poole said. "So using PGRs to control lodging is most effective in the early phase of stem elongation, before the nodes start to elongate.

"Applying PGRs early may also promote root growth, which can help reduce the risk of root lodging."

Later PGR applications have been observed to help prevent head loss, a problem that led to reduced yields in FAR Australia's Hyper Yielding Crops barley trials - a GRDC investment - in 2020.

But Mr Poole said that PGRs have their limits.

"They will not be able to remediate severe canopy problems, especially if the canopy density is the result of the wrong variety being sown at the wrong seed rate and sowing date," he said.

Where possible, Mr Poole said grazing was the best PGR of all, working particularly well in early-sown crops.

"Grazing crops before stem elongation offers growers a number of benefits, starting with saving the time and cost of applying a chemical PGRs and additional fodder benefit," he said

"Unlike chemical options, grazing removes foliage rather than just slowing development, so it helps relieve the problems of excessive canopy density such as water use and disease pressure."

Grazing up until early stem elongation can also cause a delay in flowering, which gives growers a control option for spring wheats that are developing too quickly or were planted too early.

Mr Poole urged growers to manage their crop canopy proactively rather than reactively, using pre-planting decisions.

"Keep records of this year's crop and work with your agronomist to match soil fertility, plant available water and climate to variety selection, time of sowing, row spacing and seeding rates for next season," he said.

"It's easy to lose control of your canopy through something as simple as not accounting for variations in seed size from year to year."