CROP canopy is not just a good indicator of plant health and vigour, it is also a crucial factor in achieving full yield potential, particularly in higher rainfall situations.
With that in mind, canopy management is essential for optimising available sunlight, water use efficiency and nutrient uptake.
Field Applied Research (FAR) 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 to maximise growth and align the most important development period for setting yield potential (stem elongation to head emergence) with environment," Mr Poole 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 will 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 promote high humidity levels within the crop, which increases 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, nitrogen, grazing and plant growth regulators (PGR) are the main tools for managing canopy growth problems."
Cereals use nitrogen 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 nitrogen inputs can be used to manage canopy growth once seasonal variables become clear.
Mr Poole said if a crop is underperforming during its vegetative phase, nitrogen can 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 nitrogen 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 nitrogen 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 nitrogen inputs, chemical PGR offer growers a second intervention option for limiting excessive canopy growth.
Mr Poole said PGR are primarily used to limit the risk of lodging, rather than adjusting leaf growth and area.
"During early stem elongation, PGR reduce crop height by inhibiting the effects of the plant growth hormone gibberellin," he said.
"So using PGR to control lodging is most effective in the early phase of stem elongation, before the nodes start to elongate.
"Applying a PGR 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 Grain Research and Development Corporation (GRDC) investment - in 2020.
However, Mr Poole said that PGR 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 planted at the wrong seed rate and sowing date," he said.
"It is also possible to overregulate a crop - if PGR are applied late and the plants experience heat stress within a month or so of application, for example, there may be a yield penalty."
Where possible, Mr Poole said grazing is arguably the best PGR of all and works 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 PGR 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 animals also encourage airflow within the crop canopy, which works in partnership with the foliage removal to reduce disease pressure.
"Grazing may delay the need for a foliar fungicide application and even reduce the number of applications required in a season where crops have been grazed hard," Mr Poole said.
"However, it is often important to replace the nitrogen removed by grazing, to ensure yield recovery."
Grazing is particularly well suited to high-rainfall zones where mixed farming systems are more common and water availability can encourage excessive canopy growth in warm seasons.
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.
While growers are encouraged to monitor their crop canopies, Mr Poole said clearly identifying the size of a canopy is not simple and can make management difficult - particularly as spring conditions are largely unknown at the time intervention is required.
"Even with modern crop sensors, growers will find it hard to quantify scientific canopy metrics such as green area index or green area duration for a crop," he said.
"However, a well-managed crop should be intercepting all the available sunlight as it begins stem elongation, without an excess of shaded leaves."
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."
