27 Apr 2021
In the UK winter wheat crops are largely source limited and so efficient canopies are necessary for optimising yield. In order to achieve this, fungicides and varietal resistance should be integrated to maintain a healthy canopy.
A recently developed measurement, combining the amount of accumulated sunlight and the duration of a healthy crop canopy, known as healthy area absorption (HAA) is helping to further understand the plant’s ability to capture light.
Speaking at the first in BASF’s series of Funginars, entitled ‘Canopy Management’, Dr Julie Smith, Principal Research Scientist, ADAS said, “Wheat plants intercept solar radiation (sunshine) and convert that energy to grain, and so to yield. For the crop to achieve its yield potential it needs a balanced source/sink ratio, which is dependant on the number and size of grain sites (sink) and the availability of assimilates to fill the grain sites (source).
In simplistic terms, source capacity is determined by the amount of solar radiation which can be absorbed by the post-anthesis healthy canopy, the efficiency with which light energy can be converted into dry matter and the amount of water-soluble carbohydrates the crop can store and then remobilise during grain filling if it can’t get enough energy from photosynthesis during the critical period. Source needs to be carefully balanced with the sink capacity of the crop, which is determined by the number of ears (per m2), the number of grains per ear and the potential grain weight.
In an ideal world we want the source and sink capacity to be perfectly balanced to achieve maximum yield and reflect an efficient crop. Too much source (relative to sink) is wasteful because it can’t be stored. It’s expensive to produce and maintain a healthy canopy so we don’t want a bigger canopy than is necessary. Conversely, if the crop is source limited relative to its sink it can never reach yield potential because the grain sites can’t be fully filled.
We have some control over source and sink components through good agronomy. For example, we can manipulate healthy canopy size to some extent by using an appropriate seed rate, ensuring adequate nitrogen, protecting the foliage from disease and minimising lodging. But varieties also play a big part in determining how much source and sink is available and how they balance.
Over the last 30 years there have been huge changes in modern wheat varieties. Yield has increased tremendously, largely through increases in grains/m2, so the sink capacity of the crop has been driven up by breeding efforts and although the source capacity has increased too, it has not kept pace with increases in sink. Consequently, our modern wheat varieties tend to be source limited.
The flag leaf, leaf 2, 3 and 4 all intercept solar radiation and contribute to yield production which is why it is important to protect the upper canopy from disease by timely application of appropriate fungicides. A foliar disease like septoria produces a lesion and thereby reduces the amount of green leaf area, which in turn impacts on the amount of solar radiation the leaf can absorb. We observe that disease has a bigger impact on yield than it did some years ago because of the source limited nature of our varieties.
Dry matter accumulation in wheat is directly related to the amount of solar radiation intercepted (and absorbed) by the crop. We know from Beers Law and from trial data that a canopy size of 6 GAI (green area index) units will ensure almost all the available sunlight is intercepted and growers should try to achieve this by the time the crop is flowering.
Once optimum canopy size has been achieved its essential to maintain it for as long as possible because the duration of healthy canopy is equally influential in determining yield. In our field experiments we measure both the size and duration of healthy canopy and call this our HAD value (Healthy Area Duration).
For wheat grown in the UK we find the relationship between yield and HAD is always positive and linear (more HAD = more yield). We don’t hit the plateau of the relationship because of source limitation so if we can increase HAD then we’ll get more yield out of the crop. This poses a fundamental question about how we can increase HAD; should we grow bigger canopies, should we delay senescence, can breeders increase leaf size? Its also worth noting that the Yield Enhancement Network (YEN) initiative has found an association between increased biomass and high yield, which corroborates our findings with HAD.
Whilst HAD can explain a high proportion of yield variation we can increase the robustness of the relationship further by accounting for the amount of ambient solar radiation. Think back to 2012, it was a very dull summer and yields were supressed. Even crops with optimum canopies struggled to produce good yields because there wasn’t enough sunlight available during the critical yield forming period.
So, if we know how big the canopy is, how much of it is healthy and how long it lasts for, we can measure the amount of light being intercepted by the crop and look at the amount of light available to start with (solar radiation data from the MET Office). This enables us to convert HAD to healthy area absorption (HAA). This is a measure of the accumulated solar radiation absorbed by the healthy part of the canopy during grain filling. Our data show that the relationship of yield and HAA is even stronger than yield and HAD. This model is a step forwards in understanding the negative impacts of disease on yield whilst accounting for canopy size. It enables us to compare between sites, seasons and varieties in a much more robust way than if we simply look at disease severity.
Preliminary analysis of BASF data showed that Revysol® consistently increased HAD, HAA and yield irrespective of variety, level of septoria infestation or presence of insensitive septoria strains with a yield response of approximately 0.05 t/ha per unit HAD increase.” A wider analysis of the results will be carried out in due course.
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Revysol® is the brand name for the active ingredient mefentrifluconazole. Revysol® is a registered trademark of BASF. © BASF 2021. All rights reserved.