Canopy cures
Canopy cures
How to manage wheat now is crucial, if crops are to fulfil their potential and take advantage of rising grain prices. Tom Allen-Stevens investigates.
IDENTIFYING which crop needs extra nitrogen from one that needs extra growth regulator is not as easy as it may seem. The crop that looks good and is covering the ground well now will not necessarily be your highest earner, according to UAPs southern technical adviser Peter Gould: "They may look better, but theyll be a sod to manage, expensive to grow and difficult to keep standing."
So what does a good canopy look like? Crop canopy is measured by its green area index (GAI). A GAI of one means for every square metre of ground there is a square metre of green material (leaves, stem, etc). The optimum when vegetative growth ceases in the early summer is five or six. "The key thing is that when you look down on the crop you can see the ground. If you walk into it and you cannot see the ground, you have your management all wrong," advises Mr Gould.
The main factor to getting it right is the target plant population, and this is less than you might think. "Typically a crop drilled in September with 150 seeds/sq m will now have around 500 tillers/sq m. That crop will be a doddle to grow. My philosophy is to start it off thin and build it up."
Tiller aim
But tweaking plant numbers is not now an option and growers should be looking at how to manipulate tiller populations. As a benchmark the aim should be 1,000 tillers/sq m by GS31, which would give the optimum final ear number of around 500/sq m.
"If you have more than 700 tillers/sq m by early March, your crop is getting too thick. Apply no early nitrogen and time the main dose at or just beyond terminal spikelet. The reason for that is that you want to try and kill off a few tillers. But its a dangerous game. If youre starving these crops and you get wet weather in April, you wont be able to apply any nitrogen until early May and that will impact on yield."
Less than 600 tillers/sq m is a far more beneficial state. Mr Gould suggests a dose of 40kgN/ha in early to mid March, followed by the main dose 10-14 days before terminal spikelet.
Total nitrogen applications can be calculated via one of two different routes (see box right). The purist approach is to apply sufficient amounts to reach GAI 6 with an additional 60kg to keep it greener for longer. This can be a useful approach if soil nitrogen supply is high – for dense crops and fertile soils. The more standard route is to apply according to yield potential, minus soil nitrogen supply. "In either case, take into account the effect of soil type on nitrogen efficiency," says Mr Gould. This varies from around 50% for a chalk soil to 70% for a sand.
Growth regulator use also depends on crop cover and drilling date. On an early-drilled wheat, if the canopy is large, the aim is to improve crown roots and shorten the stem. Here Mr Gould recommends a split dose at GS30 and GS31-32 of Meteor (chlormequat + choline chloride + imazaquin) or Moddus (trinexapac-ethyl) and Meteor if the risk is high or the dose delayed. "Otherwise Id avoid Moddus – it can be harsh and fickle when it comes to temperature. Meteor has more persistent effects and a better uptake at low temperatures."
Lodging risk
Another potential lodging risk is a good-looking crop drilled in November or December. "If it hasnt been rolled, crown roots cannot adhere to the soil, making the crop prone to crown root lodging." Again a split dose of Meteor and chlormequat followed by straight chlormequat at GS30 and GS31-32 is Mr Goulds suggestion.
For early-drilled canopies that have been affected by slugs or waterlogged, tiller survival is the priority, so an application of Meteor before GS30 would be more appropriate. Poor, late-drilled crops could benefit from the root stimulant Route, says Mr Gould. Route should be applied either early with herbicides or pre GS30 with Meteor.
Procams Nick Myers also advocates a more tailored approach to the use of growth regulators: "Dont just apply them for lodging control – they are also good tools for manipulating the canopy."
Drawing on lessons learned from actual farm records from Procams CMS, he believes early-sown wheats rarely warrant a split dose. However, he too has seen later drilled crops respond well: "An early application at mid-tillering boosts both tiller and root growth."
As for applying too much nitrogen too early, he also advises growers to think twice. "While it can be a good idea on second wheats or crops that are struggling, weve found poorer responses to N applied before mid-March on early-drilled or first wheats. Many growers are finding a lot of late, green tillers where theyve over-applied nitrogen early. These reduce yield and are a problem at harvest. Hold back the nitrogen so that more goes towards grain fill."
Managing the canopy
The targets:
• A green area index (GAI) by the end of the growing season of six – you can still see the ground when you look down on it
• 1,000 tillers a sq m by GS31; 600-700 tillers by early March
Nitrogen:
• Tailor applications to the crop cover
• For dense crops (>700 tillers a sq m) apply no early N and hold off main application until terminal spikelet
• For good, canopy-managed crops (<600-700 tillers a sq m) apply no early N and put on main dose at 10-14 days before terminal spikelet
• For struggling, damaged crops (600 tillers per sq m) apply 40kgN/ha in early to mid-March with the main application going on at 10-14 days before terminal spikelet
• Take into account the effect of soil type on nitrogen efficiency
Growth regulator:
1) Sept/Oct drilled crops
• For dense crops, split dose of Meteor or Moddus and Meteor at GS30/31-32
• For damaged/ waterlogged crops, apply Meteor before GS30
2) Nov/Dec drilled crops
• For dense crops, especially if the crop was never rolled, split dose of Meteor and chlormequat followed by straight chlormequat at GS30/31-32
• For struggling crops, apply 0.8 litre/ha of Route either with herbicides or before GS30 with Meteor
How much total N to apply?
Purist approach: Sept/Oct drilled wheat on clay, target is GAI 5-6:
• 30kg/ha of soil N = 1 GAI, so crop requires 150-180kg/ha soil N
• Feb N Min test reveals 60kg N in soil and crop has already reached GAI of 2
• Remaining soil N will supply 2 GAI, so another 2 GAI required = 60kg/ha applied N
• But clay is 60% efficient, so apply 100kgN/ha by terminal spikelet
• Apply extra 60kgN/ha at GS32-37 to prolong canopy life
• Total applied N = 160kg/ha
Standard approach: Sept/Oct drilled wheat on clay, target yield of 10t/ha:
• For target yield, total N in crop must be 240kg/ha minimum
• Feb N mineral test reveals 60kgN in soil and crop has reached GAI of 2, so has taken up 60kg/ha soil N
• Soil nitrogen supply is therefore 120kg/ha, assuming low organic matter
• Total N required = 240-120 = 120kg/ha
• Clay is 60% efficient so apply 200kgN/ha
• If no early N needed (600-700 tillers a sq m), apply two-thirds at 10-14 days prior to terminal spikelet and the rest at GS32-37