30 June 1995

Crop canopy is key to wheat N applications

Last weeks Velcourt/BASF open day at George Farm, Stamford, Lincs, also played host to a major exhibition of work funded by the Home-Grown Cereals Authority. Edward Long reports

WHEAT growers could improve efficiency by tailoring nitrogen rates to the needs of the crop canopy and not to a yield target determined by guesswork.

"When formulating management policy wheat growers guess the likely yield and tailor inputs to it. But this cannot be accurate, so is inefficient and wasteful," says ADAS plant pathologist Bill Clarke.

"Farmers like to see crops with a big canopy capable of trapping a lot of the suns energy. But the danger is more disease could develop. It would be far better to apply N matched to the actual needs of the crop."

Big canopies also suck up a lot of moisture and, because of respiration, burn a lot of carbohydrate.

ADAS research funded by the HGCA levy has shown that, ideally, a canopy should trap 90-95% of available radiation. To increase this to 98% would be prohibitively expensive and not give extra yield.

ADAS says five to six layers of green leaf material are needed to provide optimum interception of radiation. Canopy development is N driven and every 30kg/ha taken up by the crop adds 1 to the index. As only 60% of N is exploited by the crop, 50kg/ha has to be applied.

A crop with just five layers of leaf material may not appear to have a sufficiently dense canopy to trap radiation efficiently. But measurements taken at the Velcourt/BASF event prove it does.

Energy trapped

Just above the crop there were 1500 radiation units, with 750 just below the flag leaf and 160 at ground level. This shows 90% of the suns energy was being trapped by the crop.

Over two years of canopy management work 70% of ADAS trials have given higher yields, with an average N saving of 95kg/ha worth £30/ha. Other savings came with the use of more appropriate and cheaper fungicide and less need for growth regulator.