Oilseed rape growers are being urged to take canopy condition into account before calculating nitrogen requirements this spring, after prolonged cold weather has caused leaves to die back, reducing canopy size.
Even so, Masstock’s technical development manager Philip Marr believes the risk of over-application of nitrogen is high this year, particularly with crops drilled in good conditions in August.
“At our Brotherton Farm site in Yorkshire, canopies are generally bigger than they were last season, especially for the faster developing hybrid varieties like Excalibur,” he says.
“These crops, which already have green area indexes (GAI) of well over 1.5, won’t need much more than 100kg/ha of fertiliser nitrogen to produce a yield of 4.5t/ha. Sampling shows that there’s 60kg/ha available in the soil.”
However, Mr Marr acknowledges wide variability in the field. “At the same trials site, we’ve got slower developing types such as Flash with GAIs of well below 1.0. And later, October-drilled crops are barely at 0.1 GAI.”
These less well-developed crops will need double the amount of nitrogen to produce the same level of yield, he says. “It will be a case of fine-tuning fertiliser applications using accurate assessments of both GAI and soil mineral nitrogen.”
Plants haven’t started growing yet, says Jim Carswell of Frontier Agriculture. “The cold weather has helped to bring any forward canopies back under control and soil N monitoring is showing lower levels this year – there’s not as much in the soil.”
He doesn’t share Mr Marr’s concerns about over-application. “Few crops will need nitrogen rates to be cut back. Our trials show that the optimum amount of nitrogen for winter oilseed rape is around 200kg/ha.”
An early top-dressing should be applied as soon as conditions allow, urges Mr Carswell. “If we have a dry spring, any delay in nitrogen applications means it won’t be taken up by crops efficiently. So growers must be ready to go once it warms up a bit.”
In Hampshire, agronomist Mike Harrington reports that crops have lost a lot of green leaf area. “They’re lucky to have a GAI of 1 and the remaining leaves are badly damaged. They’re going to need building up.”
Nitrogen applications should be geared towards delivering a GAI of 3.5 at flowering, says Steve Harrison, an independent agronomist from Wiltshire.
“Crops which had canopies with GAIs of over 2 before Christmas are now back to a GAI of 1, so they haven’t been maintained. There’s less nitrogen in canopies than there was and you’ll need to get out in the field and look.”
Calculating how much nitrogen they require involves some simple arithmetic, he says.
“You need to know how much nitrogen is available from the soil and how much is already held in the crop so that this can be deducted from the 175kg/ha that the crop needs to reach a GAI of 3.5.
“Remember that each unit of GAI equates to 50kg/ha and there’s a 60% efficiency of nitrogen uptake.”
He also adds that crops with higher yield potential will need an additional 40kg/ha to perform.
“The canopy management system works well once you’ve got your eye in. It’s a case of getting out at the beginning of March when things start to grow,” says Mr Harrison.
Adopt a little and often approach with sulphur by applying it with the first two dressings, as the movement of sulphur around the oilseed rape plant isn’t very good, says Frontier’s national trials manager Jim Carswell.
Although all winter oilseed rape crops will respond to the nutrient, conventional varieties appear to be more sensitive than hybrids – both to the form and the amount of sulphur applied, he adds.
“We’ve upped our recommendation to 100kg/ha of SO3, replacing the previous guideline of 67kg/ha,” he reports. “We’ve seen yield responses of up to 0.7t/ha with hybrids, but as much as 1.2t/ha with conventionals.”
There’s also some evidence that the crop need for sulphur is higher early in the season, he notes.