Think ahead to get optimum use of N
Helping you get more
from fertiliser inputs is
the aim of this special
feature. Over the
following pages we
examine some topical
potash advice, a new
beet product, the cost
of poor spreading and
Turn to pages 76 and
77 for associated
Here, Andrew Blake
sets the ball rolling
with some new thinking on spring nitrogen
applications. Edited by
THINK hard about how winter wheat should look in June rather than spring when planning nitrogen use, advises a top ADAS crop physiologist. For former farmers weekly barometer farmer Robert Law at Royston, Herts, it is sound advice.
ADAS Boxworth-based Roger Sylvester-Bradley warns that many crops are already too thick after the mild autumn and will need careful top dressing to avoid creating wasteful leaf canopies.
In general, everyone concentrates on the immediate reaction to fertiliser they apply. But six years of HGCA-funded work with the University of Nottingham shows the critical period for producing yield is June to July, he says.
The key to optimum light interception and yield is having a large enough, but not too large, green area index or ratio of green crop to ground area at the right time, he adds. "Ideally we are looking for a figure of six by the start of flowering. In the past many growers got canopies of seven or more."
GAI increases by one unit for each 30kg/ha (24 units/acre) of available N, and there is a rule of thumb to assess it in the spring (see box). From there, it is possible, after taking soil reserves into account, to calculate the amount of N top dressing needed to achieve a GAI of six.
This season some early sown crops could have 3000 shoots/sq m by February – five times as many as required for full yield, says Dr Sylvester-Bradley. "They certainly wont need an early top dressing and growers will have to grit their teeth."
Applying nitrogen too soon to crops that already have sufficient tillers encourages lodging and disease and results in a less efficient, over-sized crop canopy. Early April should be soon enough for very lush crops, he suggests.
At Thrift Farm near Royston, most of Mr Laws wheat is not particularly advanced. Some land is in a Nitrate Vulnerable Zone, so N applications must be no higher than agronomically justified. So can canopy management principles help him boost output?
He is already on his way with crop stand. "We always used to go for a dense crop with lots of ears," says Mr Law. "But those apparently good crops never yield as well as you think they will, and often the poor-looking ones do better.
"I combined very thick Mercia one year and there was so little grain in the tank after 100 yards I thought the elevator door must have dropped off. Three years ago we had some Rialto you could easily walk through and we kept wondering where all the grain was coming from."
After that he trimmed seed rates to ensure more open stands. In contrast to other regions and years, winter cereals in his area are not over-thick this season.
His 240ha (600 acres) of winter wheat, mainly first crops after breaks on chalk, is sown in October in the main. Even with late foliar N, good proteins are hard to achieve, so he concentrates on soft milling varieties – this year Riband, Consort and Claire. Average yields since 1994 have been 8.4-10.1t/ha (3.4-4.1t/acre).
Lodging was troublesome only in 1997 when he blamed too high a seed rate. "We have cut back quite a bit since. We now start with about 280 seeds/sq m at the end of September but go up to 380 if we get into November."
"We normally begin top-dressing with 50 units/acre of N with some sulphur in the first week in March. But if the crop needs pushing we may go in February." Main reason for that level rather than any less is to avoid crop starvation if the weather turns wet and delays subsequent treatments, he says.
In the past, a further 104kg/ha (130 units/acre) was applied in two splits, the main one in April. But pressure of work last year meant it all went on in one lump with no apparent penalty.
Dr Sylvester-Bradley believes the relatively heavy first dressing is justified on Thrift Farms chalkland. "Crop recovery of applied N on chalk is only 55% compared with 70% on sand," he says.
There is also a risk that delaying the first dressing could jeopardise crops on the drought-prone soil, and their relatively backward state reduces the danger of reaching a GAI of six before June.
But Mr Laws difficulty in achieving good proteins suggests he could usefully apply rather more N overall than he normally does, provided it does not go on too early.
"I would argue that you could grow breadmaking wheats if you get your N dressings right. Yield is a very poor barometer of that, and grain protein much better."
Compared with MAFF booklet RB209 recommendations, Mr Laws use of N is low. "Id like to see you putting an extra 30-40 units on at flag leaf," he advises.
"Meeting Dr Sylvester-Bradley certainly made me think more about N rates and timings," says Mr Law. "I shall definitely try a field with some more at flag leaf and use the yield monitor to see if we get a response." *
Ground cover in early spring GAI
• Beware of over-thick stands.
• Delay N to avoid GAI >6.
• Reserve some for flag leaf.
• Case for extra N for protein?
Spring nitrogen use should be geared to producing the optimum canopy in June/July, not over-thick spring growth, says ADASs Roger Sylvester-Bradley. Herts grower Robert Law is already heading down that route.