Little need for early nitrogen in OSR this season

Hold off with the early nitrogen and be prepared to make good use of canopy manipulation techniques. That’s the advice when it comes to managing forward oilseed rape crops this year.

“There are a number of crops out there which already had a GAI of 1.5 by the middle of January,” says Rob Purvis, a Kent agronomist and technical support manager with Agrovista.

“They are quite forward in terms of development, so their nitrogen treatment from now on needs careful consideration. Otherwise there’s a risk that they’ll just grow upwards.”

Remember that the eventual aim with winter oilseed rape is to have between 6-8,000 pods/sq m, he adds. “That’s the amount that will result in the best light interception and give you the maximum seed number.”

Working backwards from there, Mr Purvis advises that this pod number comes from a GAI of 3.5 at flowering. “As 50kg of nitrogen per hectare is needed for each unit of GAI, taking photographs and uploading them onto BASF’s canopy management website will give you a GAI measurement and a good idea of how much is already in the crop.”

Nmin tests, usually done in February, will give an accurate picture of how much nitrogen is in the soil, he continues. “Then it’s a simple calculation to work out what you need to apply, remembering to take the fertiliser efficiency percentage into account.”

The traditional nitrogen application timings of late February and late March might need to be revised this year, suggests Mr Purvis. “It could be well into March before the first applications are made. Remember that there will have been minimal winter leaching, due to lack of rainfall.”

The rule of thumb of not using early nitrogen if the GAI is above 1 is very relevant his year, he notes. “Much will depend on what the canopy looks like. However, as the oilseed rape crop is not very good at transporting nitrogen around the plant, it’s important to get it on close to the time that it’s needed.”

“It takes up 3kg of nitrogen per day up until flowering, then it slows right down. So the timing is important and a long delay could backfire.”

Where canopies have a GAI of more than 1 in March or are above 2 in April, there will be a yield benefit from using a growth regulating fungicide, he continues.

“There are essentially two choices. Either go for a metconazole approach, or use tebuconazole. Metconazole is the better option for plant growth regulator activity and will improve rooting at depth. But this year’s high light leaf spot risk will complicate the PGR choice as tebuconazole is the better product on this disease.”

Where both growth regulation and disease control are required, the timing must not be a compromise, he advises. “For light leaf spot, an early application is essential, as yield is being lost for every day of delay. Early infections should be treated as soon as they are found, which is where fungicides such as prothioconazole and flusilazole come in. But for canopy manipulation, PGR needs to be applied from stem extension to the green to yellow bud stage.”

His colleague, Cotswolds agronomist Tim Bullock agrees that there appears to be little need for early nitrogen this season.

“Although it largely depends on the weather from now on, I’d be worried by any nitrogen going on before the first week of March. The crops would just grow very tall, rather than setting pods.”

At Agrovista’s Cotswolds trials site, the average GAI was already 1.88 in the middle of January, he reports. “That shows that there’s no urgency.”

When it is applied, the nitrogen should be done in two splits – the first going on when the crop starts to wake up and the main dose being applied just before the beginning of flowering, he advises.

“It looks as though it’s going to be a good year for canopy management,” says Mr Bullock. “Don’t forget that a PGR doesn’t just shorten the crop, it also involves root manipulation. In a droughty year, that could be critical.”

The timing for this application with metconazole is up to green bud, although it will vary according to crop. “A green bud application will promote side branching. As a general rule, the earlier you go on, the more manipulation you achieve. So it may have to be done on a field-by-field basis.”

See more