Growth regulators should pay dividends on many oilseed rape crops even though most are less lush than last year, say agronomists.

“You can get significant yield hits from lodging,” warns Masstock Arable‘s David Langton. “Trials show a crop that’s 50% lodged can lose about 0.3t/ha.

“Winter rape in general is more ‘normal’ than last year, but up in the Borders and beyond, and in Wales, there are some big crops with green area indices (GAI) of two already.”

But given the cold Easter forecast, applications of growth regulatory fungicides are inadvisable until the weather warms, says Mr Langton. “You won’t get the full benefit if it’s too cold.”

The decision on whether to treat, what to apply and at what dose depends on several factors, he says. “It’s not just about crop size. You need to consider plant populations, the variety and its straw strength. If you have more than 40 plants/sq m there’s a much higher lodging risk.”

Low biomass varieties are less likely to justify pgr treatment unless they are particularly thick. Caramba (metconazole), however, can help stands that are thin by boosting side-branching, he notes. “But tall, weak-strawed hybrids are very likely to respond well.”

ADAS research shows that Caramba enhances rooting more than Folicur (tebuconazole), says Mr Langton. So unless light leaf spot, against which Folicur “has the edge” is particularly evident, he advocates using it alone or in a tank-mix with a flusilazole fungicide according to specific needs.

“The highest risk crops should get the full 0.8 litres/ha. Lower risk ones could have 0.3-0.4 litres topped up with flusilazole.”

On light leachable land especially it may be worth including the trace elements boron and molybdenum, he suggests. “Demand for them is strong as crops extend.”

Severe lodging at flowering can slash output by 30%, notes ADAS‘s Pete Berry. “In general crops are smaller than last year’s, so the need for pgrs is lower. But there’s a bigger range of sizes and quite a significant proportion of crops will need one.”

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The rule of thumb for determining whether a growth regulator is needed is to check the GAI . Crops with a GAI of more than 1 in March or over 2 in April require a pgr, says Dr Berry.

“But once crops are well into the stem extension phase it’s then more difficult to assess the index because it’s more in 3D.”

Thick stands are more likely to require treating than thin ones. “If you have over 50 plants/sq m of a conventional variety with weak or moderate standing power it probably merits a pgr.”

The function of treatment is two-fold, he says.

“It’s to reduce the height and lodging, but also to reduce the size of the canopy at flowering to help improve seed set and even in short, so-called low biomass types there’s a need to control that canopy if it is large.”

Plenty of work on spray timings shows the best height control comes from treatment before the yellow bud stage, he adds. Spraying at yellow bud to early flowering has more effect on the canopy and seed set.

Standard advice is to treat when the crop is actively growing. “We have had positive yield effects from applications at the start of stem extension to mid-flowering.”

Where fields have mixtures of large and small plants the latter are likely to be out-competed. “So you should manage the pgr treatment according to the bigger ones,” he advises.