10 April 1999

Tender loving care pays off

High yields, vigour and earliness – that explains the popularity of the new spring rape hybrids. But they do benefit from extra care and attention,

as Gilly Johnson reports.

SCOTTISH growers have the most experience when it comes to spring rape. But even they should adjust their management strategies for the new hybrids, suggests Dr Elaine Booth of the Scottish Agricultural College.

"We are often asked how it is that our trial plots yield better than commercial crops. Perhaps part of the answer is that we take more care to apply inputs just when they are needed. This is what you should be doing with hybrids, to allow them to express their full potential."

Although spring rape has often been viewed as a low input, easy care crop, with hybrid varieties theres less margin for error, says Dr Booth.

Follow the seed breeders advice on sowing rate, she warns. "Although weight for weight the rate may be similar, hybrid seed tends to be much bigger and so plant populations are lower. Dont sow too early – the Canadian-bred varieties dont like the cold, wet springs weve seen in recent years." In Scotland, late March to mid-April is the norm for spring rape drilling.

Because hybrid seed is expensive – between £15-20/ha (£6-8/acre) more than conventional varieties – extra yield is needed to justify the seed bill. And with the rape market at a low of £110/t, hybrids have to give at least 0.14-0.18t/ha (1-11/2cwt/acre) more, without taking into account any extra expense on inputs.

But beware – its a mistake to raise nitrogen in an effort to boost yield, says Dr Booth. "We did expect the hybrids would need more than conventional varieties, but our trials showed that extra N is not justified. Using economic criteria (market price of crop and cost of nitrogen) the ideal rate is about 90kg N/ha (72 units/acre).

If you are growing a composite hybrid (the same as winter rape Synergy) rather than a fully restored hybrid it is particularly important to make sure that sulphur supply is adequate. "If sulphur is deficient the flowers tend to be pale and white, and less attractive to insects. This could affect pollination, which is critical for composite hybrids."

Some Scottish growers try to do without herbicide altogether on spring rape; vigorous, competitive hybrid varieties raise the odds in favour of this strategy, but not for early-drilled Hyolas.

But even where some herbicide is needed, the outlay is not great. A reduced rate of Butisan S (metazachlor) is the most popular choice, says Dr Booth. A cheaper option still for broad-leaved weed control is Treflan (trifluralin) either applied and incorporated with a rotavator before sowing, or applied and incorporated with the drill during sowing.

Doing the job at drilling saves time and soil moisture, but it is not a label-recommended method – good soil incorporation is essential for optimum performance. However, this technique has given acceptable results in SAC trials. Post-emergence and more expensive options are benazolin with clopyralid (Benazalox) for off-label control of chickweed and mayweed, and clopyralid (Dow Shield) for mayweed.

One input that must not be skimped is pollen beetle control, particularly with the composite varieties which depend on pollination from a smaller proportion of plants. For restored hybrids, pollen beetle is not such a threat, but care must still be taken that thresholds are not breached, says Dr Booth.

Pest damage

"It is the worst pest of spring rape, and sprays are needed in most seasons." Because of the damage it can do, the SAC threshold for pollen beetle has now come down to 1 beetle per plant, when crops are at the vulnerable green to yellow bud stage. Be prompt, says Dr Booth. "It pays to keep vigilant." One bonus of tackling pollen beetle is that sprays also control seed weevil. Approved products include Fastac (alphacypermethrin) and Decis (deltamethrin). A second spray may be necessary.

Disease control is relatively easy because spring rape escapes light leaf spot, the scourge of Scottish winter rape crops. Only if sclerotinia is high risk, or if alternaria pod spotting occurs and there are other sclerotinia crops in the rotation, might a fungicide be needed.

"We do see sclerotinia occasionally, but not often. Rotational control is best." Approved products for sclerotinia treatment at petal fall include Compass (iprodione with thiophanate-methyl); Konker (carbendazim with vinclozolin); Rovral (iprodione); and Ronilan (vinclozolin). MBC (carbendazim) is an option within a mix.

At harvest, most spring hybrids tend to mature earlier than conventional varieties. "Watch out for maturity – it can catch you unawares," says Dr Booth. "Although the plant breeder suggests direct combining might be possible, in our experience many crops will still need desiccating or swathing, for even maturity."