20 June 1998

VIGOUR IS KEY TO SURVIVAL

Does the answer to better rape establishment lie with seed vigour? Gilly Johnson reports.

THE ODDS are that only one in four oilseed rapeseeds drilled this autumn is likely to survive to harvest. A surprising statistic that came from a range of establishment techniques in contrasting seasons, says Dr David Stokes of Nottingham University.

A two-thirds seed loss is a high failure rate by anyones standards, and Professor Scotts team at Nottingham, helped by Dr Mike Bullard and colleagues at ADAS, is investigating why. Their work is to be funded by the HGCA and MAFF, CPB Twyford and Germains (UK) in a new three-year project.

The scientists hope to come up with answers that might enable growers to drill less seed, but with more confidence of achieving the exact plant stand required.

This ties in with recent work funded by HGCA and MAFF on canopy management in oilseed rape; his results have indicated that canopies tend to be too large to give optimum yield, says Dr Stokes.

Harvest experience adds weight to the argument. High national rape yields have often taken the industry by surprise in seasons when establishment has been poor. Dr Stokes suggests its a case of pods in a thinner crop canopy making better use of sunlight, reducing the loss of seeds in pods at the bottom of the pod canopy.

But first things first – which entails a closer look at seed quality. "We wanted to know if seed fitness and quality differed across different seed lots – and if so, did it affect establishment," says Dr Stokes.

Initial results indicated it did – "substantially" – despite this seed all being classed as being within the commercial germination standard of 85%.

Whats going wrong? In the lab, seeds are germinating as expected – but these samples then fail to emerge in the field. Dr Stokes identifies three possible scenarios in the seeds early establishment period.

&#8226 Dry, cobbly seedbeds: the seed falls through cracks in the soil, and the seedlings cant emerge easily from depths greater than 3-4cm. Heavy soils and compaction will aggravate this, says Dr Stokes. Larger seeds may have the edge in this setting, because the plant has a greater reserve of available energy.

&#8226 Slightly moist soils: the seed can take up just enough moisture to trigger the biochemical processes at the start of germination, but there isnt enough water to complete process. Some seed will then enter secondary dormancy, which could last weeks, months or even years.

&#8226 A moist soil surface layer that has been turned up following cultivations: seeds germinate, the seedling emerges, and then faces desiccation and death as the top layer dries out before the roots can penetrate to depth.

Could improvements to seed quality help overcome these problems? Not entirely, says Dr Stokes. Cultivations, soil type and weather will always play a part but better quality seed would reduce the risks, he argues.

Seed advancement is just one of the avenues to be explored. Primed seed is already a commercial option for sugar beet and vegetable seed. Dr Stokes has used this technique in rape to trim the time taken to emerge by up to a day and a half. It also makes for more even germination, because the seed is started off in more ideal, controlled conditions.

Some might question the value of speeding up emergence by just a day or so – but there are times when a head start might make the difference between a dried out soil and a surviving seedling.

"Theres always a risk at establishment because rainfall cant be predicted," says Dr Stokes. "However, if you know exactly how much soil water is available to the seed, its possible to assess if theres enough for the 5-6 days ahead."

So the closer emergence is to drilling, the less the risk of getting it wrong. Initial indications are that primed seed is better at emerging from depth – another argument in favour.

Dr Stokes primed the rapeseed by soaking in water for between 15-20 hours, at 15íC. It was then dried (at 25íC). This moves the seed towards germination, but it is dried back before the root emerges through the seed coat. The primed seed looks no different to conventional seed – and behaves just the same in the drill.

Growers should not attempt this technique themselves, he warns. "Its early days. More research is needed before we can be sure rapeseed priming is a beneficial route."

Other options open to those seeking to improve their rapeseed might be extra rigorous screening or gravity separation – but beware. Research is still needed to show the precise benefits, says Dr Stokes.

Full circle

Seed crop management is also being explored, with the collaboration of seed producers. It seems that better control of the growth of the seed crop may lead to better quality seed – which brings the research work back full circle.

"Uniform maturation and the content of protein and oil in the seed are important aspects of the seed quality. So a range of techniques for regulating crop growth will be weighed up as tools for seed crop canopy management.

"We cant promise foolproof establishment – but with a combination of small improvements, we can better the odds," says Dr Stokes.

A topic sheet identifying the physical causes of seedling failyre in autumn-sown rape on heavy soils will be published by the HGCA in July.

Helping rape plants to a head start -Dr David Stokes.