Understanding OSR Vigour13-Jan-2015
Vigour is a widely sought-after attribute in modern oilseed rape production. Particularly so to give crops the best chance of establishing strongly, resisting early pest and disease damage, and growing reliably away from the winter under increasingly variable and challenging climatic conditions.
But what does it actually mean? And how can the industry make the most of it in improving the resilience of OSR cropping across the country?
“The first thing to appreciate is that the vigour we are looking for is not necessarily an attribute of hybrid breeding,” stresses DEKALB oilseed rape breeder, Matthew Clarke. “Hybrid vigour is the genetic term used to explain the 1+1=3 effect often seen across a range of characters – including growth rate, disease resistance and general robustness – when two genetically dissimilar lines are crossed to produce a hybrid. This primarily results from more favourable combinations of a number of genes influencing different aspects of a single character.
“Some hybrids definitely seem to grow more vigorously than pure lines, either in the autumn or in the spring. However, there is as much variation between hybrids as there is between pure lines in both these respects, with some hybrids showing noticeably greater vigour in their establishment than others. So it’s a big mistake to simply equate vigour with hybrids.
“At the same time, there’s huge confusion over the measurement of vigour. All too often differences in autumn vigour scored on the basis of visual assessment actually measure variations in seed rate, germination percentage, growth habit or development speed rather than anything else. Which can be highly misleading.”
In an attempt to cut through this confusion, Matthew and his DEKALB colleagues identify two clear phases of early winter OSR performance – growth up to four leaves; and development beyond four leaves. Across these phases they define three distinct characteristics which need to be separated from one another in assessing vigour and which can together be used to improve the resilience of farm cropping.
A variety’s success in growing from germination to the four leaf stage depends on what they term its establishment ability – mainly related to the capacity to put down roots and access nutrients rapidly and robustly. Beyond four leaves, however, observed variety performance differences depend largely on a combination of early growth habit and leaf development speed.
“It’s easy to confuse early growth habit with vigour,” Matthew warns. “We have hybrids with very erect early growth like DK ExPower at one end of the scale, less erect ones such as DK Extrovert in the middle right through to very prostrate low biomass hybrids like DK Sensei. Visually the more erect types invariably seem to have established better. But detailed examination invariably shows low biomass hybrids can be every bit as well-rooted under the same conditions.
“Early growth habit can be influenced by seed rate, soil fertility and sowing time, but it’s largely genetic, allowing varieties to be selected to make the most of different conditions.
Equally valuable in this respect are the varietal differences in leaf development speed.
“Just like cereals, faster developing varieties like Excalibur, DK ExPower, DK Excellium and DK Extrovert require fewer day degrees to put on each leaf than slower-developing types like DK ExStorm and DK Explicit,” he explains.
Matthew Clarke sees the faster developing and more erect varieties he terms rapid hybrids as better suited to later drilling as well as the main window and more difficult conditions, since they can take maximum advantage of the available sunlight and temperature to develop strongly ahead of the winter.
On the other hand, he considers compact hybrids which are less erect and slower-developing from four leaves better for earlier drilling and more favourable autumn conditions because they’re less likely to come into the spring with excessive top growth.
Being almost impossible to lodge, he recommends low biomass hybrids for both the earliest sowing and high fertility conditions. However, with notably faster leaf development than other semi-dwarf types, he also reckons the latest of his low biomass hybrids are well-suited to the main sowing window. Especially so as the company’s extensive work in Scandinavia and Eastern Europe shows them to be particularly frost tolerant.
Both early growth habit and leaf development speed are, Matthew Clarke is adamant, quite distinct from the varietal differences in establishment ability to four leaves also identified in his breeding programme. More influenced by environmental conditions that either early growth habit or leaf development speed, these differences are especially valuable in ensuring the most reliable performance regardless of conditions.
“Establishment ability is a better reflection of inherent autumn vigour than the other two components,” he notes. “When we challenge varieties under extreme conditions in our early screening, it’s quite clear some are better able to survive difficulties around germination and more likely to maintain themselves as small plants over the winter than others. This ability may not be significant under the most favourable UK conditions, but it can invaluable when the going gets tough.
“By better understanding the complex of characteristics that have caused so much confusion in vigour assessments to date we have been able to focus our high output hybrid development attention on two parallel strands,” Matthew Clarke concludes.
“First, we have concentrated on building superior establishment ability into all our hybrids alongside other distinctive yield-protecting traits like RLM 7 stem canker, light leaf spot and pod shatter resistance. This provides the greatest possible assurance against the risk of poor or uneven establishment.
“At the same time, we are providing these traits in a range of rapid, compact and low biomass types, allowing growers to select the hybrids that best suit their particular conditions and agronomic regimes.”
Dekalb OSR Type
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Low Biomass Hybrids
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