Speed of oilseed rape development in the autumn is at least as important as establishment vigour in the battle against flea beetle, stresses Bayer OSR specialist, Richard Phillips. Also important in combatting damage from the larvae as well as adults, he finds in his extensive trial work across the country, is both the earliness and speed of spring regrowth.
“Too often speed of development is confused with vigour,” he notes. “However, we know they are quite separate characters; and ones that vary widely between both hybrid and conventionally-bred varieties. So, it’s vital we don’t confuse them if we want to make the best use of the genetics we have to counter the cabbage stem flea beetle threat.
“In general, it’s fair to say that hybrids are more vigorous than conventional varieties in their autumn and spring growth, making them better able to cope with challenging conditions. While establishment vigour is the prime consideration in getting the crop from germination to two true leaves, though, speed of autumn development is what really counts in its ability to grow away from autumn challenges like flea beetles and slugs before winter.
“The earliness and speed with which the crop grows away from the winter is equally important in the extent to which it can tolerate damage from flea beetle larvae – not to mention pigeon grazing. The right plant populations, of course, have a major part to play here, as does well-managed spring fertilisation and plant growth regulation. All the agronomic factors, though, fundamentally depend upon the underlying genetics.
“As well as earliness of flowering, we have long measured two key developmental characters in our Dekalb breeding programme,” Mr Phillips explains. “Together, development before winter and spring development after winter allow us to characterise the growth habits of our varieties very effectively.”
Over the years, this approach has enabled the Dekalb team to identify clear developmental differences between varieties; much as they are known to exist in cereals.
Hybrids have shown themselves to be more vigorous in both their autumn and spring growth than conventionally-bred varieties. However, as much variation has been recorded between hybrids in their autumn and spring development characteristics as between hybrids and pure lines.
“Regardless of how they are bred, some varieties grow noticeably more rapidly in the autumn and are earlier or faster to re-grow in the spring,” reports Mr Phillips. “What is more, these characters appear to be separate. So, varieties that are faster to develop in the autumn are not necessarily either earlier or faster to grow in the spring, and vice versa.
“Importantly too, studies with one of our breeding trials badly affected by flea beetle have shown varieties developing more rapidly before winter branch better, lose fewer main stems to flea beetle larvae and suffer less stunting from them than those developing less rapidly (Figure). These studies also demonstrated lower main stem losses and less stunting in varieties growing away earlier or more rapidly in the spring.
Image: Main stem losses in DEKALB breeding trials