Oilseed rape across the country has been variable over the winter. Crops that got away early are generally large and the cold snap is knocking flea beetle larvae in the petiole, making way for clean, new leaves.
For later-drilled crops that are smaller in size, growers will need to decide whether canopy management is needed this spring, depending on the level of damage from CSFB, says Georgie Wood, technical manager, Syngenta.
“OSR crops are looking variable. Those that made it through with decent plant numbers following CSFB attacks have plenty of plants there and they tend to be quite big, especially crops that went in early. You can see flower buds in some of them very low down and things have moved on very quickly if they were drilled in early August.”
These crops will almost certainly require a PGR application, says Ms Wood.
“These can be applied from early stem extension to slow down and reduce height, and we know earlier applications of PGRs have the greatest impact on crop height.”
For smaller plants that are showing potential, not applying a PGR too early will see branching benefits, which supports more pods.
“This means applying PGRs towards the end of stem extension as you get towards the green bud stage (GS55), then using PGRs for canopy management to help increase numbers of second bunches,” says Ms Wood.
Growers can assess if their crop is likely to require a PGR by measuring green area index (GAI) towards the end of February.
“If the GAI is less than 0.7 then you probably should not be using a PGR because you want that crop to grow. If it’s between 0.7-1, then the crop could benefit from a slight delay in PGR application to focus on increasing branching later on. If in late February, you have a crop with a GAI above 1, then you would be looking to regulate that early just to hold it back a bit and reduce height and lodging risk.”
History, location and fertility of the field can all act as a guide to the likelihood of lodging, says Steve Cook, agronomist, Hampshire Arable Systems.
“It really is looking at the crop to make a decision. Growers will probably want a PGR if there has been lodging in the past, they are growing in very fertile soil, or if they’re growing on a very exposed site with high winds. If they think they will get a good crop they may also wish to spend a bit more.”
If lodging risk is not high, Mr Cook says reviewing nutrient applications is another option for managing the crop architecture.
“The very big crops, which may have had some canopy management already are the easy ones to deal with. They may need a bit of growth regulation, but we have got all of February and March before things really start growing.”
Mr Cook says holding back on nitrogen and monitoring how much will be required to get to flowering and a 3.5 GAI could be more efficient.
“The nitrogen plan will be more important in my view, but if we have got something that is getting a bit leggy then we will consider a Caryx (metaconazole + mepiquat) treatment or a tebuconazole at a higher dose, which is quite a bit cheaper but not quite so effective. However, it is probably better on light leaf spot control in the spring. Tebuconazole tends to be our favoured route but usually in conjunction with nitrogen management.”
For thinner crops that were later sown and hit by flea beetle, Mr Cook says larvae could still cause a problem.
“We will have to watch them quite carefully when they get moving. It will not be for a while because it is too cold, but they will require a good hit of nitrogen in favourable weather to get them going.”