Ground cover levels in oilseed rape crops are starting to reduce quite significantly in some areas of the country.
Smaller canopies mean that these crops now have a lower Green Area Index (GAI) than would have previously been the case.
Reductions on ground cover levels can be caused by the action of growth regulators, the grazing action of pigeons, or both.
Teagasc is now advising that all efforts to stop pigeons grazing in the coming weeks need to be deployed or the canopy will be grazed further.
The effect of pigeon grazing will determine how much nitrogen (N) is needed for the crop in the coming weeks.
Nitrogen is stored in the leaves of oilseed rape and when pigeons graze leaves, they are taking away this critically important resource.
This is particularly so, given the current state of world fertiliser markets.
Oilseed rape
The size of a rape canopy can be measured by using the GAI app, which is now available on smart phones.
Where crops have a GAI of one, there is the equivalent of 50kg/ha of N already in the plants. This is worth approximately €125/ha at the current N price of around €2.50/kg.
Teagasc is strongly advising growers to accurately assess the canopy size of their rape crops before deciding upon which fertiliser regime they will follow.
Sulphur (S) is often forgotten when it comes to fertilising oilseed rape, even though the crop has a relatively high requirement for the element: 30-35kg/ha.
Teagasc has indicated that growers should use N fertilisers that have a relatively high level of S.
Ammonium Sulphate Nitrate (ASN) is a case in point. Sulphur should be applied in two splits, if possible.
Teagasc is reporting that most rape crops will have received a fungicide by this stage. However, the relatively mild winter means that it is highly likely that light leaf spot is present in many crops, especially varieties with low resistance.
Given this backdrop, growers are being advised to take sample leaves and put them in a plastic bag.
This then should be placed in a warm room (e.g., a hot press) for 24-48 hours. White, salt-like lesions on the leaves are tell-tale signs of disease.
If light leaf spot lesions are visible, then crops should be sprayed with a fungicide containing prothioconazole (e.g., Proline), metconazole (Juventus), or tebuconazole (Fezan) for control.
Prothioconazole-based products will not have a growth regulator effect on the crop, whereas the other two actives will have some level of impact in this regard.