February and early March can be frustrating, as pigeons will continue to graze crops and it is the effect of this grazing which will determine how much nitrogen (N) is needed.
A lot of damage can be caused as a result of pigeon grazing, so all efforts to stop this should be deployed.
The size of the canopy can be measured with the Green Area Index (GAI) app on an iPhone. Android user growers can use the BASF GAI website.
Over the coming weeks, growers should assess crop canopy sizes before deciding on a fertiliser regime.
Oilseed rape (OSR) is the only canopy crop grown in Ireland that takes up surplus N from the soil in the early stages of development, which can be used to meet its future growth requirements.
Nitrogen for winter OSR crops
As a consequence, calculating the Green Area Index (GAI) of winter OSR at this time of the year can be used to calculate the amount of nitrogen available within the plant’s own tissues to promote growth over the coming weeks.
It is then a case of subtracting the GAI-estimated figure from the crop’s total N requirement to work out the amount of artificial fertiliser that should be added to meet its full yield potential.
So, what is a GAI? It is the ratio of green leaf and stem area to the area of ground on which the OSR crop is growing. A crop with a GAI value of 1.0 contains the equivalent of 50kgN/ha.
An appropriate N fertiliser regime for an OSR crop with a GAI of 1.5 is as follows: The total applied N will be 190kg/ha; 40kg in early March, 90kg mid-March and 60kg at seed fill (late March/early April).
OSR crops have a relatively high sulphur (S) requirement of 30-35kg/ha. Growers should use fertilisers with relatively high levels of S in the first two splits if possible.
Disease
Teagasc is also confirming that few OSR crops received a fungicide before winter, so it is highly likely that light leaf spot is present in most at the present time.
To check for the presence of the disease, growers should take a selection of leaves and put them in a plastic bag.
This should be put in a warm place, e.g., a hot press, for 24-48 hours. Growers should then see the little white salt-like lesions on the leaves.
Where light leaf spot lesions are confirmed, it is recommended to use a fungicide containing prothioconazole (e.g., Proline), metconazole (Sirena) or tebuconazole (Fezan).
Prothioconazole-based products will not have a growth regulator effect on the crop, whereas the other two actives will have some level of effect.