Tillage: Nitrogen is the best weapon to tackle septoria in winter wheat
Published 17/02/2016 | 02:30
This time last year, field work was underway in many areas. This year, however, it is a different story. There's no need to panic though - there will be plenty of time to get the work done, and given the proportion of land that has already has been sown with winter crops, the spring workload will be lower than normal.
When the land does dry out sufficiently to bear the weight of machinery, the first crop that will need attention is winter oilseed rape crops. Many crops had very strong growth coming into the winter to the point that there would have been problems had that growth continued into the new season.
The huge population of pigeons however, sorted that out and many fields now more closely resemble carefully mown meadows than the lush greenery they sported a few weeks ago. Crops with well-established root structures will recover rapidly once they get a small amount of nitrogen and sulphur, perhaps in urea form, to replenish leaf area as soon as conditions allow.
One issue that may occur in open crops is weed occurrence. If ever there was a demonstration in selective grazing, the ability of pigeons to graze oilseed rape plants to the clay, while leaving adjacent charlock plants completely untouched, is amazing. Where weeds are a problem, it is past the date for use of propyzamide based herbicides.
However, there is now a range of herbicides available for oilseed rape for use in warm conditions. These herbicides are more selective and expensive than residual herbicides, so correct weed identification is necessary before the correct product to use can be established.
Most winter wheat crops have emerged from the winter in rude health, and although recent temperatures have been low, there is a fear that these strong crops will carry a lot of latent disease into the spring.
Given the knife edge we are told that fungicide families are on in relation to control of septoria, careful management of these crops is essential. At this early stage in the season, controlling the leaf canopy, and therefore the environment in which septoria will proliferate, is essential.
Therefore the most effective fungicide to use at this point in the season is granular nitrogen, or to be more correct, lack of granular nitrogen.
Delaying nitrogen application on to a crop will put the plant under pressure, and the crop will respond to this pressure in two ways.
Firstly it will reabsorb the weakest tillers back into the plant and secondly it will speed up what's called sequential senescence, which is allowing the lower leaves of the plant die off so that they can be used to create new leaves higher up the plant. The net effect of these reactions is that a more open canopy is created, which in turn creates a microclimate that is much less conducive to disease spread than a thick lush canopy.
Growers have got used to considering these thick lush crops as being essential for yield, but wheat responds to total dose of nitrogen applied, not the timing of the application.
By not applying what's considered the first split of nitrogen, and increasing the amount of nitrogen applied in the main or subsequent splits so that the total amount of nitrogen remains the same, the yield will be the same. Wheat has huge ability to compensate for conditions.
If you delay the nitrogen application, you reduce the number of tillers, reducing the number of ears, or wheat heads, that establish per unit area. With later application of higher nitrogen rates, the wheat plant has more nutrients available for the remaining ears that have established and in turn the plant can respond by increasing the number of viable grains that grow in each of these ears.
The essential yield component, the grains per unit area, is therefore unaffected by the timing to the nitrogen. Less ears per unit area, but more viable grains in each of the ears that do survive.
As an aside, nitrogen rarely affects the other essential yield component, weight of grain (measured as the thousand grain weight), that is more affected by disease occurrence or poor weather at grain fill.
We are continually being told about the importance of integrated pest management (IPM) in long term sustainable cereal production.
Canopy management is one of the most potent IPM tools that is currently available and the sooner we get to grips with managing nitrogen application to manipulate disease and standing capabilities of a crop, the sooner we are on the road to minimising our dependence on pesticides to do all the work for us.
Dr Richard Hackett is an agronomist based in North County Dublin and is a member of the ITCA and ACA.