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FORA
GE AND NUTRITION

Guide 20
1
7
FORAGE AND NUTRITION
Guide 2017
24
Dermot Forristal
Teagasc Oak Park
Urea is widely used on grassland farms as a source of nitrogen and its use is likely to increase as it is less expensive and has a
lower carbon footprint than calcium ammonium nitrate (CAN). However, the product is not as easy to spread and farmers need
to be conscious of spreading urea evenly, whether their own fertiliser spreader or a contractor's machine is used
Fertiliser is a hugely expensive input that impacts on grass and
crop yield. The role of a fertiliser spreader is to ensure that
the fertiliser is spread evenly across the field at the correct
rate. The evenness part is critical. Today's spreaders have a
huge challenge with farmers using bout widths from 6m to
36m wide. Broadcast spreaders rely on forming an overlapped
pattern to give an even spread where a 12m bout may require
the spreader to spread 24m, with fertiliser visible in the next
tramline/bout.
Factors which influence even spreading at a specific bout
width are:
spreader design, particularly the disc, vanes and fertiliser
delivery point;
appropriate setting of the spreader based on fertiliser
type and bout width;
fertiliser physical characteristics (density, granule size/
shape, strength); and
field conditions, specifically wind.
Urea physical characteristics
The fertiliser's physical characteristics are very important.
Urea can be more di cult to spread evenly than either CAN
or nitrogen (N), phosphorpus (P) and potassium (K) blends
because it is less dense and additionally may have smaller
particle sizes.
A fertiliser particle must have a certain mass to allow it capture
enough energy to be thrown a distance. Dense, relatively
large particles are more easily thrown as they can capture the
energy and are less likely to be slowed by wind resistance.
As an example, if you were to try to throw a table-tennis ball
12m, you would fail, whereas a golf ball would easily go that
distance and much more.
Urea has a lower density (about 20 per cent less) of that of
other fertiliser types, making it more challenging to spread
wide distances and leaving it more susceptible to wind
impacting on its spread pattern also. In addition there are
lots of di erent types of urea with di erent particle sizes
which equally impact on spreadability. The variability in size
distribution is indicated in the four graphs (Figures 1 to 4) where
three di erent urea types are compared with a conventional
NPK product.
A good particle size distribution can help achieve even
spreading over wider bout widths, although density will
always impact on spreading ability. In the past, with urea, small
low-density prills were unsuitable for bout widths much more
than 8m or 10m, but products with much larger particle size
distributions have made wider spreading possible.
Selecting a suitable urea product
To get even spreading with urea at bout widths of 12m or
greater, always look for a product with a size distribution
that has medium or large sized particles (as in Figure 3 or
4). The fertiliser supplier should be asked for the particle
size distribution of the product that they are selling. Size
distribution can be checked with the simple plastic sieve
box that many of the spreader manufacturers now supply.
Fertiliser granule strength should also be high to avoid
it being broken on the fertiliser spreader and therefore
restricting the distance it can be thrown. Again the supplier
should be asked for granule strength characteristics and
simple strength testers are available.
Spreading urea:
a bigger challenge