The current cold soils following too much rain and then more frosts
than usual in the Waikato has made most short of pasture so many will
be thinking about using N.
The decision to use N is often made fairly hastily, after it is
obvious that a shortage of pasture will occur, but in this situation
maximum growth is not achieved from the application of N. It is far
better to measure and predict future pasture growth by feed
budgeting, than to decide to use N after the feed shortage has
occurred. The old sayings "grass grows grass" and "a stitch in time"
apply when trying to get the maximum benefit from artificial N.
Applying N to paddocks which have been grazed down to 1,000 kg DM/ha
(one to two inches) or less, will result in volatilisation and/or
leaching losses, especially if no rain falls or if prolonged heavy
rain follows before the grass has had time to take it up and turn it
into growth. Applying it to pasture with about 1,600 kg DM (three to
four inches) will lose only half as much and give much better
results, but, if the decision to use N has been left too late, there
may not be many paddocks with this amount of cover. Grass growing
fast at the time of application will use more of it before leaching
losses occur.
Small frequent applications give better long term results than larger
amounts less often. It is far better to apply more to newer pastures
(ones sown in recent years) which will give a better return than old
worn out pastures which can give little response. About 40 kg of N/ha
can double growth and is the maximum suggested per application.
When soil temperature is about 5 degrees C, each kg of N in urea will
produce only about 5 kg/ha DM. At 10 degrees C about 15 kg/ha DM can
be expected, however at these low temperatures Ammonium Sulphate (AS)
also called Sulphate of Ammonia (22-0-0-24) will produce more N than
urea.
Urea is the most popular form of N because it costs less per kg of N
and is more concentrated (45%) than others so less is needed to
transport and spread. A previous advantage was that it is granulated
for easier spreading, but Ammonium Sulphate is also granulated now.
In early spring after a wet winter, sulphur (S) is likely to be
leached, so lacking. If enough elemental S has been applied in
autumn, levels can be better, but still low because the cold wet
conditions will slow its being made available. If S is low use AS
because N must have sulphur to work. AS will also give a longer
response than urea and if S is low and soils are cold it will be
faster than urea.
Some farmers have noticed that AS makes mature legumes grow faster.
This can happen when legumes are not making N or when S is low.
In about 20 comparative equal cost urea/AS trials I and farmer
clients have done in cool weather, AS has always produced more DM and
over a longer period which means you don't get the slump that occurs
after urea.
--
Best wishes,
Vaughan Jones
