Nitrogen is the major nutrient required by grass and is used to
stimulate high growth rates and is key to achieving high yields, however
if too much nitrogen is applied this can have a negative effects on
quality.
High rates of nitrogen can also cause problems with silage
fermentation due to excess nitrate having a negative impact on the
fermentation process and will produce silage that is less palatable and
the animals will be less keen to eat it.
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Nitrate is generally taken up by grasses quicker than it is
incorporated into proteins and until used this excess known as luxury
uptake is stored in the leaves. Excess nitrate will be present if
insufficient time is allowed between application and mowing and can also
occur under conditions of poor growth eg low light levels, cool
temperatures. It is also a problem if there is a dry spell after
application, when nitrate cannot be taken up by the roots, followed by a
period of wet weather that results in luxury uptake. The plant cannot
convert it to protein quickly enough so it accumulates in the plant.
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High rates of nitrogen may cause a reduction in crop sugars as they
are used to provide energy for the increased rate of plant growth and
for the manufacture of plant proteins and this increased growth rate may
in turn lead to lower crop dry matter content although in practice this
is often not significant.
Excess nitrate can be avoided by following the Grassland Rule to apply nitrogen at the rate of no more than 2.5 kg N/ha/day.
