Fertiliser 3: Grass nutrition

Course: | Last Updates: 15th October 2015

 
Ian Richards
Principal Consultant
Ecopt Consultancy
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The amount of fertilizer nitrogen needed by grass depends on end-use, stocking rate, soil condition and nutrient status and availability of nutrients from muck, slurry or arable by-products.
For explanatory purposes grassland can be split into three types: Intensively managed grassland, typically supporting a stocking rate of two cow equivalents/ha (where each cow is 600kg live weight and sheep equate to 0.15 cow) and where fertiliser is the major source of nitrogen.
Moderately intensive grassland supports 1.5-2 cow equivalents/ha and clover often makes a significant contribution to nitrogen supply.
Low input or extensive systems where the stocking rate is under 1.5 cow equivalents/ha and nitrogen supply is usually heavily dependent on clover within the sward.

Nitrogen – how much?

 

Vicon spreader

The type of livestock enterprise – stocking rate, feed policy and use of arable by-products – will determine dependence on grass and the growth, or more accurately yield, needing to be achieved.
Growth is controlled largely by the total amount of nitrogen available to the crop. It combines nitrogen made available from mineralisation or breakdown of soil organic matter, that fixed by clover, deposition from the air and applications of manures and fertiliser. Of these, fertilisers offer the most rapid and effective supply of available nitrogen to the crop and should therefore be adjusted to suit the enterprise.
It is possible to give examples of total nitrogen supply and contributions from other sources which influence the amount of nitrogen fertiliser needed (see Table 1).

Table 1. Total nitrogen needed and amounts supplied from other sources.

 
  Intensively managed Moderately intensive
Permanent grass, grazed One year cut ley, arable rotation Permanent grass/clover, grazed
Total N supply needed 450 450 340
Mineralisation/deposition 80 40 70
Clover 40 0 60
Excreta 80 0 60
Fertiliser needed 250 410 150
Source: Richards, I. Ecopt 2008.

 

Nitrogen uptake is little affected by temperature (although 5C at 10cm is desirable) and day length, unlike dry matter accumulation. Allowing time for full dry matter response was the basis for the 2.5kg/ha N a day of growth rule of thumb developed back in the 1960s and which remains a good guide today.
For example, for a 30-day rotation 75kg/ha N is a good starting point. For grass growing 50 days from fertiliser application to cutting 125kg/ha N will be about right. The same rule applies equally to set-stocked grassland, for example 60kg/ha N every 30 days.

Grazing amazing cows

Phosphate and Potash

Intensively managed grass can take up 80-100kg P205/ha a year. Soils should be analysed every three to four years to assess availability. Some phosphate is best applied in spring and the amount required varies little between cut and grazed swards.
Do not rely on grazing animals to return phosphate back to grassland through dung as typically only 8% of grass by area will be covered after a seasons grazing. Look out for deficiency shown as purpling of grass leaves and, in ryegrass, reddening of the stem base.
Demand for potash varies according to grass use. For example, silage crops can quickly deplete soil reserves and need addressing in fertiliser application plans.
Stock pass back most potash within urine but even on intensively managed grassland distribution can be patchy and soil levels should be tested every three to four years. Remember clover is more susceptible to potash deficiency than grass.
One element in fertiliser plans often overlooked is sulphur. SO3 emissions from industry have fallen to 12kg/ha SO3 a year today and, as a result, deficiency is more common and seen as yellowing of younger leaves (unlike nitrogen deficiency that shows as yellowing on older leaves).
An application of about 40kg/ha SO3 is typically required per cut of silage. Manufactured fertiliser provides guaranteed amounts unlike sulphur found in manures which can lose availability during storage to just 5-10% after six months.

Table 2. Fertiliser value of manures.
Nutrient value (£/t) Cattle slurry Cattle FYM
Nitrogen 1.04 0.52
Phosphate 0.49 1.54
Potash 1.65 4.10
Total 3.18 6.16
Source: Yara Fertiliser Manual, 2012

What's it worth?

Manures and slurry have a value as providers of nutrients to crop needs (Table 2). Typical nutrient values are well established (Table 3) and – while these can vary according to dry matter – should be taken into account when planning total fertiliser application plans. If in doubt, several organisations as well as commercial companies can test slurry and manure to give an accurate idea of nutrient content.
There is a strong risk of nitrogen loss as ammonia during application of slurry. Losses can be reduced by up to 50% where using a trailing hose, 70% for trailing shoe, 90% by injection and 80% when incorporated by cultivation.

Timing and palatability

Do not treat all fields the same, as uptake and usage of nitrogen depend to varying degrees on soil structure and temperature, quality and mix of grass types, and intended use of grass as a crop.
Manure application technique will also have a major impact on palatability of grass as a crop for livestock. Residues from manure or slurry can lead to rejection by livestock.
Ensure, where possible, a minimum of six weeks between application of manures and grazing and only apply when the crop is actively growing. This is especially important in dairy systems where the need to graze down well in the first grazing affects the quality and quantity of subsequent growth.
Likewise, applications too close to harvest can lead to high nitrate levels in herbage. A low nitrate level of under 0.1% is essential to prevent the production of ammonia nitrogen that restricts fermentation of silage in the clamp.
Although rules of thumb exist for planning of nitrogen applications remember a flexible approach will pay dividends. Fertiliser is an essential input – use it wisely.

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