ARCHIVE: Fertiliser Manual (RB209)
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Principles of Fertilising Grassland


The nutrition of grassland is more complex than that of arable crops but the main features and principles are well established. Compared to arable cropping, more nutrient sources must be taken into account and, almost always, manures are applied to part of the grassland area. Most importantly, herbage is produced to meet the needs of livestock on the farm and there is no point in producing more than can be utilised effectively. In many cases, this means that the full productive potential of grazed grassland is not needed.

Most fertiliser policies on grassland farms need to integrate the use of inorganic fertilisers with maximising the value of the readily available nutrients contained in organic manures. Manure is returned to grassland in two ways: by application as FYM or slurry and by deposition of dung and urine by grazing animals. Large proportions of the nutrients in herbage eaten by grazing animals are returned to the sward by deposition. This recycling is not efficient as application rates within dung and urine patches are much larger than grass requirement, significant losses of nitrogen as ammonia occur from urine patches and a small proportion of the sward is affected by dung or urine at any given time. However, over several years, nutrient reserves tend to accumulate in grazed land.

Slurry and FYM are valuable sources of nutrients and their correct use can result in substantial savings in inorganic fertilisers with reduced risk of causing environmental pollution. Section 2 of this Manual gives details of how to make best use of manures and how to estimate the fertiliser value of individual applications.

Nitrogen

Swards that are mainly ryegrass respond strongly to increasing nitrogen supply. Response tends to occur in two stages: firstly, nitrogen is taken up and secondly, dry-matter yield increases. Nitrogen uptake is more rapid than yield increase and is less affected by some adverse conditions such as short day length. It is necessary therefore to distinguish greening of grass (associated with nitrogen uptake) from herbage growth.

Although nitrogen has a very large effect on the growth of grass swards, target dry matter yields will only be obtained if other requirements are met:

  • adequate supply of moisture (rainfall or stored soil moisture)
  • adequate temperature
  • balanced supply of other plant nutrients
  • satisfactory soil pH
  • satisfactory sward composition

Recommendations in this Section apply where these conditions are met. Where yield is severely restricted by any of these factors nitrogen inputs from fertilisers or manures should be reduced. Summer drought is the most common cause of poor growth either due to seasonally low rainfall or where there is a drought-prone soil type. When summer growth is restricted due to lack of rain, subsequent fertiliser inputs should be reduced or omitted since previous applications will not have been fully used by the crop.

In most seasons, growth up to late spring will not be restricted by moisture stress even on the most drought-prone soils. This also coincides with the period of most rapid growth. Therefore, a high proportion of the total nitrogen use in intensive systems should be applied in spring and early summer. This is particularly important on drought-prone soils since summer growth cannot be guaranteed. Nitrogen should not normally be applied after August.

Patterns of nitrogen application must be matched to the ability of the system to utilise the grass grown and to the quality of grass required. Guidance is given as footnotes to the recommendation tables but a high degree of local judgement is also required. Grass is only of value if it can be utilised by livestock. It is not sensible to grow more grass than is needed for the livestock on the farm or if the land is too wet (e.g. in spring) to utilise the grass effectively by cutting or grazing.

In grassland systems, the amount of nitrogen supplied from soil reserves varies considerably. The fertiliser nitrogen values recommended refer to moderate Soil Nitrogen Supply status and adjustments must be made for higher or lower Soil Nitrogen Supply in a particular field. Many intensively managed swards, except newly established leys, will have a high soil nitrogen status which will result in a lower requirement for fertiliser nitrogen. This is because:

  • soil nitrogen builds up as a result of nitrogen returned in organic manures or excreta during grazing,
  • the perennial plant cover limits losses of nitrogen by leaching,
  • established grassland is not ploughed which limits nitrogen losses.

The effect of regular applications of organic manures

Livestock manures are an important source of valuable nutrients and need to be recycled to land. Careful and planned use of manures can result in large savings in purchased fertilisers.

Manures supply both readily available nitrogen and nitrogen in organic form that is released only slowly for crop uptake. The readily available nitrogen content of a manure application will usually be taken up by grass, or lost to the environment, during the first season following application. Section 2 gives information on how to calculate the available nitrogen that is equivalent to fertiliser nitrogen. The remaining organic nitrogen will be added to the soil organic matter pool and will contribute to the supply of soil nitrogen for several seasons (i.e. it will increase the soil nitrogen status). When deciding on fertiliser nitrogen use, it is important to consider both the supply of available nitrogen from recent manure applications and the available nitrogen released from older applications.

In cattle and sheep systems, most of the nitrogen ingested as conserved grass or feed, is excreted. During grazing, this nitrogen is returned to the soil and, in intensive systems, will result in the soil having a moderate or high soil nitrogen status. During housing, excreta are collected as manure. If this manure is spread equally to all of the grass conservation fields that produced the forage, then most of the nitrogen removed from the soil during the growing season will be replaced. Although this nitrogen will not all be immediately available for uptake from an individual manure application, the total available nitrogen contribution from a past history of regular manure applications will be substantial.

Thus, silage fields that receive regular average applications of manure will usually have a moderate or high soil nitrogen status. Fields which are regularly cut for silage and receive little or no manures are likely to have a low soil nitrogen status. Adjustments upwards to higher rates of nitrogen may therefore be justified.

The nutrient recommendations in the tables take account of past manure applications because these influence assessment of the soil nitrogen status of a field. The available nitrogen content of manures recently applied for the current season's growth (i.e. previous September applications onwards) must be assessed using the information in Section 2 and deducted from the table recommendation.

Phosphate and potash

The general principles of using phosphate, potash and magnesium fertilisers are described in Section 1. As described above for nitrogen, the recycling of phosphate and potash through conserved grass and back to silage fields in manure applications will commonly result in sufficient nutrients to maintain soil P and K Indices.

Fields which are regularly cut for silage and receive little or no manures will have a higher requirement for phosphate and potash. Regular soil analysis every 3-5 years and using the information in Section 2 will ensure that the nutrients in manures are used profitably and with minimal risk of causing pollution.

For grassland the following additional points should be noted:

  • Grass/clover swards are more sensitive to phosphate and potash shortages than pure grass swards.
  • Grass silage can remove large quantities of potash which must be replaced by fertiliser or organic manure application. Failure to do this can lead to rapid development of potash deficiency and low grass yields.
  • It is important to maintain a suitable balance of nutrients in grass, particularly when it is grazed. Nutrient imbalances can aggravate livestock disorders such as a shortage of magnesium (hypomagnesaemia or grass staggers). The risk of hypomagnesaemia can be reduced by applying adequate amounts of magnesium as fertiliser and/or dietary supplements, and by avoiding excessive use of potash fertiliser. Avoid applying potash fertiliser in spring to grazing land except at soil K Index 0.
  • Early spring growth can benefit from a small quantity of spring-applied phosphate. All or part of the total phosphate requirement should be applied in early spring.

Recommendations are given as kg/ha of phosphate (P2O5) and potash (K2O). Conversion tables are given in Appendix 8.

Magnesium

Although grass growth is unlikely to respond to magnesium fertiliser, it is important to maintain an adequate level of magnesium in grass herbage to help minimise the risk of livestock disorders such as hypomagnesaemia (grass staggers). Direct treatment of stock may also be needed to avoid this disorder.

At soil Mg Index 0, apply 50 to 100 kg MgO/ha every three or four years. If there is a risk of hypomagnesaemia, larger amounts may be justified to maintain soil Mg Index 2. Where liming is also needed, use of magnesian limestone may be most cost effective.

Magnesium recommendations are given as kg/ha of magnesium oxide (MgO) not as Mg. Conversion tables are given in Appendix 8.

Sulphur

Sulphur deficiency is increasingly common in grassland, especially at second and later cuts in multi-cut silage systems using high rates of nitrogen, but also sometimes at first cut. Deficiency can cause large reductions in yield. From a distance, visible symptoms are similar to those of nitrogen deficiency - poor growth and yellow tinge to leaves. However, in sulphur deficiency, the youngest leaves are pale whereas in nitrogen deficiency the older leaves are most affected. There is an increasing need for the use of sulphur fertilisers in both arable and grassland systems (see page 43 for more details).

Sulphur recommendations are given as kg/ha of sulphur trioxide (SO3) not as S. Conversion tables are given in Appendix 8.

Sodium

Sodium will not have any effect on grass growth but an adequate amount in the diet is essential for livestock health and can improve the palatability of grass. The sodium content of herbage is normally adequate for grazing livestock though it may be reduced if excess potash is applied. Herbage analysis is useful to assess the sodium status of grass. Where sodium levels are low, mineral supplements may be required for some classes of stock or a sodium containing fertiliser may be used.

Lime and micronutrients (trace elements)

Many grass species can tolerate more acid conditions than most arable crops, but grass/clover swards are less tolerant of soil acidity than all-grass swards. Clover is less likely to persist where the soil pH is below the optimum.

The optimum soil pH for continuous grassland is 6.0 (mineral soils) and 5.3 (peaty soils). In a mixed grass/arable rotation where an occasional cereal crop is grown in a predominantly grassland rotation, the soil pH of mineral soils should be maintained at 6.0 or 6.2 if barley is grown.

Over-liming should be avoided as it can induce deficiencies of trace elements such as copper, cobalt and selenium which can adversely affect livestock growth but will not affect grass growth. Where a deficiency does occur, treatment of the animal with the appropriate trace element is usually the most effective means of control, though application of cobalt and selenium to grazing pastures can be effective.

More information on the use of lime is given in Section 1.

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