Grass is the cheapest feed on most livestock farms. So optimising grass yield, quality and utilisation are key to controlling feed costs for milk and meat production, whatever the farming system.
The key to a quality grass sward is to provide all the nutrients the grass needs. But this must be done carefully, taking into account not just costs but also legal requirements, such as those for NVZs and the environment in general. Fortunately, fertilisers can be used to balance the nutrient content of manures so, with a little planning this is easy to achieve.
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A pH range of 6.0 to 6.5 is the optimum to cover availability of nutrients in grass and grass/clover swards. In the diagram below, the wider the band in each nutrient, the greater the availability.
P, K, Mg status
It all starts with the soil; knowing what fertility you have by soil testing is vital. All fields should be soil sampled every 3 or 4 years and tested for P (Phosphate), K (Potash), Mg (Magnesium) and pH (acidity).
Target index levels to optimise grass growth are:
- P index 2
- K index 2-
- Mg index 2
- pH of 6.0 to 6.5
You can tailor your fertiliser programme to achieve and maintain these targets, integrating nutrient inputs from manures and fertilisers to build fertility to target levels, or rely on soil reserves where indexes are above target. Adjust rates accordingly as shown in the Fertiliser Manual (RB209).
Nitrogen is the main driver of grass yield and has a greater impact than the other major nutrients. This can be seen in the following yields from a 2-cut silage system that received no Nitrogen, half the recommended rate, or the full rate advised in the independent Fertiliser Manual, RB209.
Nutrient uptake is limited in early spring, but increases rapidly as soil temperature at 10cm depth consistently reaches 5 to 6 oC.
Early uptake of Phosphate is critical to stimulating root development so that Nitrogen, Sulphate and Potash can be absorbed into and transferred through the grass plant. Any requirement for Phosphate in the fertiliser programme should be applied, at least in part, in the Spring. For example, applying maintenance P applications at soil indexes 2 and 3 can boost grass dry matter yield:
Potash is also crucial for the efficient uptake of Nitrogen, so make sure that grassland is maintained at soil P index 2- and receives maintenance applications, particularly in silage systems where large amounts of Potash are removed by the crop:
Sulphur is now a must in most grassland situations. Sulphur deposited from industrial pollution is so low that Sulphur-deficiency is common and reduces grass growth and quality. This is because Sulphur is needed for the grass plant to make proteins:
Nutrients removed by grass at harvest
Crop removal of nutrient becomes the base point for recommendations of Phosphate (P) & Potash (K). These are the figures that enable us to calculate what nutrients are removed and therefore, how these need to be balanced with organic manures and fertilisers.
Over the whole season, this adds up to large removals of P and K, which need to be supplied or replaced by the soil, manures and fertilisers:
While still a major nutrient, Magnesium is not typically in deficient supply. Often, historical applications of magnesium lime have resulted in too much Magnesium in soils and where soil Mg index has reached 3 or above, applying Magnesium in fertilisers should be avoided.
Each individual nutrient has a role to play in grass production, but they should not be considered in isolation. It is only when the plant is supplied with the right mix of nutrients that optimum growth is achieved.
The key to nutrient management planning is balancing the nutrients supplied by the soil with those supplied from organic manures, against the nutrients needed for target grass growth and soil reserves. The difference is made up with fertilisers:
Fertiliser requirement = crop need – (soil supply + organic manure supply)
Nitrogen (N) is the key building block for yield, because it is the primary nutrient for protein and chlorophyll. The main form of N taken up by the grass plant is nitrate (NO3).
Chlorophyll is the essential component to enable photosynthesis and therefore is key to capturing the energy of the sun, which can be converted into the proteins and carbohydrates that drive plant growth and yield. So, with Nitrogen as the core element of chlorophyll and protein, it’s easy to see why Nitrogen is tagged as the driver of grass yield.
Nitrogen is naturally present in the soil in an organic forms, which are not immediately available for plant uptake and use. These Nitrogen sources only become plant available after the process of mineralisation by soil micro-organisms, the end-point of which is nitrate (NO3).
In organic manures, the Nitrogen is in readily available and unavailable (organic) forms. This means that when manures are applied to grassland, some of the Nitrogen they contain will be immediately available for grass growth, some will enter the soil organic matter and some will release available Nitrogen when manure organic matter is mineralised.
Sulphur (S) is central to some of the essential amino acids that are the building blocks of proteins, so Sulphur-deficiency reduces protein production and grass growth.
Sulphur is more important than ever as atmospheric deposition from industrial pollution is now at a very low level. So, most grassland soils will be Sulphur-deficient, dependent to some degree on soil type and season. The benefits are not confined to DM yield; protein and sugar levels in the grass sward can also be improved and Nitrogen losses reduced.
Plants can only take up Sulphur from soil solution as Sulphate (SO3); they can’t directly use elemental Sulphur. Sulphur in the soil and organic manures can be mineralised by soil microbes to provide sulphate for grass uptake, but this is usually not enough to provide for crop requirement.
To get more detail on the use of Sulphur for grassland, refer to our booklet.
Phosphorus (P) is important for root development and early spring growth. Phosphate is also vital for energy capture by plants through photosynthesis – energy that is used for growth and quality.
Plants take up P from soil solution as Phosphate (P2O5). Within the soil, some Phosphate is bound by minerals and within organic matter; some of this can become available for plant uptake and soil analysis accounts for this by measuring the Phosphate in soil solution and the portion that is ‘readily available’.
For grass growth, the current soil Phosphate target is index 2. However, fertiliser recommendations recognise that even at soil Phosphate index 2 and into index 3, grass yield can be increased by supplying readily available Phosphate, particularly in spring. This is because the Phosphate stimulates root growth, leading to improved Nitrogen uptake and therefore growth. So make sure that Phosphate recommendations are followed carefully and apply maintenance applications:
Potash (K) is vital for Nitrogen uptake and the movement of nutrients around the plant. Potash is also important for water balance in the plant and so is essential for normal drought resistance.
As with Phosphate, within the soil some Potash is bound so that it is not immediately in soil solution and available for plant uptake; it is attached to the clay particles in the soil. Some of this can become available for plant uptake and soil analysis accounts for this by measuring the Potash in soil solution and the portion that is ‘readily available’.
So, the target soil P index is 2-. Grass growth will still respond to Potash applications at soil K index 2+, but at this index not as much is needed to balance offtake and maintain soil fertility.
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