The main yield components of improved pasture are the number of leaves per unit area and dry matter content. Higher yields come from optimising leaf numbers, maintaining a green leaf canopy and achieving a high dry matter content. A balanced fertiliser programme including all macro and micronutrients is essential to help manage all of these components.
Nitrogen is the crucial nutrient for influencing improved pasture yield. Requirements for potassium, phosphate, sulphur, calcium, and magnesium are related to the amount of applied nitrogen. There is a large demand for these macronutrients during early spring growth and to avoid limiting yield it is critical that sufficient quantities of nutrients are available for uptake when required by the plant.
Nitrogen is the major nutrient required by improved pasture. It is the key to achieving high dry matter yields and is often strategically used to increase production as it is needed. The key to achieving high yields is to apply the correct amount of nitrogen, from the right source at the right time.
This chart shows the typical nitrogen response from applications of AN or Urea based on Yara trials conducted over many years. The actual optimum will tend to vary depending on site location and grass growth potential.
Nitrogen is taken up by grass species quicker than it is incorporated into proteins and other structures so to allow optimum efficiency sufficient time must pass between application and cutting or grazing. This has led to the development of the Grassland Rule to apply nitrogen at the rate of 2.5 kg N/ha/day achieve optimum production efficiency.
Phosphorus plays a role in many plant metabolic processes and enzyme activities, so even though phosphorus demand is low compared to that of nitrogen its availability is essential. Phosphorus accelerates and improves grass growth and is important for increasing yield. The older the sward, the more important is phosphorus fertilization.
Phosphorus is very immobile in soil and its availability is limited by pH, by distance from plant roots and soil temperature.
Potassium is the nutrient taken up in the greatest quantity by pasture swards. Potassium has a wide-ranging role in the plant affecting nutrient uptake, photosynthesis, rate of growth and feed value.
Potassium is particularly important for increasing stem strength, improving drought resistance and cold tolerance and importantly for increasing yield. Potassium fertilisation is more important especially in autumn and on older grass. Nowadays potassium deficiency is becoming more of a problem, especially in some Scandinavian countries.
If adequate amounts of potassium are not available, the rate of growth and yield will be restricted, however there can be dangers to animal health if excess potassium is applied with an increased risk of hypomagnesaemia.
These data from UK show the potassium response on different K status soils.
There is also a relationship between nitrogen and potassium as the response of grass to nitrogen is dependent on an available supply of potassium to allow N uptake as nitrate and conversion into proteins.
Pasture management has a large effect on potassium requirements. Mowing regimes for silage or hay remove large quantities of potassium that needs to be replaced and it is logical to return this as manures whenever possible. Pasture regimes have very different requirements as a large proportion of the potassium is returned directly to the sward and the need for potassium will be lower.
These data show the quantities of P and K removed by mowing and balances this with returns from livestock either directly as slurries and manures.
Sulphur is essential in the formation of protein and so is crucial for growth and development. As grass grows both sulphur and nitrogen are used together so sulphur deficiency will decrease nitrogen use efficiency and so reduce yield. Historically sulphur was supplied by atmospheric deposition however as a result of reduced industrial emissions and improved air quality this has become much lower in recent years so increased responses to sulphur is now seen in grass and other crops.
These data come from a series of trials in the 1990s showing the increased yield in response to sulphur. Since then due to reduced atmospheric deposition, the response will certainly have increased.
Magnesium also needs to be considered. Magnesium is an essential nutrient, and in situations where magnesium is deficient, there will be a significant reduction in pasture yield.
Zinc and manganese have also been proven to have effects on pasture yield.