Fertiliser application and silage preparation

by Robyn McLean, GippsDairy

Early spring is a good time to assess soil fertility on your farm to ensure adequate nutrients to maximise your feedbase. The best place to start is with a soil test, ideally in August to October or before soil moisture becomes limiting. However, if you have historical soil tests, look to test at the same time of year as previously to ensure they are comparable.

Nutrient availability fluctuates throughout the year; therefore it is important to be consistent with timing of soil testing. Instead of testing every paddock, group paddocks based on soil type or management for testing. 

A soil test will indicate the most limiting nutrient of your soil with the key nutrients being potassium phosphorus and sulphur. It’s difficult to accurately test for soil nitrogen as it is highly dependent on temperature and time between sampling and testing. Instead, look for deficiencies in the plant such as yellowing on the tips of older leaves.

Potassium is one of the most important nutrients heading into spring, particularly for hay or silage paddocks. On average hay and silage remove between 17 to 27kg (more for silage, less for hay) of potassium per tonne of dry matter harvested.

Ensure potassium levels are adequate for silage growth and replenished after cutting on top of maintenance application, to avoid a deficiency in the soil.

Apply fertiliser containing potassium either six weeks before cutting the pasture for fodder conservation or apply after harvesting, particularly if the conserved fodder will be used for transition cows.  A transition diet high in potassium can impact calcium mobilisation and result in metabolic issues at calving.

Phosphorus is vital for the establishment of new plants, root development, movement of energy in the form of adenosine triphosphate (ATP) and photosynthesis.

In Victoria, Olsen P is the test used to determine plant available phosphorus in the soil. Research has shown that there is minimal response to phosphorus fertiliser when the Olsen P is above 25, therefore the ideal level is 18-22 for dairy farms. Levels above this pose a risk of losses to waterways and ground water and are unlikely to have a return on investment. If your levels are considerably elevated, consider reducing phosphorus application levels for 6-12 months and continue to monitor levels with soil tests.

When soil temperature drops below 8-10°C, it is often advisable to apply sulphate of ammonia (SOA) instead of urea as a nitrogen source. SOA also applies sulphur which can become limiting over winter due to leaching and locking up in the soil.

As it warms up, urea will likely become the most economical option for nitrogen. Economic rates of nitrogen are between 0 kg N/ha and 50 kg N/ha, and application beyond this may be less efficient. Match your pasture growth rates to rate of nitrogen. In early spring, nitrogen response rates of perennial ryegrass may be around 15-20kg DM/kg of N applied.

Moisture should not be limiting and must be available when applying nitrogen.  Applications should be timed to avoid heavy rainfall where runoff is a risk.  Pasture will only grow to the most limiting factor, so before increasing the nitrogen application rate, consider if there could be any other nutrients or soil components, including pH and calcium to magnesium balance, reducing production.


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