Keeping pastures productive

When producing hay and silage from a paddock, it is important to be aware of how much nutrient is being removed. If the nutrients are not replaced there is the risk of the pastures not performing to the standard expected and low fertility plants such as dandelions and sweet vernal grass may appear.

Phosphorus (P) and potassium (K) are the two main nutrients to consider. The following table highlights how much phosphorus and potassium is removed in every 1 tonne of dry matter.

Based on weight of nutrient per quantity of product (kg)

Where there is a range for the amount of nutrient removed (such as the hays), use the higher value if the hay is cut earlier in the season or has a large amount of green leafy material in it. Use the lower number for later cut hays.

Below are two examples of how to use this information – one for hay and one for silage.

Example 1 – Hay

Thirty (30) 5 × 4 round bales were cut off a paddock late in the season.  The harvested weight of these 5 × 4 bales was approximately 350kg each (wet weight), so 10,500kg (30 × 350kg) was harvested off the paddock.

The harvested weight needs to be converted to a dry matter weight.  Hay has a dry matter of roughly 85%, so 10,500kg × 0.85 = 8925kgDM or 8.9tDM.

So how much phosphorus and potassium was removed? The table indicates that for late cut hay, 15kg of potassium (K) and 2kg of phosphorus (P) is removed for every 1 tonne dry matter harvested.

Example:

15 kgK/tDM x 8.9 tDM = 133.5 kg potassium (K) removed in the late season cut hay in total from the paddock 2 kgP/tDM x 8.9 tDM = 17.8 kg phosphorus (P) removed in total from the paddock.

When fertiliser is spread on the paddocks, the rate applied is in kg nutrient/ha. This means when ordering nutrient to be applied as fertiliser from a supplier, it is good to also quote the amount of nutrient required as kg nutrient/ha to avoid confusion. To convert the amount of nutrient removed from the paddock by the hay to kg nutrient/ha, simply divide the amount of nutrient removed in the hay by the number of hectares it was cut from. In this case, the paddock was 3 hectares in size.

Potassium = 133.54 kg ÷ 3 ha = 44.5 kg/ha of potassium that needs to be applied. Phosphorus = 17.8 kg ÷ 3 ha = 6 kg/ha of phosphorus.

Example 2 – Silage

Fifteen (15) 4×4 round bales were cut off a paddock. The harvested or wet weight of these bales was approximately 700kg each, so 10,500kg (15 × 700kg) was harvested off the paddock.  This needs to be converted to a dry matter weight. Baled silage has a dry matter of about 40%, so 10,500kg × 0.4 = 4200 kgDM or 4.2 tDM was harvested.

So how much potassium and phosphorus has been removed? The tables indicates for silage, 27kg of potassium and 4.3kg of phosphorus is removed for every 1 tonne dry matter harvested.

Example:

27kg K/tDM × 4.2 tDM = 113kg potassium removed in the silage

4.3kg P/tDM ×  4.2 tDM = 18kg phosphorus removed

And converting into kg/ha rates of nutrients (paddock was 3 ha in size):

When contacting the local fertiliser supplier, quote the kg/ha figures you require to replace the nutrients.  They will either have a product that closely matches the nutrient content of what you require or should be able to blend a product to match.

Potassium = 113kg ÷ 3ha = 38kg/ha of potassium that needs to be applied

Phosphorus = 18kg ÷ 3ha = 6 kg/ha of phosphorus that needs to be applied

These calculations assume the fodder cut is not being fed back onto the paddock it is cut from.  If you think about 30 per cent of the fodder will be fed back onto the paddock it was cut from, you can reduce the replacement nutrient requirements by 30 per cent.  This is just replacing the nutrients that have been removed from the paddock by harvesting the fodder.  Don’t forget about annual maintenance fertiliser requirements that need to still be applied — generally in autumn

Note:  it is important to not apply more than 60 kg/ha potassium (K) in any one application.  Applying at rates higher than this could cause animal health issues such as grass tetany.

If needing to apply more than 60kg/ha K, consider applying the 60kg/ha K after silage removal, then the remainder with the normal annual maintenance fertiliser application.