Phosphorus Balance on a Dairy Farm

For many dairy farmers ‘phosphorus balance’ is a new term which has recently been discussed in relation to the Nitrates Derogation, but what is it and what does it mean to you?  For farmers exceeding the 170 KG organic N/ha and who have applied for a Derogation the phosphorus balance for the farm has to be kept below 10 KG P/ha/year.  The following article seeks to provide farmers with more information and through a worked example explain the options to be considered on individual farms.

Phosphorus is an important nutrient for the productivity of grass, crops and livestock.  For this reason it has been a major constituent of compound fertilisers and is also included in compound feeds.  However it is also a potential pollutant due to run-off or leaching when supplied in excess of plant requirements, if spread during the winter or if spread during unsuitable weather and soil conditions.  While it is estimated that only 1 KG P/ha is leached to waterways per year, this is highly significant in terms of eutrophication.  For this reason intensification of agriculture in Northern Ireland has played a role in the eutrophication of Lough Erne and Lough Neagh where phosphorus (P) levels have increased by around 50 percent in the last 10 years.  So what can dairy farmers do to help minimise the risk of further leaching of P into the waterways?

On a typical 100 cow dairy farm approximately 2 tonnes of phosphorus is brought onto the farm each year through the purchase of phosphate in fertiliser and as phosphorus in concentrates.  However, only 40 percent of this P leaves the farm in milk and livestock sold.     

Some of this surplus phosphorus is lost directly as run-off from dirty yards or fields following slurry application, but most of it builds up in the soil from where it can be subsequently leached to the drainage water.  In the last fifty years, the proportion of soil tests in Northern Ireland with excessive phosphorus has increased from less than 5 percent to almost 50 percent.  Dairy farm soils are accumulating P at an amount similar to the fertiliser P applied so by careful nutrient management, most dairy farms can eliminate P fertiliser without loss of productivity.  

Under the Nitrates Directive, phosphate fertiliser can only be applied where a crop need can be demonstrated through soil analysis.  Fields should be sampled between November and February (every four years) to get a P index with the recommendations for grassland outlined in Table 1.

Table 1:  Maximum crop requirement for grass and some crops for P₂O₅  in KG per hectare.

Phosphate can be applied up to the maximum application rate.  For example if a grazing field tests as having a soil index of 2 up to 20 KG of phosphate can be applied.  Where optimum yield is not required, as in an extensive farming system, application rates should be reduced.  On extensive grassland farms maintaining an index of 1 on grazing land is satisfactory.

The phosphorus content of concentrate feeds is currently around 0.6 percent on a dry matter basis.  However, research at AFBI Hillsborough has shown that the P content of dairy cow rations can be reduced to 0.38 percent of the total ration on a dry matter basis without affecting production.

For farmers who are over the 170 KG organic N/ha limit and have applied for a Derogation under the Nitrates Directive, a P balance of less than 10 KG P/ha/year has to be maintained.  This balances total P inputs from fertiliser, concentrates and livestock with P outputs in milk and livestock over the total area of your farm under your control.  The highest P surpluses are found on highly stocked farms with high P fertiliser usage and high levels of concentrate feeding.  

Conducting a phosphorus balance is relatively straight-forward.  An example is shown in Table 2 for a 55 hectare farm with a 100-cow herd stocked at 2.47 CE/ha and with a livestock manure loading of 208 KG N/ha.

Table 2:  Phosphorus balance for a typical 100-cow dairy herd.

In this example there is a P surplus of 22.2 KG/ha, but there are a number of options available to bring this below the 10 KG P/ha/year limit for dairy farming under a Nitrates Derogation.  For the example farm the various options and revised P balances are:

Alternatively, taking 13 hectares of conacre is sufficient to reduce the livestock manure loading to less than 170 KG N/ha, thus avoiding the need to farm under a Nitrates Derogation and the need to conduct a phosphorus balance.

A P balance can be calculated using a similar table or by using the P balance calculator  Action is required if your P balance exceeds 10 KG P/ha/year.