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Make full use of cattle slurry N, P & K

Mark Plunkett, Teagasc, Johnstown Castle, Co Wexford

Reducing chemical N use on farms will reduce nitrous oxide emissions.  Nitrous oxide is one of the main greenhouse gases we need to reduce.  It is important that where farmers are capturing more N from slurry that there is a corresponding reduction in chemical N use.  Using LESS and spreading slurry in springtime has to be matched by a reduction in chemical N use to have an impact on greenhouse gas emissions.   

Step 1. Know the value of Slurry N, P K & S

We now must look at cattle slurry the same as bag fertilizer and focus on utilising all the major nutrients as efficiently as possible. The first step to utilising the nutrients in slurry is to know how much N, P & K is in each 1,000 gallons of slurry.  Table 1 below shows the available fertilizer values (N, P & K) for a range of cattle slurries at different dry matters(DM) applied by LESS. 

Typical cattle slurry has a dry matter (DM) of 6% and a nutrient profile shown in table 1. More dilute slurry (2-4 % DM) will have reduced N, P & K values which may result in the under fertilization of crops such as grass or maize silage if the slurry is assumed to have more typical nutrient content. Take slurry DM into account and make adjustments to application rates to ensure sufficient nutrients are applied to meet crop requirements during the growing season.  The DM% of slurry can be measured on farm with a slurry hydrometer or alternatively by sending a sample of agitated slurry for nutrient analysis to a laboratory (N, P, K & DM%).

Step 2. Timing of Slurry Applications

The second step to increasing slurry N efficiency is optimizing the timing of slurry applications.  Spring applications of cattle slurry typically have higher recovery of N, by up to 50% more, compared to summer applications (Table 2).  Weather conditions in springtime will be more favorable to improving the recovery of N from the slurry for example cool (< 13⁰C), damp, overcast cast days when N loss through ammonia emissions are lowest.  To maximize utilization of slurry P and K apply close to time of crop requirements as such nutrients as P will be more available.

Step 3. Using the right equipment

Spring applications using LESS further increases N availability by 65% compared to summer applications. For example, where a grass silage crop receives 33mᶾ/ha (3,000 gallons/ac) in springtime, this will supply 33 kg/ha N fully available (~25 to 30% of the crops N requirement).  However, when applied in summertime it will only supply 20 kg/ha N.  This offers a chemical fertilizer N saving of ~ €35/ha.

Other benefits of LESS include

  1. Improved flexibility with applications as a result of reduced contamination of herbage leading to quicker return to grazing;
  2. Opportunity to apply slurry into larger grass covers which creates a wider window for application in better soil conditions particularly in spring;
  3. More even application of slurry across the spread width;
  4. Smells released during and after application are reduced.