Reducing the impact of high fertiliser prices through slurry

With fertiliser prices remaining at all-time highs in early 2023, Mark Plunkett, Teagasc Soils and Plant Nutrition Specialist, explains why cattle slurry will be a key resource on farm to reduce the impact of high nitrogen (N), phosphorous (P) and potassium (K) prices.

Firstly, retaining more slurry nitrogen (N) to grow grass in the early part of the year reduces farm chemical N requirements. Secondly, target slurry to parts of the farm that have a P and K requirement. This ensures full utilisation of all the nutrients in slurry. 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. The use of LESS (trailing shoe / band spreader) is one of the key technologies available to meet our national ammonia-gas reduction target between now and 2030.

Step 1 - Know the value of slurry N, P and K

We now must look at cattle slurry the same as bag fertiliser and focus on utilising all the major nutrients as efficiently as possible. LESS techniques help supply a larger proportion of the crop’s total N requirements than ever before. This change delivers a double dividend in that it helps to reduce agricultural emissions and our overall farm fertiliser costs. The first step to utilising the nutrients in slurry is to know how much N, P and K is in each 1,000 gallons of slurry.

On farms where dirty water / parlour washings are entering the slurry tank, a more dilute slurry is available. Table 1 below shows the available fertiliser values (N, P and 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 and K values, which may result in the under fertilisation of crops such as grass or maize silage if the slurry is assumed to have more typical nutrient content. These crops should receive a large proportion of their N, P and K in the form of slurry in 2023. 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%).

Table 1: Available N, P and K values of cattle slurry at different Dry Matter percentages in springtime by LESS application techniques

Step 2 - Timing of slurrp applications

The second step to increasing slurry N efficiency is optimising 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 favourable 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 utilisation 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 33kg/ha N fully available (~25 to 30% of the crops N requirement). However, when applied in summertime, it will only supply 20kg/ha N. This offers a chemical fertiliser N saving of ~ €35/ha.

Table 2: Available N, P and K values kg/m3 for slurry applied by LESS in spring and summer

Other benefits of LESS include:

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

This article was original published as part of a Signpost Programme newsletter.