Make full use of cattle slurry N, P & K
Dennehy Harvesting Contractors spread slurry on farmland in Clounkeen on the last day of January. The Ballyhooley-based contractors fill a mobile slurry tank via New Rock 4000 slurry tankers and pump the slurry to a dribble bar spreader. This form of slurry spreading helps to cut emissions, which is better for the environment. Picture: Andy Gibson.
With fertiliser prices, remaining at all-time highs in early 2023 cattle slurry will be a key resource on farms to reduce the impact of high N, P & 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.
We now must look at cattle slurry the same as bag fertiliser and focus on utilising all the major nutrients as efficiently as possible. Low-emission slurry spreading techniques helps supply a larger proportion of the crops 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 & 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.
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-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 & 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%).
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 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 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 summer time it will only supply 20 kg/ha N. This offers a chemical fertiliser N saving of ~ €35/ha.
