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Organic Beef Open Day - Composting cattle manure

Munoo Prasad, Composting Research and Advisory, Naas, and Michael Gaffney, Teagasc Horticultural Development, tell us how composting of manure produces a stable material free of disease and pests.

Composting is an aerobic process used to decompose and stabilise organic material into a microbiologically stable material. The end product of the composting process has benefits when applied to soil in terms of nutrients and soil structure and is free from pathogen, parasites and free from obnoxious odours.

Orla Kinane, caught up with Munoo Prasad, Composting Research and Advisory, Naas to get an insight into composting cattle manure at the Organic Beef 22 Open Day which took place on Wednesday, 28 October on the farm of John Purcell, Cashel, Co. Tipperary.

Important Compost Parameters

There are several factors which can impact the composting process and can also impact on the quality of the compost produced.   Mixing raw materials or feedstock’s for composting to achieve the desired compost characteristics requires the testing of the starting feedstock’s, particularly for carbon and nitrogen content. It is important to have the correct Carbon: Nitrogen ratio in your compost feedstock’s and perhaps when starting composting one needs to get a preliminary laboratory analysis. Cattle manure is nitrogen rich and has a C:N ratio of 20 but can vary depending on the animal diet. Indeed most manures normally contain too much nitrogen and not enough carbon for successful composting. Getting a carbon and nitrogen content from your ‘typical feedstock mix’ may help in estimating volumes to add in the future.

Cattle manure usually has a high moisture content and therefore will probably require an amendment, to get the right C:N ratio. Therefore a carbon source needs to be added, such as straw or woody materials like leaves, sawdust, wood shavings and crop residues can be used but grass alone may not be suitable. Larger woody materials, such as branches etc. may require shredding and sieving. Ultimately you are trying to achieve a carbon to nitrogen ratio of approximately 30:1. It is best to get a chemical analysis to confirm this, however a general guide may be that you need to add usually 2 or 3 times the volume of amendment for volume of manure. 

Correct particle size (size of the fragments within the compost pile) is required; too coarse and there will be loss of heat; too fine and anaerobic conditions may occur in the windrow. Generally material size of 12 to 30 mm is ideal. Moisture content of 40% to 60% is generally fine for composting. Particle size, bulk density and perhaps moisture content needs to be done in the lab, but once done, and if the farmer is using similar material such as manure and straw, it can be dispensed with or done very occasionally During composting a ‘fist test’ can be done to get a rough estimate of moisture content. As mentioned, composting is an aerobic process and therefore it is important to have sufficient oxygen in the windrow (>5%) and this can be maintained by frequent turning. However if you turn the windrow too frequently, this can lead to nitrogen loss through ammonia volatilisation. The temperature within the windrow should be in the range of 40 to 60oC and not higher than 70oC. When the temperature within the compost pile does not rise significantly it indicates the compost is ready.

Why Compost?

Composting manures helps to reduce the overall volume of the material, and minimises issues with odours and fly pests. The composting process stabilises the material, and converts a lot of the quick release nitrogen in the manure into slower release forms of nitrogen. Between 10-30% of the total nitrogen content of the manure compost is available in the first year, with the phosphorus and potassium content immediately available. It has been shown that composts with slow availability of nitrogen, in contrast to material that releases nitrogen quickly, is beneficial to the growth of clover. The composting process speeds up the formation of humic substances, which are stable forms of carbon, which can take anywhere from 20 to 1000 years to degrade, rather than the carbon in crop residues which degrades more rapidly. These stable humic substances provide stable organic matter to improve soil health, soil structure and carbon sequestration. Compost itself is easier to handle and spread than un-composted organic wastes and as the material is stabilised there is less risk of run off into water bodies etc. Composting however does require an investment in time and some equipment if it is to be done correctly, and usually requires some trial and error to optimise the process. If done correctly one ends up with a viable product that can be used in the farm as an organic fertilizer/soil amendment or animal bedding and any surplus could be sold


Composting is a very suitable and effective way of managing manure. It is a simple (e.g. windrow) process and following a few key rules and combining these with regular turning of the windrow should lead to high quality, stable compost. It can help to replace inorganic fertilizer use and bring other quantifiable benefits to the soil, such as the stable carbon substances, micro elements and a diverse microbiology while reducing the chances of pollution from the material. Certain tools and processes can be helpful in optimising the composting process and final analysis is required for optimum use of the compost.

A forum took place at the OrganicBeef22 Open Day on the farm of John Purcell, Co. Tipperary on Wednesday, 28 September and was addressed by Minister Charlie McConalogue. The forum chaired by Damien O'Reilly included panel speakers Joe Bourke, Bord Bia; Catherine Roche, Irish Country Meats; Jack Nolan, Department of Agriculture, Food and the Marine; Pat Dillon, Teagasc and host farmer John Purcell

More information about the National Organic Beef Open Day