AS WE progress through summer, it is important that feedlot pens are cleaned regularly to minimise the depth of manure on the pad, especially under shaded areas.
Pen surface can have a significant impact on increasing heat load risk. A depth of 50mm of dry, compact manure can store about 140mm of water in what will become more than 150mm of wet manure. Wet pens also have a darker surface, absorb more solar radiation and become hotter than dry pads, which are a light brown to grey colour.
Wet pen surfaces also increase the humidity within the pen, reducing the ability of cattle to cool their bodies by evaporative cooling.
Frequent pen cleaning during summer promotes more rapid pen drying and helps to reduce the heat load risk, while also minimising dust during hot dry conditions.
While the regular removal of manure from feedlot pens is a substantial cost to feedlots, manure is a rich source of nutrients and organic matter.
The nutrient content of excreted manure is influenced by the class of cattle, their diet, feed intake and other factors. Gaseous losses of N as ammonia occur rapidly and about 60 to 70 per cent of the initial N can be lost. While some P and K is removed with the manure in runoff and deposited in the holding pond, these minerals are not lost as gas.
Stockpiling or composting manure can improve the handling by breaking up lumps while further reducing the total mass of manure dry matter, volatile nutrients like nitrogen and often the moisture content, but stable nutrients such as P can become more concentrated.
When designing or improving the manure stockpiling or composting area, it is important to ensure a durable, impermeable base, good site drainage and sufficient area. It is a requirement that this area be within a controlled drainage area with runoff captured in a holding pond.
Manure aging or composting is best undertaken using low windrows rather than large piles. These are more manageable and less likely to catch on fire. Windrows are typically constructed by forming manure into a long pile with a triangular cross-section, a base width of 3m to 4m and a height of 1.5m to 2m.
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The apex and sloping sides promote water-shedding and prevent the manure from becoming too wet, which can result in significant odour.
Piles that are too low will not heat up, a process which assists decomposition, pathogen deactivation and weed seed destruction.
Piles that are too high may heat up excessively, particularly if they are not well compacted or contain wet manure. Manure fires are a source of odour and smoke and can be difficult to extinguish.
Thus wet manure from drains and sedimentation systems should be stored separately and allowed to dry before being added to windrows.
Manure composting is the microbiological breakdown of organic matter into compost or humus. Aerobic windrow composting uses organisms that need oxygen to function and is preferred because it minimises odour emissions, emits carbon dioxide rather than methane (lower net GHG emissions) and produces heat.
The benefits of composting manure include:
• more friable and consistent manure which is more easily handled and spread,
• possibilities of value adding on or off site,
• reduced viable weed seeds and pathogens,
• nutrients stabilised into a slow release form,
• reduction in temporary nutrient draw-down that can occur when raw manure is spread on soil,
• reduced nitrogen losses on spreading,
• increased concentration of phosphorus, and
• less odour release during aerobic composting.
Composting consists of an active stage and a curing stage. In the early part of the active stage, readily digestible sugars and starches are rapidly broken down and the temperature within the pile rises to over 40 degrees (typically 50 to 60 degrees). The temperature stays high for several weeks providing there is sufficient nitrogen.
Next, the more resistant materials such as lignin are broken down and pathogens are suppressed. Finally, the decomposed organic matter is converted into humus.
The correct moisture, temperature and turning of the material are important processes in composting.
The composting material has the ideal moisture content if it appears moist but little water can be squeezed from a handful.
The compost pile should be turned only after at least three consecutive days of high temperatures (more than 55 degrees). To kill pathogens and weed seeds, the pile should be turned at least three times during the active phase which may take three months or more.
Fortnightly turning will minimise labour while creating good quality compost but the pile can be turned more frequently if it has heated sufficiently and equipment and labour are available.
A strong temperature rise after turning indicates that active composting is still occurring. If the temperature does not rise markedly (with the correct moisture content), the compost can be kept in a windrow or formed into a stockpile where it can cure for at least a month.
Curing is important since immature compost may have high organic acid levels, a high carbon to nitrogen (C:N) ratio and other properties that can be detrimental to crops.
A number of feedlots have differentiated their compost by ensuring their process meets the requirements of AS 4454: 2012 Composts, soil conditioners and mulches (Standards Australia Limited 2012). This is necessary to market material as compost and may also attract a premium price, particularly in niche markets.
Further information on the removal, handling and further processing of manure or effluent is available in the Meat and Livestock Australia (MLA) Beef cattle feedlots: waste management and utilisation manual available for download from the MLA website.
Contact Jeffrey House, jeff.house@ feedlots.com.au or 0419 262 207.