All animal farms produce manure. Each dairy cow can produce 20+ gallons of manure and urine every day.
Even with only a few cows that is a lot of manure. Managing manure and other dairy by-products produced on the farm is an important task. After manure is produced it must be collected, can be processed and stored, before being applied to the land to provide nutrients for crops.
Decisions about how to manage manure are commonly based on cost, agronomics, regulations, equipment availability, and operational needs. But the complexity doesn’t stop there. Farms are now challenged to manage manure sustainably. There are many aspects to sustainability, this includes climate change and manure management impacts to greenhouse gas emissions as well as adapting farm practices to these challenges while remaining financially viable.
There is no catch-all solution. Manure systems are all differ and are dependent upon design, location, climate, regulations, soil type, access to capital and labor, among others. Let’s start by looking at the manure management process, which itself is part of the larger scope of a dairy farm. We’ll highlight variables that can be changed by the farmer to produce different outcomes, and point you to scientific papers detailing studies that have been done to analyze different aspects of this process.
After the manure is produced it can be collected, processed, stored, and land applied.
Manure can be collected from a barn or other farmstead location manually with skid steers, using mechanical systems such as alley scrapers or gutter cleaners, by gravity using slatted floors, or by flush systems that wash manure to a collection point.
Selection of a collection method is typically based on cost, bedding type, barn design, labor availability, and operational preference. See additional information on how this fits into the entire manure system.
Manure processing can be incorporated into a system to alter manure characteristics to gain an environmental, operational, or economic benefit.
There are many forms of processing, but advanced processing systems can be costly and may only make economic sense on large farms that can afford the investment. However, there are low cost processing techniques that can be adopted by facilities with less access to capital.
A few common manure processing technologies include sand separation, composting, solid-liquid separation, and anaerobic digestion.
Sand separation involves separating sand bedding (that the cows lay on) from collected manure. The recycled sand gets reused as bedding and greatly reduces downstream clogging and settling problems.
Composting is a process where microorganisms that naturally occur in manure degrade organic matter producing a humic substance. This humic substance still contains vital nutrients for crops but has less volume, reduced odors, and reduced pathogen content among other benefits.
Composting can be done in many different ways, check out this video to see a dairy which uses a windrow process.
Solid-liquid separation divides manure into multiple streams, typically a solid and liquid stream.
Manure can be separated mechanically by screening or pressing out solids, or by simply letting solids settle using gravity. The benefit of processing manure in this way is that each stream has characteristics that can be managed more effectively, which creates advantages for the farmer and the environment.
When manure solids are separated from the liquids, the solids are commonly used as animal bedding, sold, or spread on the land as fertilizer. In addition to being richer in nutrients, one truck load of solids weighs less than a truck load of liquids, so these nutrients can be hauled to far away fields more cost effectively. In some cases, solids can be land applied to certain fields to reduce environmental impacts. For example, if a field has very steep slope, liquid or slurry manure may runoff easily, making solid manure application more desirable on these fields. There are many site specific land application strategies that have to be evaluated for each individual farm.
To understand the best time and best way to apply the processed manure fertilizer to the crops or cropland, we need to study nutrient uptake of crops throughout their lifecycle, immobilization or volatilization of nitrogen in the soil, and proper management of croplands throughout the year under different climatic and weather conditions, among other things. For a primer on that science, check out our page on Crops and Soils.
In some cases, solids can be land applied to certain fields to reduce environmental impacts. For example, if a field has very steep slope, liquid or slurry manure may runoff easily, therefore solid manure applications may be more desireable on these fields. There are many site specific land application strategies that have to be evaluated for each individual farm. To understand the best time and best way to apply the processed manure fertilizer to the crops or cropland, we need to study nutrient uptake of crops throughout their lifecycle, immobilization or volatilization of nitrogen in the soil, and proper management of croplands throughout the year under different climatic and weather conditions, among other things.
For a primer on that science, check out our page on Crops and Soils.
Separated liquids are usually stored until they are land applied as fertilizer. Because they have fewer solids, separated liquids are easier to agitate and handle using pumping equipment, and can provide farmers with more options during land application.
Anaerobic digestion is a process where microorganisms, naturally occurring in manure, degrade organic matter in the absence of oxygen to produce biogas. Biogas contains methane which can be burned for heat, combusted in a generator to produce electricity, or cleaned and compressed to use as vehicle fuel among other applications. While digesters extract methane, the manure that entered the digester still has to be handled as it exits (also known as digestate) and is managed much like undigested manure. Digesters have many additional benefits including reducing greenhouse gas emissions, pathogens and odors.
Sound interesting? We think so too! Gain some additional details from US EPA AgSTAR.
Here are two types of common digesters:
Image Credit: Rebecca Larson, UW Madison
Manure storages are used to help farmers land apply manure less often at times that are beneficial to the environmental or the operation, particularly to reduce runoff during land application. For example, manure can be stored during the winter and applied in the spring when it can infiltrate into the soil and be used by crops.
There are many types of manure storage systems. The type, size, and design of the system are really important to the environmental impacts and to how the operation handles their manure, particularly to reduce odors, emissions, and impacts to groundwater below.
The final stop for manure is almost always land application. Nutrients in manure are vital to crop production and applying manure to the land can provide great benefits to soil health.
However, managing manure application timing is critical to reduce runoff, nutrient loss, and gaseous emissions of manure components that can be harmful to the environment or humans. There are many things that go into land applying manure including developing nutrient management plans, analyzing the manure and soil, selecting equipment, and timing.