USDA-ARS staff measure runoff from manure application trials.
Manure is a byproduct of animal production and could be best characterized as the expression of an animal’s inefficient use of nutrients. If an animal was totally efficient, there would be no residual material excreted. Of course, that’s not possible or feasible in a biological system. The end result, then, is the excrement (manure) is full of carbon, nitrogen, phosphorus and potassium, plus other nutrients.
Evolution of Manure
In tracing the evolution of American agriculture, we find that animals were used as a resource to produce the manure needed to provide nutrients for crops and serve as an amendment to increase soil quality.
Manure was a valuable resource in the agricultural system in the 20th century, when farms were a prime example of a complete recycling effort, and it remains so today as farmers increasingly opt for manure vs. high-cost commercial fertilizer.
Manure is a component of swine production in which we need to apply our imaginations to utilize this resource in a way that enhances agricultural production, increases the quality of our soils and provides a potential energy source from biofuel production.
Manure Nutrients, Value
Total inventory of hogs of all classes has ranged from 59 million in 2003 to about 66 million head, currently.
If we assume this population is primarily grow-finish animals, their manure production is 13.8 billion pounds annually. The composition of this manure is roughly 1.1 billion pounds of nitrogen (0.4 billion pounds of NH3), 0.6 billion pounds of phosphorus (P2O5) and 0.33 billion pounds of potassium (K2O) per year.
The value of these nutrients is $304,700,000 for nitrogen (N) using the average 2012 price for nitrogen forms, $208,500,000 for phosphorus (P) forms, and $106,755,000 for potassium (K) forms, or nearly $620 million for the nutrient value of hog manure. Equivalent nutrient value from all animal production in the United States is estimated to be nearly $3.5 billion. Figures are based on current N, P and K manure values.
These economic values for nutrients do not capture the full value of manure as a soil amendment and for restoring soil productivity and soil organic matter. Extensive observations from international research indicate the addition of manure enhances the soil and, in particular, the soil organic matter and nutrient cycling capability.
The value of manure in restoring soil quality hasn’t been quantified. In reality, the economic return may be larger due to the value-added increase of organic matter in the soil, which in turn leads to an increase in soil water-holding capacity and more soil water availability.
These effects on crop yields are not always easily recognized. However, in years like 2012, the value of good soil and improved water-holding capacity can be the difference between economically viable crop yields and crop failure. Increased soil moisture allows the value of the nutrients to be enhanced because of the increased effectiveness of nutrient utilization in crop production.
Another value of manure is to increase the aggregate stability in the soil, which means soil particles are more stable and don’t break apart under wet conditions. The end result is a soil that maintains its infiltration rate under intense rainfall events and is more stable under wet conditions so that it doesn’t erode as quickly.
With the increase in more intense rainfall events in the spring, these changes create soils that are more resilient to extreme events. Less erosion translates into increased soil value and less pollution in streams, rivers and lakes.
If these effects could be more accurately quantified, producers could have a more realistic view of the value of manure as a soil amendment and nutrient source. While this challenge remains, there is ample evidence that the value of manure on the long-term sustainability of the soil resource needs to be considered in the use of manure from animal production systems.
Capturing the value of manure is a multifaceted process. Manure has value in providing:
1. Nutrients for crop production,
2. Components for enhancing and maintaining soil health,
3. Potential energy sources through generation of biofuels, and
4. A raw source of nutrients and carbon for a new generation of biotic fertilizers, including composting.
To truly capture the value of manure, we need to begin to consider an overall production system in which we balance the value of manure as a resource against the potential environmental impacts of water, soil and air quality metrics.
This balancing approach should not be isolated to manure but instead should be applied to all aspects of agriculture. Such an approach would require agricultural production systems to simultaneously evaluate production and environmental impacts in the context of long-term sustainability.
The value-added opportunities using manure include biofuel production, such as methane generation or composting.
Composting involves mixing manure with additional carbon sources to create a balanced product that can be applied to the soil.
It is possible to use solid or liquid manure in composting by mixing it with a carbon source such as straw, corn stalks or landscape trimmings. For each pound of manure, add 1 lb. of a carbon source to the mixture. The final product supplies nutrients that produce the same yield as inorganic fertilizers, with the added value of improving soil health.
Composting reduces the volume of the manure and can be utilized in any size of production unit. In addition, composting inactivates pathogenic organisms. It can also be sold as a value-added product.
Biotic fertilizers are emerging as a form of more controlled composting, with nutrients added to create a fertilizer designed to enhance and promote the soil biological system. This technology has yet to be applied to hog manure, but it has been successful with poultry manure, and it offers another method to enhance the value of the manure in soils and provide nutrients for plant growth.
Development of biofuels has researchers evaluating the potential for methane production from manure through anaerobic digestion.
With anaerobic digesters, the system controls and captures the methane generated and releases it as a potentially usable product.
In general, methane generation from hog manure is a net of 70 Btu/hr/150-lb. pig. To supply the energy requirements of a 1,500-sq.-ft. home would require the manure from over 500 pigs.
One of the disadvantages of methane generated from manure is the lack of purity. Therefore, the methane requires conversion technology capable of handling “dirty” sources of gas vs. more common clean sources.
Still, generating methane from a more controlled system of handling manure provides a way to add value through biofuels.
An aspect not often considered is that the nutrient value of the manure source for crop production has not decreased. The nitrogen, phosphorus and potassium are still available. Pursuit of biofuels from manure management systems offers promise to help increase the value of manure through capture of the gases from the digestion process with subsequent utilization of the nutrients to meet crop requirements.
Animal production operations continue to be subject to environmental regulations to ensure their environmental impact is limited. These regulations primarily focus on the Clean Water Act to prevent discharge from production facilities into nearby bodies of water. The regulations apply to all manure, litter and wastewater generated from concentrated animal feeding operations (CAFOs).
Energy production from animal production systems provides a potential for adding value to manure and will require an additional infrastructure for equipment.
CAFOs continue to come under greater scrutiny under the assumption that the environmental impact is more significant from concentrations of animals compared to smaller facilities. However, air emissions or water quality impacts have to be evaluated on a per-animal-unit basis, and these studies are lacking.
Manure application to the land is covered under manure management plans, which vary by state. Generally, the plans are designed to prevent runoff from fields into nearby bodies of water and to reduce odor following manure application.
The study at the National Laboratory for Agriculture and the Environment, conducted to evaluate the impact of manure applications following an approved manure management plan, showed there was complete utilization of the manure nutrients by the crop with no residual amounts remaining in the soil profile at the end of the growing season. Analysis of soil test records from fields with manure applied at recommended nutrient rates for over 10 years showed there was no increase in the phosphorus levels in the soil. There is a need for more of these studies to document the changes in soil nutrient status.
Animal production operations will continue to be considered as potential environmental liabilities because of isolated incidents leading to environmental problems. In the future, application of the Clean Water Act and Clean Air Act to animal operations will become more commonplace. Animal operations will be required to provide more documentation about the management and utilization of manure.
This change alone will increase the need for enhanced documentation of manure-handling practices. However, documenting the manure handling and utilization process will open up new value-added opportunities and help reduce the potential environmental impact. For example, composting can reduce the volume of manure. Incorporating the compost after application maximizes the nutrient captured in the soil and minimizes the potential for runoff.
Another concern often expressed by producers is how to handle manure application in conservation tillage or cover crop systems where incorporation is not possible without disrupting the surface. Development of application equipment suited to this type of surface cover application would further increase the potential for manure utilization in these farming systems. It would also allow manure to be used in a wider range of conservation practices and cover crops designed to prevent surface runoff.
Tools to Boost Value
Support tools designed to help producers capture more value from manure, while minimizing environmental impact, are being developed.
Weather plays a huge role in determining the final impact of the manure application process. For example, manure applied to wet soil is more likely to run off with a moderate to heavy rain within one or two days after application. Developing guidelines for producers to consider, such as soil moisture and short-term (1-2 day) precipitation forecasts, would provide guidance to minimize the chance of runoff and ensure the nutrient value of manure is retained in the soil.
These tools would also provide information and documentation needed to address legal challenges about application times, should they arise. Legal challenges associated with manure management and application likely will not decline.
There are emerging technologies that will allow the value of manure as a nutrient source to be captured while eliminating potential water and air quality issues. These technologies have to be perfected, but they are achievable and will be cost-effective as manure values continue to increase.
An existing method of increasing the value of manure is to develop a comprehensive manure management plan that addresses a variety of environmental quality concerns and impact assessments. The value of this exercise provides a thorough evaluation of the overall production system and ultimately will provide a framework for training, assessment and documentation of the animal production system.
The development of ISO14000 type of standards and the documentation of this process may provide a way of adding value to manure management. Development and implementation of methods to document manure application procedures and amounts, capable of withstanding an auditing process, provides evidence that the manure management plan is being implemented.
There are no easy solutions to manure management. A manure management plan that considers all aspects of the manure value chain will provide valuable insights into where the value of manure can be captured as part of the hog production enterprise.
We have the opportunity in hog production systems to showcase the fact that these enterprises are prime examples of a green technology and each component has value.
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