Critical Management Point: Manure Composting

Critical Management Point: Manure Composting

Transcript (with additional commentary)

What do farmers do with the manure their cattle produce? Barns and milking facilities are cleaned frequently.  The manure and water used to wash down the floor can go into a lagoon, from which is can then be sprayed on surrounding fields according to the farm’s nutrient management plan.

Some farms are able to collect manure and use it to build compost piles.  When compost piles heat up, excess nutrients are assimilated and pathogens are killed.  The compost can then be spread on fields or smaller garden plots to enrich the soil.

Composting is a process of breaking down animal manure and plant material into organic matter that provides usable nutrients, enhances soil water holding capacity, and builds soil structure. Composting is used in agriculture to accelerate the same process nature uses to decompose materials.

There are several important factors to consider in a controlled compost system. The bacteria and fungi that accelerate composting require specific ratios of carbon and nitrogen to grow and reproduce.  So, having different sources of carbon and nitrogen, like that found in plant matter and manure, is important so the microbes can decompose them. Optimal water, oxygen, and specific temperature ranges are also crucial for microbial decomposition to occur.

The FDA’s Food Safety Modernization Act of 2013 provides “standards for the safe growing, harvesting, packing, and holding of fruits and vegetables grown for human consumption.” The guidelines recommend two strategies for composting – static and dynamic composting.

A static compost pile is not turned, so various methods are used to ensure appropriate oxygenation occurs. Since there is no physical turning, this method requires careful monitoring to ensure that the outside of the pile heats up as much as the core of the pile, and that a target temperature of at least 55°C is reached.

Dynamic composting–also called aerated or turned composting–involves forming organic waste into rows of long piles, called “windrows”, and aerating them periodically by either manually or mechanically turning the piles. Turning helps supply more oxygen and provide consistent moisture, thereby generating more heat and obtaining the target temperature of 55°C.
Composting has previously been examined by Dr Pruden and others as an approach to reduce antibiotics and certain antibiotic resistance genes in manure. The findings suggest that maximum temperature and duration of that high temperature are critical factors, but the ideal parameters have yet to be determined, especially with respect to a broad range of antibiotic resistance genes corresponding to antibiotics relevant to human health.

[Amy Pruden, Department of Civil and Environmental Engineering, Virginia Tech]:

“So here the question is, ok even if we have manure with antibiotic resistant bacteria and resistance genes in it, maybe that’s ok, if you compost it maybe those levels go down.”

In this research, manure generated from the previous dairy and beef cattle experiments were composted using static and dynamic small-scale composters.

Specifically, static composts were maintained under aerobic conditions to maintain a target temperature of 55°C for 3 days.

For dynamic compost, fresh manure was mixed with small particles of alfalfa hay and hardwood chips and turned 5 times to maintain aerobic conditions and a target temperature of 55°C for 15 days.

During the composting process, the temperature and pH were monitored and analyzed for levels of antibiotics, antibiotic resistant bacteria, and antibiotic resistance genes.

The Kirby Bauer method was used to determine which bacteria are resistant to which antibiotics.  To do this method, paper discs impregnated with different antibiotics are placed onto the surface of agar plates that have been inoculated with bacteria taken from the original plates.  The antibiotics diffuse into the agar and the plates are incubated overnight and then read.  Bacterial colonies are visible on the surface of the agar.  If the antibiotics stop the bacteria from growing, there will be a clear zone around the disc.  Many factors influence the size of this inhibition zone, but generally if the bacteria continue to grow and are visible around the disc, they are not influenced by the antibiotic and are resistant.

This study will provide guidance for livestock producers and produce growers on which manure composting method best reduces the load of antibiotics, antibiotic resistant bacteria, and antibiotic resistance genes.

Glossary of Terms

  • Antibiotic resistance – Antibiotic resistance is the capability of some bacteria to survive antibiotic treatments. Some bacteria are intrinsically resistant, for example an antibiotic that specifically targets Gram positive bacteria will not affect Gram negative bacteria. However, bacteria can also acquire resistance, either through mutation or through sharing of antibiotic resistance genes, in which much higher doses of the antibiotic are needed to have the desired effect.
  • Antibiotic resistance genes – Pieces of DNA carried by bacteria that encode functions that allow bacteria to survive and grow in the presence of antibiotics.
  • Antibiotic resistant bacteria – Bacteria that are not controlled or killed by an antibiotic with a spectrum that includes that type of bacteria.  An antibiotic spectrum is the range of bacteria susceptible to that drug. (CDC, Antibiotic Resistance, )
  • Dynamic composting – Dynamic or turned composting consists of placing the mixture of raw materials in long narrow piles called windrows that are agitated or turned and watered on a regular basis. The turning operation mixes the composting materials and enhances passive aeration. (FAO, Large scale composting,
  • Inhibition zone – In the Kirby Bauer method, if an antibiotic stops the bacteria from growing or kills the bacteria, there will be an area around the disc where the bacteria have not grown enough to be visible. This is called a zone of inhibition. The size of this zone can be measured to determine if the bacterium is resistant to antibiotics. (Agar diffusion test, )
  • Kirby Bauer method – A laboratory method to test antibiotic sensitivity of bacteria.
  • Static composting – Piles or rows of compost are not actively turned. Air can be supplied to the composting materials through perforated pipes embedded in each row, by a blower, or by keeping the piles smaller. Because the raw materials are not turned after the rows or piles are formed, they must be mixed thoroughly beforehand. (FAO, Large scale composting, )


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