Compost dairy barns – A closer look
By Wayne Schoper
By Wayne Schoper
Dairy producers have a wide
variety of options to choose from when they are making decisions
regarding the housing needs of their dairy herds.
Dairy producers have a wide variety of options to choose from when they are making decisions regarding the housing needs of their dairy herds.
One housing option that has been popular for the past several years is the compost dairy barn. In reality, the barn consists of a bedded pack on which the cows rest and ruminate. The barn construction is usually a loose housing constructed unit with the resting area for the cows left open. Bedding is then added and the cows alternately rest on the pack or eat feed that is placed in a feed manger that runs the length of the barn. The cows only leave the barn to be milked.
What sets these barns apart is the fact that sawdust is used as the primary bedding material. Sawdust has many unique characteristics that make it an excellent bedding material. It absorbs both urine and manure and holds it until the material is applied to fields for crop production purposes. Other bedding, materials such as corn stalks, will work for awhile but then break down and release much of the moisture that they have been holding. Sawdust absorbs moisture up until the material is around 60 percent water and then more sawdust must be added to keep the pack fresh and clean.
The following are some observations and fundamentals of compost dairy barn management.
Particle size and type of material
Dry fine wood shavings or sawdust seems to do the best job. Wood products have significant amounts of lignin, which resists microbial breakdown and lasts longer that other bedding materials. Fine grained material has more surface area and is conducive to good tilling. Wood chips or sawdust with large chunks of wood does not do well at all. We also have to be careful that the sawdust does not contain non-biodegradable material such as plastic or metal fragments. These will not break down and can cause problems at cleanout. Fine grained sawdust breaks down faster and is ready to be field applied and serve as an excellent source of nutrients for growing crops.
One of the keys to making these barns work the way that they should is twice-daily tilling of the bedded pack surface. We know that good tilling is important for a variety of reasons. First of all, tilling incorporates all of the manure and urine that has accumulated on the pack surface. Tilling also incorporates oxygen into the pack and enhances microbial activity, which is vital to composting. The barn is started with a fresh load of sawdust spread on the packed clay floor of the barn. The cows are allowed to roam and rest on the pack for a day or two before tilling and stirring begin. Once the tilling has started, we recommend that tilling should be happening twice a day while the cows are out of the barn being milked. Many producers like the way that the surface of the pack looks when it has fresh sawdust on it. However, it is the stirring and mixing that happens when the pack is tilled that brings up material from further down in the pack that helps get the bacteria working and gets the barn heating and breaking down the sawdust. Some producers have tried tilling just once per day and have ended up with dirty cows and less microbial activity in the pack. The tilling operation itself only takes a few minutes per day and leaves behind a nice comfortable surface for cows to lie on. We recommend tilling to a depth of 10 to 12 inches for best results. Depth of tillage is critical to introduce oxygen into the pack to aid in the composting process.
Barn ventilation and location
In the warm months adequate ventilation is needed to remove cow heat as well as the heat and moisture produced by the biologically active pack. Sufficient air exchange in cold weather is needed to remove moisture from the pack keep a
dry resting surface. Current recommendations show that a 16 foot sidewall is needed on top of a four foot concrete wall to hold the composted area. The open area above the pack is necessary for good aeration and space for cleaning and incorporation.
Manure test results
Over the past several years we have sampled the material coming out of these barns. On the average, we are seeing an analysis of 22-7-15 representing N, P, and K coming out of the manure profile. There has been some variability in the tests depending on the ration that the cows were consuming. For example, cows consuming a high alfalfa diet excreted more nitrogen into the manure pack. A characteristic of sawdust bedding is that it seems to hold nutrients such as Nitrogen very well until it is applied to the corn field. We also have noticed that we need to pay attention to the carbon-nitrogen (C/N) ratio when working with sawdust bedding materials. Raw sawdust has a C/N ratio of 400/1 and when applied as a fertilizer to corn fields will tie up nitrogen for a long period of time thus severely restricting nitrogen availability to the growing corn crop. When we clean out the compost dairy barns we find that the material coming out of these barns typically has a C/N ratio of under 20:1 and is ready to serve as an excellent source of nutrients to the growing corn crop.
Wayne Schoper is a University of Minnesota Extension educator for Brown and Nicollet counties.