Fundamentals of compost barn management
Published in Dairy Star November 25, 2006
We have been working with the concept of the compost dairy barn for a few years now and it seems that we learn something new just about every day. That might be an exaggeration, but we have learned a few things that seem to be cornerstones of good bedded pack management.
Research is ongoing at the University of Minnesota and that data will be published as time goes on. However, we have been able to establish that good quality, dry sawdust still works the best for a bedding material. Many producers have tried alternative materials such as corn stalks and various types of straw. The corn stalks hold water up to a certain point and then the cell walls deteriorate and release the water and create a real mess. Some producers have tried sawdust from cedar trees which did not work at all because of the material in the wood that inhibits the growth of bacteria. Research is being conducted to look at other possibilities such as soybean residue in the mix to reduce the expense of using pure sawdust.
So what are the fundamentals that we know at this point? What are some of the things that we know to work consistently in all barns? Following are some observations.
Particle size and type of material
Dry fine wood shavings or sawdust seem to do the best job. Wood products have significant amounts of lignin which resists microbial breakdown and lasts longer. The fine material has more surface area and is conducive to good tilling. Wood chips or material with large chunks of wood does not work well at all. We have to be careful about non-biodegradable material in the sawdust such as plastic and metal fragments. These do not break down and can cause problems at cleanout. Small particle size also breaks down easier and is ready to be field applied and serve as an excellent source of nutrients for growing crops.
We know that good tilling of the pack is important for a variety of reasons. First of all, tilling incorporates oxygen into the back and enhances microbial activity. One mistake to avoid is waiting too long before tilling the pack after a fresh load of sawdust. Current recommendations are to maybe wait a day or so after getting a fresh load to begin tillage. Many producers like the way the barn looks with new sawdust on top, but tilling and bringing up pack material from further down in the pack is what makes the system work. Once tilling does start, a twice-per-day program seems to work the best. Going to once a day incorporation or even less leads to dirty cows and less microbial activity in the pack. Tilling also reduces compaction and provides a comfortable resting area for the cows. We recommend stirring to a depth of 10 to 12 inches for best results. Depth of tillage is critical because oxygen is needed in the pack to get the composting action going. The barns where we saw the best heating often had good, deep tillage with a good mixing action.
Barn ventilation and location
In the warm months adequate ventilation is needed to remove cow heat and moisture as well as the heat and moisture generated by the biologically active pack. Sufficient air exchange in cold weather is needed to remove moisture from the pack and extend time between bedding addition. If your compost barn is naturally ventilated, location becomes very important. Build the barn in an open area where summer winds can blow through the structure. Current recommendations show that a 16 foot sidewall is needed to allow for a four foot concrete wall to hold the composted area. The open area above the wall allows for good aeration and space for cleaning and incorporation.
2006 manure test results
We conducted manure tests on a number of barns to see what was in the manure at cleanout. On the average, we are seeing an analysis of 22-7-15 representing N-P-K in the manure profile. There was some variability in the tests depending on the ration that the cows were consuming. For example, a high alfalfa forage diet seemed to show more nitrogen in the manure analysis. Since nitrogen is a building block of protein, this may be telling us that some protein is going through the cow and showing up in the manure as nitrogen. However, in some of the barns with lower nitrogen numbers, we may have seen some of the nitrogen lost to the atmosphere because of inferior bedding materials such as sawdust that contained adhesives or larger particle size. Another question that we were attempting to answer concerns carbon-nitrogen (C/N) ratios. Raw sawdust has a C/N ratio of 400/1. If this was applied to a field that was going to be planted to corn, we would see a lot of yellow corn as the raw sawdust tied up much of the nitrogen as it broke down in the soil. We need to see that ratio around 30/1 to work well as a fertilizer source in the soil profile. Manure from the barns that we sampled this fall showed C/N ratios averaging around 19/1. This indicates that the sawdust was fully decomposed and ready for crop uptake. There are remaining questions that we will be conducting research on to come up with more conclusive answers.
Compost dairy barns are an excellent choice for housing dairy cows because of the excellent cow comfort afforded by these facilities. Sawdust prices have climbed in recent years and currently stand at around $900 for a semi load. However, a recent analysis shows that amounts to around $.55 per cow per day. Even with ten dollar milk, this amounts to about five pounds of milk to pay for the bedding and the kind of cow comfort that we do not get in other facilities.