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Compost dairy barns: The carbon/nitrogen ratio

Wayne Schoper

Published in Dairy Star December 28, 2007

Compost dairy barns, otherwise known as sawdust or bedded pack barns, have been around for a few years now. They have proven to be a good choice for many producers as they have solved many cow comfort problems that have plagued conventional freestall barns for years. The producers that have these barns have been challenged by increased costs of bedding materials, namely sawdust. However, prices have stabilized for now and current costs are around $0.65 per cow per day. Producers have really appreciated the easy daily maintenance and cleanout of these barns. Since these barns also serve as the containment system, they can also save the cost of constructing a separate manure storage system.

The University of Minnesota has recently conducted research on the utilization of manure from these barns. The use of the word "compost" is a bit of a misnomer, as the barns really do not do a complete job of composting. Temperatures in the pack have tended to vary widely and generally do not reach true composting temperatures. The material that comes out of these barns is usually an earthen colored material that has a consistent moisture content of around 60%. These facilities are usually cleaned out in late fall and the material applied to fields that will be planted to corn during the next cropping season. We have tested the nutrient levels of manure from these barns and have been able to make preliminary application recommendations based on those test results.

One phenomenon that has shown up is the presence of yellow corn during the early part of the growing season on some fields, indicating a shortage of nitrogen. We do know that sawdust and other organic material can immobilize nitrogen for a period of time as it breaks down in the soil. The course of decomposition of organic matter is affected by the presence of carbon and nitrogen. The carbon/nitrogen (C/N) ratio represents the relative proportion of the two elements. A material for example, having 25 times as much carbon as nitrogen is said to have a C/N ratio of 25:1, or more simply, a C/N ratio of 25. The ratio of available carbon to available nitrogen is the most important relationship because there may be some carbon present so resistant to biological attack that its presence is not significant.

Organisms that decompose organic matter use carbon as a source of energy and nitrogen for building proteins and enzymes. They need more carbon than nitrogen, in part because they respire carbon dioxide as they 'burn' these carbon foods. However, if there is too much carbon, decomposition slows when the nitrogen is used up by the micro-organisms, leaving little for the crop. Raw sawdust has a C/N ratio ranging from 100 to 500. If we incorporated a few tons of this sawdust into a field and planted it to corn, the whole field would turn yellow until the soil micro-organisms could catch up. Other organisms in the soil will form new cell material using their stored nitrogen. In the process, more carbon is burned. Thus, the amount of carbon is reduced while nitrogen is recycled and becomes available.

Organic matter needs to have a C/N ratio of less than 30 to avoid tying up nitrogen in the soil nutrient profile. Generally speaking, a C/N ratio of 20, where C and N are the available quantities, is the upper limit at which there is little risk of tying up plant-available nitrogen. Samples from bedded pack barns have had C/N ratios ranging from about 10 to 20. Theoretically, these materials should not tie up nitrogen, but evidence from the field and laboratory show that some compost dairy barn manures immobilize soil nitrogen for one to two months. We do not know why this occurs yet, but farmers should be aware that some of the material coming out of these barns may tie up nitrogen in the soil profile for a period of time as it breaks down.

How do we combat this situation? First of all, any manure, especially bedded pack manure, needs to be incorporated as soon as possible. This preserves the ammonium portion of the nitrogen that is quickly lost to the atmosphere if left on the soil surface. This form of nitrogen needs no further conversion and is ready to be used by the crop. Low rates of application based on the crop need for phosphorus will decrease the effect of the manure on nitrogen supply, and fertilizer nitrogen can make up the difference. Also, manure spreaders should be calibrated and manure should be tested to determine how many pounds of nutrients have been applied per acre. Then it is important to use this information to make an informed decision about total fertilizer needs and application rates for the fields.

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