Compost dairy barns: What did the bedding analysis tell us?
Published in Dairy Star September 9, 2006
Last summer, my graduate student, Abby Barberg, and I conducted a descriptive study on 12 compost dairy barns in Minnesota. One of the aspects we analyzed was the bedding. The compost barns in this study were generally bedded with dry fine wood shavings or sawdust. According to the questionnaire responses, a semi-load of bedding was added every one to five weeks, varying by season, weather conditions and cow density. Fresh bedding was added when the bedding particles became moist enough to adhere to the cows.
Each compost barn pack was subdivided into 12 equal areas from which 1-quart bedding samples were collected. Bedding samples were taken at two depths in each area with a hand-operated soil auger. Bedding samples were analyzed for moisture, ammonia, pH, total Carbon (C), Nitrogen (N), Phosphorus (P), Potassium (K), and electrical conductance (soluble salts) concentrations.
The moisture content of the bedding material ranged from 28.0% to 78.9% across the two depths in all barns. The average moisture content was 54.4%, within the recommended range for composting of 50% to 60% moisture. The moisture content varied based upon time since the last addition of fresh bedding, weather and cow density in the sampling area. The average pH was 8.5, slightly above the recommended pH level for composting of 6.5 to 8.0.
The average total N of the bedding material in all compost barns was 2.54% with a range of 0.57% to 4.22%; the average P was 3,247 parts per million (ppm) with a range of 378 to 6,668 ppm; and the average K was 15,270 ppm with a range of 2,568 to 29,570 ppm. These compare to typical manure of lactating dairy cattle consisting of 5.1% N, 8,764 ppm of P, and 11,573 ppm of K. Sawdust, on average, has an N content of 0.24%. The average carbon to nitrogen (C:N) ratio of all barns and depths was 19.5:1, which is below the preferred range of 25:1 to 30:1 for composting. A C:N ratio below 25:1 may emit ammonia odor, which may influence the ammonia levels in the compost barns. The ammonia-nitrogen concentrations were greater deeper into the pack than in the top 6 inches. One barn seemed to have poorer air quality, possibly higher ammonia levels, compared to the other barns based on subjective evaluation. This barn had the lowest space per cow and poor air movement. Electrical conductance averaged 9.6 mmhols/cm (millimhols per centimeter -- a measure of electrical conductivity used to estimate the amount of soluble salts) across depths, which is only slightly below the 10 mmhols/cm maximum concentration desired for composting.
Bedding analysis on 12 farms using a compost bedded pack dairy barn
|Bedding temp (° F)||108||76–138||130–150|
|Electrical conductance (mmhols/cm)||9.6||2.4–20.5||10 maximum|
Samples were collected immediately prior to milking time and analyzed for pathogenic mastitis-causing bacteria concentrations. The total bacteria concentrations averaged 9,122,700 cfu/cc (colony forming units per cubic centimeter -- measures bacteria counts in bedding) with a range of 2,035,562 to 22,562,604 cfu/cc. Sawdust bedding and other organic bedding materials often contain greater than 1,000,000 cfu/cc of coliform bacteria when used as bedding for dairy cattle. This level is known to cause an infection. The high concentrations of bacteria found in compost barns emphasize the importance of excellent cow preparation procedures at milking time. The average DHIA SCC for all farms was 325,000 cells/ml, and the range was 88,000 to 658,000 cells/ml.
Pack temperatures were taken twice, one week apart, at each of 12 locations across the pack, at various depths (6, 12, 24 and 36 inches) with a compost thermometer. The average bedding temperature was 108°F, with a range of 76 to 138°F. The pack surface temperatures were similar to the ambient temperature. Temperatures tended to be lower on three farms that utilized larger particle wood shavings rather than sawdust for bedding. Temperatures were greater in the areas of the pack that were fluffier, that were not as heavily soiled or packed by the cows. The ideal temperature for composting is between 130°C and 150°F.
One concern expressed by some producers and their veterinarians was the presence of dust, which could cause eye irritation or pneumonia. We also are wondering about the air quality in these barns and we plan on conducting follow-up studies that will include air quality analysis.
For the producers we interviewed, bedding availability was their main concern. Most producers inquired about what other sources of bedding besides sawdust could be used. Our compost dairy barn team has started a follow-up study to investigate what other materials could work.
Results so far have shown that compost barns can be a very good alternative for housing dairy cows. Like any system, they require optimum management to work properly. Special attention should be given to cow preparation procedures at milking time and maintenance of minimum space per cow (ideally more than 85 sq. ft/cow) to avoid high levels of somatic cell counts.
We thank the Minnesota Department of Agriculture Sustainable Agriculture Grant Program for partial financial support of this study.
Cows enjoying the comfort of a compost bedded pack barn