May 27, 2009 | General

Pathogen Reduction By Complete Mix Dairy Digesters

BioCycle May 2009, Vol. 50, No. 5, p. 43
Study at four dairies using digested manure solids as cow bedding found a 99 percent reduction in fecal coliforms after anaerobic digestion.
Geoff Teigen and Mark Moser

DAIRY operators continually strive to reach and maintain peak milk production in their herds by keeping the cows healthy and comfortable. These objectives go hand in hand with respect to cow bedding choices. Proper bedding, while providing cows with comfort, should also minimize the potential of pathogen transmission.
Bedding material varies in both cost and availability by location. Wood shavings, sand, digested and separated dried manure solids (DMS), and lime are the most commonly used varieties. Wood shavings can be very expensive and hard to obtain in some regions. Sand is similar in this respect but is often favored because, when fresh, it provides no nutrients for pathogens to multiply. DMS is frequently used due to its cost (often free) and availability.
It may seem counterproductive to strive for cleanliness while at the same time bedding with DMS. However dairies with anaerobic digesters regularly take advantage of this abundant resource with much success. Anaerobic digestion of dairy manure has been repeatedly shown to reduce pathogen levels by over 99 percent. Studies, including a 2008 report by the Cornell Waste Management Institute, “Using Manure Solids as Bedding” (prepared for New York State Energy Research and Development Authority), have shown no correlation between mastitis and the use of DMS.

Mastitis is a general term for an inflammation (not an infection necessarily) in the udders of dairy cows and other mammals. It may be caused by any of a number of different pathogens including bacteria, fungi or physical trauma to the specific area. Every dairy contains some level of mastitis at any one point in time. However, most cases are subclinical and not outwardly apparent or overly detrimental to the cow’s production capabilities (although there are clinical cases where reactive strategies are implemented to limit and reduce its spread throughout the herd).
The primary concern with mastitis is an associated loss of milk production when the frequency of occurrence in a herd becomes too high. As this occurs, there typically is a linear loss in production throughout the herd. A reduction in milk quality also may occur and thereby lower its market value relative to milk from otherwise healthy dairies. Dairies tend to operate within tight margins, therefore regular precautions are taken to restrict the incidence of mastitis throughout the herd.
Mastitis is difficult to detect directly as no one pathogen is responsible for the majority of cases. However, it can be accurately tracked indirectly through the monitoring of somatic cell count (SCC) – a quantitative measure of the number of leukocytes in a specific volume of milk (usually in ml). Leukocytes are a type of cell found naturally in animals that play a major role in the immune response. The SCC method has been adopted to track and maintain mastitis at manageable levels in most herds. It has been shown that each doubling of SCC may result in a 5 percent decrease in milk production, presenting an obvious problem when extrapolated over the entire herd.
A number of potentially pathogenic organisms are relatively ubiquitous in dairy manure and may result in elevated SCC. Escherichia coli, Streptococcus, Klebsiella, Enterobacte, Salmonella, Staphylcoccus and Mycobacterium avium paratubercu-losis (MAP or Johnes disease) are some of the more common groups under frequent observation.
Maintaining low SCC counts essentially revolves around keeping the cows clean and comfortable. With respect to mastitis, a key is to restrict the transmission of pathogens through the teats. A major area of concern for such passage is in the freestall beds, where the cows spend more than half of their time.
Dairy cows, regardless of bed type, will get a percentage of manure on their beds either through their hooves or directly from the source. This manure provides a haven and rich nutrient source for pathogens to rapidly reproduce. Therefore, locating a clean and cost-effective bedding material and regularly replacing aged material is a key to the long-term health of the herd.

Digested DMS from complete mix digesters provides a clean and cost-effective bedding material. Digester mixing occurs via mechanical agitation to produce a homogenous mixture. RCM International undertook a study to determine the effectiveness of pathogen reduction during mesophilic anaerobic digestion of dairy manure in four complete mix digesters. The digesters studied each contained established populations of microbes and had been operating anywhere from 1 to 4 years. Three of the dairies (Reinford Family Farms, Wanner’s Pride-n-Joy, and Brubaker Farms) are in Pennsylvania; Patterson Farms is in New York.
At each farm, samples were taken of raw manure before the digester (influent) and of separated digested solids. Analyses were run to determine the presence of fecal coliforms in each of the samples. Results, shown in Table 1, are reported in colony forming units per gram (CFU/g).
The results fall in line with pathogen levels found in other common bedding materials (prior to use). Furthermore, it should be noted that 50,000 CFU/g is the upper limit for accuracy in this particular test and therefore the actual concentration prior to anaerobic digestion is undoubtedly higher. A quick review of the literature strongly suggests a value well over 500,000 CFU/g. A study by Cornell University researchers (“Reduction of Selected Pathogens in Anaerobic Digestion,” presented at a 2003 conference) measured over 3.5 million CFU/g in raw manure during an analysis of pathogen destruction in a New York dairy digester. This number appears to be well within the expected range of contamination.
While materials before and after digestion show such a great difference in terms of pathogen counts, there is another point worth considering. The 2008 study by the Cornell Waste Management Institute noted almost identical bacterial levels in all bedding types after a couple days of use. Sand bedding, while starting out slightly cleaner, showed significantly higher levels of some bacteria after the same amount of use. Their study indicates that any significant differences in pathogen levels correlated more with the general cleanliness of the stalls rather than the bedding type used. Physical characteristics of the bedding may also play a larger role in transmission of mastitis than pathogen content. By continuing a regular bedding replacement regime, a dairy can avoid the pitfalls of soiled bedding.
All four dairies in the RCM study use DMS for bedding and all report excellent results and overall cow comfort. There have been no spikes in SCC levels outside of normal ranges thereby allowing dairy operators to adopt the practice with peace of mind. In addition, many farms attribute reduced hock lesions to DMS bedding. More farms will continue to turn toward such sustainable strategies as additional data and time prove their relative value for the farms.
Mark Moser is founder and President of RCM International LLC. Geoff Teigen is an Environmental Scientist with RCM.

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