LOOKING FOR PROFITS IN MILK QUALITY

LOOKING FOR PROFITS IN MILK QUALITY Richard L. Wallace TAKE HOME MESSAGES Begin monitoring milk quality practices by recording bulk tank data, DHIA somatic cell count (SCC) information, and clinical mastitis treatments. High bulk tank bacteria counts usually indicate improper milking machine sanitation or poor teat hygiene prior to milking unit attachment. Elevated bulk tank SCC indicates herd level infection status with mastitis pathogens. Keep track of clinical mastitis episodes. About 88% of the losses associated with each case of clinical mastitis are due to discarded milk and decreased milk production. INTRODUCTION Many dairy producers have implemented the NMC ten step approach to mastitis control. The NMC program includes the three basic principles of mastitis control: elimination of existing infections, prevention of new intramammary infections (IMI) and monitoring infection status. Several steps deal with elimination of existing infections. These steps involve effective dry cow management (Step 3), appropriate treatment of clinical mastitis during lactation (Step 4), and culling chronically infected cows (Step 5). Prevention practices are found elsewhere. Steps 1 and 2 (proper maintenance and use of milking equipment, and employment of proper milking procedure) deal with preventing new IMI during the milking process. Step 7 (maintenance of a clean environment) deals with controlling new infections between milking times. Segregating infected cows, screening herd replacements and improving cow resistance factors are other preventive measures mentioned within the first seven steps. Monitoring procedures are found in Steps 6, 8, 9, and 10. Good record keeping, regular evaluations of udder health status, periodic review of the mastitis control program and goal setting for udder health status are the essential ingredients of these steps. The monitoring process is commonly the weakest link in most mastitis control programs. Yet, the success of any control program rests on the ability and desire to monitor progress. MONITORING MILK QUALITY Monitoring milk quality is essential for production of a safe food product that meets consumer acceptance standards. Specific traits such as milk composition, flavors and odors, bacterial content, somatic cell count (SCC), and drug residues are important to maintain consumer demand for dairy products. Most milk processors pay additional premiums for high quality milk. Break points are different between processors, but in this case, lower are better. Federal milk order pricing also includes a small bonus for low somatic cell count milk. Goals for quality milk are presented in Table 1. 52

MONITORING BACTERIAL CONTENT Monitoring bacteria in milk is routinely performed by regulatory agencies through dairy cooperatives and milk processing plants. Although these procedures are quantitative in nature, some attempt is made to differentiate between psychrotrophic and thermoduric bacteria. Psychrotrophic bacteria are cold-enduring organisms. They secrete enzymes that are proteolytic and lipolytic. These organisms cause spoilage in refrigerated milk and reduce shelf life. Pseudomonas is the most common of the non-pathogenic bacteria found, with Enterobacter, Acinetobacter, and Flavobacterium also isolated. Pathogenic psychrotrophic bacteria include Yersinia enterocolytica and Listeria monocytogenes. These organisms have been implicated in food-borne disease outbreaks. Most psychrotrophic bacteria are readily destroyed by pasteurization but their proteolytic and lipolytic enzymes can remain active. The most common sources of these organisms are water, soil, contaminated teat ends and milking equipment, and post-pasteurization contamination. Control measures include proper teat preparation, cleaning and sanitizing milk contact surfaces on farm and at the processing plant, appropriate temperature controls, and prevention of contamination from environmental sources from the farm to the store shelves. Thermoduric bacteria are proteolytic organisms from the genera Bacillus and Clostridium. These bacteria are capable of surviving pasteurization temperatures but their enzymes are destroyed in the process. These organisms are found when poor sanitary practices are employed. Mastitis pathogens can also be found in milk, but unlike psychrotrophic bacteria, they only grow at near body temperatures. In general, when bulk tank milk shipped from dairy farms contains environmental mastitis pathogens the source is not from the milk but rather from improper sanitation. Milk processors can use several quantitative methods to evaluate raw milk. The particular objectives of these procedures are to indicate the total number of organisms per milliliter of raw milk in order to reflect the level of sanitation, storage, and handling practices performed at the farm level. The Standard Plate Count (SPC) is the accepted regulatory test. With the dilution factor considered, the SPC is reported as bacteria/ml of raw milk. The Grade A regulations requires that on-farm milk contain less than 100,000 bacteria/ml. Most producers should be able to maintain levels below 5,000 to 10,000 bacteria/ml. Preliminary Incubation (PI) count is designed to encourage growth of psychrotrophic bacteria. After the raw milk has been incubated for 18 hours at 55F, another SPC is performed. The PI count cannot be interpreted accurately without a concurrent SPC. The ratio of bacteria detected before and after incubation is closely associated with milking system sanitation. The Laboratory Pasteurization Count (LPC) helps detect the level of thermoduric bacteria in raw farm milk. Initially the milk sample is pasteurized at 145F for 30 minutes, then the SPC is performed and again the ratio of before and after is determined. Since these organisms are routinely found in the cows' environment, the LPC is considered a good indicator of farm sanitation. These regulatory tests make no attempt at qualitative evaluation of the organisms found in raw farm milk. A bulk tank culture would provide that information. 53

MONITORING SOMATIC CELLS Monitoring somatic cells can demonstrate the dairy producer's ability manage clean cows and produce quality milk. Somatic cells in milk above 50,000 per ml are an indication of injury or inflammation. Milk with elevated SCC has higher proteolytic and lipolytic activity. Plasmin, a proteolytic enzyme from serum can reduce casein levels in secreted milk. Casein is the primary protein utilized in dairy products. Lipases from white blood cells can react with milk triglycerides causing release of free fatty acids, which leads to off flavors. Several different measures of somatic cells are available both at the farm level and at the level of the milk processing laboratory. The California Mastitis Test (CMT) utilizes a reagent that causes somatic cells to rupture then forms a gel with the cellular DNA. The thickness of the gel indicates the amount of somatic cells present. A score of "zero" (no gel formation) signifies a SCC range from 0 to 100,000/ml. A score of "trace" (slight gel formation) indicates a SCC range from 200,000 to 400,000/ml. Scores of 1, 2, or 3 (distinct gel formation) are all representations of high SCC. Too much emphasis is placed on distinguishing the difference between these positive scores. It is well established that cows with SCC greater than 400,000/ml have either subclinical or clinical mastitis. For this reason, CMT scores are best interpreted as; negative, trace or positive. Most milk processing laboratories and DHIA centers use automated, electronic cell counters (ECC). The use of computerized records and ECC has greatly enhanced the ability to monitor SCC and milk quality. MONITORING UDDER HEALTH / SUBCLINICAL MASTITIS Although subclinical mastitis cannot be seen by the producer, it represents the largest single loss (65-70%) of the total mastitis cost. Evaluation of SCC and microbiologic culturing of milk are the primary monitoring methods to evaluate udder health. The availability of accurate records will determine the depth and breadth of the monitoring program. Many dairy producers are not enrolled in DHIA and the SCC program. Monitoring subclinical mastitis in these herds will be more difficult, but is not impossible. Some dairy cooperatives offer individual cow SCC data and milk culturing on an as needed basis. If the herd veterinarian is not performing in-house culturing, there are laboratories available to accomplish this task. The use of a CMT paddle can offer much information if used at milking time. MEASURES AT THE BULK TANK The most basic level of measurement of subclinical mastitis and udder health is the bulk tank somatic cell count (BTSCC). This value is reported to the dairy producer several times each month. Often milk processors can make BTSCC available from each milk pick up. This data is essential for initial evaluation of herd udder health. The legal limit for BTSCC in milk sold for Grade A purposes is 750,000/ml. Herds with BTSCC above 600,000 have approximately 80% of their cows infected and are losing between 12 and 15 % in milk production. These herds have increased incidence of clinical cases and are discarding milk due to antibiotic residues. Even at low BTSCC levels (200,000 to 299,000/ml), one third of the herd is most likely infected. Graphing these values is simple and herd level trends can be evaluated. Monthly bulk tank culturing has proven useful in monitoring udder health, particularly with regard to contagious pathogens (Staphylococcus aureus, Streptococcus agalactia and 54

Mycoplasma bovis). As stated previously, sanitation problems should be suspected if bulk tank milk contains multiple environmental pathogens (Coliforms and fecal Streptococci). The sensitivity of a single bulk tank culture for contagious organisms is fairly low, especially when the herd prevalence of contagious mastitis is low as well. In other words, often one bulk tank sample will be culture negative for S. aureus or S. agalactia even though a herd may have cows infected with one or both of these organisms. The specificity of bulk tank cultures for contagious organisms is high (94%). So it is rare that a bulk tank culture will be positive when in reality no cows in the herd have contagious mastitis. Multiple sampling will improve the sensitivity of bulk tank culturing, particularly with intermittently shedding organisms like S. aureus. Most dairy herds have every-other-day milk pick up. Serial testing can be performed by having the producer aseptically collect an agitated bulk tank sample in a sterile container. This procedure can be repeated every other day when the bulk tank contains four milkings. The samples can be frozen immediately after they are obtained and delivered to the testing facility once each month. Screening for contagious pathogens can be performed on the samples and management practices can be modified as needed. HERD LEVEL EVALUATION Individual cow data has more significance when grouped and evaluated on a herd basis. Initially the proportion of cows harboring mastitis pathogens needs to be determined (defined as prevalence). Once control measures are implemented, the monitoring process determines the number of new infections occurring (defined as incidence) and old infections resolving or eliminated. Vital to this analysis is regular milk culturing and monthly SCC data. Somatic cell count data from DHIA is conveniently grouped by parity and stage of lactation. Somatic cell score (SCS) is the logarithmic transformation of the SCC. Each doubling of the SCC increases the SCS by one score. The relationship between lost milk production and SCS becomes linear and easier to evaluate. Beginning at an SCS of 3.0 (mid-point SCC of 100,000/ml) mature cows will lose 1.5 lb of milk production per day. As the SCS increases by one log score, the loss increases by 1.5 lb/day, so that an SCS of 6.0 would be associated with a milk loss of 6.0 lb/day. Values for lost milk production in first calf heifers is half that of mature cows. At least 90% of first calf heifers should have SCS of 4.0 or less. Eighty percent of mature cows should be in the low SCS range (4.0 or less). A herd goal is 85% with low SCS. This value can be graphed over time just like the BTSCC discussed previously. Whole herd, quarter milk samples may be necessary for research purposes but are impractical for field monitoring of subclinical mastitis. Using individual cow SCS to determine which cows to culture helps streamline the process. Keep in mind that one high score may not be indicative of a chronic infection. Cows with multiple scores above 4.0 are most likely to be infected. The monthly DHIA somatic cell data is generated from a composite sample. Average SCC when one quarter is infected is 500,000/ml. When 2 or 3 quarters are infected the average SCC can reach 700,000 or 1,500,000/ml, respectively. Once cows to sample are selected, the CMT can be used to determine which quarter(s) to culture. If SCC data is unavailable then the CMT paddle can be used on the entire herd. Individual quarter samples are preferred to composite milk samples. Even under the best conditions many composite samples become contaminated. Contaminated milk samples are impossible to interpret and a waste of resources. A small (3-5 ml), sterile quarter sample is preferable to a voluminous contaminated one. Cultures should be immediately 55

chilled and if microbiologic procedures are to be delayed, the samples should be frozen. Culturing all cows and heifers at 2 to 3 weeks postpartum may be helpful. This will assess the efficacy of the dry cow program and offer assurance that reservoirs of contagious organisms are not introduced to the herd. MONITORING CLINICAL MASTITIS Of all the costs associated with clinical mastitis, 88% of the losses are due to milk withheld because of drug residues and decreased milk production. Other costs include increased labor, veterinary services, treatment expenses, culling and death loss. Typically, cows with more genetic merit are higher milk producers, are under more stress during early lactation, and are more susceptible to clinical mastitis. Cows that develop clinical mastitis are two to three times more likely to be culled. Premature culling can be costly in terms of genetic progress as well as cash flow. Availability of accurate, up-to-date farm records is essential to an effective clinical mastitis monitoring program. These records are also vital to the Milk and Dairy Beef Quality Assurance Program. Changes in the incidence of clinical mastitis can be evaluated. The total number of days that milk was discarded and an estimation of lost milk production can be extremely revealing. DHIA records can offer some useful information. The percent of cows with clinical mastitis that died or were culled will indicate the extent of any existing problems. An obvious goal for death loss due to clinical mastitis is zero. Cull rates should be below 5%. On a monthly basis, no more than 3% of the herd should be affected by mastitis. Clinical cases should be cultured to determine the causative agent of the infection. Once the cause(s) is determined, management changes can be recommended versus treatment options. Treatments used for clinical mastitis are the most common causes of antibiotic residues in raw milk. Cows with repeat or recurring cases of clinical mastitis have been shown to be responsible for the majority of the discarded milk. Therapeutic efficacy of antibiotics for most mastitis pathogens has been questioned. Ultimately, the success of mastitis therapy should not be measured by the elimination of bacteria, the reduction of SCC, or reducing the cow's rectal temperature, but by the ability to increase the amount of quality milk shipped from the farm. Table 1. Interpretation of bulk tank analysis for bacteria and SCC in raw milk. Test Procedure Excellent Good Concern SPC 1,000 10,000 20,000 PI 10,000 20,000 50,000 LPC 100 300 500 SCC 100,000 200,000 400,000 Contagious pathogens None None Present Source: Shearer, J. et. al. 1992 56