Mastitis complex; no simple solutions are available for its control. Some aspects are well-understood and documented in the scientific literature. Others are controversial, and opinions often are presented as facts. The information and interpretations presented here represent the best judgments accepted by the National Mastitis Council.
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|The Role of Milking Equipment in Mastitis|
The goal for any dairy farm should be to deliver a high-quality product that has consumer appeal. The objective of any herd’s milk management program should include: (1) maximize yield secreted by the mammary gland, (b) milk cows out in a short period of time, (c) prevent damage to teats, teat ends, and the udder, and (d) have no adverse effect on chemical composition of milk. It must be recognized that factors other than milking equipment, such as milking practices, can influence milking performance and quality.
Relationship between Milking Equipment and Development of Mastitis
The teat canal is the primary defense against bacterial invasion of the udder. The teat canal is lined with a keratin layer which plays an important role in preventing infections. The keratin layer can be removed by overmilking, high vacuum levels, and hard liner squeezes, all of which increase infection rates, especially with higher production.
Milking machines may adversely affect udder health by damaging or changing the resistance of the cow’s first line of defense: teat skin, teat canal, and mucosal tissue. As the teat cup liner opens, a reverse pressure gradient can be created across the teat canal that causes bacteria-contaminated milk droplets or slugs of milk to move backward and impact against the teat end.
Bacteria or bacteria-contaminated milk droplets present at the time penetrate the teat canal and enter the teat. These impacts are caused by air admission into teat cups or claws (liner slips). The frequency of impacts increases with sudden large air admission into the cluster due to liner slips (enhanced by wet cow milking), improper application or removal of teat cups, vigorous machine stripping, and inadequate positioning of the milking unit under the cow. Milk droplet impacts can be minimized by avoiding abrupt vacuum loss through adequate reserve air flow, proper milk line sizes, sensitive vacuum regulation, proper unit application and removal, and adequate air flow into the claw vent.
According to the National Mastitis Council (1996), there are four ways in which the milking machine can be involved with the development of mastitis.
A vacuum pump creates the vacuum inside the teat cup liner by removing air in the system of pipes and hoses between the pump and the liner interior. The vacuum level at the teat end depends on the degree of vacuum drop introduced by each component in the path from the pump to the teat end. The teat end vacuum should be at a level and degree of stability compatible with rapid, complete milk extraction and minimal tissue trauma. Both experimental and field experience have shown that a vacuum level of 10.5 to 12.5 inches at the teat end during peak milk flow offers the best combination of rapid, complete milk removal with minimal physical harm and highest milk quality (Stewart et al., 1996). Nominal system vacuum settings of 12.5 to 13.5 inches for low lines, or 14 to 15 inches for high lines, will result in a vacuum level in the claw within 10.5 to 12.5 inches during peak milk flow. Lower values may result from excessive milk line height, restrictions in the milk tubes, excessive vacuum drop across ancillary components, blocked air vents, excessive air admission through air vents, or air leaks into the cluster. Claw vacuum during peak milk flow should be about 12 to 12.5 inches to milk cows as quickly as possible but still maintain gentle milking conditions. An average vacuum fluctuation of less than about 2 inches is considered desirable for a low line system and less than about 3 inches for a high line system. Higher vacuum fluctuations may indicate either a blocked air vent or excessive air flow rate through air vents or air leaks. Increasing the system vacuum level, e.g., from 13.5 to 15 inches, results in faster milking time which may be offset by higher strip yields, higher incidence of hyperkeratosis at the teat orifice, and more machine-induced teat congestion and edema, unless cow preparation procedures are excellent.
Vacuum drops occur for several reasons, including admission of air intentionally or unintentionally into the system, friction in plumbing when moving air, solid slugs of milk being conveyed in the milk pipeline, friction in long milk hoses when moving an air and milk mixture, and expenditure of energy to overcome gravity when lifting milk (Table 1). The sources of these drops need to be identified to maintain teat end vacuum in the desired range while operating the pump at the most energy-efficient and power-efficient (i.e., lowest) vacuum level.
Table 1. Sources of vacuum drops from vacuum pump to teat end (Stewart et al., 1996).
Since environmental bacteria usually do not colonize the teat canal, an active force is needed in order for these bacteria to penetrate the canal (Rasmussen et al., 1994). Proper premilking teat preparation lowers the number of bacteria at the teat end before attachment of the milking unit. Some bacteria pass through the teat canal during milking. The risk of reverse pressure gradients can be lowered by milking well-prepared cows that let their milk down before attachment of the milking unit and properly removing the unit as soon as cows are milked out.
Frequency of Slipping or Falling Teat Cups
A problem exists if more than five to 10 slips or fall-offs per 100 cow-milkings require correction by the milker. Slipping or falling off early during the milking process often results from low vacuum level, blocked air vents, or restrictions in the short milk tube. Slips occurring during late milking can be due to poor cluster alignment, poor liner condition, or uneven weight distribution in the cluster (Mein and Reid, 1996).
Milking System Performance Analysis
In maintaining or trouble-shooting milking equipment, the following components or tests are important in order to minimize or eliminate the concerns or problems that were discussed above. Milking system tests should be conducted on a regular basis by a trained service person using procedures outlined by the National Mastitis Council. These tests should include:
Vacuum levels and regulation: Sensitive regulators should be installed in clean, dry, dust-free, and convenient locations, either off the distribution tank or the main airline as near the sanitary trap as possible (ASAE, 1996). They should be cleaned at least monthly (twice a month is preferred) and filters changed as needed. Vacuum level and fluctuation at the teat end should be checked. An adequately designed milking system with proper vacuum pump capacity and low milk lines or weigh jars should have a vacuum level of 11 to 13 inches at the teat end at peak milk flow with no more than a 2- to 3-inch fluctuation during several pulsation cycles.
Teat cup liners should be replaced according to manufacturer’s recommendations. When liners were used excessively, bulk tank milk in 10 Ohio herds over 52 consecutive weeks had higher bacteria counts, especially staphylococci (Hogan et al., 1988). Teat cup liners, when not sanitized after milking infected cows, may transfer infections to uninfected cows and cause new mastitis infections. Mastitis-infected cows should be milked last or into separate milker units used only for infected cows. Studies at Virginia Tech have found that sanitizing units after milking infected cows is another effective control measure that reduces incidence of new infections (Grove and Jones, 1992). The use of automatic teat cup backflushing is another alternative that may be cost effective. Of special concern is the milking of first lactation cows after milking infected cows; milk clean, uninfected cows first.
Other milking system components that should be changed every three to four months include milk hoses, short air tubes, gaskets, and rubber plugs.
Pulsation lines should be cleaned on a regular basis as condensation, milk droplets, and airborne contaminants are frequently drawn into vacuum lines (as well as milk from split teat cup liners). Clean vacuum lines help to maintain optimum airflow within the milking system. A standard pipeline cleaner used monthly is adequate. Vacuum lines should be looped with valves and tees provided for slug washing of the vacuum line. Vacuum lines need to be sloped to drain. All low spots should have automatic drain valves and clean-out plugs.
Stray voltage: In recent years, it has become evident that small electrical currents may come in contact with dairy cows and may cause significant behavior problems, accompanied by losses in milk production and certain health problems. If the voltage during milking exceeds 0.5 volts when measured at the barn service entrance ground or at various cow contact points (stalls, feeders, waterers, milk pipeline), a more detailed analysis should be conducted to isolate the source and determine whether it is on-farm (milk tank compressor, water heater, silo unloader, lights, electric fence, house, telephone, etc.) or off-farm (power line). For additional details, see Virginia Cooperative Extension Publication 404-250, “Stray Electricity on Dairy Farms” (1992).
Milking system evaluation should be conducted at regularly scheduled intervals by a trained service representative. This evaluation should include:
Preventative Machine Maintenance
Milking machines are used for five or six or more hours every day. Broken-down machines or machines operating inefficiently cost in reduced milk volume, time, damaged udders, and reduced milk quality. Regular service inspection will allow performance at high efficiency. There are certain maintenance checks that should be performed by the operator at every milking, and every 50 and 250 hours.
Every 4 to 6 Weeks:
Every 6 Months:
American Society of Agricultural Engineers. 1996. Milking machine installations — construction and performance standards. ASAE S518.2 Jul96. Bray, D.R., P.A. Fowler, F.B. Fialho, R.A. Bucklin, S. Yeralan, T. Tran, and R.K. Braun. 1998. An automated system for monitoring milking system parameters. Pages 127-136 in Proc. 37th Annual Meeting National Mastitis Council, Madison, Wis. Grove, T.M., and G.M. Jones. 1992. Use of an enzyme-linked immunosorbent assay to monitor the control of Staphylococcus aureus mastitis. J. Dairy Sci. 75:423-434. Hogan, J.S., K.L. Smith, K.H. Hoblet, P.S. Schoenberger, D.A. Todhunter, W.D. Hueston, D.E. Pritchard, G.L. Bowman, L.G. Heider, B.L. Brockett, and H.R. Conrad. 1988. Bacterial and somatic cell counts in bulk tank milk from nine well managed herds. J. Food Protection 51:930. Mein, G., and D.A. Reid. 1996. Milking-time tests and guidelines for milking units. Pages 235-244 in Proceedings 35th Annual Meeting, National Mastitis Council, Madison, Wis. National Mastitis Council. 1996. Current Concepts of Bovine Mastitis, 4th ed. Madison, Wis. Rasmussen, M.D., E.S. Frimer, and E.L. Decker. 1994. Reverse pressure gradients across the teat canal related to machine milking. J. Dairy Sci. 77:984-993. Stewart, S., R. Farnsworth, G. Mein, D.A. Reid, A.P. Johnson, G. Beehler, and J. Paasch. 1996. Field measurement of vacuum levels using a portable liquid flow simulator. Pages 214-227 in Proceedings 35th Annual Meeting, National Mastitis Council, Madison, Wis.