What is a Milking Evaluation and How Can it Help Improve Milk Quality?

The number of somatic cells, or somatic cell count (SCC), in milk is a measure used throughout the dairy industry as an indicator of udder health and milk quality. According to the Grade "A" Pasteurized Milk Ordinance, the current regulatory bulk tank SCC limit to sell milk in the U.S. is 750,000 cells/mL (Department of Health and Human Services USA, 2019). On an individual cow level, a SCC greater than 200,000 cells/mL is indicative that an infection is present. When a cow has a high SCC (> 200,000 cells/mL) this means that a clinical or subclinical mastitis infection is present. A clinical mastitis infection is when visible changes in the milk or udder occur, such as reddening and/or hardening of the udder, flakes, or clots in the milk, etc. A subclinical infection happens when there are no visible signs of mastitis, but the cow has a high SCC. Although not harmful to humans, a high SCC can significantly reduce the processability of milk and reduce the shelf life of the dairy product (O’Brian, 2023).

In addition to the quality of the product, monitoring and controlling SCC levels on a dairy farm is important for a greater milk yield, improved cow health, increased quality premiums, and decreased treatment expenses. There are many different variables that could be contributing to a high SCC including improper milking protocols, timing of milking unit attachment, bimodal milk flow, housing, etc. If a farm is struggling with a high SCC or just wants to improve their milking practices, Penn State Dairy Educators can help producers reach their milk quality goals with a milking evaluation.

Why is Bimodal Milk Ejection a Problem?

Milk flow dynamics follows a pattern of four phases of intensity: incline, plateau, decline, and overmilking (et al., 2007). In between daily milkings, milk is stored in the udder tissue in two different tissue "compartments": more than 80% of milk is stored in the alveolar compartment and less than 20% is stored in the cisternal compartment (Figure 1). The milk in the cisternal compartment is available before oxytocin (the milk letdown hormone) is released. Pre-stimulation (pre-dipping, forestripping, wiping) is what prompts the oxytocin release which triggers the contraction of the myoepithelial cells to move the milk in the alveolar compartment to the cisternal compartment of each quarter before the attachment of the milking unit (Tuor et al., 2023). Preparation lag time is the lag time between the first udder stimulation and attachment of the milking unit. Improper preparation lag time can lead to delayed milk ejection, bimodal milk flow during the incline phase, impaired teat health, and possibly reduced milk flow (Moore-Foster et al., 2019). To allow for the alveolar milk to be letdown and to have a continuous milk flow, the preparation lag time should -be between 60 to 120 seconds. A lag time that is less than 60 seconds or more than 120 seconds contributes to a delayed milk ejection and separate removal of the cisternal and alveolar milk, causing a bimodal milk flow pattern (Tuor et al., 2022). 

Milking Evaluation

The Penn State Extension Dairy Team offers milking evaluations to help dairy producers in PA improve their milk quality and identify any issues that could be contributing to an increased SCC. The evaluation consists of an on-farm visit during milking time, where the Extension Educator will connect a VaDia digital vacuum recorder (Biocontrol, Rakkestad, Norway) to two individual milking units. The VaDia recorder measures simultaneous vacuum events during milking in four channels while attached to the milking cluster (Erskine et al., 2019). These recorders can reliably assess bimodal milk ejection in cows. Bimodal milk flow is defined when an increasing milk flow rate is followed by a decreasing flow rate during the first two minutes of milking. In addition, the educator will make visual observations about the milking routine and protocols and any other management practices that could be contributing to decreased milk quality. A follow-up report is developed with the results and vacuum graphs generated from the VaDia recorder and a set of personalized recommendations regarding the milking protocols.

Before the evaluation visit, if the farmer participates in monthly milk testing with DHIA, the educators will ask to access their records through PCDART and use the data to better evaluate the farm and make more informed recommendations. The predefined udder health graphs can provide data that may not be observed with just one farm visit. One graph that is commonly used by educators is the "Prlc Last SCC vs 1st SCC." This graph displays all of the fresh cows' last test day SCC from their previous lactation versus their first test day SCC from the current lactation. This can help educators begin to assess the dry cow program. Checking out the predefined herd udder health graph, "Avg SCC Score by Lact," can also be helpful. This graph displays the average SCC of cows by lactation number over a set amount of time. Using this graph can help educators understand any problem areas that may be present in a specific group of cows.

Interpreting VaDia Milk Flow Graphs

Digital vacuum is recorded by inserting the silicon tubing into four channels on a milking cluster: 1) short pulsation tube, 2) short milk tube, 3) a rear quarter mouthpiece liner, and 4) a front quarter mouthpiece liner. The VaDia is secured onto the milking cluster during the entire milking time (Image 1). Once milking is completed, the data from the VaDia can be uploaded to a computer and graphs are generated that look like the below examples. Each graph represents one cow during the milking time. The horizontal axis represents the time that the milking unit was on that one cow and the vertical axis is vacuum level measured in Kilopascal (kPa). The blue line is the short milk tube vacuum, the green line is the front quarter mouthpiece vacuum, and the black line is rear quarter mouthpiece vacuum. Optimum milk flow will appear like figure 2, with a rapid increase in flow rate (blue line) and a rapid drop of vacuum in the front and rear mouthpiece liners (green and black lines).

Figure 3 is an example of bimodal milk flow. This occurs when there is a drop-in flow rate (blue line) and an increase in vacuum in the front or rear mouthpiece liners (green and black lines).

Along with the VaDia vacuum recorders, a few other areas the educators focus on during the milk include the following:

1. Gloves: Clean, disposable gloves should be worn on both hands whenever milking. Everyone who is going to touch an udder/teat should have gloves on, even if they are just helping with one or two cows. This is very basic—but farmers and farm employees can become complacent and may stop wearing gloves while performing certain tasks because they do it all the time. Wearing gloves is the easiest way to prevent bacteria transfer from cow to cow. It is also important to change gloves when they become contaminated. 
2. Contagious mastitis cows- Milking cows last that are known to have mastitis caused by a contagious pathogen such as Staphylococcus aureus, can help reduce the contamination of the milking units throughout milking time.
3. Pre-dip: Teats should be pre-dipped (disinfected) with a U.S. Food and Drug Administration (FDA)-approved, sanitizing teat dip. Using a pre-dip that has an identified appropriate time to leave on the teats for it to be effective is recommended. The "kill time" for the pre-dip should be on the label of the product. If a farm has been using the same pre-dip for an extended period, it may be time to switch to a different formula to be sure the pre-dip is still effective.
4. Fore stripping: All teats should be fore stripped (3 to 5 squirts of milk from each quarter) while the pre-dip is disinfecting the teat. Fore-stripping serves three purposes, 1. teat stimulation for oxytocin let down, 2. clinical mastitis detection, and 3. removal of the highest SCC milk from the teat cistern. To have proper stimulation, forestripping each cow for a total of 10 seconds is recommended. This will help with manual stimulation as well as reducing SCC in the herd. Clinical mastitis detection is also very important while stripping. Checking the milk that is stripped out of each teat for abnormalities such as flakes, clumps, discoloration, etc., can help detect mastitis early. Using a strip cup can make it easier to see the small changes in milk that you will not see on the stall or parlor surface. Using a strip cup can be very important when contagious mastitis is present in a herd. The contagious bacteria can easily be spread on your boots, hands, and milking units from cow to cow.
5. Wiping/Drying: After 20 to 30 seconds (depending on the pre-dip), the pre-dip can be wiped off the teats with a disposable paper towel or recently washed and disinfected, dried cloth towel before attachment of the milking claw. If cows are really dirty when beginning the milk prep process wipe with a clean, dry towel before pre-dipping. More than one towel for each cow may be necessary.
6. Lag Time: It is very important to maintain a proper lag time for udder health.  Improper lag time can easily affect SCC and cause teat-end hyperkeratosis (damage) due to increased direct vacuum flow on the teat ends. The recommended lag time is 60 to 120 seconds.
7. Post-dip: The last step of milking is post-dip. The purpose of post-dip is to disinfect teats immediately after the milking claw is removed to kill most contagious bacteria present and thereby preventing the establishment of new infections during the post-milking period. In most post-dips there is also a moisturizing agent that will help prevent teats from getting chapped and sore. After milking, the teat sphincter muscle takes 30 minutes to close. It also takes 30 minutes for the post-dip to dry. If a cow lies down while the sphincter is still relaxed or before the teat dip has dried, it can lead to bacteria entering the teat end and lead to potential infection. It is estimated that 50 percent of new contagious mastitis infections can be prevented by complete and consistent post-milking teat dipping and allowing the dip to dry. A good way to keep cows standing after milking is to have fresh feed available during or right after milking.
8. Housing- It is important to keep adequate bedding for all groups so that they are always clean and dry. If animals are coming in from the pasture or the housing areas with caked on manure on their udder, flank, and legs, then more bedding is needed in those areas. People should be comfortable to go out into the cows or heifers’ pens or pasture at any time and lay down. When they stand up, their clothes should be clean and dry. Three to 4 inches of bedding is recommended to provide comfort for cows and to keep them clean and dry. Scraping stalls and adding additional bedding as needed is very important. Increasing the frequency that you add bedding to the stalls will help with keeping stalls clean and dry and to keep them more comfortable.
9. Fly control- Is something that should be considered as well. Improving the ventilation and air movement over the cows in the warmer months will greatly decrease the flies in the dairy barn. Using sticky tape and other fly traps around the farm can help reduce flies. Managing flies appropriately often requires multiple management strategies—you may have to utilize fly traps and sticky tapes, spray-on solutions, improved ventilation, etc. to control the flies. There are both organic and non-organic methods of fly control available.
10. Heat stress- All animals should have 24/7 access to clean water. Regularly cleaning (bi-weekly or monthly) water bowls and water troughs can help encourage water intake. Water plays an important role in thermoregulation to help keep cows cool. In periods of heat stress, cows will double their water intake but if easily accessible water space isn't available, they will not be able to meet their needs. Check water troughs around the barn and in the pasture to be sure they aren't leaking and are filling properly. If possible, rotate water troughs around so that the cows don't have to walk too far to access the water. High producing dairy cows can start experiencing heat stress at a temperature humidity index (THI) of 65, so the goal is to keep them as comfortable and cool as possible, so they don't start experiencing visible symptoms of heat stress. Shade should always be provided for animals on pasture. If there are not multiple shade trees in each paddock, then consider building some portable shade structures to move around as the cows move to different paddocks. Providing enough shade for cows on pasture can help keep them comfortable and reduce the severity of the "summer slump" in production and reproduction.

The follow-up reports will also include information about how to identify problem cows using various tools such as a CMT paddle, on-farm culturing, or just by using DHIA records. The educators always encourage producers to have an established veterinarian-client-patient relationship (VCPR) with their herd veterinarian to get appropriate prevention and treatment recommendations.

This program has impacted more than 70 dairy farmers in PA over the past three years. If you are a producer in PA or work with dairy producers in PA and are interested in a milking evaluation, you can contact any of these Penn State Extension Dairy Educators:

Carly Becker- Lancaster County Dairy Educator, cab7033@psu.edu, 717-283-2596

Daniela Roland- Franklin County Dairy Educator, djr6158@psu.edu, 717-809-2194 

Cassie Yost- Huntingdon County Dairy Educator, clm275@psu.edu, 814-643-1660

Emily Fread- Union County Dairy Educator, evf5337@psu.edu, 570-556-4748

Resources

Dariexnet. 2019. Are U.S. Dairy Farms Ready for the Drop in SCC Legal Limit? Dairy Cattle Extension.

Department of Health and Human Services USA. 2019. Grade "A" Pasteurized Milk Ordinance..

Erskine, R. J., B. Norby, L.M. Neuder, R.S. Thomson. 2019. Decreased milk yield is associated with delayed milk ejection. J. Dairy Sci. 102: 6477-6484.

Moore-Foster, R. B. Norby, R.L. Schewe, R. Thomson, P.C. Bartlett, R.J. Erskine. 2019. Herd-level variables associated with delayed milk ejection in Michigan dairy herds. J. Dairy Sci. 102: 696-705.

O'Brian, B. 2023. Milk Quality, Mastitis, and SCC. Teagasc Agriculture and Food Development Authority.

Tancin, V. A. H. Ipema, P. Hogewerf. 2007. Interaction of somatic cell count and quarter milk flow patterns. J. Dairy Sci. 90: 2223-2228.

Tuor, M.,B. Jenni,O. Wellnitz, R. M. Bruckmaier. 2022. Reduced liner-open phase and vacuum instead of prestimulation increase parlor efficiency in dairy cows. J. Dairy Sci. 105: 1533–1541.

Tuor, M., O. Wellnitz, R. M. Bruckmaier. 2023. The interplay of continuous milk ejection and milking system with and without prestimulation at different vacuum settings. J. Dairy Sci. 106: 3615-3624.