Bulk Tank Somatic Cell Counts

Agdex#:	410/662
Publication Date:	06/84
Order#:	84-031
Last Reviewed:	09/96
History:	New
Written by:	R. Stiles/Dairy Farmers of Ontario; J. Rodenburg - Dairy Cattle Specialist/OMAFRA

Table of Contents

1. Introduction
2. Milk Production Loss
3. Somatic Cell Count Testing
4. Additional Information
5. Related Links

Introduction

All Ontario dairymen receive monthly milk quality reports which includes a somatic cell count (SCC). Somatic cell counts taken on bulk tank milk are a good indicator of the general state of udder health in the dairy herd. Somatic cells in milk consist mainly of white blood cells produced by the cow to destroy mastitis causing bacteria that enter the udder and to repair damaged udder tissue. These cells are always present in milk but when an infectious agent enters the udder or when the udder is damaged, the number of somatic cells shed by the individual cow increases. Tissue damage and the increased SCC resulting from mastitis infection can block the tiny milk ducts in the udder, resulting in lower production when the milk secreting cells above the blockage are dried off.

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Milk Production Loss

The amount of milk production lost is related to the somatic cell count. Table 1 shows average milk production losses for individual cows based on their individual SCC in Ontario DHI herds. Because production does not decrease a uniform amount with each unit change in somatic cell count, it is impossible to translate these individual cow losses directly to bulk tank cell counts. For example, assuming the same production level, four cows at 100,000 SCC plus one cow at 1.6 million would give the same bulk tank average as five cows at 400,000 SCC while production losses would be quite different for these two groups of cows.

An estimate of milk production lost as predicted from bulk tank somatic cell count is given in Table 2. Based on this table, herds with cell counts over 500,000 SCC could be producing from 8 to 20% below potential because of the presence of sub-clinical mastitis infections. Other losses associated with high cell counts include lower milk quality and decreased cheese yield.

Although in general, herds with high bulk tank somatic cell counts have a larger number of cows with subclinical mastitis infections, predicting the percent of infected quarters or infected cows from the bulk tank somatic cell count is quite inexact. Reasons for this include variation in cell count among both infected and uninfected cows with stage of lactations, season of the year and age as well as wide variation among cows in their SCC response to infection. Considering these factors, bulk tank counts should only be used as a guideline providing general indications of overall udder health and only consistent trends over 4 to 5 months should be considered to reflect a change in the udder health status of the herd. Research done on herds in the Quinte area of Ontario indicated that the ability of bulk tank counts to predict the quarter infection rate of herds was approximately doubled (45.5% versus 80% ) when the interpretation was based on 6 previous monthly bulk tank samples instead of a single test.

In spite of only moderate predictability in the number of quarters or cows infected, bulk tank SCC's do clearly indicate that herds over 500,000 cells have a major reason for concern about udder health and should implement a program to reduce cell counts and the level of mastitis infection through improved sanitation, milking equipment management, milking procedures and a treatment and culling program. Herds consistently over 200,000 cells could also decrease mastitis incidence and realize increased milk production through a good mastitis control program. Field experience has indicated that a logical goal for a well managed dairy herd is to maintain a bulk tank count consistently below 200,000 cells per millilitre.

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Somatic Cell Count Testing

Individual cow SCC's available through milk recording programs are useful in assessing infection status and production losses of individual cows and these programs are highly recommended. In these programs, a herd average somatic cell count for the day on which cows were tested is calculated. Because of the day to day variation in herd average somatic cell count (as illustrated for one herd in Figure 1), the test date SCC herd average from a milk recording program will not always agree with the bulk tank SCC from the corresponding month.

Figure 1. Daily Bulk Tank Variation Somatic Cell Count for One Herd Over 30-day Period.

Additional Information

More information on mastitis prevention is available in OMAF Factsheets:

1. Somatic Cell Counts: Interpretation For Individual Cows, Agdex 410/662
2. Mastitis in the Dairy Cow, Agdex 410/662
3. Mastitis Prevention: Environmental Control, Agdex 410/662
4. Mastitis Prevention: Milking Management Procedures, Agdex 410/662
5. Dry Cow Therapy, Agdex 410/662
6. Mastitis Prevention, Milking Equipment Management, Agdex 410/662

For individual assistance in assessing milking equipment, milking procedures and the environment of the herd, the Dairy Farmers of Ontario provides an Udder Health Management Program. Dairymen should also work very closely with their veterinarian to develop an effective mastitis treatment and prevention program for their herd.

Related Links

• Somatic Cell Counts: Interpretation For Individual Cows
• Mastitis Prevention: Environmental Control
• Dry Cow Therapy

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