Research Summary - 2
Variation of monthly cow SCC compared to weekly cow SCC
| Date/Time: |
9/12/2025
14:00
|
| Author: |
Roger Saltman |
| Clinic: |
RLS Management Solutions LLC |
| City, State, ZIP: |
Cazenovia, NY 13035 |
R. L. Saltman, DVM, MBA
1
;
B. Sudarsan, MSc
2
;
M. Kibbanahalli, MSc
2
;
J. Beltran, MPP
2
;
1RLS Management Solutions LLC, Cazenovia, NY 13035
2SomaDetect LLC, Buffalo, NY14226
Introduction:
The use of Somatic Cell Count (SCC) as an indicator of udder health in dairy cows has evolved significantly since it was first introduced in the 1970’s It has become a cornerstone in milk quality assessment and mastitis management. Typically, SCC has been done on individual cow milk collected during the monthly DHI test. The results are received by a dairy 2 to 7 days later Various “rules of thumb” have evolved to evaluate this data which then inform the creation of various protocols for improving udder health in cows and for lowering Bulk Tank Somatic Cell Count (BTSCC), often to try to capture milk quality premiums. Milk with SCC< 200k cells/ml has generally been considered to be from uninfected cows. Cows with SCC ≥200k but <800k or ≥800k were said to be “moderately” or “seriously” infected respectively.
The Objective of this study was to determine how much variability existed in a cow’s SCC between monthly tests and if testing SCC more frequently (weekly) might provide more meaningful information for improving individual cow milk quality as well as BTSCC.
Materials and methods:
The study was conducted in a large free stall herd that was on a monthly DHI testing program. The herd aggressively used SCC results to check high cows more closely with the California Mastitis Test (CMT). Positive CMT cows (Score of 2 to 3) were considered to be infected and were treated with intramammary antibiotics. However, BTSCC remained in the 250k to 300k range for over 2 years. We hypothesized that they might be missing medium to high SCC cows in between the tests and initiated 5 consecutive weeks of DHI testing to compare month-to-month variation versus week-to-week variation. Milk test results from 1,434 cows were assigned to categories as follows: SCC< 200k cells/ml were Category 1, SCC ≥200k but <800k were Category 2, and SCC≥800k were Category 3. Analysis was done for intra-monthly variation (Week 1 to Week 5) as compared to intra-weekly variation (Weeks 1 to 2, 2 to 3, etc.).
Results:
Fifty-five cows (4%) whose Weeks 1 and 5 results were Category 1 or 2 had at least 1 intervening week in Category 3. 157 (11%) cows with Weeks 1 and 5 in Category 1 had at least 1 intervening week in Category 2. In total, 212 (15%) cows had a Category increase between Weeks 1 and 5 that they would not have known about using monthly DHI SCC testing. Analyzing individual cow SCCs, the Average Intra-monthly SCC Variance was 692,468 and the Average Intra-weekly SCC Variance was 128,868. Using the F Test to determine the statistical probability that those two values are different yielded a P Value = 1.7545E-200, indicating strong evidence of the two values being different.
Significance:
The results of this study show that there can be considerable variation in a cow’s SCC in the intervening weeks between monthly DHI tests. Most increases in SCC are an immunological response to the invasion of the mammary gland by bacteria in which neutrophils are recruited as a first line of defense. If the neutrophils are successful at neutralizing the bacteria, additional recruitment is no longer necessary. The total number of cells/ml of milk returns to a more normal homeostatic state. The absence of SCC data during the intra-monthly interval may also provide insight into why the many “rules of thumb” that compare a cow’s SCC from one month to another may lead to spurious conclusions about changes in the health of an animal’s udder. This may also suggest why aggressive use of a monthly DHI SCC list that includes just the monthly use of the CMT for “discovering” truly infected cows may not be sufficiently rigorous to meaningfully reduce BTSCC.
