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AIPL RESEARCH REPORT
SCC6 (3-05)

Somatic cell counts of milk from Dairy Herd Improvement herds during 2004

R.H. Miller, H.D. Norman, and A.H. Sanders
Animal Improvement Programs Laboratory, ARS-USDA, Beltsville, MD 20705-2350
301-504-8334 (voice) ~ 301-504-8092 (fax) ~ rlaipl@aipl.arsusda.gov ~ http://aipl.arsusda.gov

Test-day data from all herds enrolled in Dairy Herd Improvement (DHI) somatic cell testing during 2004 (93% of all DHI herds) were examined to assess the status of national milk quality. All test-day data within a herd were included regardless of breed; data from owner-sampler herds were included as well. Data from cows with records that failed Animal Improvement Programs Laboratory (AIPL) editing procedures (for example, no sire identification) were excluded. However, an earlier report showed that the somatic cell counts (SCC's) of milk from such cows did not differ appreciably from those from the other cows in the herd. Somatic cell score (SCS) is reported to AIPL as part of an individual cow's test-day yield information and was converted back to SCC in cells/milliliter [SCC = 2(SCS–3) × 100,000] for calculating herd and State averages.

Table 1 shows the total number of herd test days, the average number of cows (with usable records) per herd on test day, the average milk yield and SCC, and the percentages of herd test days that exceeded four levels of SCC (750,000, 600,000, 500,000, and 400,000 cells/ml) for each State during 2004. The current legal limit for bulk tank SCC is 750,000 cells/ml for Grade A producers; the lower limits shown have been proposed as future maximums. State and national average daily milk yields were weighted by the number of cows per herd test day. Herd, State, and national average SCC's were based on SCC's from individual cows that were weighted by milk yield on test day. Percentages of herd test days by level of SCC are for herd test days that included records of at least ten cows.

Nationally, average SCC during 2004 was 295,000 cells/ml, which is the third consecutive year of decline in the national average. Corresponding averages for 1995 through 2003 (Table 2) ranged from 304,000, to 322,000. The percentage of herd test days that exceeded 750,000, 600,000, 500,000, and 400,000 cells/ml during 2004 was 4.7, 9.4, 15.5 and 26.4, respectively. Thus, 4.7% of test days from DHI herds that participated in somatic cell testing are higher than the present legal limit for SCC of bulk tank milk. This percentage may overestimate the percentage of herds that shipped milk exceeding the legal limit for bulk tank SCC on test day because the milk of cows treated for mastitis is excluded from the bulk tank but included in DHI test data. Also, the percentage of herd test days that exceeded the legal limit would have been higher than the percentage of herds that were rejected from the market because market exclusion only occurs after repeat violations.

Variation among States (Table 1) was large. Average SCC's were often lower in the far West and higher in the Southeast; a finding consistent with previous reports. Although climatic conditions (temperature and humidity) probably contributed to regional SCC differences, SCC differences between geographically adjacent states were substantial, which suggests that mastitis-control regimens have an impact under similar climatic conditions. The overall percentage of herd test days exceeding 750,000 cells/ml was lower in 2004 than in 2003 (Table 2), but more states had greater than 5% of herd test days over this limit, showing that although progress has been made in some areas, improvement is needed in others.

Herd size and average daily milk yield have been increasing since 1995 (Table 2). The results for SCC are encouraging, but perhaps it is too early to conclude that a continued decline is expected due to genetic selection or improved management practices. The trends for increasing percentage of herd test days above three thresholds (600,000, 500,000 and 400,000 cells/ml) no longer show statistically significant increases as they have in the past. Nevertheless, greater efforts to improve milk quality should continue. State and national SCC's will continue to be summarized annually so changes in SCC and implications to milk quality can be monitored.

Table 3 provides milk yield and SCC by estimated herd size. As herd size increased, average daily milk generally increased, while average SCC declined. Possibly the most surprising figure was for the percentage of test days with greater than 750,000 cells/ml. For herds with <50 cows, this occured 7% of the time. It occured on <2% of the test days for herds with 50 to 99 cows and on <1.2% of the test days for those herds with >100 cows. Table 4 provides the same statistics for all herds by month. The seasonal pattern of milk quality is illustrated in Figure 1. Milk yield and quality decline during the summer months, and the highest quality milk is produced in late fall to early winter.


Table 1. Characteristics of DHI herd test days for milk yield and SCC of milk by State during 2004.

SCC characteristics
  Herd test days1 Cows2 per herd Average daily milk yield Average SCC Herd test days3 with SCC greater than
State (no.) (no.) (lbs) (cells/ml, 1000's) 750,000 cells/ml
(%)
600,000 cells/ml
(%)
500,000 cells/ml
(%)
400,000 cells/ml
(%)
Alabama 297 99.1 51.4 455 4.4 15.2 30.3 52.9
Arizona 339 795.8 66.6 317 2.9 7.1 13.0 24.5
Arkansas 339 78.0 57.2 404 8.8 16.8 23.9 37.8
California 10,052 517.7 72.5 289 3.5 6.5 11.2 20.7
Colorado 430 243.5 75.3 298 1.9 3.3 7.2 14.2
Connecticut 840 74.5 69.3 288 3.8 7.7 13.7 21.2
Delaware 301 92.8 64.6 351 7.6 12.0 17.9 29.6
Florida 309 173.2 68.6 475 15.9 31.1 46.0 61.2
Georgia 1,214 79.3 62.0 418 8.2 19.3 32.9 51.6
Idaho 1,782 313.3 73.8 278 4.9 8.6 12.9 22.1
Illinois 4,906 57.1 65.8 323 4.5 9.5 16.0 27.8
Indiana 3,753 45.3 65.3 356 8.3 14.7 23.6 36.2
Iowa 9,800 51.3 67.5 309 4.9 9.5 16.4 28.2
Kansas 2,512 58.3 65.5 349 9.5 16.6 24.0 38.2
Kentucky 1,867 49.0 62.5 383 9.7 17.7 28.1 42.7
Louisiana 494 57.1 55.8 449 17.4 27.9 45.1 62.1
Maine 1,167 55.5 64.1 275 2.7 6.4 11.1 21.4
Maryland 3,665 67.0 64.6 315 3.9 8.1 15.2 27.4
Massachusetts 896 62.5 66.3 291 1.2 5.1 11.4 23.4
Michigan 7,648 73.6 72.5 294 5.2 10.0 15.6 26.3
Minnesota 27,561 60.0 68.0 316 5.0 10.7 18.2 31.0
Mississippi 470 85.6 62.7 425 10.0 20.4 32.1 51.5
Missouri 3,597 44.3 60.5 394 8.8 16.4 25.1 38.9
Montana 420 92.7 73.8 215 0.7 0.7 2.9 7.6
Nebraska 1,820 68.3 67.6 350 7.4 13.4 20.8 32.9
Nevada 119 335.8 72.5 354 5.0 9.2 13.4 21.0
New Hampshire 890 73.5 70.2 241 1.9 4.3 8.2 15.2
New Jersey 567 67.6 68.0 318 5.1 10.9 18.0 32.8
New Mexico 312 592.4 72.0 327 3.2 7.1 14.1 26.0
New York 21,407 76.3 68.6 260 3.5 7.3 12.5 22.1
North Carolina 1,836 83.4 65.2 365 7.9 15.2 23.2 37.9
North Dakota 480 64.7 66.2 268 2.5 5.4 10.6 19.2
Ohio 9,191 61.1 64.4 300 5.6 10.4 16.6 26.9
Oklahoma 680 66.4 61.1 357 6.2 12.2 20.6 35.3
Oregon 2,358 131.6 68.5 261 2.5 5.0 8.3 14.8
Pennsylvania 43,431 45.2 66.7 309 4.5 9.2 15.5 26.6
Puerto Rico 640 49.7 39.9 459 17.0 28.6 36.9 50.0
Rhode Island 59 33.6 61.0 233 0.0 1.7 1.7 8.5
South Carolina 578 130.4 59.7 390 4.8 12.6 21.8 39.6
South Dakota 1,552 59.9 67.1 333 7.3 13.7 20.9 32.2
Tennessee 1,833 56.0 59.7 469 13.4 26.9 40.4 58.2
Texas 1,468 200.3 64.1 308 1.4 4.8 10.3 21.5
Utah 1,625 120.5 72.1 255 3.9 7.0 12.2 21.6
Vermont 3,992 65.0 65.4 254 2.1 5.1 8.9 16.6
Virginia 4,693 80.0 66.5 336 4.3 10.1 18.1 32.6
Washington 2,010 202.0 74.3 256 2.4 4.0 6.6 12.3
West Virginia 526 63.8 59.8 375 10.6 19.6 29.8 43.2
Wisconsin 54,185 58.3 71.3 269 4.0 7.7 12.5 21.5
Wyoming 27 122.4 69.7 253 11.1 33.3 33.3 44.4
United States 240,938 85.7 69.4 295 4.7 9.4 15.5 26.4

1All herd test days with usable records.
2Cows with usable records (less than total cows on test).
3Herd test days with ≥10 usable records.


Table 2. National averages of herd test days for milk yield and SCC of milk by year.

SCC characteristics
  Herd test days1 Cows2 per herd Average daily milk yield Average SCC Herd test days3 with SCC greater than
Year (no.) (no.) (lbs) (cells/ml, 1000's) 750,000 cells/ml
(%)
600,000 cells/ml
(%)
500,000 cells/ml
(%)
400,000 cells/ml
(%)
1995 265,844 50.0 65.3 304 4.1 9.3 16.0 27.2
1996 255,039 55.5 64.7 308 4.1 9.2 16.1 27.8
1997 287,789 57.4 66.4 314 4.2 9.5 16.6 28.8
1998 283,695 60.8 66.8 318 4.5 10.1 17.8 30.3
1999 273,364 67.0 68.2 311 4.3 9.7 17.1 29.8
2000 260,139 73.3 69.1 316 4.1 9.4 16.8 29.5
2001 244,940 79.1 69.0 322 4.9 10.6 18.2 31.1
20024 267,809 77.5 69.5 320 5.6 11.0 18.1 30.0
2003 251,182 80.5 69.6 319 5.6 11.2 18.4 30.4
2004 240,938 85.7 69.4 295 4.7 9.4 15.5 26.4
Annual trend -2,912 4.0*** 0.6*** 0.3 0.1* 0.1 0.1 0.1

1All herd test days with usable records.
2Cows with usable records (less than total cows on test).
3Herd test days with ≥10 usable records.
4Summary characteristics for 2002 were recalculated to include more complete data than was available at the time of initial reporting.
*The linear regression was significant with P ≤ .05
***The linear regression was significant with P ≤ .001


Table 3. Characteristics of DHI herd test days for milk yield and SCC during 2004, by herd size.

SCC characteristics by herd size  
Herd size1 Herd test days2 Cows3 per herd Average daily milk yield Average SCC Herd test days4 with SCC greater than
(cows) (no.) (no.) (lbs) (cells/ml, 1000's) 750,000 cells/ml
(%)
600,000 cells/ml
(%)
500,000 cells/ml
(%)
400,000 cells/ml
(%)
<50 138,524 25.6 62.5 327 7.0 12.7 19.4 30.0
50 - 99 62,107 68.1 66.8 306 2.0 5.9 12.1 24.1
100 - 149 15,389 120.2 67.9 296 1.2 4.1 9.1 20.3
150 - 199 6,983 171.9 69.4 287 1.0 3.4 7.6 17.4
200 - 299 6,326 242.4 70.7 282 0.9 2.7 6.8 15.8
300 - 499 5,435 385.2 73.2 279 1.0 2.6 5.6 13.8
500 - 999 4,024 696.5 74.0 286 1.2 3.4 7.0 14.9
1000 - 1999 1,745 1299.9 73.6 287 0.5 2.0 4.9 13.2
2000 - 2999 290 2418.9 77.1 264 1.0 1.0 2.4 7.6
>3000 115 3720.9 70.2 269 0.0 0.0 0.9 5.2
All herds 240,938 85.7 69.4 295 4.7 9.4 15.5 26.4

1Number of cows reported in milk.
2All herd test days with usable records.
3Cows with usable records (less than total cows on test).
4Herd test days with ≥10 usable records.

Table 4. Characteristics of DHI herd test days for milk yield and SCC during 2004, by month.

SCC seasonal characteristics
  Herd test days1 Cows2 per herd Average daily milk yield Average SCC Herd test days3 with SCC greater than
Month (no.) (no.) (lbs) (cells/ml, 1000's) 750,000 cells/ml
(%)
600,000 cells/ml
(%)
500,000 cells/ml
(%)
400,000 cells/ml
(%)
January 20,505 84.8 70.4 301 4.1 8.4 14.3 24.8
February 20,151 85.3 70.7 306 4.7 9.2 15.1 25.3
March 20,700 85.6 70.6 313 5.1 9.9 16.3 27.4
April 20,351 86.5 70.8 298 4.5 8.9 14.8 25.3
May 20,629 86.1 70.8 294 4.5 8.9 14.6 25.0
June 19,665 85.1 70.1 305 5.3 10.6 17.6 29.5
July 20,554 84.8 68.6 313 5.9 11.8 19.1 31.5
August 19,097 85.7 68.3 313 6.1 12.3 19.7 32.0
September 19,703 85.9 67.2 297 5.1 10.3 17.0 29.5
October 20,431 85.8 67.4 269 3.8 7.9 13.3 23.2
November 18,741 86.1 68.0 267 3.7 7.0 12.1 21.5
December 20,411 87.2 69.5 265 3.8 7.7 12.5 21.6
Yearly 240,938 85.7 69.4 295 4.7 9.4 15.5 26.4

1All herd test days with usable records.
2Cows with usable records (less than total cows on test).
3Herd test days with ≥10 usable records.

Figure 1. Average daily milk yield and Average SCC during 2004, by month.

SCC seasonal characteristics