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AIPL RESEARCH REPORT
SCC8 (2-07)

Somatic cell counts of milk from Dairy Herd Improvement herds during 2006

R.H. Miller, H.D. Norman, and L.L.M. Thornton
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 2006 (94% 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 some Animal Improvement Programs Laboratory (AIPL) editing procedures were excluded. Somatic cell score (SCS) is reported to AIPL as part of an individual cow's test-day yield information. It 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 test-day 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 2006. The current legal limit for bulk tank SCC is 750,000 cells/ml for Grade A producers; the lower limits shown have been proposed on a number of occasions as future maximums by the National Mastitis Council. 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 2006 was 288,000 cells/ml, which is considerably lower than last year.

Variation among States (Table 1) was large. State average SCC's were often lower than the national average in the Northeast and the far West and higher in the Southeast, a finding consistent with previous reports. Although climatic conditions (temperature and humidity) surely 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.

Corresponding averages for earlier years 1995 through 2005 are in Table 2. The percentage of herd test days that exceeded 750,000, 600,000, 500,000, and 400,000 cells/ml during 2006 was 3.9, 8.3, 14.3, and 25.2, respectively. The overall percentage of herd test days exceeding all levels were slightly lower than in 2005. Thus, 3.9% 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 even though 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.

Herd size and average daily milk yield have been increasing since 1995 (Table 2). However, the large increase in cows per herd for 2006 over 2005 (115.5 vs. 90.3) is primarily due to a new policy of accepting records missing sire identification into the national database. The trend for SCC over the last five years are encouraging; hopefully mean SCC will decline in the future due to both genetic selection and 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 in the past. The concerted effort to improve milk quality needs to continue. State and national SCC's in DHI 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, and average SCC declined until herd size exceeded 1000 cows. The most disturbing result was the percentage of test days with over 750,000 cells/ml. For herds with less than 50 cows, this occurred 6.4% of the time. It occurred on 2.2% of the test days for herds with 50 to 99 cows and on 1.1% of the test days for the remaining herds.

Table 4 provides the same statistics for all herds by month. The seasonal pattern of milk quality is illustrated in Figure 1. Milk yield per cow was highest in the late winter and spring. Milk yield and quality decline during the summer months, and the highest quality milk is produced from October to June.


Table 1. Characteristics of DHI herd test days for milk yield and SCC of milk by State during 2006.

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 276 117.8 52.4 432 3.6 14.5 28.6 50.4
Arizona 266 1276.5 69.0 283 0.4 1.9 7.1 14.3
Arkansas 377 110.2 57.4 457 13.0 20.4 33.7 48.0
California 9,721 653.8 73.0 291 4.0 6.9 11.4 20.7
Colorado 337 550.9 72.2 257 0.3 2.1 5.9 12.8
Connecticut 816 87.7 67.4 288 3.8 6.3 12.3 21.6
Delaware 243 113.4 69.2 291 1.6 3.3 7.0 17.3
Florida 208 655.8 69.0 319 11.1 25.5 40.4 55.8
Georgia 1,218 123.7 61.3 428 7.1 17.7 30.7 49.7
Idaho 1,712 581.1 75.4 282 5.0 8.7 13.5 21.4
Illinois 4,707 77.5 68.6 311 3.5 7.4 13.8 26.4
Indiana 3,867 77.2 68.8 327 6.5 13.5 21.4 33.9
Iowa 9,666 81.4 70.8 295 3.6 8.1 15.1 27.6
Kansas 2,201 84.5 68.5 327 5.0 9.9 18.5 32.4
Kentucky 1,798 76.1 63.0 395 7.6 16.8 28.4 46.1
Louisiana 509 97.6 52.7 456 10.6 24.8 44.0 69.0
Maine 1,185 69.9 63.4 279 3.2 7.3 13.8 23.7
Maryland 3,752 77.3 66.0 296 2.8 6.5 12.2 24.0
Massachusetts 876 78.9 69.6 263 1.9 6.3 11.3 20.7
Michigan 7,883 133.5 77.7 268 3.7 7.4 12.6 21.4
Minnesota 26,139 75.1 69.6 319 5.2 11.3 19.1 32.2
Mississippi 318 141.6 64.3 368 6.9 14.8 25.8 44.3
Missouri 3,779 60.1 60.5 365 6.7 12.6 20.9 34.9
Montana 433 108.4 73.0 221 0.0 0.2 1.6 8.3
Nebraska 1,748 116.4 68.4 320 5.3 11.1 17.7 29.8
Nevada 121 448.1 76.6 276 5.8 8.3 10.7 18.2
New Hampshire 892 84.2 69.3 243 2.0 4.8 9.2 17.5
New Jersey 574 66.8 66.2 334 3.5 8.7 18.6 32.1
New Mexico 284 1295.6 74.2 269 4.6 7.7 11.3 18.0
New York 20,867 103.1 70.9 256 2.5 6.0 11.1 21.0
North Carolina 1,751 115.7 67.4 355 4.9 11.5 22.3 37.7
North Dakota 431 84.6 70.6 294 2.6 5.1 10.4 19.5
Ohio 8,882 82.3 68.7 309 4.3 9.1 15.4 27.0
Oklahoma 711 112.2 63.7 333 3.9 8.4 15.3 31.6
Oregon 2,372 149.4 67.9 223 1.9 3.5 5.7 10.2
Pennsylvania 43,327 57.4 69.5 306 3.6 8.1 14.5 26.0
Puerto Rico 730 30.6 39.8 443 24.0 30.7 36.0 44.1
Rhode Island 52 58.8 66.1 135 1.9 1.9 1.9 1.9
South Carolina 537 161.5 62.5 383 3.5 9.5 20.3 38.5
South Dakota 1,487 121.5 69.6 308 5.7 12.7 21.6 37.9
Tennessee 1,696 82.4 59.9 463 10.3 23.8 39.2 59.0
Texas 1,667 359.7 64.1 282 1.3 4.1 8.5 18.5
Utah 1,577 159.9 70.8 255 3.8 7.4 13.1 20.7
Vermont 3,654 95.3 67.7 250 1.5 3.9 8.3 16.7
Virginia 4,302 103.7 68.8 331 2.6 7.3 14.6 28.5
Washington 1,855 230.4 75.1 251 2.0 3.1 4.6 9.3
West Virginia 452 79.9 60.3 323 4.2 10.4 17.5 31.0
Wisconsin 53,898 74.4 74.3 258 3.5 6.7 11.2 19.9
Wyoming 37 120.8 73.7 277 0.0 0.0 2.7 2.7
United States 236,191 115.5 71.2 288 3.9 8.3 14.3 25.2

1All herd test days with usable records. This includes records missing sire identification but having acceptable information in other field.
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
2002 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
2005 234,585 90.3 71.6 296 4.7 9.2 15.3 25.8
2006 236,191 115.54 71.2 288 3.9 8.3 14.3 25.2
Annual trend -3,611 4.8*** 0.6*** -1.3 0.1 -0.0 -0.1 -0.2

1All herd test days with usable records.
2Cows with usable records (less than total cows on test).
3Herd test days with ≥10 usable records.
4Cows per herd included those records missing sire identification in 2006; they were not included in prior years.
***The linear regression was significant with P ≤ .001

Table 3. Characteristics of DHI herd test days for milk yield and SCC during 2006, 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
(%)
<50 110,767 28.9 62.9 323 6.4 12.1 18.8 29.7
50 - 99 71,237 69.0 67.3 303 2.2 6.1 12.4 24.2
100 - 149 19,970 120.4 68.5 288 1.2 4.1 9.5 20.3
150 - 199 9,154 171.8 70.5 287 1.3 3.7 8.6 18.9
200 - 299 8,552 242.2 72.1 278 0.8 2.7 6.6 16.1
300 - 499 7,328 384.8 74.2 278 1.3 2.9 6.2 13.7
500 - 999 5,516 686.3 75.7 278 0.7 2.4 5.9 14.3
1000 - 1999 2,737 1353.4 75.5 288 0.7 2.3 5.4 15.6
2000 - 2999 601 2415.8 75.0 251 0.3 2.0 3.0 6.8
>3000 329 4085.2 74.8 282 0.3 0.6 0.6 8.2
All herds 236,191 115.5 71.2 288 3.9 8.3 14.3 25.2

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 2006, 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 20,213 110.0 72.4 303 4.2 8.7 14.9 25.4
February 19,348 113.0 73.3 292 3.9 7.9 13.8 23.7
March 21,324 116.3 73.2 289 3.7 7.8 13.3 23.8
April 20,109 118.2 73.0 290 3.5 7.6 13.1 23.5
May 19,613 116.1 73.0 282 3.2 6.8 11.8 21.7
June 20,064 114.7 72.2 290 3.6 7.8 13.4 24.8
July 18,862 114.5 70.1 297 4.1 9.0 15.9 27.7
August 19,148 116.0 69.1 319 6.0 12.1 20.2 33.4
September 19,436 117.1 69.0 302 5.1 10.4 18.0 30.6
October 19,074 115.0 68.6 281 4.0 8.4 14.3 25.9
November 19,121 117.2 69.4 261 3.0 6.7 12.3 22.0
December 19,879 117.6 70.9 255 2.9 6.2 11.4 20.7
Yearly 236,191 115.5 71.2 288 3.9 8.3 14.3 25.2

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 2006, by month.

SCC seasonal characteristics  
Plot of mmilk * mos