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SCC12 (2-11)

Somatic cell counts of milk from Dairy Herd Improvement herds during 2010

H.D. Norman, T.A. Cooper, and F.A. Ross, Jr.
Animal Improvement Programs Laboratory, Agricultural Research Service, USDA, Beltsville, MD 20705-2350
301-504-8334 (voice) ~ 301-504-8092 (fax) ~ duane.norman@ars.usda.gov ~ http://aipl.arsusda.gov

Test-day data from all herds enrolled in Dairy Herd Improvement (DHI) somatic cell testing during 2010 (95% 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 editing procedures were excluded. Somatic cell score (SCS) of milk is reported to the Animal Improvement Programs Laboratory as part of an individual cow's test-day yield information. The SCS was converted back to a milk somatic cell count (SCC) in cells per milliliter [SCC = 2(SCS − 3) × 100,000] for calculating herd and State averages.

Table 1 shows total number of herd test days, average number of cows with usable records per herd on test day, average test-day milk yield and herd SCC, and 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 (excluding Alaska and Hawaii but including Puerto Rico and Mexico) during 2010. The current legal limit for bulk tank SCC is 750,000 cells/mL for Grade A producers; lowering the limit to 400,000 cells/mL has been proposed as a future maximum by the National Mastitis Council and National Milk Proders Federation. State and national average daily milk yields were weighted by number of cows per herd test day. Herd, State, and national average SCCs were based on SCCs from individual cows that were weighted by milk yield on test day. Percentages of herd test days by SCC level are for herd test days that included records from at least 10 cows. Nationally, average test-day herd SCC during 2010 was 228,000 cells/mL, which was lower (by 5,000 cells/mL) than last year's average. Thirty-two States and Puerto Rico had lower average SCC than reported last year; 14 States had higher averages. A few large Mexican herds tested through the U.S. system were included for the first time.

Variation among States (Table 1) remains large. State average SCC generally was lower than the national average for mountain and western States and often higher for southeastern States (Figure 1). Although climatic conditions (temperature and humidity) surely contributed to regional SCC differences, differences between adjacent States were substantial, which suggests that herd size and mastitis-control practices, including genetic selection, are impacting State differences as well.

Corresponding national averages for 1995 through 2010 are in Table 2. National average test-day herd SCC has declined every year since 2005 and every year except one since 2001, which had the highest SCC at 322,000 cells/mL. The percentage of herd test days that exceeded 750,000, 600,000, 500,000, and 400,000 cells/mL during 2010 was 2.7, 5.7, 10.0, and 18.0, respectively, which was lower for 3 of the 4 levels than during 2009. The 2.7% of 2010 DHI herd test days that were higher than the present legal limit for bulk tank SCC may overestimate the percentage of herds that shipped milk exceeding the legal limit because milk of cows treated for mastitis is excluded from the bulk tank even though included in DHI test data. The percentage of herd test days that exceeded the legal limit also would have been higher than the percentage of herds that were rejected from the market because market exclusion only occurs after repeated violations.

Herd size and average test-day milk yield have been increasing since 1995 (Table 2). The large increase in cows per herd from 2005 (90.3) to 2006 (115.5) resulted partly because of a policy change to allow records with missing sire identification in the national database. The increase in cows per herd continued from 2009 (136.1) to 2010 (145.2). The large decline in SCC since 2001 is encouraging. Hopefully, this trend will continue as a result of improved management practices, genetic selection, and directed culling. The trend for lower percentage of herd test days above the thresholds is likewise encouraging. The concerted effort to improve milk quality will need to continue, especially now that the SCC standards are expected to be lowered. State and national SCCs for DHI herds will continue to be summarized annually so that changes in SCCs and implications for milk quality can be monitored.

Table 3 reports average test-day milk yield and herd SCC during 2010 by estimated herd size. As herd size increased, milk yield generally increased as SCC declined. The most challenging concern remains the percentage of test days for small herds over the current and proposed limits. For example, if the limit is lowered to 600,000 cell/mL on January 1, 2012, 9.6% of monthly tests would be above the limit for herds with fewer than 50 cows, 4.1% for herds with 50 to 99 cows, and 2.3% for herds with 100 to 149 cows. Because small herds no longer contribute as much to the total milk supply (see herd-average reports), the trend for increased herd size improves milk quality because large herds have lower SCCs on average.

Table 4 reports average test-day milk yield and herd SCC during 2010 by month, and the seasonal pattern for milk quality is shown in Figure 2. Milk yield per cow was highest in the spring and declined through the summer months; SCC increased from May through August and then declined quickly from September through November. The highest quality milk was produced in November and December.


Table 1. Characteristics of test-day milk yield and somatic cell count (SCC) from Dairy Herd Improvement herds by State during 2010

SCC characteristics  
  Herd test days1 Cows2 per herd Average daily milk yield Average SCC Herd test days3 with SCC greater than
State (no.) (no.) (lb) (cells/mL, 1,000s) 750,000 cells/mL
(%)
600,000 cells/mL
(%)
500,000 cells/mL
(%)
400,000 cells/mL
(%)
Alabama 217 125.0 48.5 415 1.4 10.1 24.4 47.0
Arizona 165 1,646.5 65.5 189 0.6 0.6 0.6 4.2
Arkansas 236 100.6 53.0 421 14.0 23.3 31.8 48.3
California 7,908 804.0 74.0 200 0.9 1.8 3.0 6.3
Colorado 264 671.1 75.2 223 2.3 3.0 4.2 6.4
Connecticut 721 106.6 69.4 211 2.4 3.9 7.1 11.1
Delaware 218 120.1 69.6 257 0.0 0.5 2.3 7.3
Florida 215 755.9 68.6 274 5.6 15.3 24.7 35.3
Georgia 995 185.7 64.0 337 4.7 12.5 23.8 41.5
Idaho 1,515 866.2 74.2 170 1.1 2.8 4.2 7.2
Illinois 3,656 95.8 70.3 263 3.2 7.3 12.1 22.4
Indiana 3,045 100.8 70.5 258 3.3 7.2 13.2 21.6
Iowa 7,446 123.1 74.2 247 3.6 7.4 12.8 22.7
Kansas 1,671 87.8 65.9 308 2.5 8.2 15.8 28.5
Kentucky 1,443 81.6 65.2 313 7.3 14.1 21.7 34.0
Louisiana 282 129.2 53.8 380 5.3 13.8 24.8 44.3
Maine 1,253 79.6 65.6 219 1.5 4.2 8.0 14.8
Maryland 2,981 87.9 67.7 239 2.1 4.3 8.0 15.4
Massachusetts 694 68.6 65.6 238 3.3 5.8 9.2 16.7
Michigan 6,240 177.4 77.7 182 0.9 2.3 4.2 8.3
Minnesota 21,800 94.8 72.7 279 3.4 7.6 13.8 24.9
Mississippi 281 164.8 62.9 290 3.6 10.0 17.8 34.5
Missouri 2,883 74.9 60.4 366 9.9 17.0 25.3 38.8
Montana 251 138.0 75.2 199 0.8 1.6 2.8 5.6
Nebraska 1,133 156.3 69.5 261 3.3 7.4 13.9 25.9
Nevada 78 486.6 80.3 220 0.0 0.0 0.0 7.7
New Hampshire 765 105.0 72.4 216 0.8 2.0 4.8 11.8
New Jersey 424 62.5 65.3 302 2.6 7.8 13.2 24.8
New Mexico 204 1,969.7 70.3 217 0.0 0.0 2.0 5.9
New York 17,377 140.1 74.2 216 1.2 3.3 6.5 12.8
North Carolina 1,286 145.1 66.2 279 3.1 7.1 13.5 26.4
North Dakota 279 117.3 67.9 296 2.2 5.7 13.3 20.8
Ohio 7,548 109.0 71.1 223 1.4 3.7 7.0 13.8
Oklahoma 477 85.8 59.7 346 6.9 12.8 23.5 38.2
Oregon 1,789 194.1 65.4 173 1.1 2.1 3.0 5.8
Pennsylvania 39,523 62.4 70.8 247 2.1 4.8 8.8 16.8
Puerto Rico 871 42.4 38.6 356 10.9 13.5 18.5 27.9
Rhode Island 71 48.5 62.3 190 1.4 1.4 1.4 5.6
South Carolina 511 156.9 63.3 349 2.3 7.4 16.6 33.3
South Dakota 1,124 206.0 71.9 250 3.6 8.0 15.1 27.4
Tennessee 1,127 97.3 60.7 396 9.3 20.0 30.6 47.6
Texas 1,102 596.1 62.4 233 0.5 2.3 5.8 13.5
Utah 1,100 217.9 70.9 197 2.5 5.2 7.9 12.7
Vermont 3,137 110.5 68.5 190 0.9 2.2 4.3 9.2
Virginia 3,717 123.2 70.0 285 3.3 7.3 13.7 25.3
Washington 1,569 294.7 74.0 191 1.4 2.4 4.0 7.8
West Virginia 305 89.2 58.5 309 2.3 6.9 16.7 27.5
Wisconsin 46,270 100.8 77.1 239 3.2 6.1 10.0 17.2
Wyoming 21 225.7 71.0 176 0.0 0.0 9.5 9.5
United States 198,218 145.2 72.7 228 2.7 5.7 10.0 18.0
Mexico 30 675.8 68.2 285 0.0 0.0 0.0 10.0

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 for test-day milk yield and somatic cell count (SCC) from Dairy Herd Improvement herds 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.) (lb) (cells/mL, 1,000s) 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
2005 234,585 90.3 71.1 296 4.7 9.2 15.3 25.8
20065 236,191 115.5 71.2 288 3.9 8.3 14.3 25.2
2007 227,626 125.1 71.4 276 3.5 7.6 13.4 24.0
2008 222,245 131.7 71.7 262 3.4 7.1 12.5 22.4
2009 204,195 136.1 72.1 233 2.7 5.8 10.3 18.9
2010 198,218 145.2 72.7 228 2.7 5.7 10.0 18.0
Annual trend -4,879 6.3*** 0.5*** -5.0*** -0.1 -0.2** -0.4*** -0.7***

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 were available at the time of initial reporting.
5Cows per herd included records with missing sire identification starting in 2006; those records were not included in prior years.
**Significant (P ≤ 0.01) linear regression.
***Significant (P ≤ 0.001) linear regression.


Table 3. Characteristics of test-day milk yield and somatic cell count (SCC) from Dairy Herd Improvement herds during 2010 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.) (lb) (cells/mL, 1,000s) 750,000 cells/mL
(%)
600,000 cells/mL
(%)
500,000 cells/mL
(%)
400,000 cells/mL
(%)
<50 84,002 28.9 62.5 286 5.1 9.6 14.9 23.8
50 - 99 59,558 69.3 67.3 265 1.3 4.1 8.6 17.5
100 - 149 19,354 120.3 69.1 251 0.7 2.3 5.8 13.9
150 - 199 8,701 171.8 71.0 245 0.4 1.7 4.8 11.8
200 - 299 8,313 243.3 73.2 233 0.2 1.0 3.3 9.2
300 - 499 7,448 387.0 76.0 219 0.2 0.8 1.9 5.3
500 - 999 6,062 694.6 77.2 217 0.2 0.7 1.5 4.5
1000 - 1999 3,349 1,371.6 76.8 207 0.0 0.0 0.3 2.3
2000 - 2999 837 2,420.6 76.2 198 0.0 0.0 0.1 2.0
>3000 594 4,478.5 73.9 184 0.0 0.0 0.0 0.7
All herds 198,218 145.2 72.7 228 2.7 5.7 10.0 18.0

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 test-day milk yield and somatic cell count (SCC) from Dairy Herd Improvement herds during 2010 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.) (lb) (cells/mL, 1,000s) 750,000 cells/mL
(%)
600,000 cells/mL
(%)
500,000 cells/mL
(%)
400,000 cells/mL
(%)
January 17,079 142.3 72.6 229 2.7 5.6 9.7 17.5
February 15,927 143.8 73.2 233 2.9 6.0 10.2 17.9
March 17,270 141.4 73.6 236 3.0 6.3 10.6 18.9
April 16,818 146.0 74.7 222 2.3 4.9 8.7 16.3
May 16,771 146.8 74.7 221 2.1 4.5 8.1 15.4
June 16,325 144.8 74.4 231 2.7 5.8 10.3 18.9
July 16,764 144.6 72.8 243 3.3 7.2 12.8 22.0
August 15,699 144.2 71.5 251 4.3 9.0 15.1 25.8
September 16,341 144.9 71.1 237 3.2 6.9 12.2 21.5
October 16,855 147.9 70.6 218 2.0 4.2 8.1 15.9
November 15,556 147.2 71.4 207 1.9 3.9 7.0 13.2
December 16,813 148.1 72.1 208 1.9 4.0 7.0 13.2
Yearly 198,218 145.2 72.7 228 2.7 5.7 10.0 18.0

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 test-day somatic cell count from Dairy Herd Improvement herds during 2010 by State



Figure 2. Average test-day milk yield and somatic cell count (SCC) from Dairy Herd Improvement herds during 2010 by month