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SCC11 (2-10)

Somatic cell counts of milk from Dairy Herd Improvement herds during 2009

H.D. Norman, R.H. Miller, and T.A. Cooper
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 2009 (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 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) during 2009. The current legal limit for bulk tank SCC is 750,000 cells/ml for Grade A producers; the lower limits have been proposed as future maximums by the National Mastitis Council. California's current legal SCC limit is 600,000 cells/ml. 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 2009 was 233,000 cells/ml, which was considerably lower (by 29,000 cells/ml) than last year's average. Most States (42) had lower average SCC than reported last year; only 6 States had higher averages.

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 2009 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 2009 was 2.7, 5.8, 10.3, and 18.9, respectively, which was lower for all levels than during 2008. The 2.7% of 2009 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 to 2006 (90.3 versus 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 2008 to 2009 (131.7 versus 136.1). The large decline in SCC since 2001 is encouraging as well as surprising considering the annaul changes before 2001. Hopefully, this trend will continue because of improved management practices and genetic selection. The trend for lower percentage of herd test days above all thresholds is likewise encouraging. The concerted effort to improve milk quality needs to continue. 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 2009 by estimated herd size. As herd size increased, milk yield generally increased as SCC declined. The most disturbing result still remains the percentage of test days with over 750,000 cells/ml: 5.0% for herds with less than 50 cows, 1.4% for herds with 50 to 99 cows, 0.7% for herds with 100 to 149 cows, and an average of 0.3% for herds with 150 cows or more. 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 2009 by month, and the seasonal pattern for milk quality is shown in Figure 2. Milk yield per cow was highest in the winter and spring and declined during the summer months; SCC increased from April through July and then decreased from July 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 2009

SCC characteristics  
  Herd test days1 Cows2 per herd Average test-day milk yield Average test-day herd SCC Herd test days3 with SCC greater than
State (no.) (no.) (lb) (cells/ml, 1000s) 750,000 cells/ml
(%)
600,000 cells/ml
(%)
500,000 cells/ml
(%)
400,000 cells/ml
(%)
Alabama 237 118.6 50.0 440 3.4 16.0 29.1 51.5
Arizona 186 1305.4 68.0 174 0.0 0.0 0.0 0.0
Arkansas 243 99.7 54.8 421 11.9 19.3 25.5 40.3
California 8,056 763.7 73.1 198 0.9 1.7 2.9 6.2
Colorado 263 612.0 76.6 219 0.0 0.0 0.0 1.9
Connecticut 719 104.3 67.8 233 1.7 3.3 7.5 14.2
Delaware 226 119.9 70.5 291 0.4 1.3 4.9 13.3
Florida 189 648.2 67.9 323 8.5 14.8 26.5 40.7
Georgia 1,032 179.3 64.2 350 4.4 14.5 24.8 43.4
Idaho 1,493 821.3 72.5 192 1.4 2.8 4.2 8.5
Illinois 3,859 92.2 70.0 272 3.6 7.2 12.6 23.8
Indiana 3,187 87.7 69.1 260 3.3 6.9 12.2 21.8
Iowa 7,821 113.8 74.2 257 3.7 7.3 12.6 23.9
Kansas 1,809 83.6 66.8 292 2.3 5.6 12.8 25.3
Kentucky 1,486 81.3 65.4 317 5.5 11.2 18.8 30.1
Louisiana 312 122.0 55.3 412 7.1 17.3 34.9 52.6
Maine 1,283 74.3 64.0 248 1.7 5.5 9.9 17.8
Maryland 3,228 83.9 66.9 267 3.3 6.9 12.6 21.3
Massachusetts 758 69.6 65.2 246 2.5 6.1 10.2 17.8
Michigan 6,556 164.6 76.2 189 1.0 2.4 4.9 10.3
Minnesota 22,492 88.5 72.3 280 3.5 7.6 13.9 24.8
Mississippi 285 150.8 61.8 320 3.9 10.5 20.4 36.1
Missouri 3,150 72.2 60.6 345 6.2 12.1 19.4 30.8
Montana 273 139.5 74.5 197 0.0 0.0 0.0 3.3
Nebraska 1,250 138.1 68.7 263 3.5 7.0 11.4 21.6
Nevada 103 499.5 79.6 197 0.0 0.0 0.0 2.9
New Hampshire 820 96.5 70.7 224 2.7 5.5 8.9 13.3
New Jersey 459 66.8 67.2 319 2.0 8.5 15.7 28.1
New Mexico 227 1802.4 72.7 241 2.2 4.0 4.8 6.6
New York 17,837 125.8 73.4 230 1.8 4.2 8.1 15.8
North Carolina 1,360 132.6 66.5 307 2.7 6.8 14.9 28.8
North Dakota 310 103.6 65.4 297 4.5 8.4 14.2 26.5
Ohio 7,689 109.9 70.7 222 1.6 4.0 7.4 14.9
Oklahoma 480 93.5 59.9 321 6.3 11.0 19.0 33.1
Oregon 1,905 198.7 65.0 183 1.7 2.6 4.2 7.9
Pennsylvania 40,202 59.9 70.3 264 2.6 5.7 10.5 19.5
Puerto Rico 713 79.4 39.5 368 9.1 14.9 19.9 33.4
Rhode Island 75 51.9 62.7 208 4.0 5.3 5.3 14.7
South Carolina 510 157.3 64.1 343 1.4 6.1 15.1 29.8
South Dakota 1,179 198.5 71.9 257 4.4 10.7 17.0 27.9
Tennessee 1,197 93.7 61.7 375 6.1 13.4 24.1 42.1
Texas 1,202 478.6 62.3 232 0.3 1.8 4.8 12.4
Utah 1,066 216.3 69.5 193 2.7 5.0 7.5 11.7
Vermont 3,173 104.4 67.5 206 1.3 3.0 5.4 10.1
Virginia 3,766 118.6 69.6 287 2.1 6.2 12.7 24.9
Washington 1,570 258.1 73.2 194 0.8 1.6 2.8 6.9
West Virginia 376 91.2 58.1 317 3.2 6.4 13.3 29.3
Wisconsin 47,566 95.1 76.3 239 2.8 5.6 9.5 17.0
Wyoming 17 190.4 69.4 174 0.0 0.0 0.0 0.0
United States 204,195 136.1 72.1 233 2.7 5.8 10.3 18.9

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 test-day milk yield Average test-day herd SCC Herd test days3 with SCC greater than
Year (no.) (no.) (lb) (cells/ml, 1000s) 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
Annual trend -5,016 6.4*** 0.5*** -5.4*** -0.1* -0.3** -0.5*** -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 ≤ 0.05) linear regression.
**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 2009 by herd size

SCC characteristics by herd size  
Herd size1 Herd test days2 Cows3 per herd Average test-day milk yield Average test-day herd SCC Herd test days4 with SCC greater than
(cows) (no.) (no.) (lb) (cells/ml, 1000s) 750,000 cells/ml
(%)
600,000 cells/ml
(%)
500,000 cells/ml
(%)
400,000 cells/ml
(%)
<50 89,043 28.8 62.4 289 5.0 9.4 15.0 24.4
50 - 99 61,452 69.2 67.3 271 1.4 4.1 8.9 18.5
100 - 149 19,240 120.3 68.8 258 0.7 2.8 6.4 14.7
150 - 199 8,678 172.2 70.7 250 0.5 2.1 5.5 12.7
200 - 299 8,257 242.5 72.8 239 0.3 1.0 3.2 9.7
300 - 499 7,193 385.9 75.4 224 0.2 1.0 2.2 6.3
500 - 999 5,888 690.9 76.6 216 0.2 0.6 1.4 4.0
1000 - 1999 3,136 1368.8 76.2 209 0.1 0.4 1.0 3.1
2000 - 2999 817 2427.5 74.8 199 0.0 0.0 0.0 1.6
>3000 491 4143.4 73.6 195 0.2 0.2 0.2 0.2
All herds 204,195 136.1 72.1 233 2.7 5.8 10.3 18.9

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 2009 by month

SCC seasonal characteristics  
  Herd test days1 Cows2 per herd Average test-day milk yield Average test-day herd SCC Herd test days3 with SCC greater than
Month (no.) (no.) (lb) (cells/ml, 1000s) 750,000 cells/ml
(%)
600,000 cells/ml
(%)
500,000 cells/ml
(%)
400,000 cells/ml
(%)
January 18,338 137.1 72.0 251 3.1 6.7 11.7 20.2
February 17,055 135.4 72.8 241 3.4 6.4 11.4 19.9
March 17,808 133.8 72.9 233 2.9 6.0 10.5 19.0
April 17,492 136.4 73.5 224 2.3 5.1 9.4 17.4
May 17,886 135.3 74.0 225 2.3 4.9 9.0 17.1
June 16,385 137.2 73.5 232 2.5 5.3 9.6 18.3
July 17,383 133.3 72.3 249 3.0 7.0 12.4 22.3
August 16,065 137.2 71.0 248 3.5 7.2 12.4 22.6
September 16,406 136.3 70.3 239 2.9 6.2 11.3 20.5
October 16,982 136.6 70.1 224 2.2 5.1 9.3 17.5
November 15,507 138.4 70.4 214 2.0 4.4 8.4 16.0
December 16,888 136.2 71.8 216 2.1 4.7 8.5 16.0
Yearly 204,195 136.1 72.1 233 2.7 5.8 10.3 18.9

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 2009 by State



Figure 2. Average test-day milk yield and somatic cell count (SCC) from Dairy Herd Improvement herds during 2009 by month