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SCC16 (2-15)

Somatic cell counts of milk from Dairy Herd Improvement herds during

H.D. Norman and L.M. Walton
Council on Dairy Cattle Breeding, Beltsville, MD 20705-2350
301-525-2006 ~ duane.norman@uscdcb.com ~ https://uscdcb.com

Test-day data from all herds enrolled in Dairy Herd Improvement (DHI) somatic cell testing during 2014 (98% of all DHI herds and 96% of DHI cows) 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 Council on Dairy Cattle Breeding editing requirements were excluded. Somatic cell score (SCS) of milk is reported to the Council on Dairy Cattle Breeding 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 (also includes Mexico) during . 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 periodically as a future maximum by several dairy organizations (Norman et al., 2011). 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 2014 was 200,000 cells/mL, which was only slightly higher (by 1,000 cells/mL) than last year's average.

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 a few southeastern states (Figure 1). Although climatic conditions (temperature and humidity) surely contributed to regional SCC levels, differences between adjacent states were substantial, suggesting herd size and mastitis-control practices are impacting state differences as well.

Corresponding national averages for 1995 through are in Table 2. National average test-day herd SCC had 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 2014 was 1.6, 3.4, 6.0, and 11.6, respectively. The 1.6% of 2014 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 (Table 2). The increase in cows per herd continued from 167.8 in 2013 to 177.3 in 2014. Test-day milk yield was up from 75.2 to 76.2 pounds. The large decline in SCC since 2001 has been beneficial. Hopefully, a reduction will continue as a result of improved management practices and directed culling. Genetic selection is expected to contribute more to future improvement in milk quality than in the past. The trend for lower percentage of herd test-days above the thresholds is likewise encouraging. Milk quality is benefitting from the European Union standards which are being forced upon many of the US dairy producers. 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 by herd size. As herd size increased, milk yield generally increased and 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 400,000 cell/mL, 16.9% of monthly tests would be above the limit for herds with fewer than 50 cows, 10.9% for herds with 50 to 99 cows, and 8.4% for herds with 100 to 149 cows. In contrast, only 1.1% of the monthly tests were above for herds with over 3,000 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 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 April through July and then declined quickly from August through November. The highest quality milk was produced in January and November.


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

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, 1000's) 750,000 cells/ml
(%)
600,000 cells/ml
(%)
500,000 cells/ml
(%)
400,000 cells/ml
(%)
Alabama 180 93.5 53.7 349 1.1 10.0 16.7 35.6
Arizona 132 1767.0 67.9 189 0.0 2.3 3.0 8.3
Arkansas 168 94.8 54.5 371 8.9 17.3 26.2 40.5
California 6,948 913.7 77.6 178 0.7 1.4 2.1 4.0
Colorado 221 1314.9 74.0 210 0.0 2.3 3.2 10.0
Connecticut 777 121.8 74.0 177 0.8 1.8 3.5 6.2
Delaware 179 173.7 75.4 225 0.0 0.0 1.7 3.9
Florida 251 970.9 74.2 233 1.6 4.0 8.0 15.5
Georgia 907 224.6 68.8 271 1.5 5.0 10.5 20.3
Idaho 1,289 1237.2 78.6 183 0.9 1.7 3.0 5.5
Illinois 3,062 118.4 74.0 231 1.5 3.5 6.6 13.4
Indiana 2,493 120.7 75.1 213 1.8 3.7 7.2 13.5
Iowa 5,942 150.6 77.3 216 2.1 4.9 8.2 15.0
Kansas 1,371 98.9 69.4 262 2.0 5.3 10.9 20.9
Kentucky 1,290 86.0 73.3 234 3.4 6.0 9.9 17.7
Louisiana 273 102.3 55.9 409 10.6 21.2 33.3 52.7
Maine 1,210 100.6 70.6 187 0.7 1.7 3.5 7.4
Maryland 2,609 101.0 70.8 201 0.9 2.1 4.4 8.5
Massachusetts 684 62.7 65.5 191 2.3 4.4 5.8 9.2
Mexico 62 1036.1 68.1 238 0.0 0.0 0.0 0.0
Michigan 6,146 234.1 81.2 167 0.7 1.3 2.5 5.2
Minnesota 17,803 113.1 75.4 246 2.4 5.2 9.3 17.5
Mississippi 252 179.2 63.6 251 2.4 7.1 16.3 27.4
Missouri 2,423 79.3 64.5 285 3.7 7.0 11.2 19.5
Montana 220 159.4 75.4 193 0.0 0.0 1.8 6.4
Nebraska 839 230.3 75.1 231 1.0 3.3 8.2 17.9
Nevada 55 530.2 76.7 160 0.0 0.0 0.0 0.0
New Hampshire 710 116.5 71.8 193 2.0 3.0 4.6 8.3
New Jersey 415 61.6 66.1 247 1.7 2.7 6.3 13.0
New Mexico 121 2754.3 75.6 158 0.0 0.0 0.0 0.0
New York 16,876 168.0 76.9 191 0.6 1.7 3.7 8.3
North Carolina 1,169 161.4 71.3 263 0.5 2.2 5.5 13.3
North Dakota 183 191.5 72.7 265 2.7 4.4 9.3 16.9
Ohio 6,961 114.8 72.5 205 0.8 2.0 4.0 8.7
Oklahoma 402 78.2 62.8 317 2.5 8.7 14.2 21.9
Oregon 1,374 249.7 65.6 158 1.0 1.2 1.2 3.0
Pennsylvania 35,934 69.0 73.7 211 1.5 2.9 5.3 10.5
Rhode Island 63 45.7 65.6 200 4.8 9.5 12.7 15.9
South Carolina 342 172.3 63.0 337 0.3 4.1 11.4 26.0
South Dakota 903 375.6 75.9 253 2.7 5.0 9.9 17.6
Tennessee 936 101.4 64.2 319 3.8 9.9 16.0 30.7
Texas 960 1064.5 65.2 194 1.0 3.0 5.5 11.4
Utah 805 198.2 73.0 173 2.4 3.6 4.5 6.8
Vermont 3,125 132.2 72.1 162 0.4 1.0 2.1 6.3
Virginia 3,345 140.0 72.9 253 1.1 3.5 7.1 15.6
Washington 1,366 389.1 75.0 167 1.3 1.3 3.0 5.6
West Virginia 284 80.9 61.9 240 3.9 6.3 10.2 22.2
Wisconsin 39,197 130.4 81.5 202 2.3 4.2 6.8 12.1
Wyoming 3 476.7 58.9 180 0.0 0.0 0.0 0.0
United States4 173,260 177.3 76.2 200 1.6 3.4 6.0 11.6

1All herd test days with usable records.
2Cows with usable records (less than total cows on test).
3Herd test days with ≥10 usable records.
4US Average includes records from Mexican herds.


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, 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
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
2011 191,375 154.6 73.1 217 2.3 4.8 8.5 15.7
2012 184,927 161.8 74.2 200 1.5 3.3 6.1 12.0
2013 177,944 167.8 75.2 199 1.5 3.2 5.9 11.6
2014 173,260 177.3 76.2 200 1.6 3.4 6.0 11.6
Annual trend -5,680 7.0*** 0.5*** -7.0*** -0.2*** -0.4*** -0.6*** -1.0***

1All herd test days with usable records.
2Cows with usable records (less than total cows on test).
3Herd test days with ≥10 usable records.
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 ≤ 0.001) linear regression.


Table 3. Characteristics of test-day milk yield and somatic cell count (SCC) from Dairy Herd Improvement herds during 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, 1000's) 750,000 cells/ml
(%)
600,000 cells/ml
(%)
500,000 cells/ml
(%)
400,000 cells/ml
(%)
<50 68,187 28.2 63.3 242 3.5 6.4 10.2 16.9
50 - 99 50,376 69.8 68.7 228 0.7 2.2 4.8 10.9
100 - 149 17,886 120.8 71.4 218 0.3 1.2 3.1 8.4
150 - 199 8,515 172.0 73.7 213 0.2 0.9 2.6 7.3
200 - 299 8,162 242.2 75.9 205 0.1 0.7 1.9 5.6
300 - 499 7,991 384.9 79.2 197 0.2 0.7 1.6 4.3
500 - 999 6,575 691.6 81.1 192 0.0 0.2 0.8 2.1
1000 - 1999 3,710 1378.8 81.7 185 0.0 0.0 0.2 1.2
2000 - 2999 953 2455.2 79.3 187 0.0 0.2 0.6 1.8
>3000 905 5083.6 76.2 183 0.0 0.0 0.0 1.1
All herds 173,260 177.3 76.2 200 1.6 3.4 6.0 11.6

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 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, 1000's) 750,000 cells/ml
(%)
600,000 cells/ml
(%)
500,000 cells/ml
(%)
400,000 cells/ml
(%)
January 14,227 174.0 76.0 187 1.5 2.9 5.0 9.6
February 13,999 177.1 76.4 195 1.7 3.2 5.7 11.0
March 15,070 176.7 76.7 196 1.9 3.6 6.3 11.9
April 14,677 178.5 77.2 193 1.5 3.2 5.8 11.0
May 15,073 175.4 77.7 194 1.4 3.0 5.1 10.4
June 14,108 177.4 77.4 203 1.7 3.6 6.3 12.1
July 14,627 173.6 76.4 215 2.1 4.5 8.1 14.9
August 14,426 177.8 75.9 215 2.1 4.3 7.7 14.6
September 13,894 179.9 74.5 212 1.9 4.0 7.2 14.0
October 14,724 179.2 74.9 200 1.5 3.0 5.5 10.9
November 14,054 179.3 75.4 190 1.2 2.6 4.7 9.2
December 14,381 179.4 76.2 194 1.3 2.9 5.2 9.8
Yearly 173,260 177.3 76.2 200 1.6 3.4 6.0 11.6

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 by state



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