AIPL RESEARCH REPORTSCC14 (2-13) |
Somatic cell counts of milk from Dairy Herd Improvement herds duringH.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 2012 (98% 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 requirements 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 . 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 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 2012 was 200,000 cells/mL, which was lower (by 17,000 cells/mL) than last year's average. Forty-two states had lower average SCC than reported last year; 5 states had higher averages. A few large Mexican herds tested through the U.S. system were included for the third 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 a few 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 are impacting State differences as well.
Corresponding national averages for 1995 through 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 2012 was 1.5, 3.3, 6.1, and 12.0, respectively, which was lower for all levels than during 2011. The 1.5% of 2012 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 2011 (154.6) to 2012 (161.8). The large decline in SCC since 2001 is encouraging. Hopefully, this trend will continue as a result of improved management practices and directed culling. Genetic selection is likely 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. The effort to improve milk quality is likely being influenced by the more restrictive 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 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 400,000 cell/mL, 17.4% of monthly tests would be above the limit for herds with fewer than 50 cows, 11.2% for herds with 50 to 99 cows, and 8.2% for herds with 100 to 149 cows. In contrast, none 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 May through August and then declined quickly from September through November. The highest quality milk was produced in November and December.
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Table 1. Characteristics of test-day milk yield and somatic cell count (SCC) from Dairy Herd Improvement herds by State during |
| 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 | 205 | 120.8 | 49.4 | 391 | 0.5 | 2.9 | 12.2 | 34.6 |
| Arizona | 157 | 1532.3 | 65.5 | 184 | 1.9 | 1.9 | 2.5 | 3.2 |
| Arkansas | 189 | 107.4 | 53.0 | 316 | 4.8 | 10.6 | 17.5 | 32.8 |
| California | 7,520 | 863.9 | 75.3 | 174 | 0.6 | 1.2 | 2.1 | 4.4 |
| Colorado | 243 | 851.9 | 76.1 | 198 | 0.0 | 0.0 | 0.8 | 2.5 |
| Connecticut | 728 | 119.1 | 69.8 | 210 | 1.4 | 3.6 | 5.5 | 10.2 |
| Delaware | 199 | 146.1 | 71.8 | 242 | 0.0 | 2.0 | 3.5 | 7.5 |
| Florida | 321 | 755.7 | 68.4 | 267 | 2.2 | 4.0 | 10.3 | 24.3 |
| Georgia | 929 | 196.4 | 67.1 | 280 | 1.6 | 5.4 | 11.0 | 23.9 |
| Idaho | 1,427 | 1074.8 | 77.1 | 159 | 0.3 | 0.8 | 1.2 | 2.9 |
| Illinois | 3,335 | 108.1 | 72.9 | 217 | 1.3 | 2.8 | 5.4 | 10.8 |
| Indiana | 2,629 | 129.6 | 74.4 | 212 | 1.6 | 3.5 | 6.3 | 12.9 |
| Iowa | 6,624 | 137.7 | 75.3 | 207 | 1.3 | 2.9 | 5.2 | 12.7 |
| Kansas | 1,580 | 90.7 | 66.7 | 253 | 0.9 | 2.8 | 6.5 | 15.1 |
| Kentucky | 1,409 | 82.2 | 66.5 | 251 | 3.0 | 5.7 | 10.7 | 19.4 |
| Louisiana | 328 | 124.1 | 54.0 | 347 | 4.0 | 10.1 | 17.4 | 37.5 |
| Maine | 1,320 | 82.9 | 67.8 | 190 | 1.3 | 3.0 | 6.3 | 11.4 |
| Maryland | 2,822 | 92.9 | 67.0 | 234 | 1.6 | 3.4 | 6.5 | 12.8 |
| Massachusetts | 647 | 63.4 | 64.7 | 204 | 0.8 | 2.0 | 4.2 | 8.3 |
| Mexico | 51 | 681.0 | 73.9 | 265 | 0.0 | 0.0 | 0.0 | 9.8 |
| Michigan | 6,304 | 205.7 | 79.3 | 161 | 0.3 | 0.9 | 1.8 | 4.9 |
| Minnesota | 19,697 | 102.7 | 73.7 | 242 | 1.8 | 4.4 | 8.6 | 16.6 |
| Mississippi | 275 | 166.8 | 62.9 | 245 | 1.8 | 2.2 | 5.5 | 12.4 |
| Missouri | 2,619 | 82.1 | 61.6 | 271 | 3.9 | 7.6 | 12.9 | 21.6 |
| Montana | 224 | 153.8 | 76.9 | 185 | 0.0 | 0.0 | 0.4 | 2.2 |
| Nebraska | 959 | 194.8 | 73.3 | 204 | 1.0 | 2.5 | 5.2 | 10.8 |
| Nevada | 84 | 565.2 | 80.3 | 180 | 0.0 | 0.0 | 0.0 | 0.0 |
| New Hampshire | 733 | 107.8 | 70.6 | 196 | 0.7 | 0.7 | 3.5 | 7.1 |
| New Jersey | 402 | 60.4 | 64.5 | 276 | 1.2 | 4.2 | 10.0 | 19.4 |
| New Mexico | 148 | 2361.6 | 71.1 | 161 | 0.0 | 0.0 | 0.0 | 2.0 |
| New York | 17,176 | 153.9 | 75.9 | 206 | 0.9 | 2.5 | 5.2 | 11.0 |
| North Carolina | 1,236 | 152.1 | 67.9 | 272 | 1.9 | 4.9 | 9.5 | 18.3 |
| North Dakota | 208 | 133.9 | 71.7 | 244 | 0.5 | 1.0 | 1.9 | 3.8 |
| Ohio | 7,234 | 113.2 | 71.6 | 206 | 0.7 | 1.9 | 4.0 | 9.8 |
| Oklahoma | 430 | 81.0 | 61.2 | 283 | 3.0 | 5.1 | 10.0 | 18.6 |
| Oregon | 1,593 | 206.7 | 64.6 | 161 | 1.1 | 1.8 | 3.3 | 5.2 |
| Pennsylvania | 37,201 | 65.1 | 71.0 | 222 | 1.6 | 3.3 | 6.0 | 11.9 |
| Puerto Rico | 1 | 3.0 | 50.3 | 13 | 0.0 | 0.0 | 0.0 | 0.0 |
| Rhode Island | 74 | 47.5 | 64.3 | 170 | 1.4 | 1.4 | 1.4 | 6.8 |
| South Carolina | 434 | 167.2 | 63.3 | 334 | 2.3 | 4.8 | 12.4 | 29.0 |
| South Dakota | 1,006 | 241.7 | 75.2 | 222 | 2.0 | 5.1 | 8.8 | 14.9 |
| Tennessee | 1,077 | 96.7 | 61.2 | 323 | 4.8 | 8.9 | 16.2 | 30.2 |
| Texas | 1,009 | 856.3 | 64.2 | 207 | 1.1 | 2.5 | 4.8 | 12.7 |
| Utah | 947 | 217.3 | 70.2 | 169 | 3.3 | 3.9 | 5.0 | 7.9 |
| Vermont | 3,226 | 120.3 | 69.2 | 161 | 0.7 | 1.3 | 2.8 | 6.1 |
| Virginia | 3,595 | 132.8 | 69.0 | 261 | 1.3 | 3.2 | 6.9 | 16.9 |
| Washington | 1,479 | 345.5 | 72.7 | 162 | 1.2 | 1.2 | 2.0 | 4.1 |
| West Virginia | 304 | 84.7 | 59.5 | 296 | 2.6 | 6.6 | 13.8 | 25.7 |
| Wisconsin | 42,577 | 113.4 | 79.4 | 208 | 2.0 | 4.0 | 6.6 | 11.8 |
| Wyoming | 12 | 297.0 | 76.9 | 161 | 16.7 | 16.7 | 16.7 | 16.7 |
| United States | 184,927 | 161.8 | 74.2 | 200 | 1.5 | 3.3 | 6.1 | 12.0 |
| 1All herd test days with usable records. |
| 2Cows with usable records (less than total cows on test). |
| 3Herd test days with ≥10 usable records. |
| 4US Average includes records from Mexican herds. |
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Table 2. National averages for test-day milk yield and somatic cell count (SCC) from Dairy Herd Improvement herds by year |
| 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 |
| Annual trend | -5,377 | 6.7*** | 0.5*** | -6.4*** | -0.1** | -0.3*** | -0.6*** | -0.9*** |
| 1All herd test days with usable records. |
| 2Cows with usable records (less than total cows on test). |
| 3Herd test days with ≥10 usable records. |
| 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. |
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Table 3. Characteristics of test-day milk yield and somatic cell count (SCC) from Dairy Herd Improvement herds during 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 | 74,897 | 28.6 | 62.2 | 250 | 3.2 | 6.2 | 10.3 | 17.4 |
| 50 - 99 | 54,956 | 69.7 | 67.4 | 232 | 0.6 | 2.0 | 4.7 | 11.2 |
| 100 - 149 | 18,574 | 120.6 | 69.9 | 222 | 0.2 | 1.0 | 2.7 | 8.2 |
| 150 - 199 | 8,695 | 171.9 | 71.8 | 214 | 0.2 | 0.7 | 2.0 | 6.7 |
| 200 - 299 | 8,394 | 242.9 | 74.1 | 208 | 0.1 | 0.7 | 2.1 | 6.0 |
| 300 - 499 | 7,771 | 386.5 | 76.9 | 198 | 0.0 | 0.3 | 0.9 | 3.5 |
| 500 - 999 | 6,502 | 697.1 | 79.1 | 195 | 0.0 | 0.1 | 0.4 | 2.3 |
| 1000 - 1999 | 3,330 | 1362.7 | 79.1 | 184 | 0.0 | 0.0 | 0.2 | 1.4 |
| 2000 - 2999 | 1,055 | 2438.2 | 78.0 | 180 | 0.0 | 0.0 | 0.1 | 0.5 |
| >3000 | 753 | 4686.4 | 75.0 | 168 | 0.0 | 0.0 | 0.0 | 0.0 |
| All herds | 184,927 | 161.8 | 74.2 | 200 | 1.5 | 3.3 | 6.1 | 12.0 |
| 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. |
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Table 4. Characteristics of test-day milk yield and somatic cell count (SCC) from Dairy Herd Improvement herds during by month |
| 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 | 15,631 | 158.6 | 74.4 | 200 | 1.5 | 3.3 | 6.0 | 12.1 |
| February | 15,612 | 162.9 | 75.3 | 198 | 1.4 | 3.2 | 5.6 | 11.5 |
| March | 16,617 | 162.8 | 75.6 | 198 | 1.5 | 3.4 | 5.9 | 11.5 |
| April | 15,250 | 164.1 | 75.7 | 197 | 1.4 | 3.0 | 5.7 | 11.4 |
| May | 15,754 | 159.7 | 75.8 | 199 | 1.4 | 2.9 | 5.5 | 11.1 |
| June | 15,822 | 162.7 | 75.2 | 203 | 1.6 | 3.4 | 6.3 | 12.4 |
| July | 14,495 | 162.4 | 73.0 | 216 | 2.1 | 4.6 | 8.3 | 16.3 |
| August | 15,480 | 160.2 | 72.8 | 222 | 2.3 | 5.2 | 9.6 | 17.9 |
| September | 15,120 | 161.8 | 72.0 | 205 | 1.6 | 3.4 | 6.6 | 13.1 |
| October | 15,003 | 162.1 | 72.3 | 191 | 1.1 | 2.6 | 4.9 | 10.0 |
| November | 15,112 | 161.4 | 73.2 | 186 | 1.2 | 2.3 | 4.2 | 8.5 |
| December | 15,031 | 162.8 | 74.6 | 189 | 1.3 | 2.4 | 4.2 | 8.9 |
| Yearly | 184,927 | 161.8 | 74.2 | 200 | 1.5 | 3.3 | 6.1 | 12.0 |
| 1All herd test days with usable records. |
| 2Cows with usable records (less than total cows on test). |
| 3Herd test days with ≥10 usable records. |
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Figure 1. Average test-day somatic cell count from Dairy Herd Improvement herds during by State |
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Figure 2. Average test-day milk yield and somatic cell count (SCC) from Dairy Herd Improvement herds during by month |
