AIPL RESEARCH REPORT SCC5 (2-04)

Somatic cell counts of milk from Dairy Herd Improvement herds during 2003 R.H. Miller, H.D. Norman, and A.H. Sanders 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 2003 (93% 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 (AIPL) editing procedures (for example, no sire identification) were excluded. However, an earlier report showed that the somatic cell counts (SCC's) of milk from such cows did not differ from those from the other cows in the herd. Somatic cell score (SCS) is reported to AIPL as part of an individual cow's test-day yield information and 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 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 2003. The current legal limit for bulk tank SCC is 750,000 cells/ml for Grade A producers; the lower limits shown have been proposed as future maximums. 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 2003 was 319,000 cells/ml, which was lower than the national average of 320,000 cells/ml for 2002. Corresponding averages for 1995 through 2002 (Table 2) ranged from 304,000, to 322,000, and the overall trend has been for an increase in SCC of 1700, annually. The percentage of herd test days that exceeded 750,000, 600,000, 500,000, and 400,000 cells/ml during 2003 was 5.6, 11.2, 18.4 and 30.4, respectively. Thus, 5.6% 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 exceeded the legal limit for bulk tank SCC on test day because the milk of cows treated for mastitis is excluded from the bulk tank but 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.

Variation among States (Table 1) was large. Average SCC's were lower in the far West and higher in the Southeast; a finding consistent with previous reports. Although climatic conditions (temperature and humidity) probably 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. Although the overall percentage of herd test days exceeding 750,000 cells/ml was the same in 2002 as 2003 (Table 2), fewer states had greater than 5% of herd test days over this limit, showing that progress has been made in some areas.

Herd size and average daily milk yield have been increasing since 1995 (Table 2). Overall, the results for SCC raise concern that the quality of U.S. milk is not improving. Although changes from 2002 to 2003 were small, the trends for increasing percentage of herd test days above all four thresholds (750,000, 600,000, 500,000 and 400,000 cells/ml) were significant at P ≤.01. Greater efforts to improve milk quality are essential. State and national SCC's will continue to be summarized annually so that changes in SCC and implications to milk quality can be monitored.

Table 1. Characteristics of DHI herd test days for milk yield and SCC during 2003, by State.

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 of herd test days for milk yield and SCC of milk by year.

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 was available
at the time of initial reporting.

**The linear regression was significant with P ≤ .01

***The linear regression was significant with P ≤ .001