AIPL RESEARCH REPORT SCC2 (2-01)

Somatic cell counts of milk from Dairy Herd Improvement herds during 1999 and 2000 R.H. Miller and H.D. Norman 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 1999 and 2000 (91% 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. Cows with records that failed Animal Improvement Programs Laboratory (AIPL) editing procedures (for example, no sire identification) were excluded. However, 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 and was converted back to SCC in cells/milliliter [SCC = 2^(SCS–3) × 100,000] for calculating herd and State averages.

Table 1 shows for each State in 1999 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 on test day, and the percentages of herd test days that exceeded four levels of SCC: 750,000, 600,000, 500,000, and 400,000 cells/ml. 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. Information from Alaska and Hawaii was not included because of sparse data. Table 2 shows corresponding data for 2000.

Nationally, average SCC during 2000 was 316,000 cells/ml, which was higher than the national average of 311,000 cells/ml for 1999 . Corresponding averages for 1995-1998 were 304,000, 308,000, 314,000 and 318,000. The percentages of herd test days that exceeded 750,000, 600,000, 500,000, and 400,000 cells/ml during year 2000 were 4.1, 9.4, 16.8 and 29.5, respectively. Thus, 4.1% of test-days from DHI herds that participated in somatic cell testing exceeded the present legal limit for SCC of bulk tank milk. This percentage may overestimate the percentage of herds that would have exceeded the legal limit for SCC of bulk tank milk on test day because the milk of cows treated for mastitis is excluded from the bulk tank but not excluded from the 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 was large. Average SCC's were lower in the far West and higher in the Southeast. 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 regimes have an impact under similar climatic conditions.

These results show no improvement in national SCC. Greater efforts to improve milk quality are needed. State and national SCC's will continue to be summarized annually so that changes in SCC and trends in milk quality can be monitored.