AIPL RESEARCH REPORT SCC7 (3-06)

Somatic cell counts of milk from Dairy Herd Improvement herds during 2005 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 2005 (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. Nevertheless, an earlier report showed that the somatic cell counts (SCC's) of milk from such cows did not differ appreciably 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. It 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 test-day 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 2005. The current legal limit for bulk tank SCC is 750,000 cells/ml for Grade A producers; the lower limits shown have been proposed on a number of occasions as future maximums by the National Mastitis Council. 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 2005 was 296,000 cells/ml, which is slightly higher than last year, following three consecutive years of decline.

Variation among States (Table 1) was large. State average SCC's were often lower than the national average in the Northeast and the far West and higher in the Southeast, a finding consistent with previous reports. Although climatic conditions (temperature and humidity) surely 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.

Corresponding averages for earlier years 1995 through 2004 are in Table 2. The percentage of herd test days that exceeded 750,000, 600,000, 500,000, and 400,000 cells/ml during 2005 was 4.7, 9.2, 15.3 and 25.8, respectively. The overall percentage of herd test days exceeding 750,000 cells/ml was the same in 2005 as in 2004 (Table 2) while those exceeding the other levels were slightly lower. Thus, 4.7% 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 shipped milk exceeding the legal limit for bulk tank SCC on test day because the milk of cows treated for mastitis is excluded from the bulk tank even though 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.

Herd size and average daily milk yield have been increasing since 1995 (Table 2). The trend for SCC over the last four years are somewhat encouraging, but it is too early to determine whether mean SCC will decline in the future due to genetic selection or improved management practices. The trends for increasing percentage of herd test days above three thresholds (600,000, 500,000 and 400,000 cells/ml) no longer show statistically significant increases as in the past. The concerted effort to improve milk quality needs to continue. State and national SCC's in DHI will continue to be summarized annually so changes in SCC and implications to milk quality can be monitored.

Table 3 provides milk yield and SCC by estimated herd size. As herd size increased, average daily milk generally increased, and average SCC declined until herd size exceeded 500 cows. Possibly the most disturbing result was the percentage of test days with over 750,000 cells/ml. For herds with less than 50 cows, this occurred around 7.0% of the time. It occurred on 2.1% of the test days for herds with 50 to 99 cows and on 1.2% of the test days for all other herd size groups.

Table 4 provides the same statistics for all herds by month. The seasonal pattern of milk quality is illustrated in Figure 1. Milk yield per cow was highest in the spring. Milk yield and quality decline during the summer months, and the highest quality milk is produced in late fall to early winter.

Table 1. Characteristics of DHI herd test days for milk yield and SCC of milk by State during 2005.

Table 2. National averages of herd test days for milk yield and SCC of milk by year.

Table 3. Characteristics of DHI herd test days for milk yield and SCC during 2005, by herd size.

Table 4. Characteristics of DHI herd test days for milk yield and SCC during 2005, by month.

Figure 1. Average daily milk yield and Average SCC during 2005, by month.