AIPL RESEARCH REPORT SCC8 (2-07)

Somatic cell counts of milk from Dairy Herd Improvement herds during 2006 R.H. Miller, H.D. Norman, and L.L.M. Thornton 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 2006 (94% 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 some Animal Improvement Programs Laboratory (AIPL) editing procedures were excluded. 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 2006. 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 2006 was 288,000 cells/ml, which is considerably lower than last year.

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 2005 are in Table 2. The percentage of herd test days that exceeded 750,000, 600,000, 500,000, and 400,000 cells/ml during 2006 was 3.9, 8.3, 14.3, and 25.2, respectively. The overall percentage of herd test days exceeding all levels were slightly lower than in 2005. Thus, 3.9% 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). However, the large increase in cows per herd for 2006 over 2005 (115.5 vs. 90.3) is primarily due to a new policy of accepting records missing sire identification into the national database. The trend for SCC over the last five years are encouraging; hopefully mean SCC will decline in the future due to both genetic selection and 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 1000 cows. 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 6.4% of the time. It occurred on 2.2% of the test days for herds with 50 to 99 cows and on 1.1% of the test days for the remaining herds.

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 late winter and spring. Milk yield and quality decline during the summer months, and the highest quality milk is produced from October to June.

Table 1. Characteristics of DHI herd test days for milk yield and SCC of milk by State during 2006.

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 2006, by herd size.

Table 4. Characteristics of DHI herd test days for milk yield and SCC during 2006, by month.

Figure 1. Average daily milk yield and Average SCC during 2006, by month.