AIPL RESEARCH REPORT SCC9 (3-08)

Somatic cell counts of milk from Dairy Herd Improvement herds during 2007 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 2007 (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 2007. 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. An interesting development is that California lowered their state standard for legal milk to 600,000 cells/ml. 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 2007 was 276,000 cells/ml, which is considerably lower (by 12,000 cells/ml) than last year. A majority of states (27) had lower average SCC than last year; 21 states had higher.

Variation among States (Table 1) was large. State average SCC's were often lower than the national average in the Northeast, upper Midwest, and the far West and higher in the Southeast, Mid-Atlantic, and Central states, a finding consistent with previous reports. Although climatic conditions (temperature and humidity) surely contributed to regional SCC differences, differences between adjacent states were substantial, suggesting mastitis-control practices are impacting State differences as well.

Corresponding averages for earlier years 1995 through 2007 are in Table 2. This is the third year in a row that SCC has declined. It has improved for five of the last six years, declining from a high of 322,000 cells/ml in 2001. The percentage of herd test days that exceeded 750,000, 600,000, 500,000, and 400,000 cells/ml during 2007 was 3.5, 7.6, 13.4, and 24.0, respectively. The overall percentage of herd test days exceeding all levels were lower than in 2006. In 2007, 3.5% of test days from DHI herds that participated in somatic cell testing were 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 over the period reported (Table 2). The large increase in cows per herd for 2005 to 2006 (90.3 vs. 115.5 was due in part to a change in policy of accepting records missing sire identification into the national database. The increase in cows per herd between 2006 and 2007 was substantial (115.5 vs. 125.1). The decline in SCC over the last six years is encouraging; hopefully average SCC will continue to decline in the future due to both improved management practices and genetic selection. The trends for lower percentage of herd test days above three thresholds (600,000, 500,000 and 400,000 cells/ml) is likewise encouraging. 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 3000 cows. The most disturbing result is the percentage of test days with over 750,000 cells/ml. For herds with less than 50 cows, this occurred 5.9% of the time. It occurred on only 2.0% and only 1.3% of the test days for herds with 50 to 99 and 100 to 149 cows, respectively, and averaged 0.7% of the test days for the remaining large 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 declined during the summer months. SCC increased from May through August, then decreased from September through December. The highest quality milk was produced in October, November, and December.

Table 1. Characteristics of DHI herd test days for milk yield and SCC of milk by State during 2007.

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 2007 by herd size.

Table 4. Characteristics of DHI herd test days for milk yield and SCC during 2007 by month.

Figure 1. Average daily milk yield and SCC during 2007 by month.