AIPL RESEARCH REPORT SCC6 (3-05)

Somatic cell counts of milk from Dairy Herd Improvement herds during 2004 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 2004 (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 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 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 2004. 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 2004 was 295,000 cells/ml, which is the third consecutive year of decline in the national average. Corresponding averages for 1995 through 2003 (Table 2) ranged from 304,000, to 322,000. The percentage of herd test days that exceeded 750,000, 600,000, 500,000, and 400,000 cells/ml during 2004 was 4.7, 9.4, 15.5 and 26.4, respectively. 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 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 often 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. The overall percentage of herd test days exceeding 750,000 cells/ml was lower in 2004 than in 2003 (Table 2), but more states had greater than 5% of herd test days over this limit, showing that although progress has been made in some areas, improvement is needed in others.

Herd size and average daily milk yield have been increasing since 1995 (Table 2). The results for SCC are encouraging, but perhaps it is too early to conclude that a continued decline is expected 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 they have in the past. Nevertheless, greater efforts to improve milk quality should continue. State and national SCC's 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, while average SCC declined. Possibly the most surprising figure was for the percentage of test days with greater than 750,000 cells/ml. For herds with <50 cows, this occured 7% of the time. It occured on <2% of the test days for herds with 50 to 99 cows and on <1.2% of the test days for those herds with >100 cows. Table 4 provides the same statistics for all herds by month. The seasonal pattern of milk quality is illustrated in Figure 1. 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 2004.

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 2004, by herd size.

Table 4. Characteristics of DHI herd test days for milk yield and SCC during 2004, by month.

Figure 1. Average daily milk yield and Average SCC during 2004, by month.