In 2020, U.S. dairy herds’ somatic cell count averaged 178,000 cells/mL. A year earlier, the average sat at 187,000 cells/mL. The Council on Dairy Cattle Breeding (CDCB) analyzed test-day data from all herds enrolled in Dairy Herd Improvement (DHI) somatic cell testing (98.4 percent of all DHI herds and 96 percent of DHI cows) to calculate these averages.
Unfortunately, 1.2 percent of U.S. dairy herds’ test-day SCC exceeded 750,000 cells/mL. More than 8 percent of herds’ average SCC surpassed 400,000 cells/mL.
Table 1 in CDCB’s summary shows total number of herd test days, average number of cows with usable records per herd on test day, average test-day milk yield, fat and protein percentages, and herd SCC. The current legal limit for U.S. bulk tank SCC is 750,000 cells/mL for Grade A producers. Several dairy organizations, including the National Mastitis Council, have supported lowering the limit to 400,000 cells/mL. A few states have lowered their SCC limits beyond federal requirements. State and national average daily milk, fat and protein yields were weighted by number of cows per herd test day. Herd, state and national average SCCs were based on SCCs from individual cows that were weighted by milk yield on test day.
Northern states do better than Southeast
Variation among states differs substantially. In general, northern states’ average SCC was lower than the national average. Nearly all states in the Southeast exceeded the national average. Although climatic conditions (temperature and humidity) contributed to regional SCC levels, differences between adjacent states were large, according to CDCB. This suggests that herd size and mastitis-control practices are influencing state differences, also.
Corresponding national averages for 1995 through 2020 are in Table 2 of CDCB’s summary. National average test-day herd SCC declined every year – except three – since 2001, which had the highest SCC at 322,000 cells/mL. The 2020 DHI herd test days that were higher than the present legal limit for bulk tank SCC overestimate the actual percentage of herds that shipped milk exceeding the legal limit because milk from cows treated for mastitis is excluded from the bulk tank, yet it is included in DHI test data.
U.S. dairy herd size continues to increase. In 2020, the average U.S. dairy herd had 252 cows. Test-day milk yield increased from 78.3 pounds (35.5 kg) to 79.5 pounds (36.1 kg).
CDCB officials contribute the large SCC decline since 2001 to improved management practices, including directed culling and genetic selection. Genetic selection is contributing more to milk quality improvement than in the past, according to CDCB.
Larger herds outperform smaller herds
Table 3 of CDCB’s summary reports average test-day milk yield and herd SCC during 2020 by herd size. As herd size increased up to 1,000 cows, milk yield increased. SCC declined as herd size increased – up to 500 cows. A remaining concern is the percentage of test days for herds with less than 50 cows over the current and possible future limits. If the limit was set at 400,000 cell/mL, 13.9 percent of monthly tests would sit above the limit for herds with fewer than 50 cows, 7.6 percent for herds with 50 to 99 cows, and 5.0 percent for herds with 100 to 149 cows. Only 0.1 percent for herds with more than 4,000 cows are above the limit.
Because small herds contribute less each year to the country’s milk supply, the general trend is improved milk quality. Typically, large herds produce and market milk with lower SCC.
Table 4 in CDCB’s summary reports average test-day milk yield, fat and protein percentages, and herd SCC during 2020 by month. Milk yield per cow was highest in April and June, and was lowest in October. However, the highest and lowest months only differed by 1.7 pounds (0.8 kg). SCC increased from May to August and then declined through December. The milk with the lowest SCC and highest fat and protein percentages was produced in December. July’s milk yielded the lowest fat and protein percentages.
