Somatic cell counts of milk from 2008 DHI herds

AIPL RESEARCH REPORT
SCC10 (2-09)

Somatic cell counts of milk from Dairy Herd Improvement herds during 2008

H.D. Norman, R.H. Miller, and F.A. Ross, Jr.
Animal Improvement Programs Laboratory, ARS-USDA, Beltsville, MD 20705-2350
301-504-8334 (voice) ~ 301-504-8092 (fax) ~ inquiry@aipl.arsusda.gov ~ http://aipl.arsusda.gov

Test-day data from all herds enrolled in Dairy Herd Improvement (DHI) somatic cell testing during 2008 (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 somatic cell count (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 2008. 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 its 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 10 cows. Nationally, average SCC during 2008 was 262,000 cells/ml, which is considerably lower (by 14,000 cells/ml) than last year. Most States (41) had lower average SCC than last year; only 8 States had higher averages.

Variation among States (Table 1) remains large. State average SCC's were lower than the national average in the far West and higher in the Southeast, Mid-Atlantic, and Central States. Although climatic conditions (temperature and humidity) surely contributed to regional SCC differences, differences between adjacent States were substantial, which suggests that herd size and mastitis-control practices including genetic selection are impacting State differences as well.

Corresponding averages for earlier years 1995 through 2007 are in Table 2. This year is the third in a row during which SCC declined, and SCC has improved for 6 of the last 7 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 2008 was 3.4, 7.1, 12.5, and 22.4, respectively. The overall percentage of herd test days exceeding all levels was lower than in 2007. In 2008, 3.4% 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. That 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 repeated violations.

Herd size and average daily milk yield have been increasing since 1995 (Table 2). The large increase in cows per herd for 2005 to 2006 (90.3 versus 115.5) resulted partly because of a policy change to allow records with missing sire identification in the national database. The increase in cows per herd between 2007 and 2008 was substantial (125.1 versus 131.7). The decline in SCC over the last 6 years is encouraging; hopefully, average SCC will continue to decline because of 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 that changes in SCC and implications for 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 generally declined. 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 6.0% of the time. However, it occurred on only 1.7% and 1.0% of test days for herds with 50 to 99 and 100 to 149 cows, respectively, and averaged 0.6% of 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 winter and spring and declined during the summer months; SCC increased from May through July and then decreased from August through November. 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 2008.

SCC characteristics

	Herd test days¹	Cows² per herd	Average daily milk yield	Average SCC	Herd test days³ with SCC greater than
State	(no.)	(no.)	(lb)	(cells/ml, 1000's)	750,000 cells/ml (%)	600,000 cells/ml (%)	500,000 cells/ml (%)	400,000 cells/ml (%)
Alabama	210	119.9	52.2	417	3.3	13.3	27.6	49.0
Arizona	247	1443.6	69.9	276	1.2	3.2	6.9	13.8
Arkansas	328	107.5	55.0	424	9.5	14.9	26.5	43.0
California	9,022	740.4	74.2	243	2.1	3.4	6.3	12.7
Colorado	349	646.0	75.6	251	1.4	3.2	5.4	12.3
Connecticut	845	91.9	66.3	269	3.0	7.1	11.4	20.0
Delaware	261	113.4	68.9	313	4.2	7.3	14.6	27.2
Florida	205	813.0	67.5	304	3.4	9.8	17.6	41.5
Georgia	1,153	152.7	62.6	350	3.9	10.6	23.3	40.9
Idaho	1,680	720.6	74.7	228	1.7	3.8	6.4	12.4
Illinois	4,107	90.5	68.8	284	3.4	7.6	13.1	24.3
Indiana	3,473	87.0	68.6	268	3.7	7.3	12.9	24.8
Iowa	8,455	103.2	71.7	288	4.6	9.9	16.3	28.1
Kansas	1,961	86.8	65.8	353	5.4	12.0	21.2	35.5
Kentucky	1,641	79.4	63.4	356	7.7	15.1	22.8	36.7
Louisiana	418	112.3	52.1	442	10.5	23.2	42.3	62.0
Maine	1,255	74.6	64.2	261	2.6	7.2	12.0	21.9
Maryland	3,519	81.3	66.6	278	2.6	6.5	11.8	22.0
Massachusetts	846	76.4	66.8	258	3.8	6.9	11.9	19.3
Michigan	7,346	162.6	75.4	216	1.8	3.7	6.5	12.1
Minnesota	23,891	82.4	70.7	310	4.5	9.7	17.2	30.0
Mississippi	310	168.9	64.5	332	4.5	14.8	29.4	50.3
Missouri	3,355	69.9	57.9	360	7.5	14.9	22.3	35.2
Montana	342	132.1	74.1	208	0.0	0.0	2.6	6.7
Nebraska	1,407	131.4	66.7	303	7.0	11.3	19.1	31.1
Nevada	111	490.9	81.8	265	0.9	1.8	1.8	5.4
New Hampshire	895	92.9	70.8	252	2.3	5.1	8.9	16.3
New Jersey	540	66.9	67.0	322	4.3	10.7	18.3	28.7
New Mexico	273	1555.0	71.4	280	5.1	5.1	7.3	15.0
New York	20,306	116.7	72.7	250	2.3	5.9	10.8	20.0
North Carolina	1,542	127.8	66.1	309	3.4	7.7	14.6	27.8
North Dakota	349	94.8	66.0	274	2.9	2.9	7.7	13.8
Ohio	8,414	103.2	69.4	265	2.7	6.0	11.3	21.4
Oklahoma	576	126.1	61.8	303	6.4	14.2	22.2	34.5
Oregon	2,067	161.7	66.3	220	3.4	4.8	7.3	11.4
Pennsylvania	42,501	59.9	70.2	290	3.5	7.3	13.0	23.7
Puerto Rico	667	130.4	37.8	417	7.2	17.2	31.8	53.2
Rhode Island	62	66.0	60.3	216	3.2	3.2	4.8	9.7
South Carolina	534	158.4	63.1	337	0.6	3.7	10.1	28.1
South Dakota	1,307	200.3	71.7	282	4.8	10.2	17.2	31.5
Tennessee	1,400	91.3	60.3	365	3.4	10.0	21.1	38.6
Texas	1,547	423.8	63.0	267	1.6	4.1	8.9	20.7
Utah	1,225	198.5	68.9	220	3.4	5.7	9.3	15.0
Vermont	3,507	105.7	68.3	235	1.6	4.0	7.3	15.0
Virginia	4,049	112.7	68.4	290	2.2	5.9	11.2	22.2
Washington	1,803	250.5	73.4	234	1.3	2.2	3.9	9.0
West Virginia	422	88.1	59.2	325	4.3	8.5	15.6	29.6
Wisconsin	51,504	86.8	74.6	252	3.2	6.3	10.7	18.8
Wyoming	18	227.4	75.0	230	0.0	0.0	0.0	0.0
United States	222,245	131.7	71.7	262	3.4	7.1	12.5	22.4

¹All herd test days with usable records.

²Cows with usable records (less than total cows on test).

³Herd test days with ≥10 usable records.

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

SCC characteristics

	Herd test days¹	Cows² per herd	Average daily milk yield	Average SCC	Herd test days³ with SCC greater than
Year	(no.)	(no.)	(lb)	(cells/ml, 1000's)	750,000 cells/ml (%)	600,000 cells/ml (%)	500,000 cells/ml (%)	400,000 cells/ml (%)
1995	265,844	50.0	65.3	304	4.1	9.3	16.0	27.2
1996	255,039	55.5	64.7	308	4.1	9.2	16.1	27.8
1997	287,789	57.4	66.4	314	4.2	9.5	16.6	28.8
1998	283,695	60.8	66.8	318	4.5	10.1	17.8	30.3
1999	273,364	67.0	68.2	311	4.3	9.7	17.1	29.8
2000	260,139	73.3	69.1	316	4.1	9.4	16.8	29.5
2001	244,940	79.1	69.0	322	4.9	10.6	18.2	31.1
2002⁴	267,809	77.5	69.5	320	5.6	11.0	18.1	30.0
2003	251,182	80.5	69.6	319	5.6	11.2	18.4	30.4
2004	240,938	85.7	69.4	295	4.7	9.4	15.5	26.4
2005	234,585	90.3	71.1	296	4.7	9.2	15.3	25.8
2006⁵	236,191	115.5	71.2	288	3.9	8.3	14.3	25.2
2007	227,626	125.1	71.4	276	3.5	7.6	13.4	24.0
2008	222,245	131.7	71.7	262	3.4	7.1	12.5	22.4
Annual trend	-4,050	5.9***	0.5***	-3.0**	-0.0	-0.1	-0.3*	-0.4*

¹All herd test days with usable records.

²Cows with usable records (less than total cows on test).

³Herd test days with ≥10 usable records.

⁴Summary characteristics for 2002 were recalculated to include more complete data than were available at the time of initial reporting.

⁵Cows per herd included records with missing sire identification starting in 2006; those records were not included in prior years.

*Significant (P ≤ 0.05) linear regression.
**Significant (P ≤ 0.01) linear regression.
***Significant (P ≤ 0.001) linear regression.

Table 3. Characteristics of DHI herd test days for milk yield and SCC during 2008 by herd size.

SCC characteristics by herd size

Herd size¹	Herd test days²	Cows³ per herd	Average daily milk yield	Average SCC	Herd test days⁴ with SCC greater than
(cows)	(no.)	(no.)	(lb)	(cells/ml, 1000's)	750,000 cells/ml (%)	600,000 cells/ml (%)	500,000 cells/ml (%)	400,000 cells/ml (%)
<50	99,511	28.9	62.2	310	6.0	11.2	17.6	27.8
50 - 99	66,434	69.1	66.7	289	1.7	5.1	10.8	21.8
100 - 149	20,168	120.5	67.9	275	1.0	3.2	7.9	17.4
150 - 199	9,218	172.4	69.9	267	0.7	2.5	6.3	15.2
200 - 299	8,671	242.3	71.8	262	0.6	2.1	5.0	13.7
300 - 499	7,520	384.8	74.2	249	0.5	1.3	3.6	9.8
500 - 999	6,083	689.3	76.0	250	0.4	1.1	3.4	9.3
1000 - 1999	3,287	1367.8	76.5	247	0.7	1.0	3.0	7.6
2000 - 2999	857	2425.2	74.8	247	0.9	1.4	1.9	6.0
>3000	496	4069.2	75.8	226	0.0	0.0	0.0	3.0
All herds	222,245	131.7	71.7	262	3.4	7.1	12.5	22.4

¹Number of cows reported in milk.

²All herd test days with usable records.

³Cows with usable records (less than total cows on test).

⁴Herd test days with ≥10 usable records.

Table 4. Characteristics of DHI herd test days for milk yield and SCC during 2008 by month.

SCC seasonal characteristics

	Herd test days¹	Cows² per herd	Average daily milk yield	Average SCC	Herd test days³ with SCC greater than
Month	(no.)	(no.)	(lb)	(cells/ml, 1000's)	750,000 cells/ml (%)	600,000 cells/ml (%)	500,000 cells/ml (%)	400,000 cells/ml (%)
January	19,076	127.9	72.1	276	3.3	6.8	11.9	21.3
February	18,444	131.7	72.7	280	3.6	7.5	12.7	22.5
March	19,296	131.7	72.9	276	3.7	7.6	13.1	23.2
April	18,906	133.7	73.2	261	3.4	6.8	11.9	21.2
May	19,386	131.9	73.4	257	3.0	6.2	11.0	20.3
June	17,995	134.0	72.7	261	3.3	6.9	12.5	22.5
July	18,812	129.1	71.1	278	4.1	8.9	15.4	26.8
August	18,542	132.0	70.6	276	4.3	9.4	16.2	28.2
September	17,790	131.6	69.6	264	3.7	8.1	14.3	25.2
October	18,480	132.7	69.7	249	3.0	6.6	11.7	21.8
November	17,827	132.3	70.3	230	2.3	4.9	9.2	17.4
December	17,691	132.2	71.3	235	2.8	5.8	10.2	18.7
Yearly	222,245	131.7	71.7	262	3.4	7.1	12.5	22.4

¹All herd test days with usable records.

²Cows with usable records (less than total cows on test).

³Herd test days with ≥10 usable records.

Figure 1. Average daily milk yield and SCC during 2008 by month.