Somatic cell counts of milk from 2009 DHI herds

AIPL RESEARCH REPORT
SCC11 (2-10)

Somatic cell counts of milk from Dairy Herd Improvement herds during 2009

H.D. Norman, R.H. Miller, and T.A. Cooper
Animal Improvement Programs Laboratory, Agricultural Research Service, USDA, Beltsville, MD 20705-2350
301-504-8334 (voice) ~ 301-504-8092 (fax) ~ duane.norman@ars.usda.gov ~ http://aipl.arsusda.gov

Test-day data from all herds enrolled in Dairy Herd Improvement (DHI) somatic cell testing during 2009 (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 editing procedures were excluded. Somatic cell score (SCS) of milk is reported to the Animal Improvement Programs Laboratory as part of an individual cow's test-day yield information. The SCS was converted back to a milk somatic cell count (SCC) in cells per milliliter [SCC = 2^{(SCS − 3)} × 100,000] for calculating herd and State averages.

Table 1 shows total number of herd test days, average number of cows with usable records per herd on test day, average test-day milk yield and herd SCC, and 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 (excluding Alaska and Hawaii but including Puerto Rico) during 2009. The current legal limit for bulk tank SCC is 750,000 cells/ml for Grade A producers; the lower limits have been proposed as future maximums by the National Mastitis Council. California's current legal SCC limit is 600,000 cells/ml. State and national average daily milk 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. Percentages of herd test days by SCC level are for herd test days that included records from at least 10 cows. Nationally, average test-day herd SCC during 2009 was 233,000 cells/ml, which was considerably lower (by 29,000 cells/ml) than last year's average. Most States (42) had lower average SCC than reported last year; only 6 States had higher averages.

Variation among States (Table 1) remains large. State average SCC generally was lower than the national average for mountain and western States and often higher for southeastern States (Figure 1). 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 national averages for 1995 through 2009 are in Table 2. National average test-day herd SCC has declined every year since 2005 and every year except one since 2001, which had the highest SCC at 322,000 cells/ml. The percentage of herd test days that exceeded 750,000, 600,000, 500,000, and 400,000 cells/ml during 2009 was 2.7, 5.8, 10.3, and 18.9, respectively, which was lower for all levels than during 2008. The 2.7% of 2009 DHI herd test days that were higher than the present legal limit for bulk tank SCC may overestimate the percentage of herds that shipped milk exceeding the legal limit because milk of cows treated for mastitis is excluded from the bulk tank even though included in DHI test data. The percentage of herd test days that exceeded the legal limit also 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 test-day milk yield have been increasing since 1995 (Table 2). The large increase in cows per herd from 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 continued from 2008 to 2009 (131.7 versus 136.1). The large decline in SCC since 2001 is encouraging as well as surprising considering the annaul changes before 2001. Hopefully, this trend will continue because of improved management practices and genetic selection. The trend for lower percentage of herd test days above all thresholds is likewise encouraging. The concerted effort to improve milk quality needs to continue. State and national SCCs for DHI herds will continue to be summarized annually so that changes in SCCs and implications for milk quality can be monitored.

Table 3 reports average test-day milk yield and herd SCC during 2009 by estimated herd size. As herd size increased, milk yield generally increased as SCC declined. The most disturbing result still remains the percentage of test days with over 750,000 cells/ml: 5.0% for herds with less than 50 cows, 1.4% for herds with 50 to 99 cows, 0.7% for herds with 100 to 149 cows, and an average of 0.3% for herds with 150 cows or more. Because small herds no longer contribute as much to the total milk supply (see herd-average reports), the trend for increased herd size improves milk quality because large herds have lower SCCs on average.

Table 4 reports average test-day milk yield and herd SCC during 2009 by month, and the seasonal pattern for milk quality is shown in Figure 2. Milk yield per cow was highest in the winter and spring and declined during the summer months; SCC increased from April through July and then decreased from July through November. The highest quality milk was produced in November and December.

Table 1. Characteristics of test-day milk yield and somatic cell count (SCC) from Dairy Herd Improvement herds by State during 2009

SCC characteristics

	Herd test days¹	Cows² per herd	Average test-day milk yield	Average test-day herd SCC	Herd test days³ with SCC greater than
State	(no.)	(no.)	(lb)	(cells/ml, 1000s)	750,000 cells/ml (%)	600,000 cells/ml (%)	500,000 cells/ml (%)	400,000 cells/ml (%)
Alabama	237	118.6	50.0	440	3.4	16.0	29.1	51.5
Arizona	186	1305.4	68.0	174	0.0	0.0	0.0	0.0
Arkansas	243	99.7	54.8	421	11.9	19.3	25.5	40.3
California	8,056	763.7	73.1	198	0.9	1.7	2.9	6.2
Colorado	263	612.0	76.6	219	0.0	0.0	0.0	1.9
Connecticut	719	104.3	67.8	233	1.7	3.3	7.5	14.2
Delaware	226	119.9	70.5	291	0.4	1.3	4.9	13.3
Florida	189	648.2	67.9	323	8.5	14.8	26.5	40.7
Georgia	1,032	179.3	64.2	350	4.4	14.5	24.8	43.4
Idaho	1,493	821.3	72.5	192	1.4	2.8	4.2	8.5
Illinois	3,859	92.2	70.0	272	3.6	7.2	12.6	23.8
Indiana	3,187	87.7	69.1	260	3.3	6.9	12.2	21.8
Iowa	7,821	113.8	74.2	257	3.7	7.3	12.6	23.9
Kansas	1,809	83.6	66.8	292	2.3	5.6	12.8	25.3
Kentucky	1,486	81.3	65.4	317	5.5	11.2	18.8	30.1
Louisiana	312	122.0	55.3	412	7.1	17.3	34.9	52.6
Maine	1,283	74.3	64.0	248	1.7	5.5	9.9	17.8
Maryland	3,228	83.9	66.9	267	3.3	6.9	12.6	21.3
Massachusetts	758	69.6	65.2	246	2.5	6.1	10.2	17.8
Michigan	6,556	164.6	76.2	189	1.0	2.4	4.9	10.3
Minnesota	22,492	88.5	72.3	280	3.5	7.6	13.9	24.8
Mississippi	285	150.8	61.8	320	3.9	10.5	20.4	36.1
Missouri	3,150	72.2	60.6	345	6.2	12.1	19.4	30.8
Montana	273	139.5	74.5	197	0.0	0.0	0.0	3.3
Nebraska	1,250	138.1	68.7	263	3.5	7.0	11.4	21.6
Nevada	103	499.5	79.6	197	0.0	0.0	0.0	2.9
New Hampshire	820	96.5	70.7	224	2.7	5.5	8.9	13.3
New Jersey	459	66.8	67.2	319	2.0	8.5	15.7	28.1
New Mexico	227	1802.4	72.7	241	2.2	4.0	4.8	6.6
New York	17,837	125.8	73.4	230	1.8	4.2	8.1	15.8
North Carolina	1,360	132.6	66.5	307	2.7	6.8	14.9	28.8
North Dakota	310	103.6	65.4	297	4.5	8.4	14.2	26.5
Ohio	7,689	109.9	70.7	222	1.6	4.0	7.4	14.9
Oklahoma	480	93.5	59.9	321	6.3	11.0	19.0	33.1
Oregon	1,905	198.7	65.0	183	1.7	2.6	4.2	7.9
Pennsylvania	40,202	59.9	70.3	264	2.6	5.7	10.5	19.5
Puerto Rico	713	79.4	39.5	368	9.1	14.9	19.9	33.4
Rhode Island	75	51.9	62.7	208	4.0	5.3	5.3	14.7
South Carolina	510	157.3	64.1	343	1.4	6.1	15.1	29.8
South Dakota	1,179	198.5	71.9	257	4.4	10.7	17.0	27.9
Tennessee	1,197	93.7	61.7	375	6.1	13.4	24.1	42.1
Texas	1,202	478.6	62.3	232	0.3	1.8	4.8	12.4
Utah	1,066	216.3	69.5	193	2.7	5.0	7.5	11.7
Vermont	3,173	104.4	67.5	206	1.3	3.0	5.4	10.1
Virginia	3,766	118.6	69.6	287	2.1	6.2	12.7	24.9
Washington	1,570	258.1	73.2	194	0.8	1.6	2.8	6.9
West Virginia	376	91.2	58.1	317	3.2	6.4	13.3	29.3
Wisconsin	47,566	95.1	76.3	239	2.8	5.6	9.5	17.0
Wyoming	17	190.4	69.4	174	0.0	0.0	0.0	0.0
United States	204,195	136.1	72.1	233	2.7	5.8	10.3	18.9

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 for test-day milk yield and somatic cell count (SCC) from Dairy Herd Improvement herds by year

SCC characteristics

	Herd test days¹	Cows² per herd	Average test-day milk yield	Average test-day herd SCC	Herd test days³ with SCC greater than
Year	(no.)	(no.)	(lb)	(cells/ml, 1000s)	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
2009	204,195	136.1	72.1	233	2.7	5.8	10.3	18.9
Annual trend	-5,016	6.4***	0.5***	-5.4***	-0.1*	-0.3**	-0.5***	-0.7***

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 test-day milk yield and somatic cell count (SCC) from Dairy Herd Improvement herds during 2009 by herd size

SCC characteristics by herd size

Herd size¹	Herd test days²	Cows³ per herd	Average test-day milk yield	Average test-day herd SCC	Herd test days⁴ with SCC greater than
(cows)	(no.)	(no.)	(lb)	(cells/ml, 1000s)	750,000 cells/ml (%)	600,000 cells/ml (%)	500,000 cells/ml (%)	400,000 cells/ml (%)
<50	89,043	28.8	62.4	289	5.0	9.4	15.0	24.4
50 - 99	61,452	69.2	67.3	271	1.4	4.1	8.9	18.5
100 - 149	19,240	120.3	68.8	258	0.7	2.8	6.4	14.7
150 - 199	8,678	172.2	70.7	250	0.5	2.1	5.5	12.7
200 - 299	8,257	242.5	72.8	239	0.3	1.0	3.2	9.7
300 - 499	7,193	385.9	75.4	224	0.2	1.0	2.2	6.3
500 - 999	5,888	690.9	76.6	216	0.2	0.6	1.4	4.0
1000 - 1999	3,136	1368.8	76.2	209	0.1	0.4	1.0	3.1
2000 - 2999	817	2427.5	74.8	199	0.0	0.0	0.0	1.6
>3000	491	4143.4	73.6	195	0.2	0.2	0.2	0.2
All herds	204,195	136.1	72.1	233	2.7	5.8	10.3	18.9

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 test-day milk yield and somatic cell count (SCC) from Dairy Herd Improvement herds during 2009 by month

SCC seasonal characteristics

	Herd test days¹	Cows² per herd	Average test-day milk yield	Average test-day herd SCC	Herd test days³ with SCC greater than
Month	(no.)	(no.)	(lb)	(cells/ml, 1000s)	750,000 cells/ml (%)	600,000 cells/ml (%)	500,000 cells/ml (%)	400,000 cells/ml (%)
January	18,338	137.1	72.0	251	3.1	6.7	11.7	20.2
February	17,055	135.4	72.8	241	3.4	6.4	11.4	19.9
March	17,808	133.8	72.9	233	2.9	6.0	10.5	19.0
April	17,492	136.4	73.5	224	2.3	5.1	9.4	17.4
May	17,886	135.3	74.0	225	2.3	4.9	9.0	17.1
June	16,385	137.2	73.5	232	2.5	5.3	9.6	18.3
July	17,383	133.3	72.3	249	3.0	7.0	12.4	22.3
August	16,065	137.2	71.0	248	3.5	7.2	12.4	22.6
September	16,406	136.3	70.3	239	2.9	6.2	11.3	20.5
October	16,982	136.6	70.1	224	2.2	5.1	9.3	17.5
November	15,507	138.4	70.4	214	2.0	4.4	8.4	16.0
December	16,888	136.2	71.8	216	2.1	4.7	8.5	16.0
Yearly	204,195	136.1	72.1	233	2.7	5.8	10.3	18.9

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 test-day somatic cell count from Dairy Herd Improvement herds during 2009 by State

Figure 2. Average test-day milk yield and somatic cell count (SCC) from Dairy Herd Improvement herds during 2009 by month