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
