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.
