|Wright, Janice - Jan|
Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/17/2003
Publication Date: 7/1/2004
Citation: Miller, R.H., Norman, H.D., Wiggans, G.R., Wright, J.R. 2004. Relationships of test-day somatic cell scores with test-day and lactation milk yields. Journal of Dairy Science. 87(7):2299-2306. Interpretive Summary: Previous studies estimated lactation milk loss due to elevated lactation average SCS not individual test day values. The effects of test-day SCS on current and subsequent test-day milk yield and on 305-2x-ME yield were estimated. Elevated SCS in early lactation were associated with greatest milk loss in first lactations, but milk losses were substantial for elevated SCS in all stages of second lactations. Mastitis control is critical for heifers prior to and immediately after first calving.
Technical Abstract: To determine usefulness of test-day somatic cell score (SCS) in predicting test-day and lactation total milk yield, records from Holstein first and second calvings between 1995 and 2002 were examined. Initial selection required that lactations have at least the first four test days with recorded milk yield and SCS. Least-squares analyses were conducted for milk yields on test-days 2-10 within herd and cow. The model included regressions on both current test day SCS and mean SCS of all previous test days, with separate estimates by parity; effects for parity and calving year also were included as well as a regression on days in milk on test-day 1. Corresponding analyses were conducted omitting regression on current test-day SCS. An additional analysis of lactation total milk yield was performed, with regressions on all 10 test-day SCS, using a similar model. Error degrees of freedom ranged from 143,748 to 214,526. A cow's highest SCS was most often on test-day 1 in first lactation (22.5%), but was on test-day 10 in second lactation (18.5%). In first lactation, mean of previous test-days' SCS had greatest independent usefulness in the latter half of lactation (maximum regression -.346 on test-day 9). In second lactation, mean of previous test-days' SCS was most useful in earlier stages of lactation (maximum -.366 on test-day 4). All regressions on current test-day SCS were significant and tended to be greater for test-day milk in later lactation. Relative usefulness of mean of previous test-days' SCS was greater in first lactation. Regressions of 305-2x-ME milk yield on all 10 test-day SCS were negative and significant in second lactation. For first lactation 305-2x-ME yield, regressions were negative and significant only for test-days 1-6. To minimize milk losses, mastitis control is most important immediately pre- and post-calving for first lactation, but is uniformly important throughout second lactation.