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1 Department of Dairy Science,
2 Department of Statistics and Department of Forest Ecology and Management, and
3 Department of Computing and Biometry, University of Wisconsin, Madison 53706
Corresponding author: M. A. Wattiaux; e-mail: wattiaux{at}wisc.edu.
In this study, 400,729 Dairy Herd Improvement (DHI) records collected on 77,178 cows in 692 Midwest herds over 29 mo (January 1999 to May 2001) were used to analyze milk urea nitrogen (MUN) as collected the day of the test in 6 breeds. Records of Holsteins, Jerseys, and Brown Swiss were subjected to stepwise backward elimination analysis with a model including parity (primiparous vs. multiparous cows), sample type (morning vs. evening), milking frequency (2x vs. 3x [Holstein only]), season (winter, spring, summer, and fall), yield of fat-corrected milk (FCM) classified into 1 of 3 FCM categories (FCMc) and all possible higher-order interactions. Results indicated that FCMc contributed to test-day MUN variation in multiparous, but not primiparous, Holsteins. Sample type and season were significant in both parity groups; milking frequency was not significant, but milking frequency x season and milking frequency x FCMc were significant in both parity groups. The nature of these interactions differed for each parity group. For Jersey and Brown Swiss data analyzed by sample type separately, parity was not significant but tended to interact with FCMc, whereas season, FCMc, and season x FCMc were generally significant. Mean test-day MUN was 12.7, 14.6, and 14.4 mg/dL, with 24, 45, and 42% of records above 14.5 mg/dL in Holsteins, Jerseys, and Brown Swiss in single-breed herds, respectively. In Holsteins, MUN peaked at 7 to 10 d in milk (DIM), declined until 28 to 35 DIM, and rose again thereafter. In primiparous Holsteins, MUN did not change with FCM 
42 kg/d, but for higher FCM yield, MUN declined linearly by 0.05 mg/dL per kilogram of FCM. In multiparous Holsteins, MUN increased by 0.06 and 0.03 mg/dL per kilogram of FCM as FCM yield increased from 5 to 29 and from 30 to 59 kg/d, respectively, but decreased by 0.06 mg/dL as FCM yield increased from 60 to 85 kg/d. The use of adjustment coefficients may facilitate interpretation of test-day MUN on commercial herds. Research should focus on the biological significance of the pattern of change in MUN the first few weeks postpartum and the drop in MUN in unusually high-producing cows.
Key Words: environment • Dairy Herd Improvement • nitrogen • hierarchical backward elimination
Abbreviation key: FCMc = fat-corrected milk category.
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