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Assessment of within- and between-day variability of individual cow milk urea nitrogen.

K. F. Reed




Assessment of within- and between-day variability of individual cow milk urea nitrogen.
K. F. Reed*, E. M. Wood. Cornell University Ithaca, NY.

Milk urea nitrogen (MUN) is a non-invasive metric that has potential to inform cow nutrition and reproduction management decisions because of its demonstrated relationship to plasma urea nitrogen and urinary urea nitrogen. However, before recommendations for individual cow management decisions based on MUN can be developed, we need to understand the extent to which this metric varies within and between days. Previous work suggests PUN/MUN measurements taken within the same day could differ by ≥2 mg/dL but the authors are not aware of any estimates of between day variation in individual cow MUN. Thus, our objective is to estimate the within day and between day variability of MUN throughout lactation. Milk samples were collected 3 x per day for 7 consecutive days from 16 multiparous cows at 2 periods in lactation (average DIM 40, 140). Samples were preserved and refrigerated before they were sent to a commercial lab for midinfrared spectroscopy analysis of MUN within 48 h. We fit linear mixed models of induvial cow MUN observations averaged daily (Mod1) and from each milking (Mod2) including random effects for day and cow within period. A fixed-effect parameter for the change in CP content of the diet between periods was included in both models. In addition, the fixed-effect of milking time was include in Mod1. Parameter estimates in the table indicate average MUN in P1 was similar for both models (~8.2 mg/dL, β0) with about 1 mg/dl change for each percentage point change in diet CP (% DM, βCP). Estimates for the random effects of cow and day were also similar between models and suggest that MUN samples taken from the same cow on different days is ~1 mg/dL with 40% (Mod2) and 50% (Mod1) of the variation attributable to the cow. The fixed effect of milking time suggests the 2nd milking (M2) is expected to be 0.44 mg/dL higher than the 1st (M1) and 3rd (M3) milking (βM2).Table 1. Parameter estimates (SD) of linear mixed model results

Mod18.35 (0.298)1.05 (0.387)0.8800.5000.742
Mod28.20 (0.306)1.07 (0.391)0.439 (0.0947)0.0403 (0.0963)0.9090.5030.975

Keywords: MUN, variation.