Our objective was to assess the relationship between milk urea nitrogen (MUN, mg/dL) and blood urea nitrogen (BUN, mg/dL), and the excretion of urinary nitrogen (UN, g/d), and urinary urea nitrogen (UUN, g/d) in late lactation dairy cows. Holstein cows (means ± SD; 736 ± 18 kg BW; 224 ± 54 DIM at the start of the trial) were used in a 16-pen study with 8 cows per pen, and fed one of 4 TMR containing either 11.9, 13.4, 14.4, or 16.3% CP (DM basis) for 12 weeks. Cows (n = 4 per pen) were sampled for BUN on wk 4, 8 and 12. Daily MUN (infrared analysis) was derived from sampling 4 cows per pen in 2 consecutive milkings in wk 3, 7 and 11. In the same 3 weeks, urine volume was estimated using creatinine concentration in spot urine samples collected 6 h before and 6 h after feeding in groups of 4 randomly selected cows in each pen. Linear models were built using PROC REG and differences in slopes were tested using PROC GLM in SAS. Week of sampling did not affect the slope between BUN and MUN (P = 0.11, Table 1), but affected the slope of the relationship between UN and MUN (P = 0.042) and between UUN and MUN (P = 0.029). Increased dietary CP resulted in increases in UUN/UN ratio (0.54, 0.68, 0.74, and 0.81). However, there was no difference in slope when UN or UUN were regressed against MUN. Excretion of non-urea-N was constant (40.8 g/d). Four published models showed a slope bias and over-predicted UN when applied to our data set (227, 214, 176 and 125 g/d compared with 116 g/d observed UN). The week effect observed here, and the inadequacy of previous models suggested that best equations to predict UN from MUN should consider stage of lactation. Table 1.

Y variable	X variable	Week	Intercept	SE	Slope	SE	R2	RMSE
BUN	MUN	3	0.191	0.79	0.96	0.076	0.94	0.94
		7	1.701	1.23	0.75	0.116	0.75	1.72
		11	−1.991	2.20	1.18	0.214	0.68	1.81
UN	MUN	3	47.4	7.3	6.95b	0.704	0.87	8.65
		7	29.3	9.1	8.15ab	0.084	0.90	12.50
		11	3.701	11.4	10.90a	1.106	0.87	9.32
UUN	MUN	3	5.801	5.98	7.93b	0.576	0.93	7.07
		7	−12.2	9.46	9.62ab	9.46	0.90	13.01
		11	−35.5	10.6	11.54a	1.032	0.90	8.70

^abSlopes among weeks for each set of regressions with a different superscript differ at P < 0.05. ¹Intercept is not different from zero at P < 0.05.