Our preliminary data suggested that, using DHIA data only, the ratio of milk protein production (MPP, g/d) to milk urea nitrogen production (MUNP, g/d) might be a reliable and inexpensive indicator of nitrogen (N) use efficiency of dairy cows on commercial dairy farms. Although milk urea N (MUN, mg/dL) is an indicator of N intake and urinary urea-N excretion, it does not reflect milk protein synthesis. Our main objective was to compare 5 methods of calculation of MPP:MUNP ratio with the hypotheses that the ratio would be altered neither by calculation methods that use fewer DHIA variables nor by dietary CP level. Data were from samplings collected from 2 consecutive milkings (pm and am) in wk 12 of a study with 122 Holstein cows (mean ± SD: 303 ± 55 DIM; 761 ± 77 kg BW) fed a 16.2, 14.4, 13.1 or 11.8% CP (DM basis) TMR for 12 weeks. Methods of calculation were as follows: A) daily MPP divided by daily MUNP, where daily values were calculated using 6 DHIA values (milk production, milk protein %, and MUN for am and pm sampling, respectively); B) milk protein % divided by MUN (averages of am and pm for both); C) method B using am values only; D) method B using pm values only; and E) average of methods C and D. Ratios were analyzed in SAS 9.3 with PROC MIXED and single df orthogonal contrasts to compare methods B, C, D and E to method A. Overall, there was no difference between methods A and B, or A and E, but method C overestimated, and method D underestimated the MPP:MUNP ratio relative to method A (see Table 1.). These differences, however, were associated with high MPP:MUNP ratios observed when dietary CP were 13.1 and 11.8%. In this study, the MPP:MUNP ratios remained similar when calculated with average concentrations alone or average concentrations weighted for am and pm milk production. Table 1. Comparison of 5 methods of calculation of MPP:MUNP ratio

Dietary CP	Method						Contrast P-value
A	B	C	D	E	A vs. B	A vs. C	A vs. D	A vs. E
16.2% of DM	291	289	309	274	291		0.906	0.222	0.239	0.982
14.4% of DM	333	329	364	304	334		0.838	0.119	0.150	0.946
13.1% of DM	494	480	627	397	512		0.760	0.009	0.043	0.686
11.8% of DM	763	744	1042	647	844		0.884	0.048	0.383	0.536
Overall	470	461	585	405	495		0.596	<0.001	<0.001	0.164