The effect of inclusion of soybean meal (SBM), canola meal (CM), or an excess of RDP on N metabolism was determined on 6 multiparous Holstein cows (663 ± 25 kg BW) using a replicated 3 × 3 Latin square, balanced for residual effects, with 3 21-d periods. The cows were fed grass silage-based diets, including the following protein sources 1) SBM (14.5% DM), 2) CM (21.1% DM) and 3) CM plus 317 g of urea (CMU). The SBM and CM diets were planned to be isoenergetic (1.60 Mcal/kg) and isonitrogenous (16.5% CP), and balanced to meet RDP requirements according to NRC (2001). On the last 4 d of each experimental period, DMI, and milk production and composition were determined. On the last 3 d (2 cows per d), 2 rumen samples were collected to determine NH₃ concentrations and 3 spot samples of urine were collected. Creatinine and purine derivatives concentrations were analyzed in urine to estimate urine volume and duodenal microbial nitrogen flow (MNF). Data were analyzed using the Mixed procedure of SAS (treatment, and period as fixed effects, cow as random effect). The multiple comparison test Tukey-Kramer was used to compare the treatment means. Despite lower estimated MP supply with CM vs SBM diets (2504 vs. 2765 g/d), milk and milk protein yield were not affected by protein source. Inclusion of CM in the diet numerically decreased MUN and the proportion of urinary urea-N compared with SBM but did not affect rumen ammonia concentration. Excess RDP supply did not penalize milk and milk protein yields, but decreased the efficiency of N utilization, increased MUN, the proportion of urinary urea-N, and rumen ammonia concentration. Estimated MNF was not affected by treatments. Table 1.

Item	Treatment			SEM	P-value
	SBM	CM	CMU
DMI, kg/d	24.7	25.0	26.0	1.2	0.33
N intake, g/d	651a	635a	799b	24	<0.001
Milk yield, kg/d	36.9	37.6	38.2	1.5	0.18
True protein yield, g/d	1,040	1,091	1,106	55	0.39
Milk N / N intake, %	26.4a	28.7ab	24.2b	1.1	0.02
MUN, mg N/dL	13.0a	12.0a	23.0b	0.4	<0.001
Urinary urea/total N, %	65.8a	59.9a	72.6b	2.0	<0.01
Rumen ammonia, mg/100 mL	18.7a	18.6a	42.9b	2.3	<0.001
Duodenal microbial N flow, g/d	418	415	434	29	0.86

^a,bMeans within rows with different superscripts differ (P < 0.05).