To study DMI effect on N metabolism and urea kinetics, 4 dairy cows were used in an incomplete replicated 3 × 3 Latin square (21-d periods). Three diets were formulated to provide similar supply of NE_L (34.7 Mcal/d) and MP (2300 g/d) through 3 DMI levels: 74% (Low), 86% (Med), and 98% (High) of ad libitum intake. Providing similar supply of MP and NE_L required adjusting RUP to estimated microbial-N, thus affecting N intake. On d 13, cows were infused with [¹⁵N¹⁵N]-urea (0.5 mmol/h) in a jugular vein for 3 d. Enrichments of [¹⁵N¹⁵N]- and [¹⁵N¹⁴N]-urea in urine and total ¹⁵N in feces were measured to estimate urea kinetics (Lobley et al., 2000, BJN 84: 459). Urine and feces were totally collected from d 14 to 19. Data were analyzed using MIXED procedure of SAS, evaluating DMI effect with polynomial contrasts. The LSM are given for Low, Med, High ± SEM, respectively. Despite a similar MP supply, increased DMI (16.5, 19.2 and 21.8 ± 0.2 kg/d) linearly increased N intake, fecal excretion, and balance but decreased urinary N. Although urea entry rate (UER) was not affected by DMI, urinary urea-N (UUN) linearly decreased with increased DMI, as did the ratio UUN/UER (P = 0.04: 0.53, 0.43, 0.36 ± 0.04). Urea recycled to the gut (GER) tended to linearly increase with DMI, as did the ratio of GER/UER (P = 0.04: 0.47, 0.57, 0.64 ± 0.04). The proportion of GER returned to ornithine cycle (ROC) was unaffected. Urea-N to feces (UFE) linearly increased with increased DMI, as did the ratio of UFE/GER (P = 0.01: 0.07, 0.09, 0.12 ± 0.01). Although GER was elevated with increased DMI, 25% of this increment was lost in the feces. One third of the increment in fecal N with increased DMI was related to the increased UFE: other sources of incremental fecal N could be non-digested RUP or MCP, and endogenous N secretion. Table 1.

Item (g/d)	DMI levels			SEM	Contrasts
	Low	Med	High		Linear	Quadratic
N intake	514.5	550.4	574.1	9.9	0.02	0.65
Urinary N	213.4	186.2	156.1	7.9	0.01	0.88
Fecal N	118.2	145.2	170.8	3.3	0.001	0.86
Milk N	145.9	147.7	147.9	4.7	0.78	0.89
N balance	37.1	71.3	99.4	6.5	0.01	0.92
UER	332.2	341.6	342.0	13.9	0.65	0.81
UUN	173.9	145.6	119.4	7.5	0.01	0.92
GER	158.3	196.1	222.6	19.6	0.09	0.83
ROC	55.7	64.8	75.1	6.5	0.11	0.95
UFE	10.7	16.5	27.4	1.8	0.05	0.31
Anabolism	91.9	114.8	120.1	13.2	0.21	0.63