Abstract #T219

# T219
Effect of dry matter intake (DMI) on N metabolism and urea kinetics in lactating dairy cows.
S. H. Lee1, H. Lapierre2, D. R. Ouellet*2, 1Gyeongsangnam Livestock Promotion Institute, Sancheong, South Korea, 2R&D Centre, Agriculture and Agri-Food Canada, Sherbrooke, QC, Canada.

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 NEL (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 NEL 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 levelsSEMContrasts
N intake514.5550.4574.
Urinary N213.4186.2156.
Fecal N118.2145.2170.83.30.0010.86
Milk N145.9147.7147.94.70.780.89
N balance37.171.399.

Key Words: urea kinetics, recycling, dry matter intake