Effect of supplement strategy on splanchnic net fl uxes of ammonia and urea in dairy cows fed fresh grass

Different strategies for supplementation of grass with rolled barley and soya hulls were investigated as tools for improving the nitrogen effi ciency of dairy cows under restricted zero-grazing (47% grass DM) conditions. Three multicatheterized cows were subjected to 3 experimental treatments in a Latin square design. Treatments were: SAM, soya hulls fed am and barley fed pm; BAM, barley fed am and soya hulls fed pm; SBPM barley and soya hulls fed pm. The total daily ration was the same for all treatments. The arterial concentrations, net portal fl uxes and net hepatic fl uxes of ammonia, urea-N, oxygen and glucose were not affected by treatments. The cows had higher net portal fl ux of ammonia than transfer of urea-N to the gut although the diet had a low CP content (12.4%). Effi cient recycling of nitrogen to the rumen combined with decreased nitrogen intake is in theory a promising strategy for improved nitrogen effi ciency in ruminants. However, better understanding of factors affecting nitrogen recycling in ruminants is necessary for a successful utilization of this strategy.


INTRODUCTION
Approximately 25% of the nitrogen input to a dairy herd is recovered in milk and meat.The aim of the present study was to investigate the effect of three supplement strategies in a zero-grazing system on splanchnic net fl uxes of ammonia and urea and splanchnic energy metabolism in dairy cows.
Treatments were arranged in a Latin square with 1 wk periods and blood, rumen fl uid, urine and faeces were collected over a 24 h period on the last day of each experimental period.Continuous infusion of p-aminohippuric acid (pAH, 31±3 mmol/h) into the mesenteric vein was initiated at 5 am and 16 sets of blood samples (20 ml) were obtained simultaneously from the artery, portal, and hepatic catheters at 1.5 h intervals staring at 6 a.m.
The oxygen content in whole blood was determined by an ABL520 (Radiometer, Copenhagen, Denmark) and blood glucose was determined on fresh blood using an Accu-Check compact (Roche Diagnostics, Mannheim, Germany).The packed cell volume was determined by centrifugation of micro capillary tubes and plasma was separated by centrifugation at 3.000 × g, at 4ºC for 20 min.Previously described methods were used to analyse pooled plasma samples for ammonia (Rhine et al., 1998), urea (Marsh et al., 1965), and pAH (Harvey and Brothers, 1962).Ammonia in ruminal fl uid was analysed by the same method as used for plasma.
Twenty-four h treatment means were analysed by ANOVA by the GLM procedure of SAS (SAS Inst.Inc., Cary, NC, USA).The model included the effect of cow, period, and treatment.Values in the text are given as means of three cows ± SEM.

RESULTS AND DISCUSSION
The experiment was conducted in a period with abnormal spring weather from May 20 to June 10, 2004.The grass had a lower content of CP (14.3% of DM) than expected and the total ration was therefore low in CP (12.4% of DM).The DMI (14±2 kg/d) did not differ (P=0.50) between treatments.The ruminal ammonia concentration was in average 1.7±0.6 mmol/l and lower than the expected optimal level for microbial growth (3.5 to 6 mM; Parker et al., 1995).
None of the arterial variables tested were affected by treatment (P≥0.10;Table 1).In agreement with the low CP content in the diet, the plasma urea-N concentrations were low, but the arterial ammonia concentrations were relatively high in the present study compared with literature data (Reynolds et al., 2003).The portal and hepatic blood fl ow as well as the net portal and net hepatic fl uxes of ammonia, urea-N, glucose and oxygen did not differ between treatments (P>0.10;Table 2).The net portal fl ux of ammonia accounted for 22% of the Table 2. Portal and hepatic blood fl ow as well as net portal and net hepatic fl uxes of ammonia, urea-N, oxygen and glucose in dairy cows with 3 different supplement strategies in a zero-grazing system (SAM: soya hulls fed am and barley fed pm; BAM barley fed am and soya hulls fed pm; SBPM soya hulls and barley fed pm).Mean of three cows ± SEM low nitrogen intake in the present study (Lapierre and Lobley, 2001).Despite an expected negative nitrogen balance in the rumen (AAT/PBV system) the negative net portal fl ux of urea accounted for only 64% of net portal fl ux of ammonia.The estimated salivary urea fl ux was negative (-23 mmol/h; net hepatic fl ux of urea -urinary urea excretion -portal urea uptake).The negative value is probably refl ecting sampling and/or analytical errors, but does indicate that the salivary contribution with urea to the lumen of the gut was low under the present experimental conditions.

CONCLUSIONS
Dairy cows fed a grass-based diet with low nitrogen content had a higher net portal fl ux of ammonia than urea-N transfer to the gut.These observations indicate that although the ruminant system in principle has the potential to be highly nitrogen effi cient we need to know more about nitrogen metabolism of dairy cattle in order to increase the overall nitrogen effi ciency of dairy operations.

Table 1 .
Arterial packed cell volume and concentrations of pAH, ammonia, urea-N, oxygen and glucose in dairy cows with 3 different supplement strategies in a zero-grazing system (SAM: soya hulls fed a.m. and barley fed p.m.; BAM barley fed a.m. and soya hulls fed p.m.; SBPM soya hulls and barley fed p.m.).Mean of three cows ± SEM