The effects of a 6 versus 24 h ruminal urea infusion in lactating dairy cows fed a basal diet deficient in N on ruminal ammonia concentration; arterial urea-N concentration, net portal-drained viscera (PDV) urea-N flux, arterial urea-N extraction across the PDV, and renal urea-N kinetics were investigated. Three Danish Holstein cows fitted with ruminal cannulas and permanent indwelling catheters in major splanchnic blood vessels were randomly allocated to a 3 x 3 Latin square design with 21-d periods. Treatments were ventral ruminal infusion of water for 24 h (water INF), 24-h infusion of 15 g of urea/kg of dry matter intake (DMI; 24-h INF), and 6-h infusion of 15 g of urea/kg of DMI (6-h INF). The 6-h INF was initiated 0.5 h after the afternoon feeding, and ran until 2230 h. Eight sample sets of arterial, portal, and hepatic blood, ruminal fluid, and urine were obtained at 0.5 h before the morning feeding and 0.5, 1.5, 2.5, 3.5, 4.5, 5.5, and 6.5 h after feeding (i.e., 9 to 15.5 h after the 6 h infusion was terminated). A substantial decrease in DMI for 6-h INF compared with 24-h INF and water INF was observed, and it has to be recognized that DMI may have confounding effects. However, the experimental setting plan was met (i.e., to cause changes in the daily pattern of ruminal ammonia and blood urea-N concentrations). The arterial urea-N concentration for 24-h INF and 6-h INF were greater than the arterial urea-N concentration with water INF throughout the sampling window. However, the arterial urea-N concentration for 6-h INF decreased steadily with sampling time reflecting a carryover effect from the ruminal urea infusion. The ruminal ammonia concentration and net portal flux of ammonia for 6-h INF were not different from water INF; hence, no carryover effect on ruminal ammonia concentration was observed. The portal flux of urea-N was not affected by treatment (i.e., even the combination of low ruminal ammonia. and high arterial urea-N concentration with 6-h INF was not used by the cow to increase the uptake of urea-N across the PDV). Arterial urea-N extraction across the PDV was increased with water INF especially from 0.5 to 3.5 h postprandial relative to the urea infusion treatments, reflecting increased epithelial permeability for urea-N. This indicates that daily ruminal peak of ammonia or blood urea-N concentrations overruled potential signals from low ruminal ammonia concentration observed during the sampling window. In conclusion, dairy cows appear unable to increase transport of urea-N from blood to gut in periods with low ruminal ammonia concentrations, even in a situation with infrequent N supply and apparent carryover effects on blood urea-N. It is speculated that mechanisms responsible for down-regulation of epithelial urea-N transport based on daily maximum concentrations of ammonia in the rumen or urea-N in the blood suppresses any short-term signal from low ruminal ammonia during periods with low ruminal N supply.
