Abstract
Ruminants eating dry forage secrete large volumes of saliva which results in decreased plasma volume (hypovolemia) and the loss of NaHCO 3 from the blood. The present research investigated whether or not hypovolemia and the loss of NaHCO 3 from the blood in goats brought about by dry forage feeding actually depresses feed intake and saliva secretion, respectively. The present experiment consisted of three treatments (NI, ASI, MI). In the control treatment (NI), a solution was not infused. In the ASI treatment, i.v. infusion of artificial parotid saliva was initiated 1 h before feeding and continued for the entire 2 h feeding period. In the MI treatment, iso-osmotic mannitol solution was infused. The NI treatment showed that hematocrit and plasma total protein concentration were increased due to decreased circulating plasma volume brought about by feeding. In the ASI treatment, the fluid and NaMCO 3 that were lost from the blood because of a feeding-induced acceleration of saliva secretion was replenished with an intravenous infusion of artificial parotid saliva. This replenishment lessened the levels of suppression on both feeding and parotid saliva secretion. When only the lost fluid was replenished with an intravenous infusion of iso-osmotic mannitol solution in the MI treatment, the degree of feeding suppression was lessened but the level of saliva secretion suppression was not affected. These results indicate that the marked suppression of feed intake during the initial stages of dry forage feeding was caused by a feeding-induced hypovolemia while the suppression of saliva secretion was brought about by the loss of NaHCO 3 from the blood due to increased saliva secretion during the initial stages of feeding.
Highlights
Campling and Balch (1961), and Anil et al (1993) reported that when a balloon was inserted into the rumen to restrict the rumen capacity of cows fed on hay and silage, feed intake was decreased
The mechanisms working to depress dry forage feeding in the first half of the feeding period have never been investigated
In the NI control of the present experiment, a decrease in plasma volume estimated by increases in hematocrit and plasma total protein concentrations was apparent within 15 min of the commencement of feeding (Figures 3 and 5)
Summary
Campling and Balch (1961), and Anil et al (1993) reported that when a balloon was inserted into the rumen to restrict the rumen capacity of cows fed on hay and silage, feed intake was decreased. Feeding duration and feed intake of fresh grass and hay under sham feeding conditions were not significantly different from those under normal feeding conditions whereby ingested matter was allowed to enter the rumen. This result suggested that some extraruminal mechanisms exist to suppress grass intake. The equal dose (in moles) of hyper-osmotic NaCl, polyethylene glycol-400 (PEG), sodium acetate or sodium propionate produced the same respective increases in rumen fluid osmolality when intraruminally infused. These increases in rumen fluid osmolality resulted in the same sized decrease in feed intake (Grovum, 1995). The nature of rumen osmoreceptors remains to be determined (Forbes and Barrino, 1992), and there is no biological proof of the existence of osmoreceptors in rumen walls that are sensitive to physiological variations in osmotic pressure (Cater and Grovum, 1990)
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