Ionic environment of the rumen and its effect upon microbial function, volatile fatty acid production and absorption

Osmotic pressure, pH, total volatile fatty acid concentrations, calcium, magnesium, sodium, potassium, phosphate and chloride concentrations were studied with respect to time after feeding and type of diet. Three cattle fitted with rumen cannulae were fed six diets twice a day. Rumen fluid was collected every 20 minutes for the first hour after feeding, and then hourly for a 24-hour collection period. The 24-hour mean osmotic pressures for each diet were: corn and silage 316, silage 318, milo 331, hay 312 and concentrates-silage-hay 332 mOsm. The 24-hour mean total mineral concentrations for each diet were: corn and silage 199, silage 182, milo 321, hay 191 and concentrates-silage-hay 229 mmol/litre. With the addition of K at 200, 500, and 1000 mmol/litre, molar percentages of acetate and isovalerate increased whereas those of propionate and butyrate decline. Total VFA concentration declined with K concentration. With high levels of added K glucose utilization was significantly (P<0.01) depressed. Absorption rates of acetate and butyrate from the goat rumen were estimated by a tracer technique during trials where such changes were brought about by continuous infusion of KCl and by direct application of cation exchange resin. Potassium concentrations varied between 17.4 and 83.5 mmol, and sodium concentrations varied between 131 and 27.4 mmol. Acetate-2-14C was continuously infused into the rumen in seven trials for three to six hours, and sampling continued for four to eight hours. Average net acetate absorption (mmol/h) was estimated as 32.8 on control trials; 26.9 on the KCl infusion trial and -1.8 on the resin trial. Corresponding values for butyrate absorption were 16.4, 24,0 and 19.7. A notable decrease in the acetate specific activity was observed during on resin trial