Abstract
Each of three rumen-fistulated cows was fed prairie hay at 5.5, 7.3, or 9.1 kg/day during three consecutive 2-wk periods and then the same amounts of alfalfa hay for a second series of 2-wk periods. The prairie hay contained 5.7% silica and the alfalfa hay contained 0.4% silica. Reticulo-rumen fluid was saturated with silicic acid at all levels of intake of prairie hay but was unsaturated at all levels of intake of alfalfa hay, though the concentration increased with increasing intake. The concentration did not vary with time after the feeding of either type of hay. Output of silicic acid in urine tended to increase with increasing intakes of either type of hay. The concentration of silicic acid in blood plasma was higher with prairie hay than with alfalfa hay but was unaffected by level of hay intake. Neither type of hay nor level of intake affected the concentration of silicic acid in saliva. In a second experiment, silicic acid concentration in the reticulo-rumen fluid of cows on alfalfa hay was shown to be unrelated to water intake. In both experiments, the rate of excretion of silicic acid in urine of cows on alfalfa hay closely paralleled the concentration of silicic acid in reticulo-rumen fluid (r = 0.94). Estimates derived from the results of these experiments indicate that 7% of the silica in prairie hay and 16% of that in alfalfa hay was taken into solution in reticulo-rumen fluid and thus made available for absorption. Of these amounts, not more than 12% was absorbed with a diet of prairie hay and up to 45% may have been absorbed with a diet of alfalfa hay. Excretion of silicic acid was between 60 and 75% of that absorbed on the prairie hay diet and between 40 and 60% of that absorbed on the alfalfa hay diet.
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