M a C R O M I N E R a L Absorption in Sheep Fed Tetany-prone and Non-tetany-prone Hays 1

Two experiments were conducted to determine some of the factors that led to hypomagnesemic tetany associated with the feeding of two orchardgrass hays. Sixteen mature Columbia and Suffolk wethers (62 to 72 kg), four of which were fitted with ruminal and abomasal cannulae, were fed one of two tetany-prone orchardgrass hays or a non-tetany-prone bromegrass hay. In Exp. 1, 12 wethers were used in a completely random design metabolism experiment to measure apparent absorption and retention of macrominerals. In Exp. 2, four wethers with ruminal and abomasal cannulae were used in a completely random design experiment to monitor pre-intestinal mineral absorption and ruminal characteristics. This experiment was replicated once, with wethers remaining on their diets for 16 d in each replication. In the metabolism experiment, apparent absorption and retention of Mg as a percentage of intake were lower (P<.01 and P<.05) for one tetany-prone orchardgrass hay compared with the other orchardgrass hay. Pre-intestinal absorption of Mg in the cannulated wethers was greater (P<.01) for the orchardgrass hays than for the bromegrass hay. Pre-intestinal Mg absorption was higher (P<.01), both in terms of grams per day and as a percentage of intake for the orchardgrass hay with the highest Mg content. In the noncannulated wethers, the percentage of water-soluble Mg in the feces was lower (P<.01) for the tetany-prone hays, indicating that a decrease in Mg solubility in the intestine may have influenced Mg apparent absorption. Wethers consuming bromegrass hay showed negative Ca and P balances although Ca levels were above requirements. Plasma Mg, Ca, inorganic P and K were not different among wethers fed the three hays. Ruminal ammonia-N production was higher (P<.01) for the orchardgrass hays compared with bromegrass, and the two orchardgrass hays differed from one another. Volatile fatty acid concentrations were not different among treatments. There was a tendency for ruminal K concentrations to be higher (P<.06) in wethers consuming the tetany-prone hays, but rumlnal Na concentrations were not affected by treatment. These data are interpreted to suggest that the solubility in the gastrointestinal tract may be a critical factor in mineral bioavailabillty for some forages. (