INVITRO RUMINAL DIGESTION OF AVENA-SATIVA AND PASPALUM-DILATATUM IN RELATION TO HYPOMAGNESEMIA IN CATTLE

The objective of this work was to study the solubilization of minerals of importance in hypomagnesemic tetany from C3 (Avena sativa) and C4 (Paspalum dilatatum) grasses. Fresh forages were harvested at different times of the year, and weather parameters during growth and chemical composition at harvest were recorded. Release of soluble Mg, K, and Na from both forages was compared at intervals during 24-hour incubations in media containing either rumen fluid or streptomycin. Dry matter disapearance (DMD) of fresh and dried forages and the Mg, Ca, K, and Na contents of residues after the incubations were also compared. Crude protein, Na and K contents, the K/Mg + Ca ratio and humidity during growth were higher while dry matter, NDF, and ADF contents, and growth temperature were lower for oats than for paspalum. More Mg and K were released from oats than from paspalum during the 24-hour incubations in either media. Na concentrations decreased in both media during incubation of oats but did not change during incubation of paspalum. DMD of both dried and fresh forages was greater in rumen fluid medium than in streptomycin medium, and in both media DMD was greater for oats than for paspalum. In rumen fluid, dried paspalum showed greater DMD than fresh paspalum but no difference was detected between fresh and dried oats. Paspalum and oats residues had more Ca and K in rumen fluid than in streptomycin medium. K and Na contents of residues were greater for oats than for paspalum in both incubation media. Oats residues had more Ca than paspalum residues in rumen fluid medium but Ca contents did not differ in streptomycin medium. All residues had similar Mg contents. Results of this work suggest that both Mg and K might be rapidly released from oats and more gradually from paspalum soon after forage reaches the rumen. Microbial activity had a minor influence on mineral solubilization compared to influence of plant species. The tetanigenic properties of oats could be explained by the high K levels that may originate during ruminal digestion of that forage, rather than by reduced Mg solubilization in the medium.