Relative availability of 5 inorganic magnesium sources in nonpregnant nonlactating Holstein cows.

Inorganic sources of Mg are commonly used in dairy cow diets, but their availability varies significantly. This study assessed the relative availability of 4 commonly used inorganic Mg sources and a novel alkalinizing proprietary mineral blend (PMB; Multesium; GLC Minerals LLC, Green Bay, WI). The study was a duplicated 6 × 6 Latin square, with 12 nonlactating, nonpregnant Holstein dairy cows assigned to a square based on BW and parity. Cows were fed 90% of their voluntary DMI (diet contained 0.21% Mg). Each experimental period lasted 7 d. On d 2 of each period, urinary catheters were fitted. Total urine collection started on d 3 for 48 h with samples collected and measured every 12 h. On d 4, 30 g of Mg were administered as boluses with gelatin capsules: negative control (one empty capsule), magnesium oxide (MgO), magnesium sulfate (MgSO), calcium magnesium hydroxide [CaMg(OH)], calcium magnesium carbonate [CaMg(CO)], and PMB [a blend of Ca and Mg sources that includes CaMg(CO), CaMg(OH), and MgO]. Blood samples were collected at 0, 1, 2, 3, 12, and 24 h after treatment administration on d 4 of each treatment period. Urine and blood samples were analyzed for Mg and Ca concentration. Statistical analyses were conducted with PROC GLIMMIX including treatment, time, period, square, treatment × time, treatment × period, and time × period as fixed effects, and cow nested within square as a random effect in the model. Urinary Mg excretion for 4 of the Mg sources studied [PMB, MgO, CaMg(OH), and MgSO] increased significantly, representing an increase of at least 40.8% relative to control. The supplementation of CaMg(CO) did not significantly increase relative to control. There were no significant changes in blood Mg concentration with treatment, but a significant treatment × time effect was observed. Calcium-rich sources [PMB, CaMg(OH), CaMg(CO)] had lower blood Mg concentrations at 12 or 24 h after treatment than control and CaMg(CO). Based on urinary Mg excretion 24 h after treatment, 4 of the Mg sources evaluated (including PMB) showed a similar availability, however, the availability of the commercial CaMg(CO) source included in our study was similar to the negative control (nonsupplemented cows).