LAUSR

The objective of this study was to evaluate the effects of replacing magnesium oxide (MgO) with calcium-magnesium carbonate [CaMg(CO3)2] on ruminal fermentation with or without the addition of sodium bicarbonate (NaHCO3). Eight fermentors of a dual-flow continuous-culture system were distributed in a replicated (2) 4 × 4 Latin square design in a 2 × 2 factorial arrangement of treatments (magnesium sources × NaHCO3). The treatments tested were 0.21% MgO [MgO; dry matter (DM) basis; 144.8 mEq of dietary cation-anion difference (DCAD)]; 0.21% MgO + 0.50% NaHCO3 (MgO+NaHCO3; DM basis; 205.6 mEq of DCAD); 1.00% CaMg(CO3)2 [CaMg(CO3)2; DM basis; 144.8 mEq of DCAD]; and 1.00% CaMg(CO3)2 + 0.50% NaHCO3 [CaMg(CO3)2+NaHCO3; DM basis; 205.6 mEq of DCAD]. Diets were formulated to have a total of 0.28% of Mg (DM basis). The experiment consisted of 40 d, which was divided into 4 periods of 10 d each, where 7 d were used for adaptation and 3 d for sampling to determine pH, volatile fatty acids (VFA), ammonia (NH3-N), lactate, mineral solubility, N metabolism, and nutrient digestibility. The effects of Mg source [MgO vs. CaMg(CO3)2], NaHCO3 (with vs. without), and the interaction were tested with the MIXED procedure of SAS version 9.4 (SAS Institute). There was no Mg source × NaHCO3 interaction in the pH variables and mineral solubility, and Mg sources evaluated did not affect the variables related to ruminal pH and solubility of Mg. On the other hand, the inclusion of NaHCO3 increased the pH daily average, independent of Mg source, which led to a reduced time that pH was below 5.8 and decreased area under the curve. Total VFA and lactate concentration were similar among treatments regardless of NaHCO3 and Mg source; however, the molar proportion of isobutyrate and NH3-N concentration were lower in diets with CaMg(CO3)2 compared with MgO. Moreover, NaHCO3 inclusion increased NH3-N, total daily NH3-N flow, isobutyrate concentration, and acid detergent fiber digestibility. Our results showed that CaMg(CO3)2 leads to a lower NH3-N concentration and isobutyrate proportion. Therefore, because most of the tested variables were not significantly different between MgO and CaMg(CO3)2 when combined or not with NaHCO3, CaMg(CO3)2 can be a viable alternative source to replace MgO in dairy cow diets without affecting mineral solubility, ruminal pH, nutrient digestibility, total VFA, and the main ruminal VFA. Although Mg sources are known to have an alkalizing effect, NaHCO3 inclusion in diets with Mg supplementation allowed an increase in ruminal pH, as well as an increase in isobutyrate and NH3-N flow.