Magnesium in dairy cattle nutrition: A meta-analysis on magnesium absorption in dairy cattle and assessment of simple solubility tests to predict magnesium availability from supplemental sources

Supplemental Mg sources differ in bioavailability, and solubility is one of the determining factors. We explored whether and which in vitro solubility tests could reliably differentiate the quality of supplemental Mg sources. In experiment 1, we compared 3 chemical methods using an acetic acid solution (50 mL/L, termed vinegar test), a 1 M ammonium nitrate solution, and an artificial rumen buffer fluid without rumen microbiota. The Mg solubility results suggested the vinegar test was the best method due to its robustness, simplicity, and reproducibility. In experiment 2, we validated the reliability of the vinegar test using 4 MgO sources from experiment 1 and 12 new MgO sources plus a laboratory-grade MgO as a standard. Accordingly, we repeated the vinegar test with short (0.5 h) and long (3.0 h) incubation times on these sources and then conducted ruminal incubations in 24-h batch culture experiments. The repeated vinegar test resulted in similar results as in experiment 1. Linear regression across both experiments showed the soluble Mg content (g/kg) = 44.46 (±2.55) × pH - 142.9 (±14.9), root mean square error (RMSE) = 10.2, P slope <0.001, and concordance correlation coefficient (CCC) = 0.953. The predictable pH range was from 4 to 6. The equation cannot be applied to low-alkaline sources such as Mg sulfate, Mg acetate, or a group of MgO with exceptionally high alkaline properties showing a cluster of pH above 8.5. Solubility of the MgO sources in the vinegar test ranged from 5 to 35%, whereas the 24-h ruminal incubations led to more solubility (15-70%). Nevertheless, the differences among most MgO sources were parallel to the data from the in vitro rumen solubility. Next, we performed a meta-analysis of published studies (21 studies, 94 treatments) to assess the true Mg absorption in vivo and potential factors affecting Mg absorption in dairy cows. It appeared that on average dairy cows absorbed about 20% of the Mg intake (range 10-40%), regardless of their lactation status. We revealed a new strategy to predict Mg absorption relative to dietary K as follows: true Mg absorption (g/d) = 0.3395 (±0.025, P < 0.001) × Mg intake (g/d) - 1.9273 (±1.16, P = 0.11) when dietary K ≤20 g/kg DM, and 0.154 (±1.06, P = 0.05) + 0.209 (±0.026, P < 0.001) × Mg intake (g/d) when dietary K >20 g/kg DM (RMSE = 2.19). This strategy improved the accuracy of prediction as compared with the existing prediction (CCC = 0.922 vs. 0.845). Still, over- or underestimations inherent to individual studies were evident and might be related to unaccountable factors, especially the quality of supplemental Mg sources. In conclusion, the vinegar test is a useful tool to rank inorganic Mg sources with alkaline properties. Including in vitro solubility data in Mg nutrition research could help to refine the prediction of bioavailable Mg contents and increase precision in feed formulation.

Instability and apparent lack of metabolism of phomopsin A during incubation with ovine rumen fluid

To investigate the stability and possible metabolism of phomopsin A in rumen fluid, phomopsin A was incubated in ovine rumen fluid - buffer mixtures for 24 h. High performance liquid chromatographic analysis of extracted incubation mixtures demonstrated that although phomopsin A was degraded, metabolism by rumen microorganisms appears not to be important during 24 h incubation.

Effects of the antibiotic monensin and an inhibitor of methanogenesis on in vitro continuous rumen fermentations

1. The effects of a methane inhibitor, ICI 111075, and a propionate enhancer, monensin, were studied using in vitro continuous fermenters. 2. Both compounds increased the yield of substrate energy, carbon and hydrogen in volatile fatty acids (VFA). This was mainly due to an increase in the molar proportion of propionic acid. 3. Improved yields of VFA were accompanied by reductions in methane production and microbial yield. 4. Since published information showed that monensin reduced rumen dilution rate in vivo an analogous in vitro system was proposed in which a high dilution rate control fermenter was compared with a monensin treated fermenter set to run at a low dilution rate. 5. Results showed that the general intrinsic microbial activity of the chemical manipulators was not affected by changes in dilution rate. Changing dilution rate in addition to chemical treatment however resulted in substantial modifications in the net effect on the fermentation. 6. The practical implications of reducing rumen dilution rate as a side effect of chemically manipulating the rumen fermentation could involve changes in food intake, increased importance of secondary fermentations and a reduced effect of nutrients not degraded in the rumen.