Apparent Magnesium Absorption in Dry Cows Fed at 3 Levels of Potassium and 2 Levels of Magnesium Intake

Relative availability of five inorganic magnesium sources in non-pregnant non-lactating 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, USA)]. The study was a duplicated 6 × 6 Latin square, with 12 nonlactating, non-pregnant 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 (MgSO4), calcium magnesium hydroxide [CaMg(OH)4], calcium magnesium carbonate [CaMg(CO3)2], and PMB [a blend of Ca and Mg sources that includes CaMg(CO3)2, CaMg(OH)4, 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)4, and MgSO4] increased significantly, representing an increase of at least 40.8% relative to control. The supplementation of CaMg(CO3)2 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)4, CaMg(CO3)2] had lower blood Mg concentrations at 12 or 24 h after treatment than control and CaMg(CO3)2. 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(CO3)2 source included in our study was similar to the negative control (no-supplemented cows).

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.

Production, physiological response, and calcium and magnesium balance of lactating Holstein cows fed different sources of supplemental magnesium with or without ruminal buffer

The objective of this study was to evaluate the effects of dietary replacement of magnesium oxide (MgO) with calcium-magnesium hydroxide [CaMg(OH)₂] and its interaction with ruminal buffer (sodium sesquicarbonate) supplementation on production, Ca and Mg balance, and overall physiological response of mid-lactation Holstein dairy cows. Sixty cows averaging 40.5 ± 7.0 kg of milk/d were used. Treatments were assigned following a 2 × 2 factorial arrangement: (1) MgO, (2) MgO + buffer, (3) CaMg(OH)₂, or (4) CaMg(OH)₂ + buffer. Diets were formulated to have 16.5% of crude protein, 1.82 Mcal/kg of net energy for lactation, 0.67% Ca, 0.39% P, and 0.25% Mg, all on a dry matter (DM) basis. Treatments were individually top dressed. Milk production, composition, and DM intake were evaluated. A subsample of 20 cows were randomly selected for the evaluation of Ca and Mg balance, blood gases, and electrolytes. Ruminal fluid was also collected for evaluation of pH and Ca and Mg solubility. Effects of Mg source, buffer, and the interaction Mg source × buffer were analyzed through orthogonal contrasts. An interaction of Mg source × buffer was found for DM intake and feed efficiency, in which cows fed CaMg(OH)₂ had a similar feed efficiency regardless of ruminal buffer inclusion; however, when cows were fed MgO, the inclusion of buffer reduced feed efficiency. No effects on body weight and milk yield were observed. Buffer addition tended to increase the concentrations of fat, protein, and solids-not-fat, without affecting the yields of these milk components. Magnesium source and buffer did not affect ruminal fluid, blood, urine, or fecal pH; however, buffer supplementation increased urinary pH. Treatment with CaMg(OH)₂ increased blood concentration of HCO₃^-, total CO₂, and base excess compared with cows fed MgO. No differences were observed in the ruminal solubility of Ca and Mg or on milk or urinary Ca and Mg excretion. Greater plasma Mg concentration was observed for animals fed MgO compared with cows fed CaMg(OH)₂; however, both sources were above the threshold recommended in the literature for dairy cows. Also, a reduction in fecal Mg excretion was observed in animals fed CaMg(OH)₂. In summary, we provide evidence that CaMg(OH)₂ could replace MgO without affecting performance, overall physiological response, or Ca and Mg balance of mid-lactating dairy Holstein cows.

Effects of supplemental source of magnesium and inclusion of buffer on ruminal microbial fermentation in continuous culture

Magnesium oxide (MgO) is the most common supplemental source of Mg for dairy cows and a proven ruminal alkalizer when supplemented above NRC (2001) recommendations. However, overfeeding MgO may increase feeding costs, whereas the effects of alternative sources of Mg on ruminal fermentation are not well known. Moreover, it is still unclear if Mg supplementation influences the effects of bicarbonate-based buffers on ruminal fermentation. We aimed to evaluate the effect of Mg source on ruminal fermentation with diets formulated to a final concentration of 0.25% Mg, and to determine if the effect of sodium sesquicarbonate as a buffer varies with the source of Mg. We used 8 fermentors in a duplicated 4 × 4 Latin square design with a 2 × 2 factorial arrangement of treatments, by combining 2 factors: (1) Mg source: using either MgO or an alternative source consisting of a blend of CaMg(OH)₄ and CaMg(CO₃)₂ (BLN) and (2) sodium sesquicarbonate buffer inclusion, at 0 or 0.6% of dry matter intake. Based on preliminary tests of reactivity, we hypothesized that BLN plus buffer would allow for greater ruminal pH, acetate molar proportion, and NDF digestibility than diets with MgO or without buffer. Four 10-d periods were completed, where the last 3 d were used for pH measurements and collection of samples for volatile fatty acids (VFA), ammonia (NH₃-N), Mg solubility, N metabolism, and nutrient digestibility. Effects of Mg source (source), sodium sesquicarbonate inclusion (buffer), and their interaction (source × buffer) were tested with the MIXED procedure of SAS (SAS Institute Inc.). We did not find an effect of Mg source on ruminal fermentation variables; however, concentration of soluble Mg in ruminal fluid was greater for MgO compared with BLN. On the other hand, buffer supplementation increased average ruminal pH, acetate molar proportion, and branched-chain VFA molar proportion; tended to increase NDF digestibility; and decreased both area under the curve and time below pH 6.0. An interaction of source × buffer was found for propionate, butyrate, and NH₃-N, the first one decreasing and the 2 others increasing only when buffer was supplemented to the BLN diet. Our results indicate that supplementing Mg with either MgO or BLN promotes similar ruminal fermentation in diets with total concentration of 0.25% Mg. Further evaluations are needed to assess Mg availability and animal performance in dairy cows fed BLN.

Association of subclinical hypocalcemia dynamics with dry matter intake, milk yield, and blood minerals during the periparturient period

Subclinical hypocalcemia (SCH) affects many high-producing dairy cows in the postpartum period. Recent work has shown that cows experiencing prolonged or delayed SCH are at increased risk for disease and produce less milk than cows experiencing a transient reduction in or normal concentrations of plasma Ca following parturition. Our objective was to determine the association between different postpartum SCH dynamics with pre- and postpartum dry matter intake (DMI), milk yield, and blood mineral concentrations. Data were retrospectively collected from multiparous Holstein cows (n = 78), and cows were classified into 1 of 4 SCH groups based on mean blood total Ca (tCa) concentrations at 1 and 4 d in milk (DIM): normocalcemic (NC; [tCa] >1.95 mmol/L at 1 DIM and >2.2 mmol/L at 4 DIM, n = 28); transient SCH (tSCH; [tCa] ≤1.95 mmol/L at 1 DIM and >2.2 mmol/L at 4 DIM, n = 27); delayed SCH (dSCH; [tCa] >1.95 mmol/L at 1 DIM and ≤2.2 mmol/L at 4 DIM, n = 6); and persistent SCH (pSCH; [tCa] ≤1.95 mmol at 1 DIM and ≤2.2 mmol/L at 4 DIM, n = 17). Linear mixed models were created to analyze the change in pre- and postpartum DMI, milk yield, and blood mineral concentrations over time as well as differences between SCH groups. Prepartum intake was similar between groups, but the NC and tSCH cows consumed more feed than the pSCH or dSCH cows during the first 3 wk of lactation. The tSCH cows produced more milk than the other 3 groups during the first 6 wk of lactation. Postpartum blood tCa and Mg were different between SCH groups and were highest in the NC cows and lowest in the pSCH cows. Our results suggest that the high level of DMI consumed by the NC and tSCH cows in the postpartum period supported an appropriate homeostatic response to the increased Ca demands of lactation, allowing for higher milk yield compared with their counterparts experiencing delayed or prolonged episodes of SCH.

Effects of calcium-magnesium carbonate and calcium-magnesium hydroxide as supplemental sources of magnesium on microbial fermentation in a dual-flow continuous culture

Supplemental sources of Mg can also aid in ruminal pH regulation due to their alkaline properties. Magnesium oxide (MgO) is the most common source of Mg for ruminants and can help controlling ruminal pH; however, the alkaline potential of other sources of Mg has not been evaluated. We aimed to evaluate the inclusion of calcium–magnesium carbonate (CaMg(CO₃)₂) and calcium–magnesium hydroxide (CaMg(OH)₄) alone or in combination as supplemental sources of Mg in corn silage-based diets and its impact on ruminal microbial fermentation. We hypothesized that inclusion of CaMg(OH)₄ would allow for ruminal fermentation conditions resulting in a greater pH compared to the inclusion of CaMg(CO₃)₂. Four treatments were defined by the supplemental source of Mg in the diet: 1) Control (100% MgO, plus sodium sesquicarbonate as a buffer); 2) CO₃ [100% CaMg(CO₃)₂]; 3) OH [100% CaMg(OH)₄]; and 4) CO₃/OH [50% Mg from CaMg(CO₃)₂, 50% Mg from CaMg(OH)₄]. Nutrient concentration was held constant across treatments (16% CP, 30% NDF, 1.66 Mcal NEl/kg, 0.67% Ca, and 0.21% Mg). Four fermenters were used in a 4 × 4 Latin square design with four periods of 10 d each. Samples were collected for analyses of nutrient digestibility, soluble Mg, VFA, and NH₃, while pH was measured at 0, 1, 2, 4, 6, 8, and 10 h post morning feeding to estimate % time when pH was below 6 (pH-B6) and area under the pH curve for pH below 6.0 (pH-AUC). Bacteria pellets were harvested for ¹⁵N analysis and estimates of N metabolism. Treatment effects were analyzed with the mixed procedure of SAS, while effects of using either CaMg(CO₃)₂ or CaMg(OH)₄ as Mg source in comparison to Control treatment were evaluated by orthogonal contrasts. Similar pH-related variables were observed for Control, OH, and CO₃/OH treatments, which had smaller pH-AUC and pH-B6 than CO₃ (P ≤ 0.01). Butyrate molar proportion was greater in Control and CO₃/OH than in CO₃ and OH (P = 0.04). Orthogonal contrasts showed lower flow of bacterial N (P = 0.04), lower butyrate molar proportion (P = 0.08) and greater pH-AUC (P = 0.05) for diets with CaMg(CO₃)₂ in comparison with the Control. Concentration of soluble Mg in ruminal fluid (P = 0.73) and nutrient digestibility (P ≥ 0.52) were similar across treatments. Under the conditions of this experiment, using CaMg(OH)₄ alone or combined with CaMg(CO₃)₂ allowed for a less acidic ruminal fermentation pattern than a diet with only CaMg(CO₃)₂.

The effect of source of supplemental dietary calcium and magnesium in the peripartum period, and level of dietary magnesium postpartum, on mineral status, performance, and energy metabolites in multiparous Holstein cows

The objective of this study was to determine the effects of feeding different supplemental sources of Ca and Mg in the peripartum period, and different dietary levels of Mg postpartum, on plasma mineral status, performance, and aspects of energy metabolism in transition dairy cows. Multiparous Holstein cows (n = 41) were used in a completely randomized design with a 2 × 2 factorial arrangement of treatments starting at 28 d before expected parturition. Main effects were source assignments (CS = common sources of supplemental Ca and Mg, or MA = a blend of common and commercial mineral sources with supplemental minerals primarily from a commercial Ca-Mg dolomite source; MIN-AD, Papillon Agricultural Company Inc., Easton, MD) beginning at 21 d before due date; cows were further randomized within source treatments to 1 of 2 levels of Mg supplementation (LM = formulated postpartum diet Mg at 0.30% of dry matter (DM), or HM = formulated postpartum diet Mg at 0.45% of DM) beginning within 1 d after parturition. Final treatment groups included the following: common source, low Mg (CS-LM, n = 11); common source, high Mg (CS-HM, n = 11); MIN-AD, low Mg (MA-LM, n = 10); and MIN-AD, high Mg (MA-HM, n = 9). Treatment diets were fed and data collected through 42 d in milk. Postpartum plasma Mg concentrations tended to be higher for cows fed HM and cows fed CS, but no effects were observed on peripartum plasma Ca concentrations. Peripartum plasma P concentrations were higher for cows fed MA. Dry matter intake (DMI) in the prepartum period was higher for cows fed MA (CS = 15.9 vs. MA = 16.8 kg/d) and postpartum DMI was higher in some groups depending on week. Plasma nonesterified fatty acid concentrations were lower for cows fed MA during both the prepartum and postpartum periods. A source by level interaction was observed for postpartum plasma β-hydroxybutyrate (BHB) concentrations such that cows fed CS-LM had numerically higher BHB and cows fed MA-LM had numerically lower BHB (geometric means; CS-LM = 7.9, CS-HM = 6.9, MA-LM = 6.3, and MA-HM = 7.3 mg/dL) than cows fed the other 2 treatments. Higher milk fat yield, milk fat content, and fat- and energy-corrected yield during wk 1 for cows fed MA resulted in source by week interactions for these outcomes. This study demonstrated that varying supplemental Ca and Mg sources and feeding rates had minimal effect on plasma Ca status despite differences in plasma Mg and P concentrations. Effects on DMI and plasma energy metabolites suggest an opportunity for strategic use of mineral sources in the transition period to promote metabolic health.

Effect of Dietary Cation-Anion Difference and Dietary Crude Protein on Milk Yield, Acid-Base Chemistry, and Rumen Fermentation

Eight primiparous lactating Holstein cows (47 +/- 10 d in milk) fitted with ruminal cannulae were used to determine the effect of dietary cation-anion difference (DCAD) and dietary crude protein (CP) concentration on milk yield and composition, acid-base chemistry, and measures of N metabolism in lactating dairy cows. Treatments were arranged as a 2 x 2 factorial in a randomized complete block design to provide 15 or 17% CP and DCAD of 25 or 50 mEq (Na + K - Cl)/100 g of feed dry matter [15 or 39 mEq (Na + K) - (Cl + S)/100 g of feed dry matter]. High DCAD improved dry matter intake, milk yield, and concentrations of milk fat and protein. An interaction of DCAD and CP was observed for uric acid excretion, an indicator of microbial protein yield. Uric acid excretion was higher for high DCAD than for low DCAD in low CP diets and was similar for low and high DCAD with high CP. Serum bicarbonate concentration, urinary bicarbonate excretion, blood pH, and serum Na were elevated for high DCAD compared with low DCAD. Fractional excretion of Na, K, Cl, and Ca increased for high DCAD. Blood urea N and urinary urea N were greater for high than for low CP diets. No differences due to DCAD were observed for these parameters. Results of this study suggest that, in early lactation cows, blood acid-base chemistry is altered by differences in DCAD that range between the high and low ends of the desired DCAD range. Modifications of acid-base chemistry and the corresponding changes in protein metabolism may allow for more efficient feeding of protein and better nutritional management of the lactating dairy cow.

Effect of Dietary Cation-Anion Difference and Dietary Crude Protein on Performance of Lactating Dairy Cows During Hot Weather

Thirty-two lactating Holstein cows (225 +/- 63 d in milk) were used in a 6-wk trial to determine the effect of dietary cation-anion difference (DCAD) and dietary crude protein (CP) concentration on milk and component yield, acid-base status, and serum AA concentrations during hot weather. Treatments were arranged as a 2 x 2 factorial within a randomized complete block design to provide 15 or 17% CP and a DCAD of 25 or 50 mEq (Na + K - Cl)/100 g of dry matter (DM). A DCAD x CP interaction was detected for milk yield; milk yield was less for high DCAD than for low DCAD for the high-CP diets. No differences were noted at low dietary CP. Milk fat percentage was greater for high DCAD than for low DCAD, and high-CP diets supported greater milk fat percentage than low-CP diets. No differences were observed among treatments for dry matter intake or milk protein percentage. Serum total AA and essential AA concentrations and ratio of essential AA:total AA were greater for high DCAD. These results suggest that increasing DCAD improves AA availability for protein synthesis by taking the place of AA that would otherwise be used for maintenance of acid-base balance. A better understanding of the mechanisms behind this AA-sparing effect will improve management of protein nutrition in the lactating dairy cow.

Effects of Dietary Cation-Anion Difference and Potassium to Sodium Ratio on Lactating Dairy Cows in Hot Weather

Forty-two lactating Holstein cows 188 +/- 59 d in milk were used in an 8-wk randomized complete block trial with a 2 x 3 factorial arrangement of treatments. The objective was to determine the effects of high dietary cation-anion difference (DCAD) and K:Na ratio on milk yield and composition and blood acid-base chemistry. Treatments included DCAD concentrations of 45 or 60 mEq (Na + K -Cl)/100 g of feed dry matter and K:Na ratios of 2:1, 3:1, or 4:1. Mean DCAD values were later determined to be 41 and 58. Dry matter intake was similar across treatments. Yield of milk and energy corrected milk were lower for the 3:1 K:Na ratio compared with 2:1 and 4:1 ratios. Blood urea N was lower for the highest DCAD, suggesting that DCAD possibly reduced protein degradation or altered protein metabolism and retention. Mean temperature-humidity index was 75.6 for the duration of the trial, exceeding the critical value of 72 for all weeks during the treatment period. Cows maintained relatively normal body temperature with mean a.m. and p.m. body temperature of 38.5 and 38.7 degrees C, respectively. These body temperatures suggest that cows were not subject to extreme heat stress due to good environmental control. Results of this trial indicate that the greatest effect on milk yield occurs when either Na or K is primarily used to increase DCAD, with the lowest yield of energy-corrected milk at a 3:1 K:Na ratio (27.1 kg/d) compared with ratios of 2:1 (29.3 kg/d) and 4:1 (28.7 kg/d). Results also suggest that greater DCAD improves ruminal N metabolism or N utilization may be more efficient with a high DCAD.