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Nutrient balance studies require measuring urine volume, and urinary excretion can be used to assess Mg bioavailability. A less laborious method than total collection of urine could make balance studies more feasible and expand the utility of using urinary Mg as an index of bioavailability, but the method needs to be accurate and sensitive. Sampling interval can affect accuracy because excretion must be at steady state. Two experiments were conducted to (1) determine whether urinary creatinine could be used to accurately estimate urinary output of nutrients markedly excreted via urine (N, K, Na, S, and Mg; experiment 1) and (2) determine the appropriate sampling schedule to evaluate Mg excretion after abrupt diet changes (experiment 2). Experiment 1 was originally designed to evaluate the interaction of monensin [0 vs. 14 mg of monensin/kg of dry matter (DM)] and Mg source (MgO vs. MgSO 4 ; total diet Mg: 0.36% of DM) under antagonism from increased dietary K (2.11% of DM) on urinary Mg excretion. Experiment 2 evaluated the interaction of Mg concentration (basal vs. supplemental MgO; total diet Mg: 0.20 vs. 0.42% of DM) and K (basal vs. supplemental K 2 CO 3 ; total diet K: 1.60 vs. 2.57% of DM) on urinary Mg excretion over time. Using 4-d composite samples from total collection of urine (n = 34 cow-periods), the average daily excretion of creatinine was similar to previous estimates (29.0 ± 1.16 mg of creatinine/kg of body weight) but was variable among cows (root mean squared error = 2,980 mg/d; 14% of mean). Treatment-average estimated excretion of urine and urinary N, K, Na, S, and Mg were similar to actual values; however, differences between actual and estimated values could be substantial for individual cows. Using the mean creatinine excretion per kilogram of body weight for all cows to estimate urine eliminates the lack of fit variance resulting in artificially low within-treatment variation for estimated urine volume. The standard error of the mean for estimated urine volume was 23% less (1.93 vs. 2.51) than that for actual urine production. This inflated the type I error rate, and, consequently, statistical inferences on N and K excretion differed when urine output was estimated rather than measured. The standard error of the mean for excretion of Mg calculated with actual or estimated urine production were almost identical (0.92 vs. 0.97); however, similar standard error of the mean was likely caused by differences in the covariance of urinary Mg concentration with estimated or actual urine output. Based on spot sampling (experiment 2), urinary Mg reached steady state by 2 d following an increase in dietary K regardless of Mg level, whereas excretion of urinary Mg following an increase in dietary Mg continued to increase through 7 d. Estimating nutrient excretion with urinary creatinine and body weight on average is accurate, but variance is likely underestimated. Knowing the time course of urinary Mg excretion will improve the value of using urinary Mg concentration to assess diet adequacy or Mg bioavailability., (Copyright © 2018 American Dairy Science Association. Published by Elsevier Inc. All rights reserved.)

The interaction of monensin and 2 supplemental Mg sources (MgO and MgSO 4 ) on total-tract digestion of minerals and organic nutrients and milk production was evaluated in lactating dairy cattle. Eighteen multiparous Holstein cows (139 ± 35 DIM) were used in a split-plot experiment with 0 or 14 mg/kg diet DM of monensin as the whole-plot treatments and Mg source as split-plot treatments. During the entire experiment (42 d), cows remained on the same monensin treatment but received a different Mg source in each period (21 d) of the Latin square. Diets were formulated to contain 0.35% Mg with about 40% of that provided by MgO or MgSO 4 . Diets were formulated to have similar concentrations of major nutrients and K concentrations were elevated (2.1% of DM) with K 2 CO 3 to create antagonism to Mg absorption. Apparent digestibility was measured by total collection of urine and feces. Supplemental MgSO 4 decreased DMI (26.9 vs. 25.7 kg/d) and tended to decrease milk yield (40.2 vs. 39.3 kg/d), but increased the digestibility of OM (68.3 vs. 69.2%) and starch (91.9 vs. 94.4%) compared with MgO. Feeding MgSO 4 with monensin decreased NDF digestibility compared with other treatments (46.7 vs. 50.2%). That diet also had decreased apparent absorption of Mg compared with diets without monensin (15.6 vs. 19.2%), whereas MgO with monensin had greater apparent absorption of Mg (23.0%) than other treatments. Cows consuming MgSO 4 had increased apparent Ca absorption (32.2 vs. 28.1%) and balance. A diet with MgSO 4 without monensin increased the concentration of long-chain fatty acids in milk, suggesting increased mobilization of body fat or decreased de novo fatty acid synthesis in the mammary gland. Overall, when dietary Mg was similar, MgO was the superior Mg source for lactating dairy cattle, but inclusion of monensin in diets should be considered when evaluating Mg sources., (Copyright © 2018 American Dairy Science Association. Published by Elsevier Inc. All rights reserved.)