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Macromineral Digestion by Lactating Dairy Cows: Factors Affecting Digestibility of Magnesium^*

Department of Animal Sciences, Ohio Agricultural Research and Development Center, The Ohio State University, Wooster 44691

E-mail: weiss.6{at}osu.edu.

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
The objectives of this experiment were to determine dietary factors influencing the apparent digestibility of Mg by lactating dairy cows and to compare empirical apparent digestibility values with Mg absorption coefficients used in the dairy cattle nutrient requirement model of the National Research Council (NRC). Data were compiled from 8 experiments with 39 dietary treatments and 162 cows (all lactating Holsteins) in which apparent digestibility of Mg was measured using total collection of feces and urine. The concentration of dietary Mg ranged from 0.20 to 0.36% of DM (mean = 0.27%). On average, 19% of the dietary Mg came from a Mg supplement (MgO or MgSO₄). The concentration of dietary K ranged from 1.07 to 2.65% (mean = 1.60%). The mean apparent digestibility of Mg was 0.18 and ranged from –0.04 to 0.33. The average digestibility was 30% lower than the mean value calculated by the NRC model. The primary reason for the low average Mg digestibility was high concentrations of dietary K. At a dietary K concentration of 1%, empirical data agreed with NRC estimates, but apparent Mg digestibility decreased 0.075 (± 0.035)/percentage unit of K in the diet. Lactating dairy cows had to consume an additional 18 g of Mg/d for every 1 percentage unit increase in dietary K above 1% to maintain the same intake of digestible Mg as that consumed when fed a diet with 1% K.

Key Words: magnesium • potassium • digestibility

Abbreviation key: AC = absorption coefficient

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
The NRC (2001) expresses Mg requirements on an absorbable basis. The supply of available Mg is calculated by multiplying the concentration of Mg in feeds by an absorption coefficient (AC). The NRC uses an AC of 0.16 for Mg from all feedstuffs except Mg supplements. The AC for Mg from MgO and MgSO₄ in the NRC is 0.70. Cows do not appear to excrete a significant quantity of endogenous fecal Mg; therefore, apparent digestibility of Mg was used by the NRC subcommittee to estimate its AC. Apparent digestibility data for Mg from lactating dairy cows, especially for cows fed mixed forage and concentrate diets, are limited (Henry and Benz, 1995; NRC, 2001). Because of the limited database and other uncertainties, the NRC subcommittee did not use the mean apparent digestibility of Mg from published studies, rather they reduced the mean by one standard deviation.

Digestibility of Mg is usually reduced when ruminants consume diets with high concentrations of K provided by K supplements (Greene et al., 1983; Poe et al., 1985; Ram et al., 1998) or forages intrinsically high in K (Schonewille et al., 1999). Based on a summary of published data (18 treatment means), Underwood and Suttle (1999) reported a negative relationship between dietary K concentration and apparent digestibility of Mg for lactating cows fed mixed (forage and concentrate) diets. The coefficients (slope and intercept) in that equation appeared different than the coefficients for equations derived from data obtained from sheep or from cows fed only forages (they were not tested statistically). Therefore, data obtained from sheep or from cows fed all-forage diets may not be applicable to lactating cows fed mixed diets. The objective of this experiment was to quantify Mg digestibility in lactating dairy cows fed a variety of mixed diets and to determine whether other dietary factors influenced Mg digestibility.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Digestibility data from 8 experiments with 39 dietary treatments and 162 lactating Holstein cows or cow-periods (when the experiment was a Latin square) were compiled for this analysis. All data were obtained following the same protocol for total collection of feces and urine (Weiss and Wyatt, 2000). Collection periods were 4 to 6 d, and during the collection period, samples of feeds, orts, feces, and urine were collected daily (sample size was proportional to mass of the material) and composited into single samples within cow. Cows were fed experimental diets for at least 21 d before each collection period. A summary of the treatments and diets can be found in a companion paper (Weiss and Wyatt, 2004). The objectives of the various experiments included the evaluation of different forages, fat supplements, and by-product feeds. Within each experiment, diets were formulated to contain equal concentrations of Mg and to meet or exceed the NRC (1989) requirement for Mg in use at the time of each experiment. In one experiment (Ivancic and Weiss, 2001), 6 diets were formulated to have equal but excessive concentrations of Mg (0.4% of diet DM). A diverse array of feedstuffs was used, but corn silage was the predominant forage, corn grain was the predominant starch source, and soybean meal was the predominant source of supplemental protein (Weiss and Wyatt, 2004). In addition to corn silage, alfalfa hay, alfalfa silage, and orchardgrass silage was fed in some experiments. The nutrient profiles of the diets (Table 1) and production of the cows (Table 2) was diverse.

The dependent variables of interest were apparent digestibility of Mg and intake of digestible Mg (g/d). Dietary concentrations (% of DM) of CP, NDF, Ca, P, Mg, Na, and K, and the dietary Na:K ratio were used separately as independent variables. The MIXED procedure of SAS (SAS, 1999) was used to evaluate relationships between dependent and independent variables. Experiment was included in the model as a random class variable (St-Pierre, 2001) and the dependent variable was included as a continuous variable. When an independent variable contributed significantly (P < 0.10) to the variation in the dependent variable, the Estimate option was included in the model statement of PROC MIXED. Treatment means (n = 39) were adjusted for trial effects (St-Pierre, 2001). The 95% prediction interval was calculated as described by Neter and Wasserman (1974).

	RESULTS AND DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Dietary concentrations of Mg ranged from 0.20 to 0.36% of diet DM (Table 1), with an average concentration of 0.27%. Assuming typical diets and DMI, diets with approximately 0.2% Mg will usually meet Mg requirements for lactating cows (NRC, 2001). On average, 19% of dietary Mg came from supplemental sources (either MgO, MgSO₄, or both), but 6 diets contained no supplemental Mg. The average dietary concentration of K was 70% greater than the dietary concentration that will usually meet NRC (2001) requirements for K (about 1%). The vast majority of the dietary K was intrinsic to the feedstuffs. Only 11 diets contained supplemental K and, in those diets, supplemental K was <5% of total dietary K. The average concentration of Na in the diets was 0.25 (diets with 0.22% Na will usually meet NRC (2001) requirements for Na of lactating cows). However 8 diets had Na concentrations lower than NRC (2001) requirements. All diets contained NaCl, and 14 diets contained NaHCO₃.

The apparent digestibility of Mg ranged from about –0.04 to 0.33, with an average of 0.18 (Table 2). The average digestibility of Mg in this data set is similar to the average (0.21) reported by Adedui and Suttle (1999) for lactating cows fed mixed diets.

When intake of digestible Mg (g/d) was regressed on Mg intake (adjusted for trial effects), the equation was (Figure 1):

View larger version (13K): [in this window] [in a new window]   Figure 1. Relationship between Mg intake and intake of apparently digested Mg after adjusting for random trial effects. Each point represents a treatment mean (n = 39) from 8 different experiments involving 162 cows. The equation is Y = 2.5 + 0.14X.

([1])

The values in parentheses are the standard errors for the coefficients, and the standard error associated with trial (SE_Trial) was 3.9 and residual standard error (SE_Res) was 0.07. The marginal digestibility (i.e., slope) was different from 0 (P < 0.04). Although the intercept was not different from 0 (P > 0.5), it was retained in the equation because of its high standard error. The high standard error is caused in part by the large distance between the intercept (i.e., intake of 0 g/d of Mg) and the lowest intake of Mg in this data set (38 g/d). At the average intake of Mg in this data set (55 g/d), estimated Mg digestibility (equation [1]) is 0.185. Average Mg digestibility in this data set was about 30% lower than the average AC calculated using NRC (2001) coefficients for these diets (0.26).

A possible reason for the disparity between the AC used by NRC and the experimentally determined Mg digestibility was the presence of dietary antagonists to the absorption of Mg. The AC for Mg in NRC (2001) are static within a feedstuff; no adjustments are made for possible antagonists. However, K has greatly reduced Mg absorption in ruminants (Greene et al., 1983; Poe et al., 1985; Ram et al., 1998; Schonewille et al., 1999; Underwood and Suttle, 1999). In this data set, increasing concentrations of K in the diet reduced (P < 0.03) Mg digestibility (Figure 2). After adjusting for random trial effects, the relationship was:

View larger version (13K): [in this window] [in a new window]   Figure 2. The effect of increasing dietary K (% of DM) on apparent Mg digestibility (expressed as a proportion, not percentage) by lactating dairy cows (after adjusting for random trial effects). Each point is a treatment mean (n = 39) from 8 experiments and 162 cows. The equation is Y = 0.316 to 0.075X.

([2])

The SE_Trial was 0.0045 and SE_Res was 0.004. At the mean concentration of dietary K (1.6%), the 95% prediction interval was the 0.20 ± 0.12.

Equation [2] is similar to a previous equation (i.e., Mg digestibility = 0.28 – 0.052 x %K) derived from data from lactating dairy cows (Underwood and Suttle, 1999). At a dietary K concentration equal to that needed to typically meet NRC requirements for K (approximately 1%), predicted (equation [2]) Mg digestibility is 0.24, which is statistically equal to the average AC derived from NRC coefficients (0.26). Based on equation [2], Mg digestibility would equal 0 at 4.2% K. However, increased dietary concentrations of Mg can partially offset the negative effects of high dietary K on Mg absorption (Ram et al., 1998). Equation [2] does not account for the effects of variable Mg intakes on digestibility of Mg.

The approaches taken to derive equations [1] and [2] were combined to derive an equation that accounted for the negative effects of dietary concentrations of K and the positive effects of increased Mg intake. The equation was:

([3])

where intakes are in grams per day and K is as a percentage of diet DM. The SE_Trial was 15.9 and SE_Res was 1.2. The intercept was not different from 0 (P > 0.25), but the Mg intake coefficient (P < 0.01) and the dietary K coefficient (P < 0.06) differed from 0. At a dietary concentration of 1% K, predicted digestibility of Mg equals 0.24. A value that is essentially the same as the AC calculated from NRC coefficients. Because the intercept and the slope for dietary K are essentially the same, when the diet contains 1% K, digestibility of Mg is 0.24 regardless of intake of Mg. If the concentration of K in the diet is 2% and intake of Mg is 38 g/d (minimum value in this data set), predicted Mg digestibility (equation [3]) is 0.13, but when Mg intake increases to 75 g/d (maximum in this data set) predicted Mg digestibility is 0.18. Dividing the coefficient associated with dietary K (4.4) by the coefficient associated with Mg intake (0.24) in equation [3] yields 18.3. Therefore, to maintain the same intake of digestible Mg as that obtained at a dietary K concentration of 1%, intake of Mg must increase 18.3 g/d for every 1 percentage unit increase in the concentration of dietary K > 1%. For example, if a cow needed to consume 55 g of Mg/d to meet the NRC requirement for Mg when fed a diet with 1% K, consumption would need to be 73 g/d to meet the same requirement if the diet contained 2% K.

High concentrations of dietary K, low ratios of dietary Na to K, and high concentrations of dietary CP are risk factors for grass tetany (NRC, 2001). In this data set, no statistical relationships (P > 0.35) were observed between Mg digestibility and the ratio of dietary Na to K, concentration of dietary Na, or dietary CP concentration. Neither the concentration of dietary Ca or P affected Mg digestibility (P > 0.20). Rahnema et al. (1994) reported that increased concentrations of dietary fat have reduced digestibility of Mg, but others (Grace and Body, 1989) have reported no effect of fat on Mg digestibility. Of the 39 diets used in this data set, 11 contained supplemental fat (0.5 to 3.2% of diet DM). When fat was included as a class variable (supplemented vs. not supplemented) or as a continuous variable (supplemental fat, % of diet DM) in the statistical analysis, no effect of fat was observed on Mg digestibility. However, the variation in fat concentrations of diets in this data set was limited (most diets contained no supplemental fat). The effect of fat on digestibility of Mg, if any, could not be ascertained reliably from this data set.

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
For a variety of diets, Mg digestibility by lactating dairy cows was similar to NRC estimated AC when the concentration of K in the diets was approximately 1%, but digestibility decreased 0.075 for every 1 percentage unit increase in dietary K concentration. To maintain the intake of digestible Mg equal to that obtained when diets contain 1% K an additional 18 g of Mg would need to be consumed for every 1 percentage unit increase in dietary K above 1% of the diet DM.

	FOOTNOTES

 
^* Salaries and research support provided by state and federal funds appropriated to the Ohio Agricultural Research and Development Center, The Ohio State University. Manuscript No. 42-03AS.

Received for publication December 5, 2003. Accepted for publication February 23, 2004.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 


Adedui, O., and N. F. Suttle. 1999. Influence of diet type, potassium, and animal species on the absorption of magnesium by ruminants. Proc. Nutr. Soc. 58:31A. (Abstr.)

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Association of Official Analytical Chemists. 1990. Official Methods of Analysis. Vol. 1 15th ed. AOAC, Arlington, VA.

Grace, N. D., and D. E. Body. 1989. Dietary lipid and the absorption of Mg and Ca: Effect of corn oil infused via the rumen on the absorption of Mg and Ca in sheep fed white clover. N.Z. J. Agric. Res. 22:405–412.

Greene, L. W., J. P. Fontenot, and K. E. Webb, Jr. 1983. Site of magnesium and other macromineral absorption in steers fed high levels of potassium. J. Anim. Sci. 57:503–510.

Henry, P. R., and S. A. Benz. 1995. Magnesium bioavailability. Pages 201–237 in Bioavailability of Nutrients for Animals. C. B. Ammerman, D. H. Baker, and A. J. Lewis, eds. Academic Press, Inc., San Diego, CA.

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Ram, L., J. T. Schonewille, H. Martens, A. T. V. T. Klooster, and A. C. Beynen. 1998. Magnesium absorption by wethers fed potassium bicarbonate in combination with different dietary magnesium concentrations. J. Dairy Sci. 81:2485–2492.[Abstract]

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Schonewille, J. T., A. T. Van’t Klooster, H. Wouterse, and A. C. Beynen. 1999. Effects of intrinsic potassium in artificially dried grass and supplemental potassium bicarbonate on apparent magnesium absorption in dry cows. J. Dairy Sci. 82:1824–1830.[Abstract]

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Weiss, W. P., and D. J. Wyatt. 2000. Effect of oil content and kernel processing of corn silage on digestibility and milk production by dairy cows. J. Dairy Sci. 83:351–358.[Abstract]

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