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Effect of dietary cation-anion balance in mineral balance in anaerobically exercised and sedentary horses
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EFFECT OF DIETARY CATION-ANIONBALANCE ON MINERAL BALANCEIN ANAEROBICALLY EXERCISED AND SEDENTARYHORSES L. A. Baker, MS; 1 D.L. Wall, MS; D.R. Topliff, PhD; 1 D.W. Freeman, PhD; 1 R.G. Teeter, PhD; 1 J.E. Breazile, DVM; PhD2; D.G. Wagner, PhD
SUMMARY Eight geldings and four mares were randomly assigned treatrnents within three 4x4 Latin square design experiments to study the effects of dietary cation-anion balance (DCAB) on mineral balance and dry matter digestibility in sedentary and anaerobically exercised horses. Four diets with an average DCAB (calculated as meq ((Na+K+) - Cl-)/kg of diet dry matter) of +24 (Low, L), +127 (Medium Low, ML), +227 (Medium High, MH) and +352 (High, H) were fed for a 21 day adjustment period followed by a 72 hour collection period. Diets consisted of a pelleted base concentrate of com, soybean meal and cottonseed hulls fed with either native prairie grass or bermuda grass hay in a 60:40 ratio. Diet L was formed by adding calcium chloride and ammonium chloride to the base concentrate, diet ML was formed by adding calcium chloride, and diet H was formed by adding potassium citrate and sodium bicarbonate. Diet MI-I received no supplementation and served as the control. Representative samples of feed, feces and urine were analyzed for mineral content and mineral balances were calculated by difference. Fecal output was greater (p <.05), and thus, dry matter digestibilitywas lower in exercised homes consuming diet L versus diet H. Sodium balance was greater (p <.05) in sedentary horses consuming diet MH as compared to those consuming diets ML and L Sodium balance was greater (p <.05) in exercised homes consuming diet H as compared to those consuming diets ML and L Potassium balance was greater (p <.05) in sedentary horses consuming diet H as compared to those horses consuming diet ML, however, potassium balance was not affected by DCAB in exercised horses. No significant differences were detected in chloride or magnesium balances in the sedentary horses, although chloride balance was greater (p <.05) and magnesium balance was lower (p <.05) in exercised homes consuming diet L as compared to all other diets. In sedentary homes, phosphorus balance was reflective of intake with differences (p <.05) observed between all treatments. However, in exercised homes, phosphorus balance was lower (p <.05) oniy for those consuming diet L. Calcium balance decreased significantly as DCAB decreased between all treatments in sedenAuthors' Address: 1Dept. of Animal Science, Division of Agricultural Sciences and Natural Resoumes, Oklahoma State University, Stillwater, OK. 74078. 2Dept. of Physiological Sciences, College of Veterinary Medicine, Oklahoma State University, Stillwater, OK. 74078. Acknowledgement: This research was supported by the Oklahoma Agricultural Experiment Station, Project H-1964.
Volume 13, Number 10, 1993
tary horses, while calcium balance in exercised homes was greater
(p <.05) for homes consuming diet H as compared to those consuming diet L. Previous research from our laboratory has shown that both exercised and sedentary horses consuming diets with a low DCAB experience a nutritionally induced metabolic acidosis. The current data indicate that anaerobically exercised and sedentary horses consuming these diets excrete significantly more calcium in the urine resulting in decreased calcium balances. Prolonged consumption of diets with a low DCAB may lead to a significant demineralization of bone and a subsequent weakening of the skeleton. (Key Words: Equine, Mineral, Nutrition) INTRODUCTION
Sodium, potassium and chloride are the most influential ions involved in the regulation of osmotic pressure in body fluids, as well as the maintenance of acid-base balance. The equation used to calculate dietary cation-anion balance (DCAB) in this study is meq ((Na+K +) - Cl-)/kg of diet dry matter. 1.2 Highly cationic diets have been shown to be alkalogenic and highly anionic diets have been shown to be acidogenic. Feeding diets with a low DCAB has an adverse effect on the acid-base status along with various growth and production parameters in other animal species. Diets fed to most homes have a calculated DCAB near 150 meq/kg of diet dry matter and may be as low as 100 meq/kg dry matter. These values would be considered marginal for maintaining optimum blood pH and calcium retention in poultry and dairy cattle. It has been demonstrated that rats consuming diets with high amounts of ammonium chloride have increased bone resorption3 as well as decreased blood pH and increases in urinary cAMP and calcium concentrations.4 Feeding a low DCAB has been shown to decrease growth rate? blood pH and HC0 a concentrations6in chicks. It has also been shown that feeding increasing levels of sodium with a constant level of chloride results in an alkalosis, whereas feeding increasinglevels of chloride with a constant level of sodium produces an acidosis.7 Swine researchers have observed decreases in growth and feed intake when pigs were fed diets with a DCAB of -85 meq/kg as opposed to diets with a DCAB greater than 0 meq/kg. It was also noted that as the DCAB was reduced below 175 meq/kg, blood pH and HC0 a values dropped indicative of a metabolic acidosis,a Dairy researchers have shown that feeding highly anionic diets during the dry period decreased the incidence of parturient pariesis,a Furthermore, decreases in blood pH and HC0 a levels as well as increases in urinary calcium concentration have been observed in dairy cows fed diets with a lowered cation-anion balance,a Recent studies have demonstrated that exercising homes consuming diets with low DCAB have increased urinary excretion of both chloride and calcium, and that this increased calcium excretion could lead to a net loss of calcium from the body. 1°'~1 It has been reported that homes consuming highly anionic diets experienced a nutitionally induced metabolic acidosis as evidenced by decreases
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Table 1. Compostition of treatment diets for sedentary horses, as fed basis.
Ingredient (%)
L
Ground Corn Soybean Meal Cottonseed Hulls Dical Trace Mineral Salt Limestone Chromic Oxide Calcium Chloride Ammonium Chloride Sodium Bicarbonate Potassium Citrate Prairie Grass Hay
36.80 6.00 15.00 .50 .50
Treatment ML MH 37.30 6.00 15.00 .50 .50
--
--
.20 .50 .50 ~
.20 .50 ~ --
_
--
40.00
40.00
37.30 6.00 15.00 .50 .50 .50 .20 -----
40.00
H 35.90 6.00 15.00 .50 .50 .50 .20 -.40 1.00
40.00
in arterial and venous blood pH, HC0 3, and PC0 2 values as well as a decrese in urine pH at rest12 and pre and post exercise) a While short term effects of feeding such diets may or may not be noticeable, long term effects could significantly influence the health and performance of horses. If manipulating the DCAB could ~ sh.o.wn to improve calcium balance, there is potential for mmlmlzang skeletal demineralization and the associated changes in bone strength and ultimately improve longevity. The objective of these trials was to study the effects of varied DCAB on mineral balance in sedentary and anaerobically exercised horses.
MATERIALS AND METHODS
Four mature sedentary geldings, four anaerobically exercised geldings and four anaerobically exercised mares were assigned to treatments within three Latin square experiments to study the effects of DCAB on mineral balance. The concentrate portion of the diets consisted of a pelleted base concentrate of corn, soybean meal and cottonseed hulls. This concentrate was fed in a 60:40 ratio with native prairiegrass hay to the sedentary horses, and with bermudagrass hay to the exercised homes in amounts necessary to maintain constant body weights throughout the experiment. Each period consisted of a 21 day adjustment period followed by a 72 hour collection period. Treatments with an average DCAB of +24 (Low, L), +127 (Medium Low, ML), +227 (Medium High, MH) and +352 (High, H) were formed by supplementing the base concentrate with ammonium chloride and calcium chloride (diet L), with calcium chloride (diet ML), and with potassium citrate and sodium bicarbonate (diet H). Diet MH received no additional Na ÷, K + or CI" supplementation and served as the control diet in these trials (Tables 1 and 2). Diets were calculated to contain equivalent amounts of digestible energy and crude protein across treatments for the sedentary (2.5 Mcal/kg and 9.6%) and for the exercised horses (2.7 Mcal/ kg and 10.4 %). Diets were analyzed and determined to contain 558
Table 2. Composition of treatment diets for the anaerobically exercised horses, as fed basis.
Ingredient (%) Corn Soybean Meal Cottonseed Hulls Dicalcium Phosphate Limestone, ground Trace Mineral Salt Calcium Chloride Ammonium Chloride Potassium Citrate Sodium Bicarbonate Molasses, Chromic Oxide Bermuda Grass Hay
Treatment MH
L
ML
34.04 7.00 14.80 .21 -.51 .69 .27 m m 2.40 .08 40.00
34.04 7.00 15.07 .21 .21 .51 .48 ---2.40 .08 40.00
34.04 7.00 15.04 .18 .69 .51 ---.06 2.40 .08 40.00
H 34-04 7.00 13.75 .18 .69 .51 --.81 .54 2.40 .08 40.00
approximately equal amounts of calcium, phosphorus, magnesium and sulphur (Tables 3 and 4). Sedentary horses were exercised for 30 minutes daily on a mechanical walker. Horses were individually stalled and were given routine health care throughout the trial. The eight exercised horses were aerobically conditioned 6 d/wk by galloping 3.2 km/d at a heart rate of 150 beats/min for 6 weeks. During the experiment the horses were subjected to a combined exercise regimen altemating the long slow distance work with interval training 6 d/wk. The interval training program consisted of two .4 km sprints eliciting heart rates of 200- 220 beats/rain. Heart rate was allowed to recover to below 110 beats/min between sprints. Total urine collection was taken from the geldings for 72 hours using urine hamesses, and for 24 hr from the mares via bladder catheterization. Composited urine samples were acidified and frozen for later analysis. The sedentary horse samples were analyzed for Na, K, Ca and Mg by atomic absorption spectrophotometry, for C1 via potentiometric titration using an HBI Digital Chloridometer, and for P using a Sigma chemical procedure #360- W and a Gilford Spectmphotometer. The exercised horse samples were analyzed for Na, K, Ca, P, S and Mg using a Jarrel-Ash Inductively Coupled Argon Plasma Spectrophotometer, and for C1 using a QuickChem System IV Automated Ion Analyzer. Fecal grab samples were taken from all homes to represent every two hr post feeding interval. Feed samples were also taken at various times throughout the trial and both feed and fecal mineral concentrations were determined using Inductively Coupled Argon Plasma Spectmphotometry. Feed and fecal samples were analyzed for chromium on a Gilford Response Series UV-VIS Spectrophotometer. All data were analyzed using a general linear model procedure with horse, period and treatment as the main effects. Least square means were calculated for each parameter, and differences between treatment means were detected using the pdiff procedure. Minimum significance was declared at p < .05.14
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Table 3. Analyzed mineral content of treatment diets fed to sedentary horses, dry matter basis,
L
ML
Treatment MH
H
.52 .29 .15 .57 .22 .13 1.04 +21
.54 .34 .16 .57 .27 .13 .73 +125
.50 .28 .15 .57 .32 .11 .40 +231
.58 .33 .15 1.25 .40 .14 .38 +350
Constituent Calcium % Phosphorus % Magnesium % Potassium % Sodium % Sulphur % Chloride % DCAB meq((Na+K)-CI)/kg
RESULTS AND DISCUSSION
The effect of DCAB on dry matter digestibility and fecal output in exercised horses is shown in table 5. An increase (p <.05) in fecal output and thus a decrease in DM digestibilitywas observed for exercising horses consuming diet L versus diet H. Fecal output increased from 2709 g/d on diet H to 3134 g/d for those horses consuming diet L. Accordingly, DM digestibility decreased from 66.82% to 61.63%. Mineral balance data for sedentary and anaerobically exercised horses is shown in Tables 6 and 7. Both sedentary and exercised homes consuming diet H excreted more (p < .05) sodium in the urine as compared to those consuming diets ML and MH. Also, sedentary homes consuming diet L had similar urinary sodium excretions to diet H and excreted less sodium in the feces (p <.05) as compared to those consuming all other diets. No significant differences were observed in fecal sodium excretion in exercised homes. Sodium balances were significantly greater (p <.05) in sedentary horses consuming diet MH as compared to those consuming diet L. Exercised homes consuming diet H had greater (p <.05) sodium balances as compared to those homes consuming diets L and ML. These findings agree with work noting that urinary sodium excretion is directly related to sodium intake, ts However, the increased sodium excretion in sedentary horses consuming diet L may be due to the excess chloride being excreted in the urine as sodium chloride. Urinary excretion of potassium paralleled intake. Both sedentary and exercised horses consuming diet H excreted more (p <.05) potassium in the urine when compared to all other diets. This might be explained by the effect of aldosterone secreted in response to an increase concentration of potassium in the extracellular fluid. Furthermore, both sedentary and exercised homes consuming diet L had significantly greater amounts of potassium in the feces as compared to those consuming all other diets. In sedentary homes, potassium balances were similar among diets L, ML and MH, however, potassium balance was higher (p <.05) in those horses consuming diet H as compared to those consuming diet ML. No
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Table 4. Analyzed mineral content of treatment diets fed to anaerobically exercised horses, dry matter basis.
Constituent Calcium, % Phosphorus, % Magnesium, % Potassium, % Sulfur, % Sodium, % Chloride, % DCAB meq((Na+K)-CI)/kg
L
ML
Treatment MH
H
.50 .28 .15 1.12 .11 .29 1.38 +27
.53 .29 .16 1.14 .12 .27 1.00 +130
.52 .28 .15 1.13 .11 .30 .69 +223
.54 .28 .15 1.39 .13 .43 .68 +354
differences (p >.05) in potassium balance were observed in the exercised homes. Decreasing the DCAB resulted in increased urinary chloride excretion in both the sedentary and exercised horses consuming diets L and ML (p <.05) as compared to diets MH and H. DCAB did not affect fecal chloride excretion as values were similar (p >.05) across all treatments for both groups of homes. Apparently, the increase in chloride excretion in the urine of sedentary horses consuming diets L and ML was enough to offset the increased chloride intake, as chloride balance was statistically similar across all treatments. However, the exercised horses consuming diet L had higher (p <.05) chloride balances as compared to those consuming the other diets. These findings agree with those of other researchers who noted increased urinary chloride excretion in exercising mares consuming diets with a low DCAB. 1° Magnesium has been implicated as having a minor role in the DCAB equation by dairy researchers. Therefore, magnesium intakes were held constant across all treatments for both groups of horses. DCAB did not appear to affect urinary magnesium excretion as values were similar across treatments for both groups of horses. However, both sedentary and exercised horses consuming diet L had increased (p <. 05) fecal magnesium excretions compared to those consuming diets MH and H. Magnesium balance in sedentary homes was not affected by DCAB, while exercised homes consuming diet L had lower (p < .05) magnesium balances as compared to those consuming the other diets. Urinary phosphorus excretion was not affected by DCAB as values were similar across treatments for both the sedentary and Table 5. The effect of dietary cation-anion balance on dry matter digestibility in the anaerobically exercised horses.
L
DM Digestibility Fecal Output g/d
61.63 a 3134 a
ML
Treatment MH
H
S.E.
65.41ab 63.54 ab 66.82 b 1.05 2825 ab 2978 ab 2709 b 85.99
=~bMeansin rows with differentsuperscriptsdiffer (P<,05).
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Table 6. Effect of dietary cation-anion balance on mineral balance in sedentary horses.
MINERAL, g/d
L
Treatment ML MH
L
Treatment ML MH
H
S.E.
24.02 8.57 a 11.97 a 3.47 a
22.36 8.61 a 11.67 a 2..08 a
24.33 5.94 a 13.06 a 5.36 a
35.38 14.03 b 12.48 a 8.86 b
.96 .99 1.18
71.17 104.14 41.52 a 68.47 b 16.98 a 15.28 a 12.66 ab 20.47 b
POTASSIUN Intake Urine Fecal Balance
91.85 50.74 a 22.29 a 18.82 a
93.37 49.38 a 17.52 b 26.46 a
92.37 50.33 a 17.35 b 24.69 a
113.78 73.95 b 17.46 b 22.38 a
4.31 1.06 4.53
60.76 57.53 a 5.58 a -2.35 a
30.29 31.34 b 5.78 a -3.82 a
29.29 31.43 b 3.52 a -3.16 a
CHLORIDE Intake Unne Fecal Balance
112.40 67.17 a 7.58 a 37.65 a
81.36 56.14 a 8.22 a 17.00 b
56.62 33.05 b 6.49 a 17.07 b
55.17 35.39 b 7.74 a 12.04 b
4.13 .86 4.13
12.17 6.02 a 7.95 a -1.31 a
12.30 6.74 a 7.42 ab -1.01 a
11.30 6.47 a 6.76 -1.08 a
11.80 6.44 a 6.68 c -0.48 a
MAGNESIUM Intake Urine Fecal Balance
12.41 3.88 a 7.59 a .94 a
12.69 3.70 a 6.34 b 2.65 b
12.51 3.78 a 6.45 b 2.28 a
12.48 3.70 a 6.47 b 2.31 a
.31 .24 .34
23.67 .15 a 19.05 a 5.31 a
26.46 .13 a 19.10 a 8.73 b
20.97 .14 a 18.97 a 3.86 c
24.96 .16a 17.32 b 9.98 d
SULFUR Intake Urine Fecal Balance
9.20 a 7.91 a 2.54 a -1.25 a
9.49 a 9.03 a 2.35 a -1.89 a
9.26 a 8.34 a 2.31 a -1.39 a
10.39 a 8.73 a 2.23 a
1.93 .14
-0.56 a
1.91
41.50 39.81 a 15.35 a -12.20 a
42.25 31.80 b 15.76 a -2.65 b
38.26 13.99 e 19.11 b 8.31 c
44.75 3.99 d 15.92 a 28.51 d
PHOSPHORUS Intake Urine Fecal Balance
22.77 .07a 21.6 a I.Oa
23.95 .06 a 17.74 b 6.16 b
22.86 .06 a 17.18 b 5.62 b
22.94 .06 a 17.20 b 5.68 b
.01 .42 .42
CALCIUM Intake Urine Fecal Balance
40.82 20.11 a 17.45 a 3.26 a
42.98 15.71 a 15.66 a 11.61 a
42.35 12.16 a 19.53 a 10.66 a
44,22 10.33 b 21.015 12.88 b
2.12 .97 2.29
18.03 13.51a 5.66 a -0.76 a
21.59 6.20 b 13.03 b 3.71 ab
24.50 5.32 b 13.71 b 7.90 b
POTASSIUM Intake Urine Fecal Balance
73.38 34.85 a 24.05 b 12.27 ab
71.17 46.55 a 18.19 a 6.42 a
CHLORIDE Intake Urine Fecal Balance
89.37 70.59 a 4.60 a 13.16 a
MAGNESIUM Intake Urine Fecal Balance PHOSPHORUS Intake Urine Fecal Balance
30.13 11.53 a 16.56 b 5.04ab
a,b,c,dMeans in rows with different superscripts differ (p<,05).
exercised horses. In sedentary horses, fecal excretions of phosphorus were lower in those horses consumingdiet H as compared to the other diets. Furthermore, fecal excretions were higher in exercising horses consuming diet L. In sedentary horses, phosphorus balance tended to reflect intake with significant differences noted between all treatments. Phosphorus balance was significantly lower in exercising horses consuming diet L. In exercised horses, DCAB did not appear to have an effect on sulfur balances, as values for urinary and fecal sulfur excretions were similar across all treatments. Although no significant differences were detected, sulfur balances in exercised horses were negative across all treatments. The NRC 16 suggests the sulfur requirement of the exercising horses is approximately .15%, a
560
MINERAL g/d SODIUM Intake Urine Fecal Balance
SODIUM Intake Urine Fecal Balance
CALCIUM Intake Urine Fecal Balance
H
Table 7. The effect of dietary cation-anion balance on mineral balance in the anaerobically exercised horses.
a,b,c,dMeans in rows with different superscripts differ (P < .05).
minimum value provided by most high quality protein sources. In the present study, the sulfur content of the feedstuffs was apparently overestimated resulting in sulfur concentrations below the requirement (L=.ll, ML=.12, MH=.ll and H=.13%). Therefore, these horses consumed approximately 2 to 3 g/d below the suggested 12.25 g/d which could explain the negative sulfur balances observed. As DCAB decreased, urinary calcium excretion increased (p
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< .05) across all treatments in the sedentary horses. Furthermore, urinary calcium excretion was significantly higher (p < .05) in the exercised horses consuming diet L versus those consuming diet H. Fecal calcium excretion was significandy greater for the sedentary horses consuming diet MH when compared to those consuming the other treatments. The exercising horses consuming diet H had increased (p < .05) fecal calcium excretion as compared to those consuming diet ML. As the DCAB increased, calcium balance increased (p <.05) across all treatments in the sedentary horses. Furthermore, exercised horses consuming diet H had higher (p <.05) calcium balance when compared to those consuming diet L. These findings agree with published data demonstrating increased urinary calcium excretion in horses, ~0 rabbits17 and rats4 consuming diets with lowered DCAB. Parathyroid hormone has been shown to significantlyincrease renal production of 1,25-dihydroxyvitamin D in dairy cows fed highly anionic diets, thus increasing calcium absorption from the GI tract.ra Also, osteoclastic bone resorption was more responsive to PTH as plasma hydroxyproline concentration was higher in those cows fed the lower DCAB treatment. Furthermore, an inverse relationship between blood pH and serum ionized and bound calcium concentrations has been reported in other species.20 If a similar increase in plasma ionized calcium occurs in horses undergoing metabolic acidosis in response to consumption of highly anionic diets, it may be an important factor contributing to the increase in urinary calcium excretions observed in both groups of horses. Urinary excretion of minerals such as sodium and potassium is sensitive to the intake of those minerals by the animal. The kidney is also the main route of chloride excretion. In a state of chronic metabolic acidosis, one of the routes of excretion of excess hydrogen ions is in combination with chloride in the tubule lumen of the kidney. This forms the strong acid HCI which quickly combines with ammonia to form ammonium chloride (NH4CI) which is a much weaker acid and much less damaging to the kidney tubules. The results from this study suggest that both sedentary and exercised horses consuming diets with a low DCAB have mineral 49 Depending balances similar to those seen in other livestock species.. on the level of intake, these horses may be in a state of negative calcium and magnesium balance. If prolonged, this net calcium loss could lead to an osteoporotic weakening of the skeletal system as has been demonstrated in other species.4'19 Further, an increase in calcium balance was observed in those horses consuming the diet with the highest DCAB. Feedingsuch dietsmay improve longevity by minimizing potential skeletal demineralization and decrease the incidence of developmental orthopedic diseases in young growing horses. REFERENCES 1. Mongin P. Electolytes in Nutrition: Review of basic principles and practical application in poultry and swine. Proc. ThirdAnnu. Int. Miner. Confer., Orlando, FI. 1980; 1. 2. Patience JF, Austic RE and Boyd RD. Effect of dietary
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electrolyte balance on growth and acid-base status in swine. J. Anita. Sci. 1987;64:457. 3. Barzel US and Jowsey J. The effect of chronic acid and alkali administration on bone turnover in adult rats. Clin. Sci. 1969;36:517. 4. Petito SL and Evans JL. Calcium status of the growing rat as affected by diet acidity from ammonium chloride, phosphate and protein. J. Nutr. 1984;114:1049. 5. Neishiem MC, Leach RM, Jr., Zeigler TR and Serafin JA. Interrelationships between dietary levels of sodium, chloride and potassium. J. Nutr. 1964;84:361. 6. Cohen I, Hurwitz S and Bar A. Acid-base balance and sodium to chloride ratio in diets of laying hens. J. Nutr. 1972;102:1. 7. Cohen I and Hurwitz S. The response of blood ionic constituents and acid-base balance to dietary sodium, potassium and chloride in laying fowls. Poultry Sci. 1974;53:378. 8. Block E. Manipulating dietary anions and cations for prepartum dairy cows to reduce the incidence of milk fever. J. Dairy Sci. 1984;67:2739. 9. Tucker WB, Harrison GA and Hemken RW. Influence of dietary cation-anion balance on milk, blood, urine and rumen fluid in lactating dairy cattle. J. Dairy ScL 1988;71:346. 10. Topliff DR, Kennedy MA, Freeman DW, Teeter RG and Wagner DG. Changes in urinary and serum calcium and chloride concentrations in exercising horses fed varying cation-anion balances. Proc. Elevenih Equine Nutr. and Physio. Symp. Stillwater, OK.: Oklahoma State University. 1989;1. 11. Wall DL, Topliff DR, Freeman DW, Wagner DG and Breazile JE. Effects of dietary cation-anion balance on urinary mineral excretion in exercised horses. J. Equine Vet. Sci. 1991 ;12(3):168. 12. Baker LA, Topliff DR, Freeman DW, Teeter RG, and Breazile JE. The effect of dietary cation-anion balance on acid-base status in horses. J. Equine Vet. Sci. 1992;12(3):161. 13. Popplewell JC, Topliff DR, Freeman DW, and Breazile JE. Effects of dietary cation-anion balance on acid-base balance and blood parameters in anaerobically exercised horses. Proc. Thirteenth Equine Nutr. and Physio. Symp. Gainesville, FL. 1993;191. 14. SAS Institute Inc. SAS User's Guide: Statistics, Version 5 Edition. Cary, NC: SAS Institute Inc. 1985. 15. Schryver HF, Parker MT, Daniluk PD, Pagan KI, Williams J, Soderholm LV and Hintz HF. Salt consumption and the effect of salt on mineral metabolism in horses. Cornell Vet. 1987;77:122. 16. NRC. Committee on Animal Nutrition. Nutrient Requirements of Horses. National Academy of Sciences, National Research Council, Washington D.C. 1989. 17. Thacker EJ. Effect of a physiological cationanion imbalance on the growth and mineral nutrition of rabbits. J. Nutr. 1959;69:28. 18. Goff JP, Horst RL, Mueller FJ, Miller JK, Kiess GA and Dowlen HH. Addition of chloride to a prepartal diet high in cations increases 1,25 dihydroxyvitamin D response to hypocalcemia preventing milk fever. J. Dairy Sci. 1991;74:3863. 19. Halley JT, Nelson TS, Kirby LK and Johnson ZB. Effect of altering dietary mineral balance on growth, leg abnormalities and blood base excess in broiler chicks. Poultry Sci. 1987;66:1684. 20. Chew DJ, Leonard M and Muir W II1. Effect of sodium bicarbonate infusions on ionized calcium and total calcium concentrations in serum of clinically normal cats. Am. J. Vet. Res. 1987;50( 1): 145.
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EFFECT OF DIETARY CATION-ANIONBALANCE ON MINERAL BALANCEIN ANAEROBICALLY EXERCISED AND SEDENTARYHORSES L. A. Baker, MS; 1 D.L. Wall, MS; D.R. Topliff, PhD; 1 D.W. Freeman, PhD; 1 R.G. Teeter, PhD; 1 J.E. Breazile, DVM; PhD2; D.G. Wagner, PhD
SUMMARY Eight geldings and four mares were randomly assigned treatrnents within three 4x4 Latin square design experiments to study the effects of dietary cation-anion balance (DCAB) on mineral balance and dry matter digestibility in sedentary and anaerobically exercised horses. Four diets with an average DCAB (calculated as meq ((Na+K+) - Cl-)/kg of diet dry matter) of +24 (Low, L), +127 (Medium Low, ML), +227 (Medium High, MH) and +352 (High, H) were fed for a 21 day adjustment period followed by a 72 hour collection period. Diets consisted of a pelleted base concentrate of com, soybean meal and cottonseed hulls fed with either native prairie grass or bermuda grass hay in a 60:40 ratio. Diet L was formed by adding calcium chloride and ammonium chloride to the base concentrate, diet ML was formed by adding calcium chloride, and diet H was formed by adding potassium citrate and sodium bicarbonate. Diet MI-I received no supplementation and served as the control. Representative samples of feed, feces and urine were analyzed for mineral content and mineral balances were calculated by difference. Fecal output was greater (p <.05), and thus, dry matter digestibilitywas lower in exercised homes consuming diet L versus diet H. Sodium balance was greater (p <.05) in sedentary horses consuming diet MH as compared to those consuming diets ML and L Sodium balance was greater (p <.05) in exercised homes consuming diet H as compared to those consuming diets ML and L Potassium balance was greater (p <.05) in sedentary horses consuming diet H as compared to those horses consuming diet ML, however, potassium balance was not affected by DCAB in exercised horses. No significant differences were detected in chloride or magnesium balances in the sedentary horses, although chloride balance was greater (p <.05) and magnesium balance was lower (p <.05) in exercised homes consuming diet L as compared to all other diets. In sedentary homes, phosphorus balance was reflective of intake with differences (p <.05) observed between all treatments. However, in exercised homes, phosphorus balance was lower (p <.05) oniy for those consuming diet L. Calcium balance decreased significantly as DCAB decreased between all treatments in sedenAuthors' Address: 1Dept. of Animal Science, Division of Agricultural Sciences and Natural Resoumes, Oklahoma State University, Stillwater, OK. 74078. 2Dept. of Physiological Sciences, College of Veterinary Medicine, Oklahoma State University, Stillwater, OK. 74078. Acknowledgement: This research was supported by the Oklahoma Agricultural Experiment Station, Project H-1964.
Volume 13, Number 10, 1993
tary horses, while calcium balance in exercised homes was greater
(p <.05) for homes consuming diet H as compared to those consuming diet L. Previous research from our laboratory has shown that both exercised and sedentary horses consuming diets with a low DCAB experience a nutritionally induced metabolic acidosis. The current data indicate that anaerobically exercised and sedentary horses consuming these diets excrete significantly more calcium in the urine resulting in decreased calcium balances. Prolonged consumption of diets with a low DCAB may lead to a significant demineralization of bone and a subsequent weakening of the skeleton. (Key Words: Equine, Mineral, Nutrition) INTRODUCTION
Sodium, potassium and chloride are the most influential ions involved in the regulation of osmotic pressure in body fluids, as well as the maintenance of acid-base balance. The equation used to calculate dietary cation-anion balance (DCAB) in this study is meq ((Na+K +) - Cl-)/kg of diet dry matter. 1.2 Highly cationic diets have been shown to be alkalogenic and highly anionic diets have been shown to be acidogenic. Feeding diets with a low DCAB has an adverse effect on the acid-base status along with various growth and production parameters in other animal species. Diets fed to most homes have a calculated DCAB near 150 meq/kg of diet dry matter and may be as low as 100 meq/kg dry matter. These values would be considered marginal for maintaining optimum blood pH and calcium retention in poultry and dairy cattle. It has been demonstrated that rats consuming diets with high amounts of ammonium chloride have increased bone resorption3 as well as decreased blood pH and increases in urinary cAMP and calcium concentrations.4 Feeding a low DCAB has been shown to decrease growth rate? blood pH and HC0 a concentrations6in chicks. It has also been shown that feeding increasing levels of sodium with a constant level of chloride results in an alkalosis, whereas feeding increasinglevels of chloride with a constant level of sodium produces an acidosis.7 Swine researchers have observed decreases in growth and feed intake when pigs were fed diets with a DCAB of -85 meq/kg as opposed to diets with a DCAB greater than 0 meq/kg. It was also noted that as the DCAB was reduced below 175 meq/kg, blood pH and HC0 a values dropped indicative of a metabolic acidosis,a Dairy researchers have shown that feeding highly anionic diets during the dry period decreased the incidence of parturient pariesis,a Furthermore, decreases in blood pH and HC0 a levels as well as increases in urinary calcium concentration have been observed in dairy cows fed diets with a lowered cation-anion balance,a Recent studies have demonstrated that exercising homes consuming diets with low DCAB have increased urinary excretion of both chloride and calcium, and that this increased calcium excretion could lead to a net loss of calcium from the body. 1°'~1 It has been reported that homes consuming highly anionic diets experienced a nutitionally induced metabolic acidosis as evidenced by decreases
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Table 1. Compostition of treatment diets for sedentary horses, as fed basis.
Ingredient (%)
L
Ground Corn Soybean Meal Cottonseed Hulls Dical Trace Mineral Salt Limestone Chromic Oxide Calcium Chloride Ammonium Chloride Sodium Bicarbonate Potassium Citrate Prairie Grass Hay
36.80 6.00 15.00 .50 .50
Treatment ML MH 37.30 6.00 15.00 .50 .50
--
--
.20 .50 .50 ~
.20 .50 ~ --
_
--
40.00
40.00
37.30 6.00 15.00 .50 .50 .50 .20 -----
40.00
H 35.90 6.00 15.00 .50 .50 .50 .20 -.40 1.00
40.00
in arterial and venous blood pH, HC0 3, and PC0 2 values as well as a decrese in urine pH at rest12 and pre and post exercise) a While short term effects of feeding such diets may or may not be noticeable, long term effects could significantly influence the health and performance of horses. If manipulating the DCAB could ~ sh.o.wn to improve calcium balance, there is potential for mmlmlzang skeletal demineralization and the associated changes in bone strength and ultimately improve longevity. The objective of these trials was to study the effects of varied DCAB on mineral balance in sedentary and anaerobically exercised horses.
MATERIALS AND METHODS
Four mature sedentary geldings, four anaerobically exercised geldings and four anaerobically exercised mares were assigned to treatments within three Latin square experiments to study the effects of DCAB on mineral balance. The concentrate portion of the diets consisted of a pelleted base concentrate of corn, soybean meal and cottonseed hulls. This concentrate was fed in a 60:40 ratio with native prairiegrass hay to the sedentary horses, and with bermudagrass hay to the exercised homes in amounts necessary to maintain constant body weights throughout the experiment. Each period consisted of a 21 day adjustment period followed by a 72 hour collection period. Treatments with an average DCAB of +24 (Low, L), +127 (Medium Low, ML), +227 (Medium High, MH) and +352 (High, H) were formed by supplementing the base concentrate with ammonium chloride and calcium chloride (diet L), with calcium chloride (diet ML), and with potassium citrate and sodium bicarbonate (diet H). Diet MH received no additional Na ÷, K + or CI" supplementation and served as the control diet in these trials (Tables 1 and 2). Diets were calculated to contain equivalent amounts of digestible energy and crude protein across treatments for the sedentary (2.5 Mcal/kg and 9.6%) and for the exercised horses (2.7 Mcal/ kg and 10.4 %). Diets were analyzed and determined to contain 558
Table 2. Composition of treatment diets for the anaerobically exercised horses, as fed basis.
Ingredient (%) Corn Soybean Meal Cottonseed Hulls Dicalcium Phosphate Limestone, ground Trace Mineral Salt Calcium Chloride Ammonium Chloride Potassium Citrate Sodium Bicarbonate Molasses, Chromic Oxide Bermuda Grass Hay
Treatment MH
L
ML
34.04 7.00 14.80 .21 -.51 .69 .27 m m 2.40 .08 40.00
34.04 7.00 15.07 .21 .21 .51 .48 ---2.40 .08 40.00
34.04 7.00 15.04 .18 .69 .51 ---.06 2.40 .08 40.00
H 34-04 7.00 13.75 .18 .69 .51 --.81 .54 2.40 .08 40.00
approximately equal amounts of calcium, phosphorus, magnesium and sulphur (Tables 3 and 4). Sedentary horses were exercised for 30 minutes daily on a mechanical walker. Horses were individually stalled and were given routine health care throughout the trial. The eight exercised horses were aerobically conditioned 6 d/wk by galloping 3.2 km/d at a heart rate of 150 beats/min for 6 weeks. During the experiment the horses were subjected to a combined exercise regimen altemating the long slow distance work with interval training 6 d/wk. The interval training program consisted of two .4 km sprints eliciting heart rates of 200- 220 beats/rain. Heart rate was allowed to recover to below 110 beats/min between sprints. Total urine collection was taken from the geldings for 72 hours using urine hamesses, and for 24 hr from the mares via bladder catheterization. Composited urine samples were acidified and frozen for later analysis. The sedentary horse samples were analyzed for Na, K, Ca and Mg by atomic absorption spectrophotometry, for C1 via potentiometric titration using an HBI Digital Chloridometer, and for P using a Sigma chemical procedure #360- W and a Gilford Spectmphotometer. The exercised horse samples were analyzed for Na, K, Ca, P, S and Mg using a Jarrel-Ash Inductively Coupled Argon Plasma Spectrophotometer, and for C1 using a QuickChem System IV Automated Ion Analyzer. Fecal grab samples were taken from all homes to represent every two hr post feeding interval. Feed samples were also taken at various times throughout the trial and both feed and fecal mineral concentrations were determined using Inductively Coupled Argon Plasma Spectmphotometry. Feed and fecal samples were analyzed for chromium on a Gilford Response Series UV-VIS Spectrophotometer. All data were analyzed using a general linear model procedure with horse, period and treatment as the main effects. Least square means were calculated for each parameter, and differences between treatment means were detected using the pdiff procedure. Minimum significance was declared at p < .05.14
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Table 3. Analyzed mineral content of treatment diets fed to sedentary horses, dry matter basis,
L
ML
Treatment MH
H
.52 .29 .15 .57 .22 .13 1.04 +21
.54 .34 .16 .57 .27 .13 .73 +125
.50 .28 .15 .57 .32 .11 .40 +231
.58 .33 .15 1.25 .40 .14 .38 +350
Constituent Calcium % Phosphorus % Magnesium % Potassium % Sodium % Sulphur % Chloride % DCAB meq((Na+K)-CI)/kg
RESULTS AND DISCUSSION
The effect of DCAB on dry matter digestibility and fecal output in exercised horses is shown in table 5. An increase (p <.05) in fecal output and thus a decrease in DM digestibilitywas observed for exercising horses consuming diet L versus diet H. Fecal output increased from 2709 g/d on diet H to 3134 g/d for those horses consuming diet L. Accordingly, DM digestibility decreased from 66.82% to 61.63%. Mineral balance data for sedentary and anaerobically exercised horses is shown in Tables 6 and 7. Both sedentary and exercised homes consuming diet H excreted more (p < .05) sodium in the urine as compared to those consuming diets ML and MH. Also, sedentary homes consuming diet L had similar urinary sodium excretions to diet H and excreted less sodium in the feces (p <.05) as compared to those consuming all other diets. No significant differences were observed in fecal sodium excretion in exercised homes. Sodium balances were significantly greater (p <.05) in sedentary horses consuming diet MH as compared to those consuming diet L. Exercised homes consuming diet H had greater (p <.05) sodium balances as compared to those homes consuming diets L and ML. These findings agree with work noting that urinary sodium excretion is directly related to sodium intake, ts However, the increased sodium excretion in sedentary horses consuming diet L may be due to the excess chloride being excreted in the urine as sodium chloride. Urinary excretion of potassium paralleled intake. Both sedentary and exercised horses consuming diet H excreted more (p <.05) potassium in the urine when compared to all other diets. This might be explained by the effect of aldosterone secreted in response to an increase concentration of potassium in the extracellular fluid. Furthermore, both sedentary and exercised homes consuming diet L had significantly greater amounts of potassium in the feces as compared to those consuming all other diets. In sedentary homes, potassium balances were similar among diets L, ML and MH, however, potassium balance was higher (p <.05) in those horses consuming diet H as compared to those consuming diet ML. No
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Table 4. Analyzed mineral content of treatment diets fed to anaerobically exercised horses, dry matter basis.
Constituent Calcium, % Phosphorus, % Magnesium, % Potassium, % Sulfur, % Sodium, % Chloride, % DCAB meq((Na+K)-CI)/kg
L
ML
Treatment MH
H
.50 .28 .15 1.12 .11 .29 1.38 +27
.53 .29 .16 1.14 .12 .27 1.00 +130
.52 .28 .15 1.13 .11 .30 .69 +223
.54 .28 .15 1.39 .13 .43 .68 +354
differences (p >.05) in potassium balance were observed in the exercised homes. Decreasing the DCAB resulted in increased urinary chloride excretion in both the sedentary and exercised horses consuming diets L and ML (p <.05) as compared to diets MH and H. DCAB did not affect fecal chloride excretion as values were similar (p >.05) across all treatments for both groups of homes. Apparently, the increase in chloride excretion in the urine of sedentary horses consuming diets L and ML was enough to offset the increased chloride intake, as chloride balance was statistically similar across all treatments. However, the exercised horses consuming diet L had higher (p <.05) chloride balances as compared to those consuming the other diets. These findings agree with those of other researchers who noted increased urinary chloride excretion in exercising mares consuming diets with a low DCAB. 1° Magnesium has been implicated as having a minor role in the DCAB equation by dairy researchers. Therefore, magnesium intakes were held constant across all treatments for both groups of horses. DCAB did not appear to affect urinary magnesium excretion as values were similar across treatments for both groups of horses. However, both sedentary and exercised horses consuming diet L had increased (p <. 05) fecal magnesium excretions compared to those consuming diets MH and H. Magnesium balance in sedentary homes was not affected by DCAB, while exercised homes consuming diet L had lower (p < .05) magnesium balances as compared to those consuming the other diets. Urinary phosphorus excretion was not affected by DCAB as values were similar across treatments for both the sedentary and Table 5. The effect of dietary cation-anion balance on dry matter digestibility in the anaerobically exercised horses.
L
DM Digestibility Fecal Output g/d
61.63 a 3134 a
ML
Treatment MH
H
S.E.
65.41ab 63.54 ab 66.82 b 1.05 2825 ab 2978 ab 2709 b 85.99
=~bMeansin rows with differentsuperscriptsdiffer (P<,05).
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Table 6. Effect of dietary cation-anion balance on mineral balance in sedentary horses.
MINERAL, g/d
L
Treatment ML MH
L
Treatment ML MH
H
S.E.
24.02 8.57 a 11.97 a 3.47 a
22.36 8.61 a 11.67 a 2..08 a
24.33 5.94 a 13.06 a 5.36 a
35.38 14.03 b 12.48 a 8.86 b
.96 .99 1.18
71.17 104.14 41.52 a 68.47 b 16.98 a 15.28 a 12.66 ab 20.47 b
POTASSIUN Intake Urine Fecal Balance
91.85 50.74 a 22.29 a 18.82 a
93.37 49.38 a 17.52 b 26.46 a
92.37 50.33 a 17.35 b 24.69 a
113.78 73.95 b 17.46 b 22.38 a
4.31 1.06 4.53
60.76 57.53 a 5.58 a -2.35 a
30.29 31.34 b 5.78 a -3.82 a
29.29 31.43 b 3.52 a -3.16 a
CHLORIDE Intake Unne Fecal Balance
112.40 67.17 a 7.58 a 37.65 a
81.36 56.14 a 8.22 a 17.00 b
56.62 33.05 b 6.49 a 17.07 b
55.17 35.39 b 7.74 a 12.04 b
4.13 .86 4.13
12.17 6.02 a 7.95 a -1.31 a
12.30 6.74 a 7.42 ab -1.01 a
11.30 6.47 a 6.76 -1.08 a
11.80 6.44 a 6.68 c -0.48 a
MAGNESIUM Intake Urine Fecal Balance
12.41 3.88 a 7.59 a .94 a
12.69 3.70 a 6.34 b 2.65 b
12.51 3.78 a 6.45 b 2.28 a
12.48 3.70 a 6.47 b 2.31 a
.31 .24 .34
23.67 .15 a 19.05 a 5.31 a
26.46 .13 a 19.10 a 8.73 b
20.97 .14 a 18.97 a 3.86 c
24.96 .16a 17.32 b 9.98 d
SULFUR Intake Urine Fecal Balance
9.20 a 7.91 a 2.54 a -1.25 a
9.49 a 9.03 a 2.35 a -1.89 a
9.26 a 8.34 a 2.31 a -1.39 a
10.39 a 8.73 a 2.23 a
1.93 .14
-0.56 a
1.91
41.50 39.81 a 15.35 a -12.20 a
42.25 31.80 b 15.76 a -2.65 b
38.26 13.99 e 19.11 b 8.31 c
44.75 3.99 d 15.92 a 28.51 d
PHOSPHORUS Intake Urine Fecal Balance
22.77 .07a 21.6 a I.Oa
23.95 .06 a 17.74 b 6.16 b
22.86 .06 a 17.18 b 5.62 b
22.94 .06 a 17.20 b 5.68 b
.01 .42 .42
CALCIUM Intake Urine Fecal Balance
40.82 20.11 a 17.45 a 3.26 a
42.98 15.71 a 15.66 a 11.61 a
42.35 12.16 a 19.53 a 10.66 a
44,22 10.33 b 21.015 12.88 b
2.12 .97 2.29
18.03 13.51a 5.66 a -0.76 a
21.59 6.20 b 13.03 b 3.71 ab
24.50 5.32 b 13.71 b 7.90 b
POTASSIUM Intake Urine Fecal Balance
73.38 34.85 a 24.05 b 12.27 ab
71.17 46.55 a 18.19 a 6.42 a
CHLORIDE Intake Urine Fecal Balance
89.37 70.59 a 4.60 a 13.16 a
MAGNESIUM Intake Urine Fecal Balance PHOSPHORUS Intake Urine Fecal Balance
30.13 11.53 a 16.56 b 5.04ab
a,b,c,dMeans in rows with different superscripts differ (p<,05).
exercised horses. In sedentary horses, fecal excretions of phosphorus were lower in those horses consumingdiet H as compared to the other diets. Furthermore, fecal excretions were higher in exercising horses consuming diet L. In sedentary horses, phosphorus balance tended to reflect intake with significant differences noted between all treatments. Phosphorus balance was significantly lower in exercising horses consuming diet L. In exercised horses, DCAB did not appear to have an effect on sulfur balances, as values for urinary and fecal sulfur excretions were similar across all treatments. Although no significant differences were detected, sulfur balances in exercised horses were negative across all treatments. The NRC 16 suggests the sulfur requirement of the exercising horses is approximately .15%, a
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MINERAL g/d SODIUM Intake Urine Fecal Balance
SODIUM Intake Urine Fecal Balance
CALCIUM Intake Urine Fecal Balance
H
Table 7. The effect of dietary cation-anion balance on mineral balance in the anaerobically exercised horses.
a,b,c,dMeans in rows with different superscripts differ (P < .05).
minimum value provided by most high quality protein sources. In the present study, the sulfur content of the feedstuffs was apparently overestimated resulting in sulfur concentrations below the requirement (L=.ll, ML=.12, MH=.ll and H=.13%). Therefore, these horses consumed approximately 2 to 3 g/d below the suggested 12.25 g/d which could explain the negative sulfur balances observed. As DCAB decreased, urinary calcium excretion increased (p
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< .05) across all treatments in the sedentary horses. Furthermore, urinary calcium excretion was significantly higher (p < .05) in the exercised horses consuming diet L versus those consuming diet H. Fecal calcium excretion was significandy greater for the sedentary horses consuming diet MH when compared to those consuming the other treatments. The exercising horses consuming diet H had increased (p < .05) fecal calcium excretion as compared to those consuming diet ML. As the DCAB increased, calcium balance increased (p <.05) across all treatments in the sedentary horses. Furthermore, exercised horses consuming diet H had higher (p <.05) calcium balance when compared to those consuming diet L. These findings agree with published data demonstrating increased urinary calcium excretion in horses, ~0 rabbits17 and rats4 consuming diets with lowered DCAB. Parathyroid hormone has been shown to significantlyincrease renal production of 1,25-dihydroxyvitamin D in dairy cows fed highly anionic diets, thus increasing calcium absorption from the GI tract.ra Also, osteoclastic bone resorption was more responsive to PTH as plasma hydroxyproline concentration was higher in those cows fed the lower DCAB treatment. Furthermore, an inverse relationship between blood pH and serum ionized and bound calcium concentrations has been reported in other species.20 If a similar increase in plasma ionized calcium occurs in horses undergoing metabolic acidosis in response to consumption of highly anionic diets, it may be an important factor contributing to the increase in urinary calcium excretions observed in both groups of horses. Urinary excretion of minerals such as sodium and potassium is sensitive to the intake of those minerals by the animal. The kidney is also the main route of chloride excretion. In a state of chronic metabolic acidosis, one of the routes of excretion of excess hydrogen ions is in combination with chloride in the tubule lumen of the kidney. This forms the strong acid HCI which quickly combines with ammonia to form ammonium chloride (NH4CI) which is a much weaker acid and much less damaging to the kidney tubules. The results from this study suggest that both sedentary and exercised horses consuming diets with a low DCAB have mineral 49 Depending balances similar to those seen in other livestock species.. on the level of intake, these horses may be in a state of negative calcium and magnesium balance. If prolonged, this net calcium loss could lead to an osteoporotic weakening of the skeletal system as has been demonstrated in other species.4'19 Further, an increase in calcium balance was observed in those horses consuming the diet with the highest DCAB. Feedingsuch dietsmay improve longevity by minimizing potential skeletal demineralization and decrease the incidence of developmental orthopedic diseases in young growing horses. REFERENCES 1. Mongin P. Electolytes in Nutrition: Review of basic principles and practical application in poultry and swine. Proc. ThirdAnnu. Int. Miner. Confer., Orlando, FI. 1980; 1. 2. Patience JF, Austic RE and Boyd RD. Effect of dietary
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electrolyte balance on growth and acid-base status in swine. J. Anita. Sci. 1987;64:457. 3. Barzel US and Jowsey J. The effect of chronic acid and alkali administration on bone turnover in adult rats. Clin. Sci. 1969;36:517. 4. Petito SL and Evans JL. Calcium status of the growing rat as affected by diet acidity from ammonium chloride, phosphate and protein. J. Nutr. 1984;114:1049. 5. Neishiem MC, Leach RM, Jr., Zeigler TR and Serafin JA. Interrelationships between dietary levels of sodium, chloride and potassium. J. Nutr. 1964;84:361. 6. Cohen I, Hurwitz S and Bar A. Acid-base balance and sodium to chloride ratio in diets of laying hens. J. Nutr. 1972;102:1. 7. Cohen I and Hurwitz S. The response of blood ionic constituents and acid-base balance to dietary sodium, potassium and chloride in laying fowls. Poultry Sci. 1974;53:378. 8. Block E. Manipulating dietary anions and cations for prepartum dairy cows to reduce the incidence of milk fever. J. Dairy Sci. 1984;67:2739. 9. Tucker WB, Harrison GA and Hemken RW. Influence of dietary cation-anion balance on milk, blood, urine and rumen fluid in lactating dairy cattle. J. Dairy ScL 1988;71:346. 10. Topliff DR, Kennedy MA, Freeman DW, Teeter RG and Wagner DG. Changes in urinary and serum calcium and chloride concentrations in exercising horses fed varying cation-anion balances. Proc. Elevenih Equine Nutr. and Physio. Symp. Stillwater, OK.: Oklahoma State University. 1989;1. 11. Wall DL, Topliff DR, Freeman DW, Wagner DG and Breazile JE. Effects of dietary cation-anion balance on urinary mineral excretion in exercised horses. J. Equine Vet. Sci. 1991 ;12(3):168. 12. Baker LA, Topliff DR, Freeman DW, Teeter RG, and Breazile JE. The effect of dietary cation-anion balance on acid-base status in horses. J. Equine Vet. Sci. 1992;12(3):161. 13. Popplewell JC, Topliff DR, Freeman DW, and Breazile JE. Effects of dietary cation-anion balance on acid-base balance and blood parameters in anaerobically exercised horses. Proc. Thirteenth Equine Nutr. and Physio. Symp. Gainesville, FL. 1993;191. 14. SAS Institute Inc. SAS User's Guide: Statistics, Version 5 Edition. Cary, NC: SAS Institute Inc. 1985. 15. Schryver HF, Parker MT, Daniluk PD, Pagan KI, Williams J, Soderholm LV and Hintz HF. Salt consumption and the effect of salt on mineral metabolism in horses. Cornell Vet. 1987;77:122. 16. NRC. Committee on Animal Nutrition. Nutrient Requirements of Horses. National Academy of Sciences, National Research Council, Washington D.C. 1989. 17. Thacker EJ. Effect of a physiological cationanion imbalance on the growth and mineral nutrition of rabbits. J. Nutr. 1959;69:28. 18. Goff JP, Horst RL, Mueller FJ, Miller JK, Kiess GA and Dowlen HH. Addition of chloride to a prepartal diet high in cations increases 1,25 dihydroxyvitamin D response to hypocalcemia preventing milk fever. J. Dairy Sci. 1991;74:3863. 19. Halley JT, Nelson TS, Kirby LK and Johnson ZB. Effect of altering dietary mineral balance on growth, leg abnormalities and blood base excess in broiler chicks. Poultry Sci. 1987;66:1684. 20. Chew DJ, Leonard M and Muir W II1. Effect of sodium bicarbonate infusions on ionized calcium and total calcium concentrations in serum of clinically normal cats. Am. J. Vet. Res. 1987;50( 1): 145.
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