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A review of calcium, phosphorus and magnesium metabolism in the horse
Editorial A R E V I E W OF C A L C I U M , P H O S P H O R U S A N D M A G N E S I U M M E T A B O L I S M IN THE H O R S E V.B. M e a c h a m , BS
SUMMARY Mineral metabolism in the equine is a complex process involving absorption sites and interactions between the minerals themselves and other compounds. Factors such as the digestibility of the minerals and the extent of the interactions all play an important role in the mineral amounts required in the diet. Inadequate levels of minerals in the diet can lead to physiological maladies s u c h as H N S a n d R i c k e t t s . A k n o w l e d g e a n d understanding of mineral metabolism is essential to the success of equine management.
INTRODUCTION Calcium, phosphorus and magnesium are all essential macro minerals, necessary for proper bone growth and development in horses) 3 Because the horse is used as an athlete, sound bone structure is essential. There are many factors that influence mineral metabolism in the horse and a basic understanding of these factors is essential. This paper will deal with absorption, interactions between Ca, P and Mg as well as interactions with other compounds such as lactic acid and oxalic acid. The dietary requirements of each mineral and the effects of imbalances will also be conveyed.
Phosphorus Schryver 23 found that the large intestine was the major site of P absorption in the horse. This is contrary to results found in ruminants 2° and swine where the small intestine is the primary site of absorption. They suggest that the high levels of phosphate and bicarbonate found in the cecum and colon serve as a buffer for the volatile fatty acids produced by microfloral fermentation. Alexander suggested that phosphate may be the major buffer system in the dorsal colon where the concentration of bicarbonate falls and phosphate rises. 23
Magnesium In one study, s the cumulative apparent absorption of Mg was measured f r o m different regions of the intestine in ponies. Various diets containing different forage: grain ratios were fed and absorption calculated by the Cr203: nutrient ratio technique. ~° They found that 25% of the Mg present was absorbed before the digestability reached the jejunum and 46 to 56% was removed before the cecum. Little Mg was absorbed in the large intestine. The total Mg digestibility was found to be 52% in the high concentrate diet and 61% in the high forage diet. The type of diet had no significant affect on the site of absorption, s
INTERACTIONS
MINERAL ABSORPTION Calcium is actively absorbed by the horse in the small intestine in a two step process: from the lumen into the mucosa, and from the mucosa to the underlying tissues. Both steps require the presence of vitamin D and the latter is dependent on oxidative phosphorylation. It is thought that 25-hydroxycholecalciferol (a steroid) induces the synthesis of a calcium-binding protein in the duodenal mucosa. Parathormone plays a facilatory role in Ca absorption. 3 Author's address: Dept. Animal Science, VA Tech and State Univ., Blackshurg, VA.
210
Calcium In the equine, as well as other species, mineral absorption and metabolism are affected by various i n t e r a c t i o n s a n d i n t e r r e l a t i o n s h i p s b e t w e e n these inorganic substances. These may include the effects of the levels of each mineral in the diet, on the metabolism of the given mineral, as well as on the associated minerals. Other compounds, such as oxalic acid, and lactic acid can have effects on mineral absorption and metabolism. An understanding of these relationships is important when formulating diets for the horse, particularly in the growing animal. EQUINE VETERINARY SCIENCE
Calcium
- Calcium
Schryver, 22 studied the effects of Ca intake on skeletal metabolism and Ca homeostatic mechanisms in young ponies. They reported that all ponies maintained normal levels of plasma Ca when fed varying levels of Ca (1.50, .80 and .15% of the diet). The intermediate level of Ca (.80%) was about equal to that recommended by the NRC (1978). The ponies responded to the dietary levels of Ca in such a way as to maintain normal concentration of plasma Ca. When fed low levels (15%) of Ca, intestinal absorption increased, renal excretions decreased and Ca was removed from the bone at a rate greater than that which it was deposited. These ponies displayed a negative Ca balance. Opposite responses were observed in ponies fed the high level (1.5%) diet. In this experiment, the skeletal removal of calcium from bone responded directly to the homeostatic needs of the animal. In a similar test, Whitlock, 30 studied the effects of high calcium intake in horses and noted that, on a body weight basis, response of all horses were similar. In a trial conducted by Schryver, 28 the mineral composition of the whole body, liver and bone of young horses was analyzed. They reported that the calcium content in the body of the horse was significantly affected by the dietary levels of Ca. However, when analyzing the skeletal mass, they found little difference in the weight of bones relative to body weight and that the specific gravity was not significantly higher in horses fed high calcium diets. They also found that the Ca content of the bone ash was uneffected by diet. These data suggest that dietary levels of Ca are not important, in terms of body composition, to the development of bone. It is obvious that these findings lead to discrepancies between whole body composition studies and balance studies discussed earlier. These discrepancies have been noted before. Nielson TM noted that pigs at slaughter contained 15% less Ca and 39% less P than d e t e r m i n e d by balance experiments. Schryver 28 felt that these differences could be a result of two factors. The first is the inherent cumulative errors in balance studies which tend to inflate the balance. The second might be that balance studies will detect short term effects that will not effect whole body composition. Calcium
- Phosphorus
Lieb TM found that high Ca intake also affected P metabolism. In the balance trial, they fed two diets containing .78 and 2.0% Ca to 6 ponies, and measured metabolic and total phosphorus losses. Total fecal phosphorus excreted was significantly increased with increased Ca in the diet. Urinary phosphorus excretion and P absorption were decreased as the level of dietary Ca was raised from .78 to 2.0%. The metabolic fecal phosphorus was found to be maintained at a constant level (6.5 m g / k g BW/d) despite the dietary intake of Ca. They reported that true digestability of P decreased from 61.4 to 34.2% when Ca was increased from .78 to 2.0%. This occurred primarily because of the increasing loss of dietary P in the feces. They also found that high Ca intake (2.0%) tended to depress plasma P levels. In a trial conducted by Kichura Z2, diets containing various levels of Ca and P were fed to 24 mature ponies. Volume 4, Number 5
They found that the amount of dietary Ca influenced only slightly, the ability of ponies to decrease urinary P. There was an exception to this seen in yearlings. When yearlings were fed a low Ca and low P diet, they excreted significantly higher levels of urinary P than yearlings fed a high Ca, low P diet. The authors offered no explanation for this observation. Calcium
- Magnesium
Schryver 2g saw in their analysis of whole body composition that the levels of Ca fed had no influence on the whole body or liver content of magnesium. Again, this is contrary to findings of balance studies that suggest t h a t high Ca diets d e c r e a s e d the u t i l i z a t i o n of magnesium. Phosphorus
- Phosphorus
Various studies have also been conducted to determine the effects of phosphorus levels on the metabolism of minerals. Schryver 25 fed ponies, ranging in age from 6 mo to 2.5 y, varying levels of P to determine the effects on P metabolism. They discovered that renal excretion of P was the most important means of maintaining P metabolism in ponies. Renal excretion of P was low when dietary levels were low and increased as dietary levels increased. They also found that endogenous fecal P excretion did not vary with dietary levels. These findings are contrary to findings in ruminants and rats where fecal endogenous losses were dependant on the P level in the diet. In ruminants, P levels in urine tend to be consistently low and unaffected by diet, and endogenous f e c a l e x c r e t i o n is t h e m e t h o d f o r m a i n t a i n i n g homeostasis. The concentration of plasma P was affected only at very high levels (1.19% of diet) of P in the diet. In another study concerning factors influencing endogenous P losses in ponies, Kichura ~2 found that ponies absorbed higher percentages of dietary P as levels increased. However, they felt that this increase may have been attributable to the form of P in the diet. In their low P diets, corn was used as the major source of P. Phosphorus in corn is in the form of phytate P, which is less available for ponies. The P in the high level diets was primarily monosodium P which is readily available to the equine. Kichura ~2 also found that efficiency of P absorption decreased as age increased, especially in low P level diets. They also found that urinary excretion was extremely low when ponies were fed low levels of P. However, contrary to results found in rats 4 older ponies decreased P excretion in urine more effectively than did the young ponies. Across diets, endogenous fecal P and plasma P concentrations decreased with increasing age. It was also noted that plasma concentrations were poor predictors of endogenous fecal P levels in aged ponies, yearling and weanlings. Linear regression analysis showed R values of -.02, -.76 and -.65 for aged, yearling and weanling ponies, respectively. This study tended to contradict the results found by Schryver 25concerning the effect of dietary levels on endogenous fecal P loss. In this study, high dietary levels of P tended to decrease endogenous fecal P although these differences were not significant. It seemed that age of the pony set the general level of endogenous fecal phosphorus, and may be 211
modified by factors such as dietary Ca and P. Reviewing the whole body composition study done by Schryver 2s, dietary P levels did not influence levels in bone ash. However, in another study, 24 it was shown that high P diets accelerated bone turnover even when calcium levels were inadequate for balance. P h o s p h o r u s - Calcium
In a trial studying the effects of high phosphorus diets on Ca metabolism in ponies, it was shown that a high phosphate diet depressed the intestinal absorption of Ca. 42 In similar studies done with humans, conflicting results have been seen. These findings may be the result of the Ca demand of the subjects and the of supplemental P. Endogenous fecal excretion of Ca increased in ponies fed high (1.19% of diet) levels of P despite decreased Ca concentrations. These changes were very small and their significance uncertain. The amount of Ca retained in the high P diet did not differ from those receiving the low P diet (.20% of diet). Renal excretion of Ca was found to be depressed in ponies on the high P diet. This led the authors to believe that the lower renal excretion of Ca (than would be expected if excretion were considered a function of absorbed calcium) is a result of decreased plasma concentration. This decreased plasma concentration of Ca was consistently seen in ponies fed high concentrations of P. The lowered plasma Ca may have induced parathormone secretion, although blood Ca never returned to normal levels throughout the 30-day trial. This elevated level of p a r a t h o r m o n e was further suggested by an increase in the rate of calcium removal from bone. However, in all but one animal, increased calcium deposition in the bone accompanied the elevated removal rate, thus the net bone formation was dependant on the Ca content of the diet. Long-term studies involving high phosphorus intake have shown that this diet is detrimental to the skeleton of the horse. 24
absorption. Three diets containing none, 100 meg and 400 meg of lactic acid were fed. Mean true digestion coefficients for Ca were 47.14-+1.86, 49.11-+.63 and 52.41 _+2.03 for the control, low lactic acid and high lactic acid diets, respectively. These differences were not significantly different, although the difference between the control and high lactic acid treatment approached significance (P less than .06). Oxalic acid is the primary toxic substance in plants such as rhubarb (Rheum rhaponeticum), halogeton (Halogeton golmeratus), greasewood (Sacrobatus vermiculatus) and soursob (Oxalis cernva). Oxalic acid binds with divalent cations, such as Ca and Mg and thereby decreases the utilization of the minerals. 9 Ward 29 noted that 20 to 33% of the Ca in alfalfa is in the oxalate form and impaired utilization in ruminants. This poses a potentially serious threat to horse producers who rely on alfalfa as a Ca source. In the trial conducted by Hintzfl 9 two diets were fed with Ca: oxalic acid ratios 1.7 to 3.0 in alfalfa hays from differing sources. Metabolism studies were conducted with eight mature ponies being fed 33% hay and 67% oats. No significant differences were found between the diets in the true digestibility of Ca, P or Mg. It was concluded that the alfalfa hays in this study were excellent sources of Ca for ponies.
MINERAL
REQUIREMENTS TABLE 1.
Estimate of requirements of calcium, phosphorus and magnesium for horses with mature weight of 500 kg. Class
Percentage in diet Ca P Mg
Grams/day Ca P Mg
Weanlings
38
25
5
.70
.45
.1
Yearlings
35
21
6
.60
.35
.1
Mature horses
22
14
5
.35
.25
.09
Pregnant mares
30
23
7
.40
.30
.09
Lactating mares Source: NRC, 1978
50
36
9
.50
.35
.09
Magnesium - Calcium/Phosphorus
Magnesium has also been shown to interact with both Ca and P. Hintz 7 fed varying levels of magnesium (. 16, .31 and .86%) to mature ponies and evaluated the effects on Mg, Ca and P metabolism. They reported that Mg levels had a d iphasic response to Ca absorption. Both a Mg deficiency or high levels of Mg enhance Ca absorption. Phosphorus absorption was not affected by increased Mg levels. Although high dietary levels of Ca or P could presumably induce a Mg deficiency, high magnesium levels did not induce a Ca or P deficiency in this study. It was also shown that endogenous urinary loss of Mg was greater than endogenous fecal loss, and that increased urinary excretion of Mg with an increase of Mg in the diet suggests that the kidney is important in Mg homeostasis. NON-MINERALS INTERACTIONS Other non-mineral substances can have effects on mineral metabolism. Two such substances, lactic acid and oxalic acid, have been studied in balance trials in horses. In a study by Nelson, 17 six pony geldings were used to determine the effect of dietary lactic acid on Ca 212
Calcium and P h o p h o r u s
Requirements of Ca and P have been studied quite extensively in the last decade. Because the horse's primary function is that of an athlete, proper bone development and maintenance is essential to its success. The most important period for proper Ca:P levels in the diet is during the first two years of the horses life. This fact is reflected in that the dietary requirements of the young foal are relatively high at birth and decrease with i n c r e a s i n g a g e ( N R C , 1978). T h i s d e c r e a s e in requirements from birth is due to the decreasing growth potential with age. At 12 months, the foal will have already accumulated 94% of the ash (including 98% of the Ca and 94% of the P) that it will have as a two-year-old (Ott and Asquith, 1983). Realizing that 99% of the Ca and 80% of the P found in the body is contained in the skeleton (Schryver et al., 1974b), the importance of EQUINE VETERINARY SCIENCE
p r o p e r m i n e r a l n u t r i t i o n for the y o u n g horse b e c om es m o r e evident. Ca:P Ratios The C a : P ratio has been a s o u r c e of m u c h debate a m o n g horse owners a n d m a n a g e r s . T h e ' i d e a l ' range seemed t o fall within th e 1. h 1 or 2:1 range. 2~ While this range does seem to meet the r e q u i r e m e n t s f o r p r o p e r g r o w t h a n d m a i n t a i n a n c e o f b o n e in all horses, the possibility o f m i x i n g diets a n d f o r m u l a t i o n t o t a l diets that c o n s i s t e n t l y fall within this r a n g e is questionable. Variations in feeds a n d pastures f r o m season to season and year to year can be expected. F u r t h e r m o r e this C a : P ratio is s e l d o m identical to t h a t w h i c h the minerals are a b s o r b e d by the intestines or m a d e available to the tissues. 26
According to work done at Cornell University by Schryver? 6 the horse can tolerate a range of ratios very well. They had raised several groups of horses from 6 mo to 1 1/2 years with diets containing ratios from h 1 to 4:1 without adverse effects. They stressed that the real importance is the absolute levels of these minerals fed in the diet, as long as they met the minimum requirements of each and that P levels do not exceed Ca levels. In an experiment conducted by J o r d a n J z the effects of the Ca:P ratio were explored further. In their trial, 30 four-month old ponies were fed diets containing ratios from 2:1 to 6:1 for a period of 3 years. Calcium intake was 4 to 5 times that required. They found that there were no differences in height, average gain or reproductive performance between groups. Effects o f C a : P I m b a l a n c e s As was discussed earlier, excessive P inhibits Ca a bsorpt i o n . Diets high in P and (or) lo w in Ca have been shown to induce parathyroid hypertrophy and hyperplasia a c c o m p a n i e d by increased b o n e resorption. 2~ This m e t a b o l i c b o n e disorder is k n o w n as n u t r it io n al secondary hyperparathyroidism (NSH). 2 During NSH, high dietary P elevates serum p h o s p h a t e a n d depresses se rum C a content. N o r m a l c o n c e n t r a t i o n o f serum Ca is essential. F o r m e t a b o l i c f u n c t i o n s such as e n z y m e activity, n e u r o m u s c u l a r irritability a n d b l o o d clotting, therefore Ca is r e m o v e d f r o m the b o n e a n d m o b il iz e d in the b l o o d to m a i n t a i n p r o p e r levels. This c o n t i n u o u s decalcification o f the bone can lead to transitory lameness and enlarged facial bones. 23 R e v i e w i n g the findings o f J o r d a n , it was f o u n d that excessive C a in the diet was not d e t r i m e n t a l to the ponies. They saw t h a t ponies on the high Ca diets c o n s u m e d m o r e water a n d that their pens were c o n s t a n t l y wetter than those c o n s u m i n g n o r m a l level o f Ca. H e suggested t h a t this was due to excess renal f u n c t i o n to eliminate excess Ca.
C A , P, M g D e f i c i e n c i e s D u e to C a an d P roles in b o n e f o r m a t i o n and m a i n t a i n a n c e , low levels o f either o f these tw o minerals c a n c a u s e t h e o n s e t o f r i c k e t t s . R i c k e t t s is t h e m a l f o r m a t i o n o f bones due to i n a d e q u a t e calcification and is generally f o u n d in y o u n g animals. 5
Volume 4, Number 5
Deficiency signs o f M g include hyperirritability, glazed eyes, tetany, and e v e n t u a l collapse. This c o n d i t i o n is often precipitated by stress, such as in the sale barns o f large auctions. T h e d i s o r d e r seems to respond to M g - Ca injections. In y o u n g foals, M g deficiency c a n lead to calcification o f the b l o o d vessels. 6
REFERENCES 1. Alexander F 1963. Digestion in the horse. In: DP Cuthbertson (Ed) Progress in Nutrition and Allied Sciences p 259. Oliver and Boyd, London. 2. Argenzio RA, JE Lowe, HF Hintz and HF Schryver 1974. Calcium and phosphorus homeostasis in horses. J Nutr 104:18. 3. Best and Taylor 1978. Secretion, digestion, and absorption in the intestine. In: J R Brobeck (Ed) Physiological Basis o f Medical Practice p 2 to 91. Williams and Wilkins, Baltimore, London. 4. Caverzasio J,JP, Bonjour and H Fleish 1982. Tubular handling of Pi in young growing and adult rats. A m J Physio 242:F705. 5. Evans JW, A Borton, HF Hintz and LD VanVleck 1977. The Horse WH Freeman and Co., San Francisco. 6. Harrington DD, C Marroquia and V White 1971. Experimental magnesium deficiency in horses. J Anim Sei 7. Hintz HF and HF Schryver 1973. Magnesium, calcium and phosphorus metabolism in ponies fed varying levels of magnesium. J Anim Sci 37:927. 8. Hintz HF and HF Schryver 1972. Magnesium metabolism in the horse. J Anita Sci 35:755. 9. Hintz HF, HF Schryver, J Duty, C Lakin and RA Zimmerman 1984. Oxalic acid content of alfalfa hays and its influence on the availability of calcium phosphorus and magnesium to ponies. J Anita Sci 58:939. 10. Hintz HF, DE Hogue, EF Walker Jr, JE Lowe and HF Sehryver 1971. Apparent digestion in various segments of the digestive tract of ponies fed diets with varying roughage-grain ratios. J Anim Sci 32:245. 1I. Jordan RM, VS Myers, B Yoho and FA Spurrel11973. A note on calcium and phosphorus levels fed ponies during growth and reproduction. In: Proc 3rd Eq Nutr and Phys Syrup Univ Florida. Gainesville, Fla. 12. Kichura TS, HF Hintz and HF Schryver 1983. Factors influencing endogenous phosphorus losses in ponies. In: Proc 8th Eq Nutr Res Syrup Univ Kentucky, Lexington, Ky. 13. Krook L, and JE Lowe 1964. Nutritional secondary hyperparathyroidism in the horse. Pathol Vet 1 Suppl. 14. Leib S and JP Baker 1975. Effect of high calcium intake on phosphorus metabolism. In: Proc 4th Eq Nutr Res Syrup Cal Poly Pomona, CA. 15. Moore JH and C Tyler 1955. Studies on the intestinal absorption and excretion of calcium and phosphorus in the pig. Brit J Nutr 9:63. 16. NRC 1978.Nutrient Requirements o f Domestic Animals No 6. Nutrient Requirements o f Horses (4th Revised Ed) National Academy of Sciences - National Research Council. Washington DC. 17. Nelson WG, JP Baker and RE Tucker 1977. Influence of lactic acid on calcium absorption by ponies. In: 5th Eq Nutr and Phys Syrup Univ Missouri, Columbia, MO. 18. Nielson AJ 1972. Deposition of calcium and phosphorus in growing pigs determined by balance experiments and slaughter investigations. Acta Agr Scand 22:223. 19. EA and RL Asquith 1983. Influence of protein and mineral intake on growth and bone development of weanling horses. In: Proc 8th Eq Nutr and Phys Syrnp Univ Kentucky, Lexington, KY. 20. Pfeffer E, A Thompson and DG Armstrong 1970. Studies on intestinal digestion in the sheep. Brit J Nutr 24:197. 21. Reiss E and JM C a n t e r b u r y 1971. Genesis of hyperparathyroidism. Amer J Med 50:679. 22. Schryver H F, P H Craig and H F Hintz !970. Calcium metabolism in ponies fed verying levels of calcium. J Nutr 100:955. 23. Schryver HF and HF Hintz 1972. Calcium and phosphorus requirements of the horse: A review. Feedstuffs.July 10 p 35. 24. Schryver, HF, HF Hintz, and PH Craig 1971a. Calcium metabolism in ponies fed a high phosphorus diet. J Nutri 101:259. 25. Schryver HF, HF Hintz and PH Craig 1971b. Phosphorus metabolism in ponies fed varying levels of phosphorus. JNutr 101:1257. Continued
213
26. Schryver HF, HF Hintz, PH Craig, DE Hogue and JE Lowe 1972. Site of phosphorus absorption from the intestine of the horse. J Nutr 102:143. 27. Schryver HF, HF Hintz and JE Lowe 1974a. Calcium and phosphorus in the nutrition of the horse. Cornell Vet 64:494. 28. Schryver HF, HF Hintz, JE Lowe, RL Hintz, RB Harper and JT Reid 1974b. Mineral composition of the whole body, liver and bone of young horses. J Nutr 104:126. 29. Ward G, LH Harbers and JJ Blake 1979. Calcium-containing crystals in alfalfa. Their fate in cattle. J Dairy Sci 62:715. 30. Whitlock R, HF Schryver, L Krook, HF Hintz and PH Craig 1970. Calcium metabolism in horses with a varied calcium intake. Federation Proc 29:565 (abstr). 31. Young VR, GP Lofgreen and JR Luick 1966. The effects of phosphorus depletion, and of calcium and phosphorus intake, on the endogenous excretion of these elements by sheep. Brit J Nutr 20:795. Bruceila Abortus
Continued from page 199
REFERENCES
ANNOUNCING..... Step-by-step depiction of the gross anatomy and its relationship to the ultrasonic anatomy of the horse embryo and placental membranes Includes a graph for estimating the age of the conceptus by ultrasound measurements
Shipped unfolded in a rigid cylinder
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1. Collins, JD, Kelley, WR, Twomey, T, Farrelly, BT, Whitty, BT: 8rucella-associated vertebral osteomyelitis in a Thorughbred mare. Vet Rec88:321-326, 1971. 2. Cosgrove, JS: Clinical aspects of equine brucellosis. Vet Rec73: 137%1382, !961. 3. Denny, HR: A review of brucellosis in the horse.Eq Vet J 5: 121125, 1973. 4. Duff, HM: Brucella abortus in the horse. JComp Path therap 50: 151-158, 1937. 5. Gardner, GR, Nicoletti, P, Scarratt, WK: Treatments for brucellosis in horses by Florida practitioners. FL VetJ 12: 21-22, 1983. 6. Gilyard, AT, Gilyard, RT: Vaccinal therapy in fistulous withers. J A m Vet Med Assoc. 102: 204-207, 1943. 7. Millar, R: Diseases of the ligamentum nuchae associated with 8rucella abortus infection in the horse, and its treatment with strain 19 vaccine. Brit Vet J I18: 16%171, 1961. 8. Schalm, OW, Jain, NC, Carroll, EJ: Veterinary Hematology. 3rd Ed, 1975, Lea and Febiger, Philadelphia. 9. Steward, JS: Fistulous withers and poll evil. Vet Rec 15: 15631573, 1935.
ULTRASONIC A N A T O M Y OF THE EQUINE EMBRYO BY O.J. GINTHER, DVM, PhD. UNIVERSITY OF WISCONSIN
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EQUINE VETERINARY SCIENCE
SUMMARY Mineral metabolism in the equine is a complex process involving absorption sites and interactions between the minerals themselves and other compounds. Factors such as the digestibility of the minerals and the extent of the interactions all play an important role in the mineral amounts required in the diet. Inadequate levels of minerals in the diet can lead to physiological maladies s u c h as H N S a n d R i c k e t t s . A k n o w l e d g e a n d understanding of mineral metabolism is essential to the success of equine management.
INTRODUCTION Calcium, phosphorus and magnesium are all essential macro minerals, necessary for proper bone growth and development in horses) 3 Because the horse is used as an athlete, sound bone structure is essential. There are many factors that influence mineral metabolism in the horse and a basic understanding of these factors is essential. This paper will deal with absorption, interactions between Ca, P and Mg as well as interactions with other compounds such as lactic acid and oxalic acid. The dietary requirements of each mineral and the effects of imbalances will also be conveyed.
Phosphorus Schryver 23 found that the large intestine was the major site of P absorption in the horse. This is contrary to results found in ruminants 2° and swine where the small intestine is the primary site of absorption. They suggest that the high levels of phosphate and bicarbonate found in the cecum and colon serve as a buffer for the volatile fatty acids produced by microfloral fermentation. Alexander suggested that phosphate may be the major buffer system in the dorsal colon where the concentration of bicarbonate falls and phosphate rises. 23
Magnesium In one study, s the cumulative apparent absorption of Mg was measured f r o m different regions of the intestine in ponies. Various diets containing different forage: grain ratios were fed and absorption calculated by the Cr203: nutrient ratio technique. ~° They found that 25% of the Mg present was absorbed before the digestability reached the jejunum and 46 to 56% was removed before the cecum. Little Mg was absorbed in the large intestine. The total Mg digestibility was found to be 52% in the high concentrate diet and 61% in the high forage diet. The type of diet had no significant affect on the site of absorption, s
INTERACTIONS
MINERAL ABSORPTION Calcium is actively absorbed by the horse in the small intestine in a two step process: from the lumen into the mucosa, and from the mucosa to the underlying tissues. Both steps require the presence of vitamin D and the latter is dependent on oxidative phosphorylation. It is thought that 25-hydroxycholecalciferol (a steroid) induces the synthesis of a calcium-binding protein in the duodenal mucosa. Parathormone plays a facilatory role in Ca absorption. 3 Author's address: Dept. Animal Science, VA Tech and State Univ., Blackshurg, VA.
210
Calcium In the equine, as well as other species, mineral absorption and metabolism are affected by various i n t e r a c t i o n s a n d i n t e r r e l a t i o n s h i p s b e t w e e n these inorganic substances. These may include the effects of the levels of each mineral in the diet, on the metabolism of the given mineral, as well as on the associated minerals. Other compounds, such as oxalic acid, and lactic acid can have effects on mineral absorption and metabolism. An understanding of these relationships is important when formulating diets for the horse, particularly in the growing animal. EQUINE VETERINARY SCIENCE
Calcium
- Calcium
Schryver, 22 studied the effects of Ca intake on skeletal metabolism and Ca homeostatic mechanisms in young ponies. They reported that all ponies maintained normal levels of plasma Ca when fed varying levels of Ca (1.50, .80 and .15% of the diet). The intermediate level of Ca (.80%) was about equal to that recommended by the NRC (1978). The ponies responded to the dietary levels of Ca in such a way as to maintain normal concentration of plasma Ca. When fed low levels (15%) of Ca, intestinal absorption increased, renal excretions decreased and Ca was removed from the bone at a rate greater than that which it was deposited. These ponies displayed a negative Ca balance. Opposite responses were observed in ponies fed the high level (1.5%) diet. In this experiment, the skeletal removal of calcium from bone responded directly to the homeostatic needs of the animal. In a similar test, Whitlock, 30 studied the effects of high calcium intake in horses and noted that, on a body weight basis, response of all horses were similar. In a trial conducted by Schryver, 28 the mineral composition of the whole body, liver and bone of young horses was analyzed. They reported that the calcium content in the body of the horse was significantly affected by the dietary levels of Ca. However, when analyzing the skeletal mass, they found little difference in the weight of bones relative to body weight and that the specific gravity was not significantly higher in horses fed high calcium diets. They also found that the Ca content of the bone ash was uneffected by diet. These data suggest that dietary levels of Ca are not important, in terms of body composition, to the development of bone. It is obvious that these findings lead to discrepancies between whole body composition studies and balance studies discussed earlier. These discrepancies have been noted before. Nielson TM noted that pigs at slaughter contained 15% less Ca and 39% less P than d e t e r m i n e d by balance experiments. Schryver 28 felt that these differences could be a result of two factors. The first is the inherent cumulative errors in balance studies which tend to inflate the balance. The second might be that balance studies will detect short term effects that will not effect whole body composition. Calcium
- Phosphorus
Lieb TM found that high Ca intake also affected P metabolism. In the balance trial, they fed two diets containing .78 and 2.0% Ca to 6 ponies, and measured metabolic and total phosphorus losses. Total fecal phosphorus excreted was significantly increased with increased Ca in the diet. Urinary phosphorus excretion and P absorption were decreased as the level of dietary Ca was raised from .78 to 2.0%. The metabolic fecal phosphorus was found to be maintained at a constant level (6.5 m g / k g BW/d) despite the dietary intake of Ca. They reported that true digestability of P decreased from 61.4 to 34.2% when Ca was increased from .78 to 2.0%. This occurred primarily because of the increasing loss of dietary P in the feces. They also found that high Ca intake (2.0%) tended to depress plasma P levels. In a trial conducted by Kichura Z2, diets containing various levels of Ca and P were fed to 24 mature ponies. Volume 4, Number 5
They found that the amount of dietary Ca influenced only slightly, the ability of ponies to decrease urinary P. There was an exception to this seen in yearlings. When yearlings were fed a low Ca and low P diet, they excreted significantly higher levels of urinary P than yearlings fed a high Ca, low P diet. The authors offered no explanation for this observation. Calcium
- Magnesium
Schryver 2g saw in their analysis of whole body composition that the levels of Ca fed had no influence on the whole body or liver content of magnesium. Again, this is contrary to findings of balance studies that suggest t h a t high Ca diets d e c r e a s e d the u t i l i z a t i o n of magnesium. Phosphorus
- Phosphorus
Various studies have also been conducted to determine the effects of phosphorus levels on the metabolism of minerals. Schryver 25 fed ponies, ranging in age from 6 mo to 2.5 y, varying levels of P to determine the effects on P metabolism. They discovered that renal excretion of P was the most important means of maintaining P metabolism in ponies. Renal excretion of P was low when dietary levels were low and increased as dietary levels increased. They also found that endogenous fecal P excretion did not vary with dietary levels. These findings are contrary to findings in ruminants and rats where fecal endogenous losses were dependant on the P level in the diet. In ruminants, P levels in urine tend to be consistently low and unaffected by diet, and endogenous f e c a l e x c r e t i o n is t h e m e t h o d f o r m a i n t a i n i n g homeostasis. The concentration of plasma P was affected only at very high levels (1.19% of diet) of P in the diet. In another study concerning factors influencing endogenous P losses in ponies, Kichura ~2 found that ponies absorbed higher percentages of dietary P as levels increased. However, they felt that this increase may have been attributable to the form of P in the diet. In their low P diets, corn was used as the major source of P. Phosphorus in corn is in the form of phytate P, which is less available for ponies. The P in the high level diets was primarily monosodium P which is readily available to the equine. Kichura ~2 also found that efficiency of P absorption decreased as age increased, especially in low P level diets. They also found that urinary excretion was extremely low when ponies were fed low levels of P. However, contrary to results found in rats 4 older ponies decreased P excretion in urine more effectively than did the young ponies. Across diets, endogenous fecal P and plasma P concentrations decreased with increasing age. It was also noted that plasma concentrations were poor predictors of endogenous fecal P levels in aged ponies, yearling and weanlings. Linear regression analysis showed R values of -.02, -.76 and -.65 for aged, yearling and weanling ponies, respectively. This study tended to contradict the results found by Schryver 25concerning the effect of dietary levels on endogenous fecal P loss. In this study, high dietary levels of P tended to decrease endogenous fecal P although these differences were not significant. It seemed that age of the pony set the general level of endogenous fecal phosphorus, and may be 211
modified by factors such as dietary Ca and P. Reviewing the whole body composition study done by Schryver 2s, dietary P levels did not influence levels in bone ash. However, in another study, 24 it was shown that high P diets accelerated bone turnover even when calcium levels were inadequate for balance. P h o s p h o r u s - Calcium
In a trial studying the effects of high phosphorus diets on Ca metabolism in ponies, it was shown that a high phosphate diet depressed the intestinal absorption of Ca. 42 In similar studies done with humans, conflicting results have been seen. These findings may be the result of the Ca demand of the subjects and the of supplemental P. Endogenous fecal excretion of Ca increased in ponies fed high (1.19% of diet) levels of P despite decreased Ca concentrations. These changes were very small and their significance uncertain. The amount of Ca retained in the high P diet did not differ from those receiving the low P diet (.20% of diet). Renal excretion of Ca was found to be depressed in ponies on the high P diet. This led the authors to believe that the lower renal excretion of Ca (than would be expected if excretion were considered a function of absorbed calcium) is a result of decreased plasma concentration. This decreased plasma concentration of Ca was consistently seen in ponies fed high concentrations of P. The lowered plasma Ca may have induced parathormone secretion, although blood Ca never returned to normal levels throughout the 30-day trial. This elevated level of p a r a t h o r m o n e was further suggested by an increase in the rate of calcium removal from bone. However, in all but one animal, increased calcium deposition in the bone accompanied the elevated removal rate, thus the net bone formation was dependant on the Ca content of the diet. Long-term studies involving high phosphorus intake have shown that this diet is detrimental to the skeleton of the horse. 24
absorption. Three diets containing none, 100 meg and 400 meg of lactic acid were fed. Mean true digestion coefficients for Ca were 47.14-+1.86, 49.11-+.63 and 52.41 _+2.03 for the control, low lactic acid and high lactic acid diets, respectively. These differences were not significantly different, although the difference between the control and high lactic acid treatment approached significance (P less than .06). Oxalic acid is the primary toxic substance in plants such as rhubarb (Rheum rhaponeticum), halogeton (Halogeton golmeratus), greasewood (Sacrobatus vermiculatus) and soursob (Oxalis cernva). Oxalic acid binds with divalent cations, such as Ca and Mg and thereby decreases the utilization of the minerals. 9 Ward 29 noted that 20 to 33% of the Ca in alfalfa is in the oxalate form and impaired utilization in ruminants. This poses a potentially serious threat to horse producers who rely on alfalfa as a Ca source. In the trial conducted by Hintzfl 9 two diets were fed with Ca: oxalic acid ratios 1.7 to 3.0 in alfalfa hays from differing sources. Metabolism studies were conducted with eight mature ponies being fed 33% hay and 67% oats. No significant differences were found between the diets in the true digestibility of Ca, P or Mg. It was concluded that the alfalfa hays in this study were excellent sources of Ca for ponies.
MINERAL
REQUIREMENTS TABLE 1.
Estimate of requirements of calcium, phosphorus and magnesium for horses with mature weight of 500 kg. Class
Percentage in diet Ca P Mg
Grams/day Ca P Mg
Weanlings
38
25
5
.70
.45
.1
Yearlings
35
21
6
.60
.35
.1
Mature horses
22
14
5
.35
.25
.09
Pregnant mares
30
23
7
.40
.30
.09
Lactating mares Source: NRC, 1978
50
36
9
.50
.35
.09
Magnesium - Calcium/Phosphorus
Magnesium has also been shown to interact with both Ca and P. Hintz 7 fed varying levels of magnesium (. 16, .31 and .86%) to mature ponies and evaluated the effects on Mg, Ca and P metabolism. They reported that Mg levels had a d iphasic response to Ca absorption. Both a Mg deficiency or high levels of Mg enhance Ca absorption. Phosphorus absorption was not affected by increased Mg levels. Although high dietary levels of Ca or P could presumably induce a Mg deficiency, high magnesium levels did not induce a Ca or P deficiency in this study. It was also shown that endogenous urinary loss of Mg was greater than endogenous fecal loss, and that increased urinary excretion of Mg with an increase of Mg in the diet suggests that the kidney is important in Mg homeostasis. NON-MINERALS INTERACTIONS Other non-mineral substances can have effects on mineral metabolism. Two such substances, lactic acid and oxalic acid, have been studied in balance trials in horses. In a study by Nelson, 17 six pony geldings were used to determine the effect of dietary lactic acid on Ca 212
Calcium and P h o p h o r u s
Requirements of Ca and P have been studied quite extensively in the last decade. Because the horse's primary function is that of an athlete, proper bone development and maintenance is essential to its success. The most important period for proper Ca:P levels in the diet is during the first two years of the horses life. This fact is reflected in that the dietary requirements of the young foal are relatively high at birth and decrease with i n c r e a s i n g a g e ( N R C , 1978). T h i s d e c r e a s e in requirements from birth is due to the decreasing growth potential with age. At 12 months, the foal will have already accumulated 94% of the ash (including 98% of the Ca and 94% of the P) that it will have as a two-year-old (Ott and Asquith, 1983). Realizing that 99% of the Ca and 80% of the P found in the body is contained in the skeleton (Schryver et al., 1974b), the importance of EQUINE VETERINARY SCIENCE
p r o p e r m i n e r a l n u t r i t i o n for the y o u n g horse b e c om es m o r e evident. Ca:P Ratios The C a : P ratio has been a s o u r c e of m u c h debate a m o n g horse owners a n d m a n a g e r s . T h e ' i d e a l ' range seemed t o fall within th e 1. h 1 or 2:1 range. 2~ While this range does seem to meet the r e q u i r e m e n t s f o r p r o p e r g r o w t h a n d m a i n t a i n a n c e o f b o n e in all horses, the possibility o f m i x i n g diets a n d f o r m u l a t i o n t o t a l diets that c o n s i s t e n t l y fall within this r a n g e is questionable. Variations in feeds a n d pastures f r o m season to season and year to year can be expected. F u r t h e r m o r e this C a : P ratio is s e l d o m identical to t h a t w h i c h the minerals are a b s o r b e d by the intestines or m a d e available to the tissues. 26
According to work done at Cornell University by Schryver? 6 the horse can tolerate a range of ratios very well. They had raised several groups of horses from 6 mo to 1 1/2 years with diets containing ratios from h 1 to 4:1 without adverse effects. They stressed that the real importance is the absolute levels of these minerals fed in the diet, as long as they met the minimum requirements of each and that P levels do not exceed Ca levels. In an experiment conducted by J o r d a n J z the effects of the Ca:P ratio were explored further. In their trial, 30 four-month old ponies were fed diets containing ratios from 2:1 to 6:1 for a period of 3 years. Calcium intake was 4 to 5 times that required. They found that there were no differences in height, average gain or reproductive performance between groups. Effects o f C a : P I m b a l a n c e s As was discussed earlier, excessive P inhibits Ca a bsorpt i o n . Diets high in P and (or) lo w in Ca have been shown to induce parathyroid hypertrophy and hyperplasia a c c o m p a n i e d by increased b o n e resorption. 2~ This m e t a b o l i c b o n e disorder is k n o w n as n u t r it io n al secondary hyperparathyroidism (NSH). 2 During NSH, high dietary P elevates serum p h o s p h a t e a n d depresses se rum C a content. N o r m a l c o n c e n t r a t i o n o f serum Ca is essential. F o r m e t a b o l i c f u n c t i o n s such as e n z y m e activity, n e u r o m u s c u l a r irritability a n d b l o o d clotting, therefore Ca is r e m o v e d f r o m the b o n e a n d m o b il iz e d in the b l o o d to m a i n t a i n p r o p e r levels. This c o n t i n u o u s decalcification o f the bone can lead to transitory lameness and enlarged facial bones. 23 R e v i e w i n g the findings o f J o r d a n , it was f o u n d that excessive C a in the diet was not d e t r i m e n t a l to the ponies. They saw t h a t ponies on the high Ca diets c o n s u m e d m o r e water a n d that their pens were c o n s t a n t l y wetter than those c o n s u m i n g n o r m a l level o f Ca. H e suggested t h a t this was due to excess renal f u n c t i o n to eliminate excess Ca.
C A , P, M g D e f i c i e n c i e s D u e to C a an d P roles in b o n e f o r m a t i o n and m a i n t a i n a n c e , low levels o f either o f these tw o minerals c a n c a u s e t h e o n s e t o f r i c k e t t s . R i c k e t t s is t h e m a l f o r m a t i o n o f bones due to i n a d e q u a t e calcification and is generally f o u n d in y o u n g animals. 5
Volume 4, Number 5
Deficiency signs o f M g include hyperirritability, glazed eyes, tetany, and e v e n t u a l collapse. This c o n d i t i o n is often precipitated by stress, such as in the sale barns o f large auctions. T h e d i s o r d e r seems to respond to M g - Ca injections. In y o u n g foals, M g deficiency c a n lead to calcification o f the b l o o d vessels. 6
REFERENCES 1. Alexander F 1963. Digestion in the horse. In: DP Cuthbertson (Ed) Progress in Nutrition and Allied Sciences p 259. Oliver and Boyd, London. 2. Argenzio RA, JE Lowe, HF Hintz and HF Schryver 1974. Calcium and phosphorus homeostasis in horses. J Nutr 104:18. 3. Best and Taylor 1978. Secretion, digestion, and absorption in the intestine. In: J R Brobeck (Ed) Physiological Basis o f Medical Practice p 2 to 91. Williams and Wilkins, Baltimore, London. 4. Caverzasio J,JP, Bonjour and H Fleish 1982. Tubular handling of Pi in young growing and adult rats. A m J Physio 242:F705. 5. Evans JW, A Borton, HF Hintz and LD VanVleck 1977. The Horse WH Freeman and Co., San Francisco. 6. Harrington DD, C Marroquia and V White 1971. Experimental magnesium deficiency in horses. J Anim Sei 7. Hintz HF and HF Schryver 1973. Magnesium, calcium and phosphorus metabolism in ponies fed varying levels of magnesium. J Anim Sci 37:927. 8. Hintz HF and HF Schryver 1972. Magnesium metabolism in the horse. J Anita Sci 35:755. 9. Hintz HF, HF Schryver, J Duty, C Lakin and RA Zimmerman 1984. Oxalic acid content of alfalfa hays and its influence on the availability of calcium phosphorus and magnesium to ponies. J Anita Sci 58:939. 10. Hintz HF, DE Hogue, EF Walker Jr, JE Lowe and HF Sehryver 1971. Apparent digestion in various segments of the digestive tract of ponies fed diets with varying roughage-grain ratios. J Anim Sci 32:245. 1I. Jordan RM, VS Myers, B Yoho and FA Spurrel11973. A note on calcium and phosphorus levels fed ponies during growth and reproduction. In: Proc 3rd Eq Nutr and Phys Syrup Univ Florida. Gainesville, Fla. 12. Kichura TS, HF Hintz and HF Schryver 1983. Factors influencing endogenous phosphorus losses in ponies. In: Proc 8th Eq Nutr Res Syrup Univ Kentucky, Lexington, Ky. 13. Krook L, and JE Lowe 1964. Nutritional secondary hyperparathyroidism in the horse. Pathol Vet 1 Suppl. 14. Leib S and JP Baker 1975. Effect of high calcium intake on phosphorus metabolism. In: Proc 4th Eq Nutr Res Syrup Cal Poly Pomona, CA. 15. Moore JH and C Tyler 1955. Studies on the intestinal absorption and excretion of calcium and phosphorus in the pig. Brit J Nutr 9:63. 16. NRC 1978.Nutrient Requirements o f Domestic Animals No 6. Nutrient Requirements o f Horses (4th Revised Ed) National Academy of Sciences - National Research Council. Washington DC. 17. Nelson WG, JP Baker and RE Tucker 1977. Influence of lactic acid on calcium absorption by ponies. In: 5th Eq Nutr and Phys Syrup Univ Missouri, Columbia, MO. 18. Nielson AJ 1972. Deposition of calcium and phosphorus in growing pigs determined by balance experiments and slaughter investigations. Acta Agr Scand 22:223. 19. EA and RL Asquith 1983. Influence of protein and mineral intake on growth and bone development of weanling horses. In: Proc 8th Eq Nutr and Phys Syrnp Univ Kentucky, Lexington, KY. 20. Pfeffer E, A Thompson and DG Armstrong 1970. Studies on intestinal digestion in the sheep. Brit J Nutr 24:197. 21. Reiss E and JM C a n t e r b u r y 1971. Genesis of hyperparathyroidism. Amer J Med 50:679. 22. Schryver H F, P H Craig and H F Hintz !970. Calcium metabolism in ponies fed verying levels of calcium. J Nutr 100:955. 23. Schryver HF and HF Hintz 1972. Calcium and phosphorus requirements of the horse: A review. Feedstuffs.July 10 p 35. 24. Schryver, HF, HF Hintz, and PH Craig 1971a. Calcium metabolism in ponies fed a high phosphorus diet. J Nutri 101:259. 25. Schryver HF, HF Hintz and PH Craig 1971b. Phosphorus metabolism in ponies fed varying levels of phosphorus. JNutr 101:1257. Continued
213
26. Schryver HF, HF Hintz, PH Craig, DE Hogue and JE Lowe 1972. Site of phosphorus absorption from the intestine of the horse. J Nutr 102:143. 27. Schryver HF, HF Hintz and JE Lowe 1974a. Calcium and phosphorus in the nutrition of the horse. Cornell Vet 64:494. 28. Schryver HF, HF Hintz, JE Lowe, RL Hintz, RB Harper and JT Reid 1974b. Mineral composition of the whole body, liver and bone of young horses. J Nutr 104:126. 29. Ward G, LH Harbers and JJ Blake 1979. Calcium-containing crystals in alfalfa. Their fate in cattle. J Dairy Sci 62:715. 30. Whitlock R, HF Schryver, L Krook, HF Hintz and PH Craig 1970. Calcium metabolism in horses with a varied calcium intake. Federation Proc 29:565 (abstr). 31. Young VR, GP Lofgreen and JR Luick 1966. The effects of phosphorus depletion, and of calcium and phosphorus intake, on the endogenous excretion of these elements by sheep. Brit J Nutr 20:795. Bruceila Abortus
Continued from page 199
REFERENCES
ANNOUNCING..... Step-by-step depiction of the gross anatomy and its relationship to the ultrasonic anatomy of the horse embryo and placental membranes Includes a graph for estimating the age of the conceptus by ultrasound measurements
Shipped unfolded in a rigid cylinder
214
1. Collins, JD, Kelley, WR, Twomey, T, Farrelly, BT, Whitty, BT: 8rucella-associated vertebral osteomyelitis in a Thorughbred mare. Vet Rec88:321-326, 1971. 2. Cosgrove, JS: Clinical aspects of equine brucellosis. Vet Rec73: 137%1382, !961. 3. Denny, HR: A review of brucellosis in the horse.Eq Vet J 5: 121125, 1973. 4. Duff, HM: Brucella abortus in the horse. JComp Path therap 50: 151-158, 1937. 5. Gardner, GR, Nicoletti, P, Scarratt, WK: Treatments for brucellosis in horses by Florida practitioners. FL VetJ 12: 21-22, 1983. 6. Gilyard, AT, Gilyard, RT: Vaccinal therapy in fistulous withers. J A m Vet Med Assoc. 102: 204-207, 1943. 7. Millar, R: Diseases of the ligamentum nuchae associated with 8rucella abortus infection in the horse, and its treatment with strain 19 vaccine. Brit Vet J I18: 16%171, 1961. 8. Schalm, OW, Jain, NC, Carroll, EJ: Veterinary Hematology. 3rd Ed, 1975, Lea and Febiger, Philadelphia. 9. Steward, JS: Fistulous withers and poll evil. Vet Rec 15: 15631573, 1935.
ULTRASONIC A N A T O M Y OF THE EQUINE EMBRYO BY O.J. GINTHER, DVM, PhD. UNIVERSITY OF WISCONSIN
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EQUINE VETERINARY SCIENCE