Vitamin D Metabolites in Plasma of Cows Fed a Prepartum Low-Calcium Diet for Prevention of Parturient Hypocalcemia1

Vitamin D Metabolites in Plasma of Cows Fed a Prepartum Low-Calcium Diet for Prevention of Parturient Hypocalcemia 1 H. B. G R E E N , a R. L. H O R S T , a D . C. B E I T Z , 4 and E. T. L I T T L E D I K E 3 Department of Animal Science Iowa State University and National Animal Disease Center Ames 50011 ABSTRACT

calcium, phosphorus, magnesium, 1,25dihydroxyvitamin D, and hydroxyproline were seen during first few days after initiation of feeding low calcium. Thus, we propose that the preventative action of the low-calcium diet is associated with preparation of the calcium homeostatic mechanism several days before the calcium demand o f initiation of lactation.

Our objective was to characterize changes in vitamin D metabolites of plasma in Jersey cows fed a prepartum low-calcium diet. Eight cows were fed a high-calcium diet (80 g/day) and eight were fed a low-calcium diet (8 g/day) at least 14 days before parturition. Calcium concentrations in plasma decreased after initiation of feeding either diet, but cows fed low-calcium diet tended to have lower prepartum calcium and phosphorus and greater peripartal calcium in plasma. Hydroxyproline in plasma was greater during peripartal period in cows fed low-calcium diet. Prepartum 1,25-dihydroxyvitamin D in plasma tended to be greater in cows fed low calcium. Increases in 1,25-dihydroxyvitamin D were only 2 and 3 days after initiation of the lowcalcium diet; during the first 2 days after parturition, however, 1,25-dihydroxyvitamin D tended to be lower in those cows fed low calcium. As parturition neared, 24,25-dihydroxyvitamin D tended to be lower in cows fed the low calcium-diet. Usual early postpartum changes in

INTRODUCTION

Received April 21, 1980. xJournal Paper No. J-9822 of the Iowa Agriculture and Home Economics Experiment Station, Ames. Project 2185. This project was supported in part under Broad Forum Cooperative Agreement No. 12-14-3001-533 between Iowa State University and the National Animal Disease Center, Agricultural Research, Science and Education Administration, US Department of Agriculture. 2Lilly Research Laboratories, Division of Eli Lilly and Company, Greenfield Laboratories, P.O. Box 708, Greenfield, IN 46140. 3National Animal Disease Center. 4 Department of Animal Science. 1981 J Dairy Sci 64:217-226

Many dietary measures to prevent parturient paresis have been suggested. Boda and Cole (3) suggested a low-calcium and high-phosphorus diet to prevent parturient paresis. Their proposed prevention diet of oats, hay, ground barley, a n d monosodium phosphate did not meet the daily requirements for energy and protein. Another worker studied the effect of low-calcium diets fed during the prepartum period (37). Goings et al. (13) suggested a low-calcium diet of corn silage and a concentrate mixture (primarily shelled corn) that met the daily requirements of 450-kg preparturient Jersey cows for energy, protein, and phosphorus. Their low-calcium diet, when fed from 10 days to 2 wk prepartum, effectively prevented parturient paresis in Jersey cows in the Iowa State University dairy herd and in Jersey cows in several other Iowa dairy herds (38). Because little is known about the biochemical mechanism by which low-calcium diets prevent parturient paresis, this study was o f the role o f the low-calcium diet in controling (a) calcium mobilization from bone and (b) the concentration of vitamin D metabolites in blood plasma. Understanding the biochemical mechanism of prevention of parturient paresis by the low-calcium diet may lead to new knowledge about the etiology of parturient paresis and, thus, contribute to development o f

217

218

GREEN ET AL.

effective alternative methods parturient paresis.

of preventing

M A T E R I A L S A N D METHODS Animals

Sixteen pregnant Jersey cows (second lactation or later) from the Iowa State University dairy herd were divided initially into four groups. Assignment to each of four groups was on age, milk production, and history o f parturient paresis. As detailed later, the four groups of cows subsequently were combined into highcalcium and low-calcium groups. Eight cows were in each of the two groups. Two cows in each group were used for two successive parturitions, giving a total of 10 parturitions in each treatment group. Four cows in each group had parturient paresis during their previous lactations (Table 1). Prepartum Diets

The pregnant dry cows were fed orchard grass hay ad libitum and 9.1 kg of corn silage top-dressed with .045 kg of monosodium phosphate daily during the initial part of the dry period. This ration contained approximately 30 g of calcium, 38 g of phosphorus, and 23 g of magnesium. Twenty-one days before expected date of calving, cows were placed into individual stalls and fed alfalfa hay and dicalcium phosphate ad libitum for 7 days, providing at least 120 g o f calcium, 24 g of phosphorus, and 32 g of magnesium daily. Then, cows were fed one of the following four experimental diets: a) high-calcium, high-magnesium, b) high-calcium,

low-magnesium, c) low-calcium, high-magnesium, or d) low-calcium, low-magnesium. The low-calcium, low-magnesium diet was the same diet proposed by Goings et al. (13) to prevent parturient paresis and consisted o f corn silage, ground shelled corn, molasses, urea, and monosodium phosphate. This diet provided approximately 8 g of calcium, 20 g of phosphorus, and 6 g of magnesium and met the daily requirements of energy, protein, and phosphorus for maintenance and reproduction of a 450-kg cow (27). The other three diets were formulated by supplementing the lowcalcium, low-magnesium diet with either magnesium oxide or calcium carbonate (ground limestone), or both. The high-calcium diets contained 80 g of calcium (180 g of supplemental calcium carbonate); the high-magnesium diets contained 22 g o f magnesium (27 g of supplemental magnesium oxide). The four experimental diets were fed to cows twice daily beginning 14 days before expected day of parturition. Cows were fed the low-calcium diets (low or high magnesium) an average of 13.9 days before parturition; cows were fed the high-calcium diets (low or high magnesium) an average of 14.1 days before parturition (Table 1). After parturition, all cows were fed ad libitum a diet of a grain mixture and alfalfa hay that met daily calcium requirements. Blood Sampling

Venous blood samples were collected daily by jugular puncture from 14 days before expected date of parturition to 3 days postpartum. Day 0 samples were collected as soon

TABLE 1. Characteristics of two experimental groups of cows.

Prepartum dieta

No. of cows

Average age of cows at parturition

History of milk fever

Time fed diet

Milk production b (305-day, ME,2X) Milk Fat

High-calcium Low-calcium

8 8

(months) 59 63

(no. of cases) 4 4

(days) 14.1 13.9

~ (kg) 5,568 252 5,706 263

aEach dietary group contained two subgroups of cows fed high- and low-magnesium diets. Two cows in each group were used for two successive parturitions. bMilk production during lactation before feeding experimental prepartum diets. Journal of Dairy Science Vol. 64, No. 2, 1981

VITAMIN D AND PARTURIENT HYPOCALCEMIA as possible after parturition. Heparin (200 units/10 ml of blood) was used to prevent clotting, and the plasma was separated by centrifugation and frozen at - 2 0 ° C until analyzed. Parturient Paresis Treatment

A sterile, 23% calcium borogluconate solution (containing magnesium) was used to treat paretic cows. Treatment was initiated when the cow was recumbent. Plasma Analyses

Calcium and magnesium concentrations in plasma were determined by atomic absorption spectrophotometry (39, 40) with a .5% solution of lanthanum chloride to reduce phosphate and sulfate interference. Inorganic phosphate concentrations in plasma were determined by the Fiske-Subbarow method (10) adapted for use with an Auto Analyzer (Technicon Instruments Corp., Ardsley, NY). Concentrations of plasma hydroxyproline were measured by the procedure of Tepper and DeVos (35) adapted for an Auto Analyzer. Determinations of concentrations of 25-hydroxyvitamin D, 1,25-dihydroxyvitamin D, and 24,25-dihydroxyvitamin D in plasma were according to procedures described by Horst et al. (18). The D 2 and D 3 forms of these metabolites were not separated.

219

Within each dietary calcium treatment, the concentrations of each of the measured plasma constituents were not altered (P>.05) by the amount of dietary magnesium. Therefore, for additional statistical analyses of the data, all cows were grouped into either a high-calcium (10 parturitions) or a low-calcium (10 parturitions) group (Table 1). The cows were fed the high-calcium diet an average o f 14.1 days and the low-calcium diet an average of 13.9 days. Milk production and average ages of cows in the two groups were similar. One case of parturient paresis was observed in the cows fed the high-calcium diet. Also, one case of parturient paresis was observed in the cows fed the low-calcium diet; however, this cow was fed the low-calcium diet for only 7 days, which is less than the recommended time for prevention of parturient paresis (13). Plasma Calcium

Calcium concentrations of cows fed a highor low-calcium diet prepartum are in Figures 1 and 2. Calcium concentrations in Figure 1 are expressed as a function of time after initiation of feeding of the experimental diets. Data for day 8 of Figure 1 (also of Figures 3 to 11) for the cow fed the low-calcium diet that calved

10"4I

Statistical Analyses

Experiment was analyzed by analysis of variance techniques for a 2 x 2 factorial design over nine time periods beginning from initiation of feeding test diets or 5 days before calving (32).

.~ 7:z RESULTS A N D DISCUSSION

The calcium, phosphorus, magnesium, hydroxyproline, 1,25-dihydroxyvitamin D, and 24,25-dihydroxyvitamin D concentrations in plasma of a series of samples from 20 Jersey cows were measured. Concentrations of each plasma constituent are plotted as a function of a) the number of days on each diet (Figures 1, 3, 5, 7, 9, 11) and b) the number of days prepartum and postpartum on each diet (Figures 2, 4, 6, 8, 10, 12). The cows were fed one of our prepartum diets containing low or high amounts of both calcium and magnesium.

o 6.4

5.6

4.8

Figure 1. Calcium concentrations of plasma in cows fed either a high-or low-calcium diet for 8 days during prepartal period. Plus or minus SE for each mean is indicated. Solid line ( ) represents data for cows fed low-calcium diet, and dashed line ( - - -) represents data for cows fed high-calcium diet. Day 0 represents day of change to experimental diets. Journal of Dairy Science Vol. 64, No. 2, 1981

220

GREEN ET AL.

before the predicted date of parturition was deleted. Although not statistically significant (P>.05), cows fed the low-calcium diet tended to have lower mean calcium concentrations in plasma than did cows fed the high-calcium diet. Calcium had not returned to initial control after 8 days of feeding the experimental diets. In similar studies, Goings et al. (13) also observed a decrease in calcium concentrations of plasma when cows were switched to a diet lower in calcium content; they, however, observed a return of plasma calcium to control amounts after 4 days of feeding the low-calcium diet. The initial decrease in plasma calcium of cows fed the high-calcium diet in our study may have been from changing from a diet high in calcium (120 g/day) to one lower in calcium (80 g/day). This magnitude of change in dietary content of calcium at the start of the experimental diet feeding was n o t in the studies of Goings et al. (13) and may explain the differences between the two studies. To show dietary effects on calcium concentration of plasma at parturition, calcium concentration was plotted as a function of days before and after parturition (Figure 2). Cows fed the high-calcium diet had a greater

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Time During

Perlpartal Per$od ( d a y s )

Figure 2. Calcium concentrations in plasma in cows during an 8-day peripartal period. Plus or minus SE for each mean is indicated. Day 0 represents day of parturition. Solid line ( ) represents data for cows fed low-calcium diet, and dashed line (- - -) represents data for cows fed high-calcium diet. Asterisk (*) represents significant differences (P<.05) in calcium concentrations for diets on indicated days. Journal of Dairy Science Vol. 64, No. 2, 1981

decrease (P<.05) in calcium of plasma at parturition and 1 day postpartum than those fed the low-calcium diet. Linmer (20) observed protection against hypocalcemia at parturition by feeding a similar low-calcium diet. A hypocalcemic response during the peripartal period commonly occurs in cows as lactation is initiated (9, 14, 24, 25, 28). Van Soest and Blosser (36) have reported that the hypocalcemia occurred about 30 h prepartum in either normal or paretic cows. Thus,. feeding of the low-calcium diet during the prepartum period results in a delayed and less severe hypocalcemia at parturition. Plasma Phosphorus

Concentrations of phosphorus in piasma of cows fed the high calcium initially increased (% 1 mg/100 ml) above prediet concentrations of phosphorus in plasma. This initial increase was probably a result of the inhibitory effects of high calcium diets on parathyroid hormone (PTH) secretions resulting in enhanced kidney resorption of phosphorus (21, 24, 30). During the next 7 to 8 days the phosphorus in plasma declined to prediet concentration, presumably due in part to the loss of plasma phosphorus to milk formation and fetal development. The plasma phosphorus of cows fed the low-calcium diet, however, declined linearly from 6.0 mg/100 ml to 4.6 mg/100 ml during the 8 days after initiation of feeding the lowcalcium diet (Figure 3). Statistically significant differences (P<.01 and P<.05) in phosphorus of cows fed the high- and low-calcium diets were noted from 3 to 6 days after initiation of feeding the two diets. These differences in phosphorus of plasma were not the result of differences in dietary intakes of phosphorus, because the phosphorus content of the two diets was similar. Other investigators (9, 19, 25, 36) reported that changes in phosphorus parallel those of calcium in plasma as parturition nears. The decrease in phosphorus of cows fed the low-calcium diet could be explained by the phosphaturic effects of parathyroid hormone (PTH) (24, 30, 31). As noted by Goings et al. (13 ), plasma PTH concentrations increase during the first 4 days after initiation of feeding a low-calcium diet. As in Figure 4, no differences in phosphorus of cows fed the high- and low-calcium diets

VITAMIN D AND PARTURIENT HYPOCALCEMIA

221

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Z.d

1,¢ Time on D i e t

(days)

Figure 3. Phosphorus concentrations in plasma in cows fed either a high- or low-calcium diet for 8 days during prepartal period. Plus or minus SE for each mean is indicated. Solid line ( ) represents data for cows fed low-calcium diet, and dashed line ( - - - ) represents data for cows fed high-calcium diet. Asterisks represent significant differences (*P<.05 ; "*P<.O1). Day 0 represents day of change to experimental diets.

were statistically significant d u r i n g t h e peripartal period. All cows s h o w e d the typical h y p o p h o s p h a t e m i a t h a t o t h e r s have associated with p a r t u r i t i o n (9, 19, 21, 36).

M a g n e s i u m c o n c e n t r a t i o n s in p l a s m a o f cows fed t h e t w o diets were similar d u r i n g t h e first 8 days a f t e r f e e d i n g test diets (Figure 5). Average c o n c e n t r a t i o n s d u r i n g t h e 8 days were 2.3 m g / l O 0 ml for cows fed t h e h i g h - c a l c i u m d i e t a n d 2.2 f o r t h o s e fed t h e l o w - c a l c i u m diet. T h e s e are similar to r e p o r t s b y o t h e r s f o r p r e g n a n t J e r s e y cows (1). Figure 6 d e m o n s t r a t e s t h a t n o d i e t a r y effects ( P > . 0 5 ) o n m a g n e s i u m in p l a s m a were significant d u r i n g t h e p e r i p a r t a l period. As s h o w n b y o t h e r s (12, 15, 19, 21, 22, 23, 26), m a g n e s i u m t e n d e d to increase in b o t h g r o u p s as p a r t u r i t i o n n e a r e d ; this increase was m o s t m a r k e d in t h e h i g h - c a l c i u m group. By 3 d a y s a f t e r p a r t u r i t i o n , m a g n e s i u m h a d r e t u r n e d to p r e p a r t u r a status. Plasma Hydroxyproline

As has b e e n suggested b y o t h e r s (2, 7), an increase in c o n c e n t r a t i o n o f h y d r o x y p r o l i n e in b l o o d p i a s m a is associated w i t h an i n c r e a s e d m o b i l i z a t i o n of calcium f r o m b o n e . T h e r e f o r e , h y d r o x y p r o l i n e c o n c e n t r a t i o n in p l a s m a was m o n i t o r e d a f t e r i n i t i a t i o n o f feeding t e s t diets (Figure 7) a n d d u r i n g t h e p e r i p a r t a l p e r i o d

3.0

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B o

v

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"~ z.z 4.0 ; z.o : 3.z o

l,l

214 l,|

1.6 Time

Time During P e r i p l r t a l Period (days)

Figure 4. Phosphorus concentrations in plasma in cows during an 8-day peripartal period. Plus or minus SE for each mean is indicated. Solid line ( ) represents data for cows fed low-calcium diet, and dashed line (- - -) represents data for cows fed highcalcium diet. Day 0 represents day of parturition.

on Diet

(days)

Figure 5. Magnesium concentrations in plasma in cows fed either a high- or low-calcium diet for 8 days during prepartal period. Plus or minus SE for each mean is indicated. Solid line (-) represents data for cows fed low-calcium diet, and dashed line ( - - - ) represents data for cows fed high-calcium diet. Day 0 represents day of change to experimental diets. Journal of Dairy Science Vol. 64, No. 2, 1981

222

GREEN ET AL. 4.0

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z.4

///

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1.6

1.2

Time on D i e t Ttme During P e r t p a r t i l

Period

(days)

Figure 6. Magnesium concentrations in plasma in cows during an 8-day peripartal period. Plus or minus SE for each mean is indicated. Solid line ( ) represents data for cows fed low-calcium diet, and dashed line (- - -) represents data for cows fed highcalcium diet. Day 0 represents day of parturition.

(Figure 8) to d e t e r m i n e w h e t h e r the two test diets and parturition alter m o b i l i z a t i o n of bone calcium. H y d r o x y p r o l i n e in plasma t e n d e d to be greater in cows fed the low-calcium diet for 8 days after initiation of feeding the test diets (Figure 7); the differences, however, were n o t statistically significant (P>.05). Mean h y d r o x y proline tended to increase during this 8 days in b o t h groups o f cows. Lintner (20), however, d e m o n s t r a t e d a significant difference in hyd r o x y p r o l i n e during a similar p r e p a r t u m period o f cows fed similar high- and low-calcium diets. F r o m our study, even though the low-calcium diet m e t only a p p r o x i m a t e l y one-third o f the daily r e q u i r e m e n t for dietary calcium, bone resorption and calcium mobilization were n o t increased m a r k e d l y by feeding the low-calcium diet during the p r e p a r t u m period. Dietary effects on h y d r o x y p r o l i n e , however, were d e m o n s t r a t e d during the peripartal period (Figure 8). H y d r o x y p r o I i n e o f cows fed either the high- or low-calcium diets were similar on days 5, 4, and 3, but f r o m 2 days p r e p a r t u m to 1 day postpartum, h y d r o x y p r o l i n e in cows fed the low-calcium diet was greater (P>.05 or P > . 0 1 ) . Thus, b o n e resorption and calcium mobilization f r o m bone seemed to be stimulated by the low-calcium diet before the time w h e n Journal of Dairy Science Vol. 64, No. 2, 1981

{days)

Figure 7. Hydroxyproline concentrations in plasma m cows fed either a high- or low-calcium diet for 8 days during prepartal period. Plus or minus SE for each mean is indicated. Solid line ( - - ) represents data for cows fed low-calcium diet, and dashed line ( - - - ) represents data for cows fed high-calcium diet. Day 0 represents day of change to experimental diets.

cows were m o s t likely to develop parturient paresis. Lintner (20) also had m a d e a similar observation.

"°I 3.6

~3.2

2.1!

2.4

2.0 m

P-T-T-V-T-

1.6

1.2

-2 -, Ttme During Perlpertal

o

;

~

;

Per(od (days)

Figure 8. Hydroxyproline concentrations in plasma in cows during an 8-day peripartal period. Plus or minus SE for each mean is indicated. Solid line ( ) represents data for cows fed low-calcium diet, and dashed line (- - -) represents data for cows fed highcalcium diet. Asterisks represent significant differences (*P<.05; **P<.01) in hydroxyproline concentrations for diets on indicated days. Day 0 represents day of parturition.

VITAMIN D AND PARTURIENT HYPOCALCEMIA 280

240 s

o

16G

%

E

dO

0

Tlme

on

Diet

(days)

Figure 9. 1,25-Dihydroxyvitamin D concentrations in plasma in cows fed either a high- or low-calcium diet for 8 days during prepartal period. Plus or minus SE for each mean is indicated. Solid line ( ) represents data for cows fed low-calcium diet, and dashed line (- - -) represents data for cows fed highcalcium diet. Asterisks (*) represent significant differences (P<.05). Day 0 represents the day of change to experimental diets.

An increased capacity for calcium mobilization from bone seems necessary for cattle to avoid development of parturient paresis.

223

Black and Capen (2) noted that urinary excretion of hydroxyproline increased during the last month of gestation in cows that did n o t develop parturient paresis while the a m o u n t of hydroxyproline in urine and plasma did n o t increase in cows that developed parturient paresis. From our study and other work (20, 41), cows fed a low-calcium diet prepartum may be capable of greater rates of calcium mobilization from bone during the period near parturition when calcium mobilization rates usually are too low to meet calcium needs (29). Thus, cows fed the low-calcium diet are more able to maintain higher calcium concentrations in plasma during parturition and initiation of lactation. This adaptation may play a major role in prevention of parturient paresis. Plasma 1,25-Dihydroxyvitamin D and 24,25-D ihydroxyvita min D

Concentrations of 1,25-dihydroxyvitamin D and 24,25-dihydroxyvitamin D during the peripartal period of cows fed high- and lowc a l c i u m d i e t s w e r e m e a s u r e d f o r t h e role o f s e l e c t e d v i t a m i n D m e t a b o l i t e s in t h e p r e v e n t i o n o f p a r t u r i e n t paresis. P l a s m a 1 , 2 5 - d i h y d r o x y v i -

280 4.2 24C

,6i

\¢

I

3.0 m 2.4

; 1.s 1.2

.6

Time on @tet (days) Ttme D u r l n g

Pertpartal

Period

(days)

Figure 10.1,25-Dihydroxyvitamin D concentrations in plasma in cows during an 8-day peripa~a] period. Plus or minus SE for each mean is indicated. Solid line ( ) represents data for cows fed low-calcium diet, and dashed line (- - -) represents data for cows fed high-calcium diet. Day 0 represents day o f parturition.

Figure 11. 2 4 , 2 5 - D i h y d r o x y v i t ~ i n D concentrations in plasma in cows fed either a high- or lowcalcium diet for 8 days during prepa~al period. Plus or minus SE for each mean is indicated. Solid line ( ) represents data for cows fed low-calcium diet, and dashed line (- - -) represents data for cows fed high-calcium diet. Day 0 represents the day o f change to the experimental diets.

Journal of Dairy Science Vol. 64, No. 2, 1981

224

GREEN ET AL.

tamin D o f cows fed the low-calcium diet increased (P<.05) on days 2 and 3 (Figure 9). There was a t e n d e n c y for this increase to be maintained t h r o u g h o u t this p r e p a r t u m period e x c e p t on day 6. F r o m w o r k with rats, the low-calcium diet, acting by way o f stimulation o f intestinal calcium-binding protein by 1,25d i h y d r o x y v i t a m i n D, probably increases the capacity of the cows to absorb dietary calcium (4, 5, 11). The h o r m o n a l form of vitamin D is synthesized from 2 5 - d i h y d r o x y v i t a m i n D in the kidney in response to h y p o c a l c e m i a and h y p o p h o s p h a t e m i a (4, 6, 8, 33), both o f which occurred when cows were fed the low-calcium diet (Figures 1 and 3). Either of these changes in plasma calcium or phosphorus may have been responsible for stimulating 1,25-dihydroxyvitamin D p r o d u c t i o n (16, 17). A f t e r parturition, the mean 1,25-dihyd r o x y v i t a m i n D concentrations in plasma of cows fed the high-calcium diet were greater, although the difference was n o t significant (P>.05) (Figure 10). As discussed earlier, cows fed the high-calcium diet had a more severe h y p o c a l c e m i a at parturition; hypocalcemia stimulates synthesis of 1 , 2 5 - d i h y d r o x y v i t a m i n D (6). ttorst et al. (17) also observed a marked

increase in 1 , 2 5 - d i h y d r o x y v i t a m i n D concentration in paretic, older cows following parturition; n o n p a r e t i c cows s h o w e d a gradual increase with a l o w e r m a x i m a l c o n c e n t r a t i o n than in paretic cows. The increased 1 , 2 5 - d i h y d r o x y v i t a m i n D c o n c e n t r a t i o n following parturition of cows fed either o f the diets may have been caused by a c o m b i n a t i o n o f h y p o c a l c e m i a and h y p o p h o s p h a t e m i a as well as the increased PTH concentration that has o c c u r r e d in response to parturient h y p o c a l c e m i a o f cows (17, 24). Initiation of feeding the high- or the lowcalcium diets caused no significant changes in 2 4 , 2 5 - d i h y d r o x y v i t a m i n D o f plasma (Figure 11). F u r t h e r m o r e , no change in c o n c e n t r a t i o n s o f this m e t a b o l i t e occurred during the 8 days o f feeding either diet. Also, no differences in 24,25-dihydroxyvitamin D o f plasma between cows of the t w o dietary groups during the peripartal period (Figure 12) were significant (P>.05). There was a tendency, however, for c o n c e n t r a t i o n s of 24,25-dihydroxyvitarnin D to be greater during the peripartal period in cows fed the high-calcium diet. Also, no specific physiological f u n c t i o n has been defined for 2 4 , 2 5 - d i h y d r o x y v i t a m i n D (34). ACKNOWLEDGMENTS

Authors are grateful for the c o o p e r a t i o n o f Fred Foreman, Larry Madren, and T o m Heeg and the use of the Iowa State University dairy herd in these studies.

4.~ 4.1 ~3.6 3

REFERENCES

3.0

1

T l l e During P e v l p * r t a l P e r i o d

(days)

Figure 12. 24,25-Dihydroxyvitamin D concentrations in plasma in cows during an 8-day peripartal period. Plus or minus SE for each mean is indicated. Solid line ( - - ) represents data for cows fed lowcalcium diet, and dashed line (- - -) represents data for cows fed high-calcium diet. Day 0 represents day of parturition. Journal of Dairy Science VoL 64, No. 2, 1981

1 Allcroft, W. M. 1947. Observations on some metabolic disorders of cows as evidenced by chemical analysis of samples of blood from clinical cases. Vet. J. 103:30. 2 Black, t1. E., and C. C. Capen. 1971. Urinary and plasma hydroxyproline during pregnancy, parturition and lactation in cows with parturient hypocalcemia. Metabolism 20:337. 3 Boda, J. M., and H. t4. Cole. 1954. The influence of dietary calcium and phosphorus on the incidence of milk fever in dairy cattle. J. Dairy Sci. 37:360. 4 Boyle, I. T., R. W. Gray, and H. F. DeLuca. 1971. Regulation by calcium of in vivo synthesis of 1,25-dihydroxycholecalciferol and 21,25-dihydroxycholecalciferol. Proc. Nat. Acad. Sci. 68:2131. 5 Bronner, F., Y. Charnot, E. E. Golub, and T. Freund. 1976. Calcium binding protein and regulation of calcium transport. Calcif. Tiss. Res. 21:27. 6 DeLuca, H. F. 1974. Vitamin D: The vitamin and the hormone. Fed. Proc. 33:2211.

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