Calcium and phosphorus metabolism in ruminants with special reference to parturient paresis

Meta-analysis-based estimates of efficiency of calcium utilisation by ruminants

The most abundant mineral in the body of animals is Ca, which has crucial importance for the regulation of various processes. The maintenance of Ca balance has become more challenging, especially in lactating ruminants, owing to the increased milk yields and thus Ca requirement. To determine the Ca requirement, factors such as Ca secretion via milk or Ca deposition in body tissues and conception products are summed up to the net Ca requirement. Nevertheless, dietary Ca cannot be completely utilised by the animal to cover the net Ca requirement, therefore a value for the efficiency of Ca utilisation is applied, which is the maximum proportion of Ca from the feed that the animal can use for covering the net requirement. However, current estimates for the efficiency of Ca utilisation are inconsistent. Therefore, the objective of the present meta-analysis was to estimate the efficiency of Ca utilisation for ruminants, considering the Ca supply of the animal. A data set of 223 observations was compiled from 37 studies, including data on cattle and small ruminants. Standardised Ca digestibility was calculated from data on Ca intake and faecal Ca excretion, corrected for faecal endogenous losses. Furthermore, a data subset on only lactating ruminants was created. For this subset, Ca excretion via faeces and urine and standardised Ca digestibility were related to the Ca supply of the animal. An exponential function was fitted to standardised digestibility data in response to Ca concentration in the diet and Ca supply, revealing that standardised Ca digestibility decreased with increasing dietary Ca concentration and Ca supply. The median for standardised Ca digestibility was 40%, with a remarkable variation between 9% and 88%. In response to Ca supply, faecal Ca excretion increased in a strong linear manner (slope = 0.76, R² = 0.96). Excretion of Ca via urine was very low even when Ca supply was very high. To conclude, Ca digestibility is a suitable indicator for the efficiency of Ca utilisation, since excessive Ca is almost completely excreted in faeces; however, Ca digestibility has to be determined at a Ca supply level below the requirement of the animal. To date, only very limited number of data have been reported for such supply conditions. Comparative studies using various Ca sources are suggested for future studies but should be conducted using a marginal Ca supply level.

Effect of dietary potassium supplementation on the calcium absorption capacity in the rumen and abomasum and fractional excretion of urinary minerals in sheep

The aim of this study was to determine the effect of potassium (K) supplementation on the calcium (Ca) absorption capacity from the rumen and abomasum of sheep. The Ca absorption capacity from the rumen and abomasum of sheep was measured using stable strontium (Sr) as a Ca-analogue tracer method. The sheep, cannulated at either the rumen or abomasum, were randomly allocated to one of two groups (control or K-supplemented) and fed in individual metabolism pens twice daily with a diet comprising oaten hay, lucerne chaff and barley fortified with or without potassium carbonate (K2CO3). The K content of the diet of the K-supplemented animals was 3.1% of dry matter compared with 1.4% for the control animals. The animals were fed their respective diets for a period of 2 weeks. The fractional absorption capacity (FC) of Ca was estimated before, during and after the treatment period. Supplementation with K decreased the FC of Ca in both the rumen and abomasum during treatment and increased the FC of Ca in the abomasum post-treatment. Supplementation with K also increased the fractional excretion of K in the urine, but decreased the fractional excretion of Ca and magnesium (Mg) (P < 0.05), showing that K supplementation significantly affected Ca and Mg metabolism. Results suggest that renal conservation of Ca and Mg is an important mechanism controlling the Ca and Mg pool for vital functions of the body. In times of high demand for these minerals during lactation and pregnancy, high K in the diets may predispose sheep to hypomagnesaemia and hypocalcaemia, which in turn will have a negative impact on productivity and economic returns.

Effects of supplementing rice straw with Leucaena (Leucaena leucocephala) and Madras thorn (Pithecellobium dulce) foliages on digestibility, microbial N supply and nitrogen balance of growing goats

A total of 12 crossbred (Boer × Anglo-Nubian) goats were chosen from a commercial farm on the basis of similar body weight (25.0 ± 3.1 kg). The goats were housed in individual pens and allowed 3 weeks to adapt to experimental conditions. The goats were randomly allocated to three treatments in a 3 × 3 Latin square experiment (replicated four times). Within each period, each goat was given rice straw as roughage plus the respective treatment diet. The diets were iso-nitrogenous and iso-energetic containing cassava pulp, molasses, urea and commercial mineral and vitamin mix. The experimental treatments were (i) soybean meal (SBM), (ii) partial substitution of SBM with Leucaena (Leucaena leucocephala) foliage and (iii) partial substitution of SBM with Madras thorn (Pithecellobium dulce) foliage. Nutrient intakes, ruminal characteristics (pH, ammonia nitrogen and volatile fatty acids), nitrogen balances, plasma urea nitrogen and microbial N supply were not significantly different among treatments. The results of this study indicate that protein foliages from locally grown shrubs and trees can substitute imported feedstuff concentrates (e.g. SBM) as protein supplement for goat production.

Effect of anionic salt and highly fermentable carbohydrate supplementations on urine pH and on experimentally induced hypocalcaemia in cows

The objective of this experiment was to determine the effect of dietary grain on calcium homeostasis. Six rumen-fistulated dairy cows with 3 or more previous lactations and no history of parturient paresis were randomly assigned to a sequence of diets in a crossover study with 4 periods of 10 days each. Dietary treatments were: A control ration consisting of wrap grass silage alone (1), the control ration supplemented with ammonium chloride and ammonium sulphate salt solution (2), control ration following a period with supplementation (3) and control ration supplemented with increasing amounts of barley from 4 to 10 kg/cow per day, expected to produce subclinical rumen acidosis (4). Daily intake of the diets was adjusted to 14 kg DM/cow per day. On day 11, the calcium-regulating mechanisms in cows were challenged until recumbency by a standardized intravenous EDTA infusion and cows were left to recover spontaneously. Anion supplementation and the feeding of highly fermentable carbohydrate lowered urine pH below 7.0 due to subclinical acidosis. During spontaneous recovery from EDTA induced hypocalcaemia, the cows more quickly regained a whole blood free calcium concentration of 1.00 mmol/L if they had most recently been supplemented with either anionic salts or with increasing amounts of barley, as compared to the basic ration. It is concluded that so-called slug-feeding or 'steaming up' with highly fermentable carbohydrates before parturition in milk fever susceptible cows enhanced calcium homeostasis similar to the effect seen in cows on anionic diets.

Fractional excretion of electrolytes during pre- and postpartum periods in cows

In this study, fractional excretions (Fe) of sodium (Na), potassium (K), chloride (Cl), calcium (Ca), magnesium (Mg) and phosphorus (PO4) were examined with the aim to demonstrate interactions between fractional excretions of these electrolytes within each period and relate them to electrolyte metabolism in clinically normal cows at different stages of lactation and dry period. The material of this study consisted of 20 clinically healthy Holstein-Friesian cows of the same age and milk yield. Blood and urine samples were collected on 190-200th, 240-250th and 270-280th days of pregnancy and on days 1-7th, 35-45th and 75-85th after calving, altogether 6 times. An increase was observed in FeCa and FeMg during the transition from the lactation to the dry period (p < 0.05), and a decrease in FeCa (p < 0.05), FeMg (p < 0.01) in the 2nd month of the dry period. FePO4 and FeMg, respectively, increased on levels of p < 0.01 and p < 0.05, while FeCa decreased on a level of p < 0.05 after gestation compared to the level before gestation. FeNa and FeK showed a decrease of p < 0.001 and p < 0.01, respectively, between the 1st and 2nd months of the dry period, while after gestation this value showed an increase in FeNa (p < 0.05) and FeK (p < 0.01). FeCl increased significantly (p < 0.05) only from postpartum to the 1st month of lactation. There was a strong positive correlation between FeNa and FeCl in all of the periods. It was concluded that there were significant changes in the Fe of Na, K, Cl, Ca, PO4 and Mg before parturition and during lactation; these changes could have an important role in assessing renal function and electrolyte balance.

Phosphorus partitioning during early lactation in dairy cows fed diets varying in phosphorus content

The effect of dietary P content on P partitioning and excretion during early lactation was evaluated in 13 cows fed diets containing 0.34 (no supplementary P), 0.51, or 0.67% P. All cows were fed a common pre-partum total mixed ration (TMR) (0.28% P), followed by common TMR (0.51% P) for 7 d post-partum. On day 7, cows were randomly assigned to one of the three dietary P treatments. All treatment diets contained 16.6% CP, 15.2% ADF, 26.3% NDF, and 0.74% Ca. Total collections of milk, urine, and feces were conducted during weeks 3, 5, 7, 9, and 11 of lactation. Average body weight (591 kg), milk yield (47.9 kg/d), and DMI (25.2 kg/d) throughout the 10-wk trial were not affected by dietary P content. With increasing dietary P, however, linear increases in P intake (84.7, 135.2, and 161.5 g/d), fecal P (42.3, 87.5, and 108.6 g/d), urinary P (0.32, 1.28, and 3.90 g/d), and total P excretion (42.6, 88.8, and 112.5 g/d) were observed. Apparent P digestibility (49.0, 34.4, and 32.8% of P intake) decreased quadratically with increasing dietary P. Phosphorus balance was highly variable, but cows fed the 0.34% P diet were in negative P balance longer than were cows fed diets containing 0.51 or 0.67% P. With increasing dietary P, serum concentrations of inorganic P increased linearly, but serum Ca and Mg concentrations decreased. Increasing dietary P increased fecal and urinary P excretion in early lactation cows. Increased duration of negative P balance and changes in blood mineral concentrations suggest that cows fed low P diets mobilized more P from body reserves than cows on medium- and high-P diets.

Nitrogen and phosphorus partitioning in lactating Holstein cows fed different sources of dietary protein and phosphorus

To evaluate dietary N and P partitioning, 36 Holstein cows grouped by parity were assigned at calving to diets supplemented with soybean meal (S) or a combination of S and blood meal (B). Diets S and B were formulated to contain 16.2% CP and 0.35% P using mono- and dicalcium phosphate (PM) or wheat bran (WB) as the supplemental source of P. Actual dietary P contents were 0.38, 0.36, 0.34, and 0.34% for SPM, BPM, SWB, and BWB. Two-day total collections of feces, urine, and milk were conducted between 30 and 45 d in milk (DIM), then all cows were fed a control diet until 120 DIM. Between 120 and 150 DIM, cows were again fed the diet assigned at calving, then 2-d total collections of feces, urine, and milk were conducted. Milk production was similar for cows fed diets containing WB (SWB or BWB) when compared with cows fed PM. However, DMI tended to be lower, and P intake and total P excretion were lower in response to WB (20.7 kg/d, 71.9 g/d, and 40.3 g/d) compared with cows fed PM (23.0, 86.7, and 46.8 g/d). Apparent digestibility of dietary P did not differ due to source of supplemental P, averaging 45% across diets. The lower P intake by cows fed WB resulted in lower absorbed P and lower retained P (32.2 and 7.5 g/d) compared with those fed PM (40.6 and 13.4 g/d). Apparent N digestibility, urinary N, and N retention were not affected by P source. Blood meal decreased apparent N digestibility and absorbed N, and also decreased P retention compared with S. In later lactation, cows retained proportionately more absorbed N and P in body tissue and secreted less in milk than they did in early lactation. Results indicated the organic source of P (phytate-P) in WB can be used to provide a substantial portion of the P needed in dairy cattle diets after peak lactation, but the amount of WB in the diet during early lactation should be limited to prevent suppression of DMI and P retention.

Ruminal, cardiorespiratory and adrenocortical sequelae of Na2EDTA-induced hypocalcaemia in calves

A study was undertaken to provide further information on the ruminal, cardiorespiratory and hypothalamo-pituitary-adrenocortical (HPAC) physiological sequelae of hypocalcaemia in dairy calves. The functional picture observed in standing calves experiencing Na2EDTA-induced progressive hypocalcaemia showed a biphasic pattern. During the first phase (Ca2+ varying between 1.20 +/- 0.09 and 0.64 +/- 0.15 mmol/L, mean +/- SD), the animals became dull and lethargic, shifting their weight from one hind limb to the other, with cool extremities and hypersalivation. Their ventilation was slightly increased but their heart rate, thoracoabdominal pressure, pulmonary mechanics, haemoglobin and temperature remained constant. Conversely, their systemic arterial pressure (SAP) and the amplitude of their ruminal contractions (RCA) were severely decreased. During the second phase (Ca2+ < 0.64 +/- 0.15 mmol/L), there was restlessness, tachycardia, hypertension, polycythaemia and, finally, inability to stay upright. It is suggested that the diminished Ca2+ availability caused smooth-muscle and myocardial dysfunctions which could explain the RCA and SAP changes recorded during the first phase, whereas neural and/or humoral sympathetic discharge probably accounted for the reversal in SAP and heart rate when Ca2+ was decreased further. Serum cortisol increased regularly and remained significantly correlated with Ca2+ in each animal. Moreover, regression of delta cortisol/delta Ca2+ on delta Ca2+/delta Na2EDTA was significant (p < or = 0.001). It was concluded that mild asymptomatic hypocalcaemia severely impairs ruminal function, which will progressively worsen the Ca2+ deficit; that the inability to maintain posture in hypocalcaemia is not due to hypotension; and that the higher the HPAC response to hypocalcaemia, the higher the resistance to its effects. An asymptomatic periparturient cow with barely detectable ruminal activity may merit preventive calcium borogluconate therapy. Also, the physiological role of hypotension in explaining the clinical picture may be less important than other processes, such as neuromuscular failure. Finally, the present results imply a possible HPAC exhaustion in cows with periparturient paretic hypocalcaemia.

Skeletal changes in the ovariectomised ewe and subsequent response to treatment with 17 beta oestradiol

The ewe has been suggested as a suitable large animal model for assessment of therapeutic agents for the treatment of osteoporosis. In this study we asses the response of ewe bone mineral parameters, using dual energy x ray absorptiometry (DXA), after ovariectomy. Regular DXA analysis was performed over a period of 75 weeks followed by a period of dosing with 17 beta oestradiol. Total body bone mineral density (BMD) was reduced in ovariectomised animals 15 weeks post operatively. BMD remained lower than control animals for the entire pre dose period. Subsequently, dosing with 17 beta oestradiol prevented further loss of bone over a period of 38 weeks. Several complicating factors were noted during the study including seasonal BMD variation, a correlation between fat/lean ratio and total body BMD together with a reversible reduction in bone mineral content as a result of lactation. Also changes in BMD were associated with age at ovariectomy.

Vitamin D status and hypocalcemic response to protamine in exercised and non-exercised dairy cows

Bone resorption was studied in post parturient cows using intravenous injection of protamine. Protamine inhibits bone resorption and the protamine-induced hypocalcemia can be taken as a measure of this process. The studies were performed in a herd where half of the cows had been tied indoors for 2 1/2 years, the other half had been given daily exercise during the same period. All animals were fed in the same way. The hypocalcemic response to protamine was the same in both groups, which indicates that exercise did not change bone resorption. The serum levels of calcium, inorganic phosphorus and magnesium around parturition did not differ. The daily exposure to daylight increased the serum level of 25-(OH)-cholecalciferol in late summer in the exercised group.

Treatment of mineral disorders in cattle

It is difficult to identify mineral deficiencies and even harder to predict their occurrence. The best indicators of a particular disorder are the characteristic clinical signs. In their absence, biochemical tests or well-designed and properly conducted dose-response field trials are the only sure ways of defining deficient states and assessing their functional and economic significance. This article identifies treatments of the main mineral disorders in cattle and preventive strategies to minimize their occurrence. Dietary manipulations or oral treatments are indicated for chronic primary deficiencies in which there is insufficient mineral in the diet to meet cattle requirements. In severe and acute deficiency, parenteral treatment restores normal concentrations rapidly to a functional level in the animal's tissues. Advocate parenteral treatments if alimentary absorption is impaired or at times of increased mineral demand, such as late pregnancy, but be wary of toxicity. Many innovative products and delivery systems are now available for relatively cheap, effective, long-term mineral supplementation with minimal handling of cattle.

Dry cow nutrition. The key to improving fresh cow performance

Evidence supports the concept of the dry period being a critical component to lactation preparation rather than an insignificant rest period between lactations. Required nutrient amounts for the dry cow are the sum of maintenance, pregnancy, and reserve replenishment needs with additional requirements for growth during the first two pregnancies. Maintenance energy requirements can be dramatically increased by level of activity and adverse environmental conditions. A wide variety of feed ingredients can be successfully fed to dry cows as long as rations are appropriately formulated to meet energy, protein, mineral, and vitamin requirements. A early and close-up dry program best matches increasing pregnancy requirements and declining intake capacity with management capabilities. The early dry cow ration is formulated for high fiber/low energy density while the close-up ration contains higher energy density with less fiber. Both rations contain sufficient other nutrients based on determined intake. This two-group system provides maximal flexibility in managing for optimum body condition at calving. Environmental stresses and dramatic dietary changes should be minimized during the transition period from late gestation to lactation. A sound dry cow program results from integration of quality nutrition and cow management practices as described. A dry cow program that enacts these guidelines should result in reduced incidence of clinical mastitis, successfully complete pregnancy with a viable calf, maximize genetic potential for milk production, minimize incidence of health disorders, and allows cows to breed back within an economically optimum time interval. Overall, a sound dry cow program is a critical key to improved fresh cow performance.

Calcium, phosphorus, and magnesium homeostasis in ruminants

Significant advances have been made in the last 30 years in the understanding of Ca, PO4, and Mg homeostasis in ruminants. Despite these advances, the primary cellular or molecular lesions responsible for failure of homeostasis have yet to be identified. Suggested causes, such as primary hormone deficiencies, have been eliminated, and we now believe that aging and nutrition can reduce the ability of intestine, bone, and kidney to respond rapidly to the hormone signals responsible for homeostasis during rapid increases in demand for these minerals. Further research is required before these lesions can be identified and new knowledge applied to the development of economical, effective programs that prevent milk fever and hypomagnesemic tetany. Until then, diseases such as milk fever will continue to affect 8 to 9 per cent of our dairy cows, and dairy farmers will continue to spend millions of dollars a year for treatment of the primary disease and the many secondary problems that result from these diseases.

Study of osteogenesis in adult sheep on a high and a low calcium ration by means of fluorescing bone markers

The rate of osteogenesis was studied in 8 non-pregnant, non-lactating Friesian dairy sheep, 3-6 years old, by means of a treatment with 3 different bone seeking agents. Four sheep were fed a low calcium ration (LCa:1.8 g Ca/d) and four other sheep a high calcium ration (HCa:12.7 g Ca/d). The bone markers, oxytetracycline-HCl, alizarine-complexion and demeclocycline-HCl, were administered at intervals of 6 weeks, and the sheep were killed 1 week after administration of the last marker. In undecalcified cross sections from the middle of ribs 2, 10 and 12, and from the proximal and distal parts of rib 10, the numbers of labelled osteons and the number of osteons with 1, 2 or 3 markers were counted under fluorescent microscopy. In the ribs of sheep from the LCa group, the number of labelled osteons and the quantity of labels per osteon tended to be higher than those of sheep from the HCa group. When osteogenic activity was compared in the different sites of ribs analysed, lowest osteogenic activity was observed in the proximal part of the 10th rib. The use of fluorescing markers offers the possibility of studying osteogenic activity over a certain period of time in adult sheep.

Regulation of calcium and phosphorus homeostasis in the dairy cow

This paper describes the mechanism of calcium and phosphorus homeostasis in dairy cows in an effort to provide a clearer understanding of the rationale behind current management and supplementation practices. Specifically addressed is the need to keep prepartum dietary calcium intake at less than or equal to 50 g/d to minimize the incidence of milk fever. Also discussed is the need to increase National Research Council recommendations for postpartum dietary calcium from 2.7 to 3.4 g/kg milk. This is particularly important during the first 1 to 2 mo of lactation to maintain calcium balance.

Vitamin D3 toxicity in dairy cows

Large parenteral doses of vitamin D3 (15 to 17.5 x 10(6) IU vitamin D3) were associated with prolonged hypercalcemia, hyperphosphatemia, and large increases of vitamin D3 and its metabolites in the blood plasma of nonlactating nonpregnant and pregnant Jersey cows. Calcium concentrations 1 day postpartum were higher in cows treated with vitamin D3 about 32 days prepartum (8.8 mg/100 ml) than in control cows (5.5 mg/100 ml). None of the cows treated with vitamin D3 showed signs of milk fever during the peripartal period; however, 22% of the control cows developed clinical signs of milk fever during this period. Signs of vitamin D3 toxicity were not observed in nonlactating nonpregnant cows; however, pregnant cows commonly developed severe signs of vitamin D3 toxicity and 10 of 17 cows died. There was widespread metastatic calcification in the cows that died. Because of the extreme toxicity of vitamin D3 in pregnant Jersey cows and the low margin of safety between doses of vitamin D3 that prevent milk fever and doses that induce milk fever, we concluded that vitamin D3 cannot be used practically to prevent milk fever when injected several weeks prepartum.

The effect of dose rate of 1-alpha-hydroxycholecalciferol on calcium and phosphorus metabolism in sheep

1. A combination of a mineral balance and a radioisotope technique has been used to study the relationship between dose rate of 1-α-hydroxycholecalciferol (1α-OH-D3) and the magnitude and duration of its effect on the various processes of calcium and phosphorous metabolism in adult wether sheep.

2. The rate of absorption of Ca was markedly increased by treatment and maximum response occurred at the lowest dose rate.

3. Although sheep were already Ca-replete, the extra Ca absorbed was all retained and increased retention was brought about by a combination of an increase in bone accretion and a decrease in bone resorption. This finding conflicts with the generally-held belief that bone resorption is increased by cholecalciferol treatment.

4. The rates of absorption and retention of P were increased by 1α-OH-D3treatment and maximum response occurred at the lowest dose rate.

5. That P absorption could be increased by treatment suggests that not all the available dietary P was absorbed in the control period.

6. Although the loss of endogenous P in the faeces was unaltered by treatment, the secretion of P into the gut was increased, and the increase was directly related to increased serum inorganic P concentration.

7. Nearly all the extra P absorbed was retained and increased retention was achieved by a combination of an increased incorporation into and a decreased loss from the non-exchangeable pools of bone and soft tissues.

8. The time interval taken for absorption rates of Ca and P to return to normal after the end of each treatment was related to the dose rate of 1α-OH-D3. Although higher dose rates had little effect on the magnitude of response, they did prolong slightly the duration of response.

Calcium-containing crystals in alfalfa: their fate in cattle

The fate of crystals in the parenchymatous sheaths around vascular bundles in alfalfa leaves was followed through the bovine digestive tract by scanning electron microscopy. The bundle and sheath pass from the rumen largely intact. Most crystals are released from the bundle sheath postruminally. In feces, some crystals appear partially eroded and others are intact. By energy-dispersive x-ray analysis calcium is the primary crystal cation. Intact cyrstals isolated from alfalfa leaves by low-temperature ashing and from bovine feces by washing and differential specific gravity were subjected to Raman microprobe analysis. Most crystals were calcium oxalate, a few were potassium oxalate, and some contained both compounds. From 20 to 33% of calcium in alfalfa is in the form of oxalate and apparently unavailable to ruminants. Carbonate is probably in partially eroded crystals from feces. Data presented account for the poorer utilization by cattle of calcium from alfalfa than that from inorganic sources.

The effect of 1-alpha-hydroxycholecalciferol on calcium and phosphorus metabolism in the lactating ewe

1. The effect of 1-alpha-hydroxycholecalciferol (1 alpha-OH-D3) on calcium and phosphorus metabolism has been studied in ewes at peak lactation by a combination of a mineral balance and a radioisotope technique. 2. The rate of Ca absorption was substantially higher in the treated ewes than in controls. The rates of endogenous loss of Ca into urine, faces and milk, however, were only slightly higher. 3. In consequence, the net retention of Ca was increased and the loss of skeletal reserves of Ca normally associated with peak lactation, prevented. 4. Although the rate of bone accretion increased slightly, the increase in skeletal retention of Ca resulted mainly from a decrease in the rate of bone resorption. 5. This finding conflicts with the generally held belief that bone resorption is increased by cholecalciferol treatment. 6. The rates of apparent absorption and retention of P were increased by the treatment probably as a result of a direct effect of the 1 alpha-OH-D3 on P absorption. 7. These results provide a possible explanationof the beneficial effect of 1 alpha-OH-D3 in preventing parturient paresis (milk fever) in the dairy cow.

Adaptations in the calcium and phosphorus metabolism of sheep in response to an intravenous infusion of Ca

1. The effect on calcium and phosphorus metabolism of a high rate of Ca infused directly into the blood of Ca -deficient wethers, already receiving an adequate Ca intake, has been studied by a combination of balance and radioisotope techniques. 2. The rate of Ca retention, which was already high, increased only slightly during the infusion before it reached a maximum. The surplus Ca was compensated for by a decrease in the rate of absorption and an increase in the rate of urinary Ca excretion. 3. These findings support the theory that at maximum retention the rate of Ca absorption becomes regulated according to the rate at which Ca can be stored in bone. 4. results suggest that the decreased absorption was due to a decrease in the rate of active absorption and that the low rate remaining was due to diffusion. 5. The rate of P retention was increased by the Ca infusion, possibly as a result of the increased Ca retention.

The effect of dietary calcium intake of ewes in pregnancy on their Ca and phosphorus metabolism in lactation

1. A combination of a mineral balance and a radioactive technique has been used to study the effect of the dietary calcium intake of ewes in pregnancy on their Ca and phosphorus metabolism in lactation.

2. Ewes which had received a Ca-deficient diet in pregnancy absorbed Ca at a greater rate and with a greater efficiency in early lactation than did the control animals, which had received an adequate Ca intake in pregnancy. The apparent absorption of P was also higher in the Ca-deficient sheep.

3. Since both groups of sheep received the same high Ca intake in lactation, the increased rate of absorption of Ca must have resulted from an increased rate of active transport rather than an increased rate of diffusion.

4. Retention of both Ca and P was positive in the Ca-deficient ewes but negative in the control ewes. This difference in retention reflected a difference in bone metabolism.

5. The maximum rate of absorption possible, which is equal to the rate of irreversible loss of Ca from the rapidly exchangeable pool, was much greater in the Ca-deficient lactating ewes than was previously found in Ca-deficient wethers. This difference is due to the additional pathway of loss of Ca in the milk of the lactating animals.

6. The relationship between these findings and the prevention of milk fever in dairy cows by manipulation of the dietary Ca intake is discussed.