Acid-base balance and susceptibility of ewes to hypocalcaemia.

Abstract

Proc. Aust. Soc. Anim. Prod. Vol. 19 ACID-BASE BALANCE AND SUSCEPTIBILITY OF EWES TO HYPOCALCAEMIA I. McL. GRANT A. M. BINGHAM, and I. W. CAPLE University of Melbourne, Dept of Veterinary Science, Werribee, 3030. SUMMARY The effect of acid-base balance on the susceptibility of ewes to hypocalcaemia was determined in groups of ewes fed 3 diets where the fixed cation-anion balance [Na + K - Cl] was -4.1, 29.1 and 82.3 meq/lOO g DM, respectively. Ewes fed the diet with a fixed cation excess (82.3 meq/lOO g DM) had higher urine pH and lower urine Ca concentrations. They also had lower blood ionised Ca concentrations after an overnight fast, and tended to develop hypocalcaemia more rapidly during EDTA infusion than ewes fed a diet with a fixed anion excess (-4.1 meq/lOO g DM). It was concluded that dietary fixed cation-anion balance may be a factor which predisposes pregnant ewes to hypocalcaemia. Keywords: hypocalcaemia, calcium, acid-base, pH. INTRODUCTION Hypocalcaemia in pregnant ewes is a common problem in south-eastern Australia in flocks lambing in winter and spring (Grant et al. 1988). Our preliminary studies indicated that ewes grazing green pastures in winter and spring had lower urine and plasma Ca concentrations than ewes grazing dry pastures in the autumn. This seasonal occurrence of hypocalcaemia in ewes did not appear to be associated with Ca, P or vitamin D nutrition (Grant et aZ. 1988; Caple et al. 1988). Recent studies in the cow (Block 1984) and the doe (Fredeen et aZ. 1988b) have shown that acid-base balance as determined by dietary fixed ion intake (meq [Na + K - Cl]), may be more important than Ca intake in affecting Ca metabolism in ruminants. The ability of cows and goats to adapt to sudden increases in Ca demand was decreased during metabolic alkalosis induced by dietary fixed cation excess, where [Na + K - Cl] was greater than 50 meq/lOO g DM. More than 70% of samples of Victorian pastures analysed have had a fixed ion content greater than 50 meq/lOO g DM (Caple and Shovelton unpublished). These experiments were undertaken to examine how the ability of the ewes to respond to rapid changes in Ca demand, induced by intraveous EDTA infusion, was influenced by their acid-base balance as determined by the dietary fixed ion intake during pregnancy and lactation. MATERIALS AND METHODS Animals and diets Nine 2-year old and 13 5-year old Corriedale ewes were fed a basal diet consisting of (g/kg DM): oats 242, lupins 134, luceme chaff 302 and clover hay 322, from 120 days of gestation until the third week of lactation. The basal diet contained 17% CP, had a digestibility of 68%, an estimated ME of 9.6 MJ/kg DM and contained 5.7 g Ca, 3.5 g P, 3.4 g Mg and 7.7 g Cl/kg DM. The ewes were allocated at random, after stratification for age and live weight, to 1 of 3 diets prepared by adding to the basal diet (1) 1.3 g NaWlOO g DM, control diet; (2) 2 g NH&l/100 g DM (fixed anion excess) acid diet, and (3) 1 g NaCl + 2 g NaHCO,/lOO g DM (fixed cation excess) alkaline diet (Table 1). Ewes were housed in treatment groups and fed twice daily. Sample collection and analysis Venous blood and urine was sampled approximately 2 weeks before and after lambing. Blood was collected into syringes, which had been flushed with heparin saline (125 IU/mL), and samples were kept on ice for immediate measurements of blood pH and gasses. Urine samples were stored at 4OC and mineral analysis was carried out within 2 weeks of collection. Urine specific gravity was used to correct urine mineral concentrations for changes in urine water concentrations (Caple and Halpin 1985). Blood pH and gas measurements were made using an automatic acid base analyser. Ionised Ca was measured with an ion specific electrode. Calcium, Na, K and Mg concentrations were determined by atomic absorption spectrophotometry and inorganic P was measured calorimetrically. Chloride concentrations were measured using a Coming-EEL 920 chloride meter. EDTA infusion Approximately 2 weeks before and 3 weeks after lambing, ewes were deprived of food but not water overnight before being infused intravenously with sodium EDTA at the rate of 60 mg/kg liveweight per 412 Proc. Aust. Sot. Anim. Prod. Vol. 19 Table 1. The Na, K and Cl content (meq/lOO g DM) of diets hour. Blood ionised Ca was monitored in samples collected before and intervals during EDTA infusio. The EDTA infusion was continued until 0.6 mmol/L (infusion time). The infusion was then stopped and blood 30 min intervals until its concentration had returned 0.9 mmovL (recovery after fasting, and at 30 min ionised Ca had decreased to ionised Ca was monitored at time). Statistical analysis Results were analysed for significance (P < 0.05) by l-way analysis of variance, and differences between means were tested using the least significance test. Data from each ewe before and after lambing, and before and after overnight fasting were compared using a Students t-test for paired data. RESULTS Dry matter intake and animal peeorrnance The average daily DM intake of ewes averaged 1.6 kg before and 2.4 kg after lambing, and no effect of dietary treatment on intake was observed. There was no effect of diet either on ewe liveweight (mean 54 kg before lambing and 52 kg after 3 weeks of lactation), or on lamb growth rate to 3 weeks of age (mean 332 g/day). The 2-year old ewes weighed less than the 5-year old ewes before lambing (49 and 57 kg, respectively, P < O.Ol), but the mean growth rates of the lambs were similar (331 and 332 g/day, respectively). Blood and urine analysis Results of analysis on blood and urine samples are shown in Table 2. Blood ionised Ca was 0.05 mmol/L lower in lactating than in pregnant ewes ( P = 0.03). Overnight deprivation of food resulted in a 0.09 mmol/L decrease in ionised Ca in pregnant ewes (P < O-001), but no change was observed in lactating ewes. Table 2. Mean pH and mineral concentrations in blood and urine samples, blood gas and acid-base excess (ABE), in ewes fed diets with different fixed anion-cation balance during pregnancy and lactation 413 Proc. Aust. Sot. Anim. Prod. Vol. 19 Table 3. The mean + s.e.m. EDTA infusion time (time required for plasma Ca2+ to decrease to 0.6 mmol/L) and recovery time (time required for plasma Ca 2+ to return to 0.9 mmol/L after infusion) of ewes in each dietary group 2 weeks before and 3 weeks after lambing EDTA infusion In ewes fed the alkaline diet there was a tendency for blood ionised Ca to decrease to 0.6 mmol/L quicker than in ewes fed the acid or control diets. After the EDTA infusion was stopped, blood ionised Ca also tended to take longer to return to 0.9 mm01 Ca/L in ewes fed the alkaline diet (Table 3). The susceptibility of ewes to hypocalcaemia was greater in pregnant than lactating ewes based on infusion and recovery time (Table 3). Five-year old ewes were more susceptible to hypocalcaemia than 2-year old ewes. Infusion times for 2 and 5-year old ewes were 1.6 and 1.4 h before lambing (n.s.), and 3.1 and 1.9 h after lambing (P = 0.05), respectively. The recovery times of the 2 and 5-year old ewes were 3.9 and 6.7 h (P = 0.06) before lambing and 1.4 and 2.35 h (P = 0.04) after lambing, respectively. DISCUSSION Ewes fed the diet with a fixed anion excess developed a subclinical metabolic acidosis as assessed from changes in blood and urine [I-I+], and blood [HC03-] (Table 2). In ewes fed the cation excess diet the changes in these parameters were not significantly different from ewes fed the control diet. Since ewes in each treatment group had similar DM intakes it was assumed that any observed differences in calcium metabolism between treatments was due to the effects of differences in dietary fixed ion intake. Blood ionised and total plasma Ca concentrations tended to be lower in ewes fed the alkaline diet than in ewes fed the acid diet. A reduction in the exchangeable Ca pool has also been observed in sheep (Craige 1947) and cows (Dishington 1975) fed alkaline diets. In the present study, ewes fed the diet with the fixed anion excess also had higher urinary calcium concentrations than those fed the alkaline diet. Fredeen et al. (1988a, 1988b) observed that does fed diets with an excess of fixed anions (-2 meq/lOO g DM) developed subclinical acidosis with hypercalcinuria, and had increased Ca absorption from the diet and apparently increased Ca resorption from bone. Does fed diets with a fixed cation excess (77 meq/lOO g DM) had lower urinary calcium excretion and reduced absorption of dietary Ca. Thus low rates of dietary Ca absorption from certain diets fed to pregnant and lactating ewes may be associated with changes in Ca metabolism induced by alkaline diets. Younger ewes were more resistant to hypocalcaemia during EDTA infusion than older ewes both before and after lambing. The rates of bone accretion and resorption, and the size of the rapidly exchangeable Ca pool, decline with age in sheep (Braithwaite and Riazuddin 1971). The EDTA infusion times were longer and recovery times were shorter in lactating ewes compared with pregnant ewes. Similar differences between pregnant and lactating cows have also been observed during EDTA infusions (Kronfeld et al. 1974), and this is assumed to reflect a higher rate of Ca flux through the exchangeable Ca pool during lactation. Without assessments of either the acid-base balance of the sheep or the fixed anion-cation balance of the diets it is not possible to conclude how much of the seasonal variation in susceptibility to hypocalcaemia and the seasonal variation in urinary and plasma Ca concentrations is related to these factors. In our previous study it was observed that urinary Ca concentration was elevated in ewes grazing dry grass compared with ewes grazing green herbage (Grant et al. 1988). It was suggested that this may have been due to a low availability of Ca from certain fresh herbage diets. In support of this suggestion, Chrisp et al. (1989) reported that lactating ewes absorbed only 17% of ingested Ca from a ryegrass-oat herbage diet containing 6 g Ca/kg DM. Since a large base reserve is recycled between rumen and saliva, dietary factors affecting salivation may also affect acid-base balance and be more important than Ca intake in influencing Ca metabolism in grazing sheep. ACKNOWLEDGMENTS This project received support from the Wool Research Trust Fund. 414 Proc. Aust. Sot. Anim. Prod. Vol. 19 BLOCK, E. (1984). J. Dairy Sci 67: 2939-48. BRAITHWAITE, G. D. and RIAZUDDIN, S. H. (1971). Brit. J. Nuts 26: 215-25. CAPLE, I. W. and HALPIN,, C. C. (1985) In ` Dairy Cattle Production' p. 307. Proc. No. 78. The post-grad. Cttee in , Vet. Science, The University of Sydney. CAPLE, I. W., BABACAN, E., PHAM, T. T., HEATH, J. A., GRANT, I. M., VIZARD, A. L., CAMERON, S. J. and ALLWORTH, M. B. (1988) Proc. Aust. Sot. Anim. Prod. 17: 379. CHRISP, J. S., SYKES, A. R. and GRACE, N. D. (1989). Brit. J. Nuts: 61: 45-8. CRAIGE, A. H. (1947). Am. J. Vet. Res. 8: 247-52. DISHINGTON, I. W. (1975) Acta Vet. Stand. 16: 50-7. FREDEEN, A. H., DePETERS, I. J. and BALDWIN, R. L. (1988a) J. Anim. Sci. 66: 15973. FREDEEN, A. H., DePETERS, I. J. and BALDWIN, R. L. (1988b) J. Anim. Sci. 66: 174-8. GRANT, I. McL., FOOT, J. Z., BROCHUS, M. A., BINGHAM, A. M. and CAPLE, I. W. (1988). Proc. Aust. Sot. Anim. Prod. 17: 194-7. KRONFELD, D. S., MAYER, G. P. and RAMBERG, C. F. (1974) Handbook of Physiology, section 7, ~01.4. Am. Physiol. Sot., Washington. REFERENCES 415