Factors affecting plasma calcium and phosphorus concentrations in grazing ewes in spring and autumn lambing flocks in Victoria.

Abstract

194 Proc. Aust. Soc. Anim. Prod. Vol. 17 FACTORS AFFECTING PLASMA CALCIUM AND PHOSPHORUS CONCENTRATIONS IN GRAZING EWES IN SPRING AND AUTUMN LAMBING FLOCKS IN VICTORIA ** I.McL.GRANT*, J.Z. FOOT , M.A BROCKHUS** , A.M. BINGHAM* and I.W. CAPLE* SUMMARY Factors contributing to periparturient hypocalcaemia were examined in spring and autumn lambing ewes fed pasture and allocated to low and high nutritional treatments. Pasture calcium (0.38% to 0.51% DM) was adequate throughout the year, but pasture phosphorus (0.17% DM) in the autumn provided less than requirements. Mean plasma Ca and P concentrations were lower in spring lambing ewes before and after lambing. In the spring lambing flock on low nutrition, pregnant ewes had lower plasma Ca and higher P than pregnant ewes on high nutrition. Mean plasma Ca and P were lower in ewes with twins, but were unaffected by the condition score of the ewes. Plasma Ca decreased in older lactating ewes fed grain in the autumn, and 'plasma P decreased in older ewes in the spring lambing flock. It is concluded that the pregnant and lactating ewes had difficulty in maintaining calcium homeostasis when grazing short green pastures in winter and lush pastures in spring, even though these pastures contained adequate Ca concentrations according to current requirement statements. Keywords: calcium, phosphorus, season, hypocalcaemia, sheep INTRODUCTION Pregnant and lactatingewes normally mobilise bone Ca and P reserves and increase intestinal Ca absorption to meet their Ca and P requirements for the foetus and udder, and to maintain plasma Ca concentrations within defined limits. Bone reserves are replaced in late lactation or during the dry period if the ewes can absorb sufficient Ca and P (Braithwaite 1983, 1986). It is generally assumed that Ca and P deficiency does not occur'in grazing sheep in Australia, (McDonald 1968; Underwood 1981) but Ca deficiency has been reported in sheep fed grain and little roughage during droughts (Franklin 1953). However, hypocalcaemia is a common problem in older pregnant ewes in winter and spring lambing flocks grazing improved pastures in Southern Australia (Larsen et al. 1986). The aim of this study was to identify factors which may affect calcium homeostasis in grazing ewes. MATERIALS AND METHODS Eight age-structured flocks of 40: merino ewes were each set-stocked on 3.5 ha plots at the Pastoral Research Institute at Hamilton Victoria'. Replicated high and low nutrition flocks lambed in either autumn (May/June) or spring (Aug/Sept). High nutrition ewes lambing in spring grazed improved perennial rye,grass and subterranean clover pastures which were reseeded in 1983. All other groups grazed unrenovated pastures. Ewes were scored for body condition (range l- emaciated to 5 - very fat: Jefferies 1961) at intervals of 4 weeks or less. When 25% of the%ewes in the high nutrition groups were in condition score 2.3 or` below, supplementary feed was introduced and continued until not more than 2 animals were 2.3 or &low. The procedure was the same for animals in the low nutrition'group but the threshold condition score was.l.7. Observations extended frdm June 1986 to August 1987. Autumn lambing ewes were fed 200-260 g oats/head/day. Spring lambing ewes were fed 450-600 g triticale/head/day from mid April to June, and 300 g triticale + 100 g lupins/head/day from-June, until weaning in August (Fig. 1.). Calcium concentrations in plasm& and urine s&nples were measured by atomic absorption spectrophotometry; and inorganic phosphorus was measured * University of Melbourne, Dept. Vet. Clinical Sciences, Werribee, 3030 ** Dept. Agriculture and Rural Affairs, Pastoral Research Institute, Hamilton. . Proc. Aust. Soc. Anim. Prod. Vol. 17 195 calorimetrically. Urine mineral concentrations were corrected for changes in urine water concentrations (Caple and Halpin 1985). Pasture samples collected from the plots before lambing, in early lactation and at weaning were dried at 60' C and ground through a 40 mesh sieve before Ca and P content &DM) was measured using XRay Fluorescence. Fig. 1. Calendar of events in spring and autumn lambing flocks w Data were analysed using either the SPSS' program (SPSS' 1986) or the GLIM program (Baker and Nelder 1978). Analysis of variance and regression analysis w e r e used to analyse the effects of season, nutrition, age and ewe condition score on plasma Ca and P. RESULTS The mean live weights of spring and autumn lambing ewes at lambing were 40 + 0.39.kg and 47 + 0.55 kg respectively (p<&O5)land the mean condition scores werZ 2.7 + 0.04 and '3.3 + 0.04 respectively (p<O.O5). Ewes lambing to ewes joined for autuh and spring laings ,were 84% and 80% respectively (ns); and the percentages of lambs reared to ewes joined were 71% and 59% respectively (p<O.'O5). Pasture and grain Ca and P concentrations The mean pasture Caconcentrations (% DM) in April, late May, July and October were 0.38%, 0.45%, 0.39% and 0.51% respectively. The mean pasture P concentrations at these times were 0.17%, 0.30%, 0.28%, and 0.28% respectively. Samples from pastures grazed by treated and control spring lambing flocks had similar Ca and P concentrations at each collection. The oats, triticale and lupins contained 0.05% Ca, 0.30% P and 0.03% Ca,0.25% P and 0.2% Ca,O.29% P, respectively. Plasma Ca and P in spring lambing flocks (Table 1.) Pregnant ewes, compared with non-pregnant ewes, had lower plasma Ca in both the low nutrition (2.24 vs 2.42 m&p<0.05) and high nutrition groups (2.42 vs 2.57 mM; p<O.O5).'Ewes in the high nutrition group had higher plasma Ca. than the low nutrition ewes prior to lambing (2.41 vs. 2.25 mM; p=O;O3) and at lambing (2.32 vs 2.11 mM; p=O.O2). Plasma P was lower 'in the high nutritionewes before lambing (1.70 vs 2.0.4 mM; p=O.O7), ,at marking (1.49 vs 1.88 mM; p=O.O3) and at weaning (1.74 vs 2.07 mM; p=O.O2). . There was no relationship between condition score and plasma Ca or P concentrations:Ewes with twins had lower plasma Ca at lambing than ewes with single lambs (2.09 vs 2.26 mM; p=O.O3). Age had no effect on plasma Ca in the spring lambing ewes. However, mean plasma P was lover in older ewes before lwing .(2.07, 1.95, 1.79 and 1.64 mM; f-or 2, 3, 4 and 5 year old ewes respectively; 'p=O.O2), at lambing (1.68, 1.82, 1.59 and 1.45 mM; p=O.O6) and at marking (1.91, 1.79, 1;62 and 1.43 m& p=O.O5). 196 Proc. Aust. Soc. Anim. Prod. Vol. 17 Table 1. Mean plasma Ca and P concentrations in ewes Plasma Ca and P in autumn lambing flocks (Table 1.) In pregnant ewes plasma Ca was similar but plasma P was lower that in non-pregnant ewes (2..02 vs 2.26 mM'p<O.Ol). Lactating ewes had lower plasma Ca thanewes which did not,lamb (2.13 vs 2.25 mM; p=O.O13). No effect of nutritional treatment or condition score on plasma Ca or P was observed in the autumn lambing ewes. Effects of age on mean plasma Ca concentrations were apparent at marking (2.33, 2.28, 2.16, and 2.11 mM, for 2, 3, 4 and 5 year-old ewes, respectively; pro.0841 and weaning (2.33, 2.10, 2.17, and 2.05 mM, for 2-5 year-old ewes, respectively: p<O.OOlL Ewes with twins had lower plasma . P than ewes bearing single lambs prelambing (1.74 vs 2.06 mM; p=O.O03) and at lambing (1.59 vs 1.78mM; p=O.O86). Urine mineral concentrations in ewesprelambing and at weaning were 4.0 and 2.7 umol Ca/mosmole respectively for the spring lambing flock. The.urine phosphorus concentrations were less than 0.3 umol P/mosmole, In the autumn lambing flock the respective values were 9.0 and 1.6 umol Ca/mosmole (P<O.O5), and'0.43 and 4.48 urn01 P/mosmole (P<O.O5). . DISCUSSIOti Pregnant ewes in the spring lambing flocks had lower plasma Ca and P concentrations than pregnant ewes in the .autumn lambing flocks.' During the'eriod of these observations, no ewes showed signs of clinical hypocalcaemia. The spring lambing.ewes may have been`less able to increase mineral intake and absorption or to further increase mobilisation of Ca and P from bone. The spring lambing ewes were grazing short'winter pastures during late pregnancy, and those on the high nutritional treatment had higher plasma Ca, but lower plasma P than the low nutritional group. This suggests that food intake, and therefore calcium intake, was limiting during this period. This suggestion receives support 'from the observations tha$ the non-pregnant ewes in the low nutrition group also had lower plasma Ca, and that the differences between the nutritional treatments disappeared when ewes were grazing abundant spring pastures..Nevertheless, the Proc. Aust. Soc. Anim. Prod. Vol. 17 197 plasma Ca decreased even further duri w 1 ,actat ion in the spring. This suggests that the ewes absorbed Ca poorly from the lush spri ng past ure. The nature of the diet and its digestion.may,well influence calcium absorption in sheep. Sansom et al.(1982) observed that lactating ewes fed concentrate diets with approximately similar intakes of Ca (5 g/d) were able to maintain plasma Ca similar to those of non-pregnant ewes.. Grace et al. 1974 found that sheep absorbed calcium poorly from green pasture diets (0.9% Ca, intake 5g Ca/d), and that calcium absorption increased with pasture intake. The reasons for the difference between availability of calcium from concentrate and pasture diets remain to be determined. The autumn lambing ewes were grazing predominantly dry grass prior to lambing and were able to maintain concentrations of plasma Ca, but plasma P decreased. Their urine Ca concentration was also high. Therefore Ca absorption appeared to be adequate during late pregnancy even though the Ca concentration of the herbage was similar to that in the spring pastures where pregnant ewes could not maintain their plasma Ca. Plasma and urine Ca declined markedly after lambing in the autumn lambing flocks, and this may have been due to the effects of feeding grain low in Ca and an insufficient total Ca intake from the short autumn pastures. Urine P concentration increased with grain feeding. The ewes which did not lamb in this flock also showed a decline in plasma Ca, but they maintained plasma Ca higher than the lactating ewes (Table 1.). The autumn lambing ewes had similar mean plasma Ca in both nutritional treatment groups and this suggested that they were able to absorb similar amounts of Ca when supplemented with grain. Our observations indicate that pregnant and lactating ewes may not absorb sufficient Ca from pastures in winter and spring in southern Australia to maintain plasma Ca concentrations as high as those in ewes in the autumn. These seasonal differences in plasma Ca occur despite pastures containing adequate Ca and P according to current estimates of nutritional requirements (Grace 1983, Anon 1984). The reasons for the apparent low availability of Ca from winter and spring pastures remain unknown. However, any factor including interrupted grazing during inclement weather, and management procedures (such as crutching, prelambing shearing) which reduces pasture and Ca intake, is likely to increase the severity of hypocalcaemia' in pregnant and lactating ewes and predispose them to clinical disorders. REFERENCES ANON (1984). In 'Recent Advances in Animal Nutrition-1984*', p. 113; editors W. Haresign and D.J.A. Cole. (Butterworths: London)-. BAKER, R.J. and NELDER, J.A. (1978). The Glim System..Release 3. Numerical . Alogorithims Group,' Royal Statistical Society, London. : BRAITHWAITE, G.D. (1983). Br. J. Nut. 50: 711. BRAITHWA1TE;G.D. (1986). J. agric. SC, Camb..106: 271. FRANKLIN, M.C. (1953). Aust. Vet. 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