Abstract:
Proc. Aust. Soc. Anim. Prod. Vol. 16 SELENIUM RESPONSES IN GRAZING EWES AND THEIR LAMBS H. DOVE*, A. AXELSEN* and R. WATT** SUMMARY Merino and Border Leicester x Merino lambs were supplemented with selenium (Se) either by giving a 'selenised' vaccine to ewes pre-partum and lambs postpartum or by giving intra-ruminal Se pellets to ewes. Both Se sources raised blood glutathione peroxidase activities in ewes and lambs significantly above those in control animals, which were moderate to low. The effects of Se on ewe live weights were small and transient. In Merino lambs liveweight gain and fleece weight were influenced by both ewe age and lamb rearing type. Both Se supplements increased weaning weights (approx. 2 kg) and fleece weights (vaccine 9%; pellet 17%). Weight responses in crossbred lambs were either transient or negative. (Keywords: selenium, lactation, lambs, ewes) INTRODUCTION Sub-clinical responses of lamb s to selenium (Se) in south-east Australia remain equivocal and not well quantified (McDonald 1975; Paynter et al. 1979; Caple et al. 1980), particularly in relation to other factors known to affect live weight and wool growth, such as genotype, age of dam and rearing type of lamb (Donnelly et al. 1985). Our study aimed to measure these effects on ewe and lamb performance and to quantify and compare the responses resulting from the administration of Se either sub-cutaneously ('selenised' clostridial vaccine) or by intra-ruminal pellets. MATERIALS AND METHODS In late July 1984, at Ginninderra Experiment Station near Canberra, 180 Merino ewes aged either two years (44.1 kg) or seven years (51.0 kg) and mated to Merino rams were allocated into three treatment groups containing 30 ewes of each age class. A further 180 Merino ewes aged three years (53.0 kg) or six years (51.2 kg) and mated to Border Leicester rams were similarly allocated to three groups. Within each mating type, the three groups received either no supplementary Se (control), clostridial vaccine containing Se as sodium selenate (SV: ewes injected (dose 5 mg Se) two weeks pre-partum, lambs (dose 1 mg Se) at approx. four weeks of age) or Se from an intra-ruminal pellet (SP; given to ewes, with metal grinder, two weeks pre-partum). The control and SP groups received vaccine without Se. All ewes were treated for helminths two weeks pre-partum and lambs were treated at weaning on 28 November. After allocation, the six groups rotationally grazed six 10 ha plots (approx. three weeks/plot), in an attempt to minimize the effects of random variation between plots. Soils in the plots were derived from porphyritic dacite and were predominantly shallow red earths (pH 5.3-5.5) of low nutrient status (Sleeman 1979). The availability of pasture (phalaris/subterranean clover) at allocation was 1000-1200 kg DM/ha and this declined to less than 500 kg DM/ha prior to the main flush of pasture growth in mid-spring. After weaning, Merino and crossbred lambs were managed separately, though both groups grazed dry pasture on soils similar to the experiment area. *CSIRO Division of Plant Industry, Canberra, A.C.T. Goulburn, N.S.W. **N.S.W. Dept of Agriculture, 187 Proc. Aust. Soc. Anim. Prod. Vol. 16 Ewes were weighed at allocation and on 12 Sept., 10 Oct., 31 Oct. and 28 Nov.. Lambs were weighed at birth, on the above dates and at lamb shearing on 5 Feb. 1985. At all weighings except 31 Oct., jugular blood samples were taken from a sub-group of the same six lambs in each Merino group to allow blood glutathione peroxidase (GSH-Px) determinations (Paglia and Valentine 1967). Ewes were bled on Crossbred lambs were sampled only 7 Aug., 12 Sept. and in group SP, at weaning. at weaning and shearing. The effects of ewe age, number of lambs suckled or rearing type and Se status on ewe and lamb performance were analysed by regression using procedures similar to those used by Donnelly et al. (1985). An interaction between lamb genotype and Se response was noted in early analyses and, thereafter, separate analyses were conducted for each genotype. RESULTS At allocation, mean whole blood GSH-Px activity in ewes was 47 U/g Hb and did not differ between the six groups. After lambing, the activity in control ewes had fallen to 37 U/g Hb, compared with 85 U/g Hb in ewes on treatment SV (P<O.OOl) and 167 U/g Hb in ewes on treatment SP (P<O,OOl). By weaning, the activity in this last group had fallen to 107 U/g Hb. During the spring, the GSHPx activity in SP Merino lambs was about twice that in the SV lambs and about four times that in control lambs (Table 1). Levels in crossbred lambs at weaning and shearing were similar to the Merino lambs. In February, activities in all lambs had risen by about 100 U/g Hb from those found in late spring. Table 1 Blood glutathione peroxidase activity in Merino lambs a,b,c Column means with different letters differ at PcO.05 XIYJ Row means with different letters differ at PcO.05 There was no difference between the ewe mating groups in live weight response, so the results have been pooled (Table 2) and adjusted for differences in allocation weight. Results are expressed as terms to be added to our subtracted from the fitted value at the first level of all factors. After lambing, ewes suckling lambs had incurred large penalties in live weight. This effect became more pronounced as lactation progressed, but from late October, when lactation was declining rapidly, weight changes in dry and lactating ewes were not different. Three year old and six year old ewes lost more weight than the other age classes (2-2.5 kg; P<O.Ol) though this effect was confounded with genotype of lamb suckled. Selenium supplementation, especially via vaccine, had significant effects on ewe live weight early in lactation, but by weaning these had disappeared. Selenium treatment had no effect on lamb birth weight or survival. Lambsof both genotypes from older ewes were lighter at birth and small differences persisted through lactation. Twins were lighter than singles at birth and the difference increased to about 5 kg at weaning (Table 3). Merino lambs in supplemented groups grew faster than controls, were about 2 kg heavier at weaning and maintained this advantage through summer (P<O.O05). Crossbred lambs in group SV grew faster in early spring but then more slowly than control lambs. Crossbred 188 Proc. Aust. Soc. Anim. Prod. Vol. 16 lambs in group SP grew more slowly throughout lactation and were still significantly lighter at shearing (2.6 kg; P<O.OOl). Both these groups had higher faecal helminth egg counts than the equivalent Merino groups (O.lO>P>O.O5). .Table 2 Components of the live weight response in Merino ewes Supplemented Merino lambs produced more wool than control lambs (Table 3). In crossbred lambs, the small response to Se in group SV approached significance but lambs in group SP had significantly lighter fleeces than the other two groups. DISCUSSION Our results confirm the existence of sub-clinical in sheep grazing in the Southern Tablelands of N.S.W.. area, including those of the present study, are of low igneous rock; Se-responsive conditions might thus have 1975; Caple et al. 1980; Langlands et al. 1981). Se responsive conditions Many of the soils in this pH and are derived from been expected (McDonald, Prior to treatment, whole blood GSH-Px activity in the ewes was moderately low and in control lambs after lambing, was higher than the activities at which lamb growth responses have been suggested to occur (~25-30 U/g Hb; Paynter et al. 1979; Caple et al. 1980; Peter 1980). During spring, GSH-Px activity in the control Merino lambs fell to deficient levels, as in previous studies (Paynter et al. 1979; Caple et al. 1980). Sub-cutaneous injection of Se into ewes pre-partum raised their post-partum GSH-Px activity and that of their lambs. This high activity was maintained by the injection given to the lambs. The higher activities in ewes and lambs in group SP after lambing imply a greater availability of Se from intra-ruminal pellets. Activities in this group then fell during spring, though not as fast as has been reported for blood Se levels after similar treatment (Hunter et al. 1982). The high activites in all groups after the dry summer presumably reflect the high Se intakes characteristic of such periods (Hunter et al. 1982). Significant effects of Se on ewe live weight were noted, but clearly, the main factors affecting ewe live weight in lactation were ewe age and particularly, the number of lambs being suckled. As in previous studies (Donnelly et al. 1985), both ewe age and lamb rearing type contributed significantly to weaning weight (approx. 1 kg and 5 kg respectively) in both lamb genotypes. Both forms of Se treatment increased liveweight gain in Merino lambs. The 2 kg advantage in weaning weight was close to that found by McDonald (1975) and was not lost over su3nmer. The administration of Se by injection resulted in only half the wool growth response noted with Se given intra-ruminally. The 17% 189 Proc. Aust. Soc. Anim. Prod. Vol. I6 increase in Merino wool growth, equivalent to an annual increment of about 400 g greasy wool, is also similar to the increment of 14.4% (530 g) reported by McDonald (1975). The equivocal response in crossbred lambs may be related to the effect of helminths. However, given the long half-life of the red blood cell (120-150d) the possibility of 'adequate' blood GSH-Px levels but deficient tissue Se levels in these groups cannot be discounted. Table 3 Components of the live weight and wool growth response of Merino and Border Leicester x Merino lambs Our suggest Southern response results have demonstrated significant response to Se in Merino lambs and that Se supplementation of Merino flocks in susceptible parts of the Tablelands may be advisable. Further work is needed to clarify the in crossbred lambs. REFERENCES CAPLE, I-W., ANDREWARTHA, K.A., EDWARDS, S.J.A. and HALPIN, C.G. (1980). Vet. -.. 56-: .-J. -- 160. DONNELLY, J-R., McKINNEY, G.T. and MORLEY, F.H.W. (1985). Aust. J. 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