BIOLOGICAL TRACE ELEMENT RESEARCH 2, 281-290
Effect of Zinc Supplementation on the Reproductive Performance of Grazing Merino Ewes DAVID G. MASTERS*
Department of Animal Science and Production, University of Western Australia, Nedlands, Western Australia, 6009 AND
H. E. FELS Western Australian Department of Agriculture, Jarrah Road, South Perth, Western Australia, 6151 Received May 12, 1980; Accepted June 13, 1980
Abstract Two experiments were carried out in Western Australia to investigate the effect of zinc supplementation on the reproductive performance of grazing Merino ewes. We found that supplemental zinc, when provided prior to mating and throughout pregnancy, increased the number of lambs produced by 14% (P < 0.05) in both experiments. An intermediate zinc treatment, when supplementation was begun later in pregnancy gave a 9% ( P < 0.01) increase in the number of lambs in one experiment and no increase in the second. Lamb birth weights were increased by zinc supplementation in Experiment 1 and in Experiment 2, 12-20-weekold Iambs from zinc supplemented ewes were 2.1 kg heavier than those from nonsupplemented controls. Plasma zince levelsdecreased significantly during pregnancy and lactation, but were increased at some sampling dates by 20-25% by zinc supplementation. Index Entries: Zinc, and reproductive performance; reproduction, and zinc in sheep; supplementation of zinc, in reproduction; Merino ewes, and zinc in reproduction; ewes, and zinc in reproduction. 9 1980 by The Humana Press Inc. All rights of any nature whatsoever reserved. 0163-4984/80/12004)281502.00
MASTERS AND FELS
Introduction The zinc requirements of grazing ruminants are not well defined. Sporadic outbreaks of zinc-responsive conditions have been reported in Guyana (I), Greece (2), Norway (3), Finland (4), and Australia (5). These responses were observed despite the fact that pasture zinc levels were usually in excess of those normally considered to be adequate (6). In all but one of the above reports, supplementation with zinc increased live weights and/or alleviated parakeratosis or other skin lesions. However, Egan (5) significantly increased the lambing percentage of Dorset Horn ewes by drenching them regularly with zinc sulfate. The reproductive process in the female rat may be dramatically affected by severe zinc deficiency. Failure to mate, failure to continue pregnancy to term, and congenital malformations may result (7, 8). Zinc deficiency during the first few days of pregnancy causes abnormal cleavage and blastulation in preimplantation eggs (9). In cattle, Pryor (10) reported lower plasma zinc levels just prior to parturition, and Dufty et al. (11) suggested that these are associated with difficult births. Apgar and Travis (12) have reported that lowering the zinc content of the diet to < 1.0/.tg g-~ during the last 64 days of pregancy of the ewe had no effect on the number and size of lambs. A survey on the zinc status of grazing sheep in Western Australia (13) revealed that during early pregnancy ewes were often grazing pastures containing less than 20/.tg Zn g-i and that the zinc status of these sheep, as indicated by plasma and wool zinc levels, was lowest not only in early pregnancy when pasture zinc levels were low, but also in later pregnancy when pasture zinc levels were at their highest. These results are consistent with those of Williams et al. (14) who reported that the developing foetus may accumulate a high proportion of the dietary zinc absorbed by the ewe. The experiments reported here were designed to show whether pregnant grazing ewes would respond to supplemental zinc and whether levels of zinc in plasma are affected by pregnancy, lactation, and zinc supplementation. The work was undertaken in areas of Western Australia where soils were inherently low in zinc and crops and pastures may require zinc fertilizer for optimal growth.
Experiment 1 Sheep and Experimental Design Three hundred and thirty 2-3-year-old Merino ewes were allocated at random from stratified weight groups into three groups of 110. Treatments were: Group 1 (control): no supplementary zinc.
ZINC AND REPRODUCTION IN THE EWE
Group 2 (zinc throughout mating, pregnancy and lactation): ewes were given a slow release intraruminal zinc pellet and grinder (15) 16 days prior to mating and an additional zinc pellet every 6-7 weeks for the duration of pregnancy and lactation. Group 3 (zinc for part of pregnancy and lactation): ewes were given the same form of zinc supplementation as Group 2, but beginning 69 days after mating commenced. At the outset,l the ewes were joined with vasectomized rams and between I4 and 36 days later with entire rams. The entire rams were fitted with ewe marking harnesses and a different colour crayon was used every two weeks to detect services and returns to service. After 28 days, 120 ewes that had mated in the first estrous cycle were examined by laparascopy and ovulation rates were recorded, These included 40 that had received zinc pellets and 80 that had not. The ewes were kept in one group until lambing and were fed supplementary oaten hay and oats grain from day 83 to day 215. The day before lambing began, the three treatment groups were separated and the paddock fenced into three sections. During lambing the groups remained separated and were rotated around the three sections once per week. The sheep were observed twice per day, newborn lambs were weighed, tagged, and their mother identified and recorded. Dead lamb carcasses were counted and removed. At the end of lambing, the three groups were recombined, To verify the lambing data, ewes were classified as having lambed or not by the method of Dun (16) and by comparing live weights before and after lambing.
Collections and Analyses Ewes and, later, Iambs were weighed every 6-7 weeks. Subsamples of 20 ewes from each group were selected at the start of the experiment for serial sampltng of blood. Of these, nine per group were sterilized by crushing the fallopian tubes.
(a) Blood. Blood was collected by jugular puncture with sterile disposable needles into heparinized tubes and centrifuged within one hour of collection. The samples were returned to the laboratory on ice. Plasma was diluted (1/1) with 10% TCA, centrifuged, and analyzed by atomic absorption spectrophotometry. (b) Pasture. Every 6-7 weeks samples were selected and cut at ground level at approximately six sites on a diagonal line across the paddock. Samples were washed on a 1 mm sieve with deionized water and dried for 48 ~Experiment #1 commenced when the vasectomized rams were introduced on December 7. Experiment #2 commenced when entire rams were introduced on December 12.
MASTERS AND FELS
h at 80~ C. They were then ground, digested, and analyzed for zinc, as in Masters and Somers (13). During the period of supplementary feeding, samples of hay and oats were also analysed.
Experiment 2 Sheep and Experimental Design Three hundred mixed-age Merino ewes were allocated at random into three groups of 100. The ewes were mated with Poll Dorset rams for 55 days. Treatments given were: Group 1 (control): no zinc treatment. Group 2 (zinc throughout mating, pregnancy and lactation): ewes were supplemented by the same method as in Experiment 1, but treatment started on the first day of mating. Group 3 (zinc for part of pregnancy and lactation): ewes were given the first zinc supplement 108 days after the commencement of mating and again every 6-8 weeks through pregnancy and lactation. The ewes grazed as one group throughout pregnancy and lambing. Immediately after lambing the lambs and ewes were penned and each ewe was marked in front of the udder with fleece branding fluid (Si-Ro-Mark, Cooper Australia Ltd.). Three colors were used, one for each treatment group. All sheep were then released overnight and penned again the following morning. All lambs that had suckled overnight had a color mark on top of their heads. This was used to allocate them to one of the treatment groups. Ninety-five percent of lambs were easily identified from the distinctive color on their heads. Any with no color on the head or two colors were released individually, observed, and the mother identified. Lambing data were verified by the same methods as in Experiment 1.
Statistical Analysis The number of lambs born and surviving were compared by ;2 analysis. Live weights were compared using Students' t test and plasma zinc levels by a two-way analysis of variance of the 3 • 2 factorial experiment (3 zinc treatments, 2 breeding treatments).
Locations and Environment Experiment 1 was carried out at Mt. Barker Research Station (34~ 37'S, 117~ The average rainfall is 690 mm and the soil type is sandy loam over clay. The predominant available forage was grass (60-70%), the species were barley grass (Hordeum leporinum), silvergrass (Vulpia spp) and
ZINC AND REPRODUCTION IN THE EWE
ryegrass (Lolium spp). The other constituents of the pasture were clover (Trifolium subterraneum) and erodium (Erodium botrys). By the end of the experiment the proportion of clover had increased to 75%. Superphosphate was applied at the rate of 135 kg/ha in 1978 and 1979. Superphosphate contains 250-750 #g Zn g-~ as an impurity (17). The fertilizer, cultural practices, and soil types at the station are typical of farms throughout the area. A second experiment was carried out at the same time on a private property at Badgingarra (30~ 115~ This location is approximately 550 km north of Mt. Barker and has an average annual rainfall of 530 ram. The soil is yellow and grey acidic sand containing much ironstone gravel. The pasture was grass dominant, grass species were speargrass (Stipa spp) and ripgut brome (Bromus rigidus). Clover (Trifolium subterraneum) was the other major component of the forage. At both sites most of the rain falls between April and October. From October to April much of the pasture is dry and sheep are often fed supplementary grain or hay between March and May.
Results The levels of zinc in pastures during mating, pregnancy, and lactation are shown in Table 1. Lambing data are presented in Table 2 and plasma zinc data in Table 3 and Fig. 1.
Experiment 1 Relative to the group 1 controls, group 2 ewes produced 14% more lambs (P < 0.05) and group 3, 9% more lambs (P < 0.1). Treatment of ewes with zinc also increased the proportion of lambs to survive (P < 0.05). By the end of the lambing period, 70% of lambs born to the nontreated controls were alive compared with 84% of lambs born to the ewes that received zinc throughout mating and pregnancy (group 2). No significant differences in the incidence of estrus, numbers of ewes returning to service, or ovulation rates were found. Ewes treated with zinc throughout mating and pregnancy had heavier lambs (0.19 kg) than control ewes (P < 0.05), but this difference in weight had disappeared within 6 weeks. There was no significant difference between the lamb weights of the two zinc treatments.
Zinc Levels in Plasma. Plasma zinc levels were increased by the zinc pellets from June to August (P < 0.05) (see Fig. 1). The sheep in group 2 had 20-25% higher plasma zinc concentrations during this period than the controls in group 1.
MASTERS AND FELS
TABLE 1 Levels of Zinc in Pasture, Hay, and Oats* Days after start of experiment Pasture: 8 before 0 42 55 84 107 126 149 161 201 205 245 252 294 298 Oats grain Oaten hay
Mt. Barker 17.5
13.4 15.8 26.3 23.0 26.4 27.4 16.0 28.8 32.6 24.0 21.4 29.3 34.0 20.9 18.4-21.2 7.5-25.2
a/~g g-t dry matter.
Pregnancy and/or lactation reduced plasma zinc concentrations at two sampling dates immediately prior to lambing and again after seven weeks of lactation (P < 0.05). The interaction between zinc treatments and breeding treatments was significant after seven weeks of lactation (P < 0.05, see Table 3).
Experiment 2 Group 2 produced 14% more lambs than groups 1 or 3 ( P < 0.05). Inspection of mammary development of all ewes and comparison of live weights before and after lambing indicated that this difference owed to an increase in the proportion of ewes bearing lambs and not to increased survival of lambs after parturition. Although birth weights were not recorded, the lambs born to ewes in group 2 were slightly heavier (0.7 kg) than those from group I at the end of lambing. This difference had increased after another 5 weeks (I.2 kg, P < 0.05) and 12 weeks (2.1 kg, P < 0.025).
ZINC AND REPRODUCTION IN THE EWE
TABLE 2 Responses to Zinc Supplementation Experiment Exp. #1 Number lambs Number ewes P < 0.05 P