Abstract:
SOME PRACTICAL CONSIDERATIONS FOR IMPROVING THE PREGNANCY RATE OF TROPICAL MERINOS P.S. HOPKINS* and M.S. PRATT* Summary The strategic use of natural shade and iodine supplementation at the time of joining led to an improved fertility of tropical Merino ewes. This was most pronounced when ambient heat stress was greatest (p <O.Ol). A higher incidence of oestrus and a higher pregnancy rate Heat' in mated animals both contributed to the improved fertility. stress conditions experienced during summer in a tropical semi-arid environment were sufficient to cause a suppression of oestrous activity. Temporary relief from these conditions brought about by a cool change and an associated storm led to a marked synchronization of oestrus. The aberrent oestrus cycle-lengths of 'synchronized' ewes were not specifically associated with a reduction in pregnancy rate. I. INTRODUCTION The detrimental effects of heat stress at joining on the fertility of ewes have been the subject of numerous investigations. Attempts to apply some of the more basic findings to field conditions have received less attention. This paper examines the effects of the strategic use of natural shade and iodine supplementation on the pregnancy rate of tropical Merinos joined during either spring or summer. These treatments were chosen after preliminary studies with tropical Merinos in a climate chamber had substantiated suggestions that heat stress exerts a major influence on female fertility when applied during the first week after mating (Thwaites 1971) and that thyroxine (and iodine supplementation) therapy at joining improved the pregnancy rate of heat stressed ewes (Ryle 1961). Marginal to low plasma thyroxine levels of sheep in semiarid tropical regions of Queensland (unpublished data) was a further reason for including the iodine supplementation treatment. II. MATERIALS AND METHODS Eight hundred and nine locally bred Merinos were selected from a flock of 1000 mature ewes. Animals excluded from the study were over Two groups 8 yr. The sheep were randomly allocated to four groups. were joined to entire harnessed rams (3%) for a 24 d period in Oct. (spri,ng), the two remaining groups were joined for 24 d in Nov.-Dec. (Summer). Vasectomized rams were placed with all groups of ewes for two weeks before joining with entire rams. One of the groups from each of the joining periods was run in a well shaded (Acacia farnesiana) 100 ha paddock and received a weekly potassium iodide supplement (20 mg drench) during the two weeks before joini g and the week after -joining. The other (control) group from each oB the joining periods was run in an unshaded l-00 ha paddock and received no supplement. Oes trus recordings we re made at 3di nterva IS The rams were rotated between shaded and unshaded paddoc ks eve ry'three d and were *Queensland Department of Primary Industries, 'Toorak' Research Station, Julia Creek, Queensland. 4823 153 replaced after 12 d. Ewes were removed from the various flocks 7-10 d after oestrus was recorded and run together in a moderately well shaded paddock until pregnancy diagnosis in the fifth month of gestation (Pratt and Hopkins 1975). III . RESULTS Provision of shade and iodine supplementation enhanced the reproductive performance of paddock joined ewes (Table 1). The improvement was most apparent in the summer joining when environmental heat stress was highest. This improvement was attributed to both a higher incidence of oestrus (p ~0.01) and a higher pregnancy rate of mated ewes. TABLE 1 The effects of shade and iodine supplementation on the incidence of oestrus and the pregnancy rate of ewes joined during the spring and`summer The degree of heat stress experienced by the control flock during the summer joining period was sufficient to cause a suppression of oestrous activity (Table 2). The occurrence of a cool change (storm) Ted to a marked synchronization of oestrus in the last three d of the joining. The pregnancy rate of 'synchronized' ewes which showed aberrent oestrus patterns was slightly higher (77%) than that of the ewes which mated in the first 21 d of the joining (67%). The cool change caused an apparent though less pronounced synchronization in the treated ewes. The comparable values for pregnancy rates of these ewes were 82% and 80% (Table 2). 154 TABLE 2 The pregnancy rates of ewes joined during the various 3 d observation periods of the summer joining are listed in Table 3. These indicate that ram infertility was not contributing to the results described herein. TABLE 3 IV. DISCUSSION - The provision of shade and iodine supplementation led to an improved level of fertility in ewes which were joined during summer in this semiarid tropical environment. It is not possible from this experiment to determine which of these treatments was the most important, but it is practically and economically feasible to provide both in this region. 155 The propagation of Athel Pines (Tamarix aphylla) on the heavy brown soils of the plains is a simple means of shading large flocks of sheep, while the provision of a potassium iodide solution which drips into the bore drain or other watering points affords an acceptable method of providing this supplement. It is possible that these treatments would be seen to better advantage under the more extensive grazing conditions of commercial properties. This reasoning stems from the fact that the sheep which normally run in a number of scattered groups in large joining paddocks would tend to congregate on shaded watering points thereby ensuring better ram distribution and adequate supplement intake. The physiological interpretation of the synchronization of oestrus with the change in climatic conditions warrants further study. Follicular development had apparently occurred normally during the heat stress conditions, though the expression of behavioural oestrus was temporarily suppressed. The subsequent pregnancy rate associated with the` aberrent oestrus cycles was slightly higher than normal, indicating that this phenomenon was not inimical to fertilization. This pregnancy rate was not the result of a prolonged cool change at joining since the heat stress conditions returned within 24 h of the onset of the change. The practical implications of attaining a. high level of fertility during a summer joining also extend to a consideration of lamb survival. By joining in Nov. -Dec. the ewes do not experience heat stress in the last month of gestation, a factor which normally contributes to the low birth weight and high neonatal mortality rate of lambs in this environment. Furthermore, lamb survival would also be enhanced by the absence of hot weather during the first week after birth (Moule 1954). The only limitation to the adoption of this husbandry procedure.arises from the fact that it exacerbates the weaner nutrition problem normally encountered in winter. V. ACKNOWLEDGEMENT Financial support from the Australian Wool Corporation for the servicing of this project is acknowledged. VI . MOULE, G.R. (1954). REFERENCES Australian veterinary Journal. 30: 153. = Australian veterinary Journal. PRATT, M.S. and HOPKINS, P.S. `(1975). 51: X 378 RYLE, M. (1961). Journal of agricultural Science. 57: 1. B THWAITES, C.J. (1971). Australian Journal of experimental Agriculture and animal Husbandry. 11: 265. T 156