Abstract:
SYNCHRONISATION OF EWES AT THE SECOND OESTRUS AFTER PROGESTAGEN TREATMENT T. N. EDEY* and C. J. THWAITES* Summary To determine the change in distribution of ewes exhibiting oestrus at the first and second oestrous cycles after progestagen treatment, data were examined from six experiments in which 430 ewes received progestagens but experienced no fertile mating at the first post-treatment oestrus. Three progestagens, three breeds and two seasons were involved. In all experiments there were more silent heats at the first as compared with the second post-treatment cycle; hence more ewes were available for mating at the second cycle. Among 326 ewes which exhibited oestrus at both cycles, standard deviations about the mean day of oestrus ranged in the six groups from * 0.47 to t 1.13 days for the first oestrus and from t 0.85 to t 1.44 days for the second oestrus. Because of the increased dispersion at the second cycle, a higher percentage of ewes exhibited oestrus during the peak three days of the first cycle than was the case with the second. The number of heats falling within three days at the first cycle approximately equalled the numbers falling within four to five days at the second cycle. It is concluded that in some circumstances it may be advantageous to delay mating until the second cycle. I. INTRODUCTION .Reasonable efficiency of synchronisation of oestrus in the ewe has been achieved using progestagens orally and by injection (Lamond 1964) and also, more recently, intravaginally (Robinson 1964). However, fertility following synchronisation has frequently been impaired due possibly to fertilisation failure or early death of the fertilized egg (Lamond 1964), or to an increased proportion of abnormal ova (Foote and Waite 1965). A practical way to avert this possible impairment of fertility would be to delay mating until the second post-treatment oestrus, provided synchronisation persisted. Because of the normal variation about the mean cycle length, some reversion towards a random distribution would be expected at each successive cycle following synchronisation. In fact, however, published information on the rate at which dispersion occurs is fragmentary, being mainly confined to data on ewes which were m,ated but failed to conceive at the first post-progestagen oestrus. Lishman and Hunter ( 1961), Hogue, Hansel and Bratton ( 1962), and Brunner, Hansel and Hogue ( 1964) present this type of data for small numbers of ewes, without statistical analysis. Their results indicate that synchronisation may persist reasonably well to the second post-progestagen oestrus. To the authors' knowledge, the only experiment designed to study this situation is reported by Dhindsa et al. ( 1964). These workers, using groups of 36 ewes, found no significant difference in the degree of synchronisation at the first and second post-treatment oestrus. *Department of Livestock Husbandry, University of New England, Armidale, N.S.W. In a review of a number of experiments, Lishman and Hunter (1961) noted that sub-oestrus or 'silent heat' following progesterone withdrawal may effectively reduce the numbers of ewes available for mating at the first synchronised oestrus, but, since most of these ewes subsequently exhibit normal oestrus approximately 17 days later, increased efficiency may result from delaying mating until~ the second synchronised oestrus. Designs involving this latter practice have been employed in a variety of experiments at the University of New England, Armidale, and this paper presents data from these on the degree of synchronisation at the first and second posttreatment oestrous cycles. No attempt is made to compare the effectiveness of different progestagens, since conditions in the various experiments were not always comparable. II. MATERIALS During the Autumns of 196$4 and 1965, synchronisation techniques were applied to 430 ewes in six experiments for purposes other than synchronisation studies. The experiments embraced three breeds and three hormonal treatments as set out in Table 1. In Experiments 1, 2, 3 and 5 the sheep were paddock-grazed; in Experiments 4 and 6 they were hand-fed in yards. Infertile rams wearing raddle harnesses (Radford, Watson and Wood 19608) were used to mark ewes in oestrus during the first and second post-treatment cycles. Ewes were inspected every 24 hours for fresh raddle marks. III. RESULTS Table 2 shows the relative synchronisation data in the six experiments, and indicates that in three experiments there were many silent heats at the first but significantly fewer at the second post-treatment oestrus. TABLE 1 Sources of data * 17 a-acetoxy -9 Q! fluoro - 11 p - hydroxypregn. - 4 - en 3,20-dione (Searle. Chicago). +Robinson (1964). $6-Chloro - AS - 17 - acetoxyprogesterone (Eli Lilly Aust. Ltd.). - -.-.--. ..-. 39 TABLE 2 Number of ewes having synchronised oestrus or silent heats at the first und second cycles 'fNumber of ewes detected in oestrus within a 7-day period. $Number of silent heats between cycles differs significantly, when : `:+P < 0.0 1; * + +P < 0.001. Of 430 sheep treated, 326 (75.8% ) exhibited oestrus at both times. Within experiments, the range was from 60.0% to 94.4%. Synchronisation patterns of the 326 ewes which cycled twice are shown in Figure 1. Since the release times of different progestagens vary, the mean release times are not of importance in this study. However, the standard deviations of the mean interval from the end of progestagen treatment to oestrus, when subjected to variance ratio test, indicate a significant decrease in persistence of synchronisation in Experiments 1, 2, 4 and 5, a non-significant decrease in Experiment 3 and a non-significant increase in Experiment 6. A more critical examination of effectiveness of synchronisation can be made if we accept as synchronised only the ewes cycling on the three successive days of highest incidence of oestrus. Table 3 shows the relative figures at the first and second cycles. At the first cycle, percentages synchronised ranged from 80.95 to 100% ; at the second cycle, the range was 80.65 to 90.48 % , with five out of six experiments showing a decline. Table 3 also shows the percentage of ewes exhibiting oestrus on the four and five peak days at the second cycle. The four day analysis shows that in three experiments the corresponding three-day percentage at the first cycle has been equalled or surpassed. In the five day analysis, this is the case in five out of six experiments. Percen.tage of ewes symhronised within three days at the first cycle and within three, four and five days at the second cycle TABLE 3 40 DAYS AFTER END OF PROGESTAGEN TREATMENT Fig. 1.4ccurence od oestrus at the first second cycle after synchronisation. an d t Mean interval from end of progestagen treatment t S.D. $ S.D.` differ significantly, *P<O.O5; **P<O.Ol s IV. DISCUSSION These results provide evidence that once oestrus is synchronised, the degree of dispersion at the next oestrous cycle is small. This confirms the tentative conclusions, based on small numbers of ewes, of Lishman and Hunter ( 1961)) Hogue, Hansel and Bratton ( 1962), Brunner, Hansel and Hogue ( 1964) and Dhindsa et al. ( 1964). In the present work, in broad terms, the percentage of ewes synchronised to within three days at the first cycle is spread over four to five days at the second cycle. The importance of this increased dispersion where mating is delayed until the second cycle would depend on the circumstances, but in most cases would probably not be great. Where the dangers of depressed fertility at the first cycle (Lamond 1964; Foote and Waite 1965) are to be avoided, or in experiments where a harmful influence of exogenous hormones exists, the small loss of efficiency of synchronisation at the second cycle might be considered unimportant. No comparative information on fertility at first and second cycles is available from these experiments. The effect of silent heats must be considered. Lishman and Hunter ( 1961), and Robinson ( 1964) reported a considerable incidence of silent heats following the use of progesterone. The present work confirms this but also indicates that chlormadinone and to a lesser extent SC 9880 may in some circumstances suffer the same disadvantage. However, most of the ewes experiencing silent heats at the first post-treatment cycle had a normal synchronised second oestrus, and the number of ewes available for mating within a specified period at the second cycle was actually greater than at the first cycle in all six experiments (Table 2). V. ACKNOWLEDGMENTS The data were derived from experiments supported by the Wool Research Trust Fund and the University of New England. Thanks are due to Mr. P. F. May, C.S.I.R.O., Armidale, for statistical advice. VI. REFERENCES B RUNNER , M. A., H ANSE L, W., and HOGUE, D. E. ( 1964). Use of 6-Methyl-17-acetoxyproges'cerone and pregnant mare serum to induce and synchronise estrus in ewes. Journal of Animal Science 23: 32. D HINDSA , S. A., H OVERSLAND , A. S., S MIT H, E. P., and V AN H OR N, J. L. ( 1964). 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( 1964). Synchronisation of oestrus in sheep by intravaginal and subcutaneous application of progestin impregnated sponges. Proceedings of Australian Society o f Animal Production 5: 47. R ADFORD 43