Abstract:
158 Proc. Aust. Soc. Anim. Prod. Vol. 17 PMSG RESPONSE IN EWES OF DIFFERENT BREEDING VALUE FOR REPRODUCTIVE RATE AS AN AID TO SELECTION L.G. BUTLER+* and W.M.C. MAXWELL+** SUMMARY One hundred and fifty ewes with lifetime records of ovulation rate and lambing were divided into two groups on the basis of age, live weight and reproduction index (calculated from reproductive performance data). Both groups were synchronised in oestrus with progestagen sponges inserted for 14 days. At sponge removal one group received no PMSG while the other group received 400 i.u. PMSG. Eight days after sponge removal, the ovaries of all ewes were examined at laparoscopy. Ovulation rate increased from 2.04 eggs shed per ewe ovulating in the control group to 3.00 due to PMSG (P <O.OOl). There was a positive relationship between ovulation rate and reproduction index (r = 0.18) but there was no index x PMSG treatment interaction. Although this work does not support the hypothesis that, within a flock, ewes of higher breeding value for reproductive rate respond more to PMSG treatment than ewes of low breeding value, further work with larger numbers of animals is needed. Keywords: ovulation rate, PMSG, selection, reproductive rate INTRODUCTION There is evidence reviewed by Bindon (1984) tha.t ewes of high breeding value (genetic merit) for reproductive rate have a higher ovulatory response to PMSG than ewes of low breeding value. Bindon et al. (1971) and Trounson and Moore (1972) respectively found that the mean ovulatory response to PMSG of Trangie YHigh Fertility' ewes was 300% and 50% greater than that of ewes' selected against multiple births. This evidence had'been derived from established flocks exhibiting a high reproductive rate as a result of selection for this trait (Booroola, Trangie High 'Fertility, Romney High Fertility). This genetic,association between prolificacy and sensitivity to PMSG has also been noted in the Finn and D'Man breeds of sheep, and in mice and cattle (Bindon 1984). _ Responsiveness to PMSG could be a valuable tool in a selection program aimed at increasing reproductive rate, if the ovaries of ewes of high breeding value for reproductive rate are most responsive to PMSG. That is, if in comparison with ewes of low breeding value, ewes of higher breeding value for reproductive rate were distinguished by expressing higher ovulation rates. under . PMSG stimulation, then accuracy of selection would be increased. In effect this procedure could increase the realised heritability of re.productive rate and Any means of increasing 'the' .consequently the rate of genetic progress'achieved. rate of genetic progress in reproductive rate would benefit the sheep industry. , , _ \ This hypothesis has not been tested in a flock which has not been highly selected for reproductive rate over many generations. The-present experiment was designed to determine the magnitude of response in ovulation rate following treatment with PMSG of ewes of differing breeding value for reproductive rate . Proc. Aust. Soc. Anim. Prod. Vol. 17 MATERIALS AND METHOD Sheep 159 Ewes of mean live weight 59.5 kg (S.E.M. = 0.55 kg) and mean condition score 3.4 (S.E.M. = 0.07) with records of pedigree, annual ovulation, and lifetime reproduction were available from the Katanning Animal Breeding and Research Institute Fertility Flock (Butler and Morrow 1984). This flock had , been selected and' bred for reproductive rate (assessed by ovulation rate) for only 6 years from 1980 using ovulation rate as the major selection criterion. Most ewes were 6 years old and were the progeny of ewes initially screened into the flock and sired by unselected rams. Design Ewes were ranked on an index for reproductive performance by the method of Turner (1968). This method took account of the number of lambings and the heritability and repeatability of lambs born. .Effectively the ranking gave an index of 100 to a ewe which averaged one lamb per joining up to the age of observation. The ewes were then divided into two groups, A and B, balanced by using stratified lists for index, age, and liveweight. Management On February 18, 1986 progestagen sponges were inserted and removed 14 days later. At sponge removal, ewes in group A were injected with 400 i.u. PMSG and the ewes in group B received no PMSG. The ewes were then joined with 10% teaser wethers. Eight days after sponge removal, the ovaries of all ewes were examined by laparoscopy and ovulation rate was recorded. An assay of the PMSG batch'in uns'elected Merino sheep at Katanning indicated that in autumn, 400 i.u. would give.a mean ovulation rate of 3.5 (range 1 - 8). Ewes were weighed and condition score ,estimated each week and at , laparoscopy. analysis I A logit regression model with binomial errors was fitted to, the ovulation rate data. When index, treatment and index'x treatment effects were adjusted for live weight and live weight change;, there wasno effect and so live weight differences were ignored. RESULTS.,. During the 22 da.ysof the experiment,, average-live weight and condition score decreased from59.5 kg to 52.4.kg and 'from. 3.41 to 3.28 respectively. * The regression of ovulation rate'on ind.ex wassignificant (P.<O.OS) for the data for PMSG treatedeand nonFtre.ated sheep combined.(r= 0.18) and*-for.the data for the non-treated, sheep alone. {rs,,: O-,29). bI~ ?heseregressions suggest that ' the index was effective in ident&fy+ng,e,wes .,of ,diff erent .genotype-,forovulation rate. The contingency\..table (Table 1) shows. the-number, of animals .in each cell for ovulation-rate x PMSG treatment x index r,ange* and the 'a.erage,ovulation rate for each index range. I The analysis. showed that ovulation rate increased Statistical 160 Proc. Aust. Soc. Anim. Prod. Vol. 27 from 2.04 (S.E.M. = 0.09) eggs shed per ewe ovulating in the untreated group to 3.00 (S.E.M. = 0.17) due to PMSG treatment (P <O.OOl). There was no effect on response to PMSG due to index x treatment interaction. Table 1 Number of animals in each cell (index range x PMSG treatment x ovulation rate) and average ovulation rate for PMSG treatment x index range. DISCUSSION These data do not support the hypothesis that, within a flock, individual animals of different estimated breeding value for reproductive rate vary in their response to PMSG. It seems unlikely that the 12% and 4% reduction in live weight and conditionscore respectively would be entirely responsible for, the lack of interaction of index with response to PMSG, because these reductions would be expected to resultin a depression of about 0.1 eggs shed per ewe ovulating (Lindsay et al. 1975). 4 The mean ovulation rate of the PMSG'treated group (3.0) was lower than we expected (3.5). Therefore it is possible that greater stimulation of the ovaries of higher index ewes if treated with a higher dose of PMSG may have increased the proportion of ewes with high ovulation rates, sufficiently to generate a significant difference. In this respect, a sensitivity analysis of the contingency table suggested that, in order to achieve significance with 150 e w e s , 90% of high index ewes would have had to exhibit 4 or more ovulations under PMSG stimulation, compared to only 10% of untreated ewes. t Further work is required to elucidate the hypothesis: ,pmx?. &ist. Soc. Anim. PPod. VO-L. 27 161 ACKNOWLEDGMENTS We are indebted to Dr G. Hood and Mr J. Hunton for assistance with the laparoscopies and to Mrs J. Speijers for statistical assistance. REFERENCES BINDON, B.M. (1984). Aust. J. Biol. Sci. 37: 163. BINDON,B.M.,CH'ANG, T.S. and TURNER, H.N. (1971). Aust. J. Agric. Res. 22: 809. BUTLER, L.G. and MORROW, G.F. Assoc. Animal Breed. Genetics 4: 184. (1984). Aust . LINDSAY, D.R., KNIGHT, T.W., SMITH,J.F. and OLDHAM, C.M. (1975). Aust. J. Agric. Res. 26: 189. TROUNSON, A.O. and MOORE, N.W. (1972). Aust. J. Agric.1 Res. 23: 851. TURNER, H.N. (1968). Proc. - Symp: Physiology of Reproduction in Sheep, Oklahoma State University, Stillwater, Oklahoma. July 26 - 27. ed. G. Scott. p. 67.