Abstract:
THE REPEATABILITY OF OVULATION RATE IN MERINO EWES M.J. CARRICK*, C.M. OLDHAM* and D.R. LINDSAY* Summary The ovaries of 3 groups of Merino ewes were examined for corpora lutea by laparoscopy every 17 days between 28.3.74 and 16.6.75. During the 1974 breeding season the mean body weight of the ewes was maintained at 37 kg. Between the 1974 and -1975 breeding seasons the nutrition of the ewes was varied to give groups of ewes with mean body weights of 37, 41 and 46 kg; groups of ewes of low, medium and high nutritional status, respectively. These groups were maintained throughout the 1975 breeding season. Repeatability coefficients were estimated for ovulation rate both within and between breeding seasons. Within breeding season estimates were all around 0.5. However, between breeding seasons the estimate of repeatability of ovulation rate was dependent on the nutritional status of the ewes between the two seasons, low/low (r = 0.67, P < 0.001) low/medium (r = 0.42, P < 0.05) or low/high (r = 0.24, N.S.). These results suggest that selection of ewes for their ovulation rate may be practical and that the best results would be attained where ewes are joined in similar or better condition to that at selection. I. INTRODUCTION The recent introduction of laparoscopic techniques for the examination of ovarian activity in sheep has opened up the possibility of selecting animals for increased fertility on the basis of their ovulation rate. Whether or not such an approach is feasible depends on two factors. First, the correlation between twin ovulations and twin births in individual animals and, second, the repeatability of estimations of ovulation rate of individuals both within and between breeding seasons. The experiment presented here wasdesigned to study the second of these two relationships. It is well known that the nutritional status of the animal, as reflected in its body weight, influences the likelihood of twin births and therefore of twin ovulations. For this reason treatments were imposed in which the animals were fed to achieve substantial body weight differences in the second season of observation. II. MATERIALS AND METHODS (a) Animals .87 non-pregnant, 6-tooth Merino ewes were brought to the University of W.A. field station (Perth) in February 1974. They were randomly allocated to 3 groups. The ewes were in forward store condition and were 'fed a maintenance diet of oats and hay for the duration of the 1974 breeding season. The ewes were weighed weekly and were in continuous contact with vasectomised rams, over the entire period of the experiment. *On completion of the 1974 breeding season each group of ewes a different quantity of the same diet in order to establish ewes nutritional classes - high, medium and low - before the onset of breeding season. The mean body weights of the 3 groups of ewes t ii was fed in 3 the 1975 is shown Department of Animal Science E Production, University of Western Australia, Nedlands, W.A. 125 in Table 1. These were maintained through the 1975 breeding season. In the period between the breeding seasons 4 ewes died and 15 ewes of similar age were added to the experiment. (b) Examination of Ovaries The ovaries were examined for corpora lutea using the technique described by Oldham et al. (1975). The use of a 2 mm Wolf foetoscope enabled serial observation without any complications from infections or adhesions. Observation of ovarian activity was initiated on 28.3.74 and was continued every 17 days until 16.6.75. Prior to each observation the ewes were yarded away from feed and water for 24 hours. (c) Breeding Season An assumption of the estimate of repeatability is that the probability of ovulation is high (> 80 % of ewes ovulating) and does not change In 1974 four successive cycles significantly among successive cycles. were considered to satisfy this assumption and in 1975 seven successive cycles were included. Only those ewes which had ovulated at each examination were used for the estimation of repeatability. (d) Estimation of Repeatability The method of estimating repeatability of ovulation rate within a breeding season treats successive oestrous cycles as a Markov series with the following characteristics: 126 To test the necessary to use tribution with a (Sokal and Rohlf, null hypothesis Ho: r = 0 (vs Hl : Irl > o) it is a test of goodness of fit to;ompare the observed disbinomial distribution. A suitable test would be a G-test 1969) or a X2 test. If Ho is rejected we may accept Hl, and conclude that x and y are not independent and thus correlated. The repeatability of ovulation rate between breeding seasons was estimated by determining the correlation between the total numbers of eggs shed by individual ewes in each of the two breeding seasons. . III. RESULTS The mean body weights of the three groups of ewes are presented in Table 1. TABLE 1 Estimates of repeatability of multiple ovulation in 3 groups of sheep under different nutritional regimes All groups were at a similar body weight in 1974. In 1975 the 3 groups had disparate mean body weights reflecting the nutritional treatments. In Group 1 mean body weight in 1974 was similar to that recorded in 1975. Estimates of the within breeding season repeatability are shown in Table 1. Since there was no nutritional basis for separating them in 1974, the 3 groups were pooled to provide a repeatability coefficient of 0.45. This was significantly different from zero (P < 0.001). In 1975 estimates of repeatability were of the same order as 1974, however only Group 3 was statistically significant due to the smaller number of animals used for each estimate. Estimates of repeatability of ovulation rate between breeding seasons are shown for the 3 groups in Table 1. In Group 1 with consistently low nutritional was 0.67 (P < O.OOl>, and in Group 2 (low-medium) 0.42 (P < 0.05). However, in Group 3 -(low-high) efficient was low and not significantly different IV. DISCUSSION status, the coefficient the coefficient was the repeatability cofrom zero. All groups of ewes in this experiment gave estimates of repeatability for twin ovulation within breeding season around 0.5. The reliability of estimations of repeatability of ovulation rate by this method depends upon the number of animals and their ovulation rate. They are best where the number of animals is large and the ovulation rate around 127 1.50. Two of the 1975 estimates of repeatability did not reach statistical significance, although the repeatability coefficient was of the s&me order as those that did reach significance. In these two cases the number of animals was small and their ovulation rate was low.. The repeatability of the overall performance of ewes from one season to the next is dependent on the nutritional status at the time of measurement. Where body weights were similarly low in the 2 seasons the individual twin ovulating performance was highly repeatable (r = 0.67). On the other hand-, when the nutrition of the animals as reflected by increased body weight was changed for the better between breeding seasons, individual performances were not as well related. Moreover, the degree of dissimilarity-in performance was a function of difference in nutritional status between the two breeding seasons. This suggests that within the type of flock we studied 3 types of ewes can be distinguished. In the first category are those ewes that continue to have multiple ovulations even under poor nutritional conditions. The second category involves ewes that can be induced to have multiple ovulations under favourable nutritional conditions. Ewes of the third category do not appear to be capable of having a multiple ovulation. Examination of ovaries by laparoscopy can out injury to the ewe's reproductive potential the repeatability of twin ovulation within and is sufficiently high to suggest that selection ewes by this method is a possibility. be done quickly and with(Oldham et al. 1975) and between breeding seasons of potential twin lambing Our results indicate that there are a number of factors associated with the nutritional status of the ewes which must be considered. If at the time of laparoscopy the ewes are in poor condition most of the animals that have multiple ovulations will continue to ovulate twins regardless of -their subsequent nutritional status but the number of animals available for selection may not be sufficient to justify the use of this method. On the other hand, in the case of ewes in good condition the ability of many of the ewes to reproduce twin ovulations at subsequent joinings will depend on them being joined in similar or better condition to that at selection. Under these conditions and where initial fecundity is sufficiently high, selection of ewes for their ovulation rate may offer a practical alternative to the selection of ewes on the basis of their dams' lambing performance. v. .ACKNOWLEDGEMENTS and P.. Chidgzey for techmathematical assistance. Meat Research Committee and Lefroy Research Fellowship. We are grateful to S. Abbott, B. Beckers nical assistance and D. Basu and N. Khan for This project was supported by the Australian one author (M.J.C.) was holder of the E.H.B. VI. REFERENCES OLDHAM, C.M., KNIGHT, T.W. and LINDSAY, D.R. (1975). Journal of experimental Agriculture and Animal Husbandry (in press). SOKAL, R.R. and ROHLF, F.J. (1969). In 'Biometry'. (W.H. Freeman and Company, San Francisco). 128