OVULATION RATE AND LITTER SIZE OF BARBADOS, TARGHEE AND CROSSBRED EWES'

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1 OVULATION RATE AND LITTER SIZE OF BARBADOS, TARGHEE AND CROSSBRED EWES' G. E. Bradford and J. F. Quirke 2 University of California 3, Davis ABSTRACT Ovulation rate was measured in Barbados Blackbelly (United States strain; B), Targhee (T), Barbados X Targhee (B T) and Barbados X Dorset-Targhee (B X DT) ewes at first and second estrus following synchronization of cycles early in the breeding season. Body weight at sponge removal differed (P<.001) between B (30.9 kg) and T ewes (54.9 kg) and was intermediate for B T (44.9 kg) and B X DT (43.5 kg) ewes. Ovulation rate was higher (P<.O1) at first and second estrus for B (1.86, 2.04), B T (1.93, 2.04) and B X DT (1.72, 1.80) than for T (1.29, 1.40) ewes. Regressions of ovulation rate on body weight within the breed groups did not differ significantly from each other and the average was significant (b = CL/kg at first estrus and b = CL/kg at second estrus, where CL = number of corpora lutea), but differences between the groups in body weight did not explain the differences in ovulation rate. Litter size for B, B T, B X DT and T groups was 1.71, 1.84, 1.84 and 1.28, respectively. The B X T ewes were superior to the average of the B and T ewes for ovulation rate (P<.05) and litter size (P<.01); there was no direct estimate of embryo survival, but the results indirectly indicate superiority of the crossbreds for this component also. The B, B X T and B DT ewes were all exceptionally uniform in ovulation rate and litter size; 81 and 69% of the CL counts for B and B crossbred ewes, respectively, were two, and 68 and 79% of the litters produced by B and B crossbred ewes, respectively, were twins. (Key Words: Sheep, Barbados, Ovulation, Litter Size, Heterosis.) Introduction The Barbados Blackbelly breed has received considerable attention in recent years as one of the few breeds of tropical hair sheep that has a high prolificacy (Patterson, 1976; Maule, 1977; Mason, 1980; Fitzhugh and Bradford, 1983). The breed is found in several Caribbean countries (characteristically in small flocks) and a strain derived from it has evolved in the southern United States during the current century (Shehon, 1983a). The latter sheep have many of the characteristics of the breed on the island of Barbados where it originated, but dif- 1 The authors express their appreciation to G. M. Spurlock for his assistance in making available the original Barbados ewes and to D. Torell, M. Dally and J. Hays for assistance with collection of the data. Supported in part by the Small Ruminant CRSP, U.S. AID Grant DSAN-XII The Agr. Inst., Grange, Dunsany, Co. Meath, Ireland. 3 Dept. of Anim. Sci. Received July 11, Accepted November 5, fer in that rams of the U.S. strain are generally horned, there is more variation in color pattern (Levine and Spurlock, 1983; Shelton, 1983a) and more individuals with some wool. They retain at least some of the year-round breeding tendency of the original strain (Boyd, 1983; Goode et al., 1983 ; Levine and Spurlock, 1983 ; Shelton, 1983b). The breed in Barbados has a higher-thanaverage variation in litter size (Bradford et al., 1983; Patterson, 1983), suggesting that it would be of interest to know the ovulation rate of ewes of this breed and of its crosses. Apparently such information has not been reported to date. We had the opportunity to measure ovulation rate in a small flock of U.S. Barbados sheep and some of their crosses, and report the results here. Materials and Methods The Barbados sheep for this study were from the flock described by Levine and Spurlock (1983), which had been assembled from feral stocks of the breed in California, with some introductions from Texas. The flock had been under selection for litter size and short lambing 905 J. Anita. Sci :

2 906 BRADFORD AND QUIRKE interval for several years. A sample of 20 ewes and 3 rams was taken to the Hopland Field Station in late They were put into a yearround mating program beginning April 1978 in comparison with Targhee and F1 Dorset Targhee ewes, to evaluate their breeding season. The three groups of ewes were flock-mated in different pastures to Barbados rams, producing pure Barbados (B), and two types (B T and B x DT) of crossbred progeny. The Hopland Station represents an annual grassland ecosystem, with a dry season lasting approximately 6 mo, May through October, each year. Under these conditions the ewes in all three groups tended to lamb only once a year, mostly in March and April but with a few in October and November. The breeding season study was terminated in early 1981, and a total of 100 ewes of four groups: B, B x T, B DT and T was assigned to a project to determine ovulation rate. The first three groups were from the breeding season study, while the T ewes were Station-flock grade Targhees of similar breeding to the dams of the B x T ewes. The two crossbred groups contained only 1- and 2-yr-old ewes, while the Targhee ewes were 1 to 3 yr of age and the Barbados ewes 1 to 4 yr. All ewes had been maintained on dry range, without supplementation, from late May to the start of the trial. They were brought in from the range on August 17, weighed and given a ration of approximately 2 kg of alfalfa cubes per ewe per day in drylot for the following 8 wk. Estrus was synchronized by treatment with 60 mg medroxyprogesterone acetate vaginal sponges 4 for 14 d. Ewes were weighed again at sponge removal on August 31. CorPora lutea (CL) were counted by laparoscopy on September 9 and again on September 24. The ewes were flock-mated to Targhee rams in October, and returned to the range until late February when they were brought in to a barn for lambing. Data were analyzed using a least-squares program (Harvey, 1977) with breed and age (1 yr and />2 yr) included in the model. Body weight was included as a covariate in some analyses. The following a priori contrasts were evaluated using a t-test: i) B minus T, ii) B x T minus T, iii) difference between the mean of B and T and * Upjohn Co., Kalamazoo, MI. the F1 cross between these breeds (test for heterosis) and iv) B x T minus B DT. Results Body weight, ovulation rate and litter size data for the four groups of ewes are presented in table 1. The data presented are least-squares means; mean squares from the analyses of variance and the evaluation of the a priori contrasts are presented in table 2. The weight presented is the one recorded at the end of the synchronizing treatment, 2 wk after the ewes had been brought in off the range. The ewes were gaining weight at this time but had been in rather poor condition at the start of the experiment. The weights for Targhees are fairly typical of weights for ewes in this flock in late summer. The Barbados ewe weights were rather low in comparison with those recorded for other flocks (Fitzhugh and Bradford, 1983; Shelton, 1983a), and confirm the subjective impression that the straightbred Barbados sheep were not well adapted to the dry feed conditions of this experiment. The B T crossbred ewes were slightly above midparent average in weight, but also significantly lighter than the Targhees. Of the 100 ewes in the experiment, all but four had ovulated at each of the two examinations; 93 ovulated both times. The least-squares means for ovulation rate in table 1 are based on data from these 93 ewes. All 100 ewes were joined with rams; seven died or were missing before lambing, one was dry and 92 lambed. Thus, there were no appreciable differences among breed or age groups in percentage of ewes ovulating or lambing. The distributions of ovulation rates and litter sizes for all ewes are presented in table 3. The relationship between ovulation rate and body weight was examined. The linear regressions were positive in all four groups, and not significantly different among the groups, although the coefficients were higher for Barbados ewes than for the other groups. The pooled within-breed correlation coefficients were.33 and.37, and the regression coefficients were and CL/kg, for ovulation rate at first and second cycle, respectively, the latter being somewhat higher than values for a larger group of Targhee ewes in the same flock (Quirke et al., 1985). The data in table 1 were not adjusted for body weight; with the large differences in mean body weight among

3 REPRODUCTION IN BARBADOS AND CROSSBRED EWES 907 TABLE 1. LEAST-SQUARES MEANS FOR BODY WEIGHT, OVULATION RATE AND LITTER SIZE OF TARGHEE, BARBADOS AND CROSSBRED EWES Litter size Breed No. Body Ovulation rate No. or cross ewes wt, kg First Second ewes Mean Targhee (T) Barbados.(B) B T B X DT a Age 1.5 yr ~2.5 yr abarbados sire X Dorset Targhee F I dam. the groups, such adjustment would result in unrealistic mean ovulation rate values for some of the groups. Differences in ovulation rate between the two age groups were small and nonsignificant. Differences between the breed groups, on the other hand, were highly significant. Barbados ewes exceeded Targhees by about.6 CL (P<.01) at each cycle, and this superiority of the breed was transmitted to crossbred progeny. Differences among the breeds in litter size were similar to those in ovulation rate. Average within-breed repeatability of ovulation rate was.42, which may be compared with values of.27 and.25 for Targhee ewes 2.5 yr old and older and 1.5 yr old, respectively, in the same flock (Quirke et al., 1985). Discussion One of the reasons for undertaking this study was to estimate the contribution of ovulation rate to the large coefficient of variation in litter size reported for several flocks of this breed (Fitzhugh and Bradford, 1983). Surprisingly, both the Barbados and Barbados crossbred ewes in the present study were remarkably uniform in both ovulation rate and TABLE 2. ANALYSIS OF VARIANCE AND LINEAR CONTRASTS FOR BODY WEIGHT, OVULATION RATE AND LITTER SIZE OF TARGHEE, BARBADOS AND CROSSBRED EWES Source df Body wt, kg a Fi~t Ovulation rate a Second Litter size a Breed 3 10,461"** Age 1 1,031" Remainder Linear contrasts B -- T "** B X T -- T "** BX T-(B+T)/ B T- B DT "* 2.261"** 1.842"** "*.65.18"* "**.36.16".32.14".35.13"* ameansquares. *P<.05. **P<.01. ***P<.O01.

4 908 BRADFORD AND QUIRKE TABLE 3. DISTRIBUTION OF OVULATION RATE AND LITTER SIZE FOR TARGHEE, BARBADOS AND CROSSBRED EWES No. corpora lutea First estrus Second estrus Litter size Breed T B B T B DT litter size. For the B, B x T and B DT groups combined, 70, 75 and 75% of all records for first and second ovulations and litter size, respectively, had the value two, and only 6, 9 and 5% of the records were >/three. M. Shelton (personal communication) also reports exceptionally uniform reproduction performance for Barbados crossbred ewes in Texas. One Barbados ewe had five CL at the first examination, but other than that, none of the pure or crossbred Barbados ewes in this study showed the kind of variation reported in the studies of this breed referred to earlier, including 14.4% tripplets and quadruplets among over 200 second and later litters in the flock providing the Barbados stock for this experiment (Levene and Spurlock, 1983). The explanation is, presumably, either the small number of animals, giving rise to large sampling variation, or a depressing effect of the plane of nutrition in this environment on expression of their genetic potential. The nutritional handicap should not have been too great; the ewes had been on a flushing level of nutrition for over 30 d by the time of the second ovulation. Whatever the explanation, the observed distribution of litter sizes, in both the Barbados and the crossbred ewes, is a very desirable one. There are a number of reports (Land et al., 1973, 1974) of additive inheritance of ovulation rate in crosses between prolific and nonprolific breeds. In the present study the lack of reciprocal crosses precludes a precise estimate of heterosis. However, the Fl B x T ewes were significantly superior to the mid-parent average for both ovulation rate and litter size, being equal or superior to the better parent in both traits. The B x DT ewes were similar to the B x T ewes, providing additional evidence for transmission by the Barbados breed of genes with dominant effects for higher ovulation rate. McGuirk (1970) reported a high level of hetero- sis for ovulation rate in Border Leicester x Merino cross ewes, which was attributed to depression of phenotypic expression of ovulation rate of Border Leicester ewes below their genetic potential in that environment, due to health problems which did not affect the F 1 ewes. The low body weights of the Barbados ewes in this study may have held their performance below potential, in spite of the flushing treatment. Corpora lutea were not counted following the estrus at which the ewes were mated, so the data provide no direct estimate of embryo survival. However, it is possible, using the inverse of Hanrahan's (1982) equation relating litter size to ovulation rate (Y = X -.076X 2, where Y = litter size and X = ovulation rate) to obtain an estimated ovulation rate for any given litter size (Bradford et al., 1986). Using this approach, we calculated the ovulation rate expected for the T, B, B x T and B x DT groups of ewes to be 1.38, 2.02, 2.24 and 2.24, which are in quite good agreement with the values observed in the preceding cycle, i.e., 1.40, 2.04, 2.04 and 1.80, respectively. Where the predicted values differ from those observed, they are higher, indicating either that ovulation rate increased from second to third cycle in the B T and B X DT ewes, or that embryo survival was higher in the crossbred ewes than in the B and T ewes. Unless there is a large maternal effect of the Targhee dam, the results indicate appreciable heterosis for embryo survival. The results of this study do not explain the high variability in litter size observed in some Barbados breed flocks. However, the results do suggest that where a higher variability in litter size of this breed is observed, it is probably the result of variable ovulation rate with at least normal embryo survival, rather than high ovulation rate and high prenatal loss. A potentially

5 REPRODUCTION IN BARBADOS AND CROSSBRED EWES 909 more important finding is that the Barbados breed appears to transmit an above-average ovulation rate in a dominant fashion, producing, from matings with a breed with ovulation rate less than 1.5, crossbred ewes with a uniformly high twinning rate. Since the latter is frequently a goal of sheep producers, it suggests a role for the Barbados breed where crossbred progeny of this breed are adapted to the production and marketing system. Literature Cited Boyd, L. H Barbados Blackbelly sheep in Mississippi. In: H. A. Fitzhugh and G. E. Bradford (Ed.) Hair Sheep of Western Africa and the Americas. pp Westview Press, Boulder, CO. Bradford, G. E., H. A. Fitzhugh and A. Dowding Reproduction and birth weight of Barbados Blackbelly sheep in the Golden Grove flock, Barbados. In: H. A. Fitzhugh and G. E. Bradford (Ed.) Hair Sheep of Western Africa and the Americas. pp Westview Press, Boulder, CO. Bradford, G. E., J. F. Quirke and T. R. Famula Fertility, embryo survival and litter size in lines of Targhee sheep selected for weaning weight or litter size. J. Anim. Sci. 62:895. Fitzhugh, H. A. and G. E. Bradford (Ed.) Hair Sheep of Western Africa and the Americas. Westview Goode, L., T. A. Yazwinski, D. J. Moncol, A. C. Linnerud, G. W. Morgan and D. F. Tugman Research with Barbados Blackbelly sheep in North Carolina. In: H. A. Fitzhugh and G. E. Bradford (Ed.) Hair Sheep of Western Africa and the Americas. pp Westview Press, Boulder, CO. Hanrahan, J. P Selection for increased ovulation rate, embryo survival and litter size. Proc. 2nd World Congr. Genet. AppI. to Livestock Prod. V: 294. Harvey, W. R Users guide for LSML76, mixed model least-squares and maximum likelihood computer program. Ohio State Univ., Columbus. Land, R. B., J. Pelletier, J. Thimonier and P. Manleon Quantitative study of genetic differences in the incidence of oestrus, in ovulation and in the plasma progesterone concentrations of luteinizing hormones in sheep. J. Endocrinol. 58: 305. Land, R. B., W. S. Russell and H. P. Donald The litter size and fertility of Finnish Landrace and Tasmanian Merino sheep and their reciprocal crosses. Anita. Prod. 18:265. Levene, J. M. and G. M. Spurlock Barbados Blackbelly sheep in California. In: H. A. Fitzhugh Africa and the Americas. pp Westview Maule, J. P Barbados Blackbelly sheep. World Rev. Anita. Prod. 24:14. Mason, I. L Prolific Tropical Sheep. FAO Anim. Prod. and Health Paper No. 17, Rome. McGuirk, B. J Hybrid vigour in sheep production. M.S. Thesis. Univ. of New South Wales, Kensington. Patterson, H The Barbados Blackbelly Sheep. Bull. Barbados Ministry of Agr., Bridgetown. Patterson, H. C Barbados Blackbelly and crossbred sheep performance in an experimental flock in Barbados. In: H. A. Fitzhugh and G. E. Bradford (Ed.) Hair Sheep of Western Africa and the Americas. pp Westview Press, Boulder, CO. Quirke, J. F., G. E. Bradford, T. R. Famula and D. T. Torell Ovulation rate in sheep selected for weaning weight or litter size. J. Anim. Sci. 61: Shelton, M. 1983a. The Barbados Blackbelly ("Barbado") breed in Texas. In: H. A. Fitzhugh Africa and the Americas. pp Westview Shelton, M. 1983b. Crossbreeding with the "Barbado" breed for market lamb or wool production in the United States. In: H. A. Fitzhugh Africa and the Americas. pp Westview