G. E. Bradford

Length of gestation in ewes carrying lambs of two different breeds

Length of gestation was studied in pregnancies established by intrabreed, interbreed, and mixed-breed embryo transfers of Finnish Landrace (Finn; mean gestation: 144.9 days) and Targhee (mean gestation: 150.4 days) embryos to Finn cross and Targhee recipients. At least one lamb of each breed comprised mixed-breed pregnancies. There was a significant effect of the breed of lamb (Finn, Targhee, or Finn and Targhee) on the length of gestation (P<.01), but not of breed of recipient. Mixed-breed pregnancies had a mean gestation period intermediate between those for pregnancies which contained only one breed of lamb. The mean gestation period for mixed-breed pregnancies was 2.10+/-.70 (x+/-S.E.) days longer than for pregnancies with only Finn lambs (P<.01) and 2.99+/-.73 days shorter than for pregnancies with only Targhee lambs (P<.001). The delay in parturition in mixed-breed pregnancies beyond the normal gestation period for Finn lambs occurred even in litters with a majority of Finn lambs. These results demonstrate an interaction between fetuses in the processes leading to parturition. Possible mechanisms by which the Targhee lamb delayed parturition in mixed-breed pregnancies are discussed.

Resumption of ovarian function in autumn lambing Dorset, Rambouillet and Finnish Landrace ewes

The establishment of ovarian activity during lactation was studied in the postpartum period of Rambouillet, Dorset and Finnish Landrace ewes following lambing during the month of October (1981). The mean postpartum intervals to first ovulation and first estrus were 22.7 and 53.0 for Rambouillets, 25.2 and 51.0 for Dorsets, and 22.5 and 49.7 days for Finnish Landrace ewes. Estrus was not associated with the first ovulation postpartum in any breed. The number of silent ovulations prior to the first estrus was highest in the Rambouillet and lowest in Finnish Landrace breeds. Of the 18 ewes in the project, 14 had normal luteal phase lengths, 1 had a possible short luteal phase and 3 had prolonged luteal phases following the first ovulation postpartum. The first service conception rate of all ewes bred was 82% (14 17 ) at an average of 52 days postpartum. The lambing rate following the autumn breeding was higher (2.14 +/- 0.14) than the lambing rate which followed the previous spring breeding (1.28 +/- 0.11).

Year and season effects on oestrus and ovarian activity in ewes of different breeds and crosses

Abstract The dates of onset and cessation of ovulation and oestrus and the duration of the ovulatory, oestrous, anovulatory and anoestrous seasons were determined for Suffolk, Rambouillet, Dorset, Finnish Landrace and reciprocal F 1 crosses between the last three breeds in a study which extended over 3 years. Ovulation rate, duration of oestrus and cycle length were also examined. Among the purebreds the major source of variation in these traits could be attributed to differences between the Rambouillet and the remaining breeds. Rambouillet ewes commenced and ceased ovarian and oestrous activity earlier than the Finn; first ovulation also occurred earlier in the Rambouillet than in the Suffolk and the duration of ovarian activity was longer for the Rambouillet than for either of these breeds. The oestrous season was 27 days shorter for the Rambouillet than for the Dorset, due to a higher incidence of ovulation without oestrus in the Rambouillet. The anoestrum was between 27 and 65 days longer for the Rambouillet than for any of the other purebreds. The period of anovulation was longest for the Suffolk breed. There was little evidence for heterosis in the duration of the breeding and non-breeding seasons. The repeatability of components of the breeding season was generally low. Ovulation rate increased slightly from early to mid-season and then declined significantly as the breeding season advanced, in all breeds.

Cryopreservation of embryos as a means of germ plasm conservation in sheep

Embryos were collected from 4 lines of Targhee sheep between 1986 and 1990. The lines were selected for preweaning growth rate (Lines DH and HW) or for multiple births (Line HT); Line C served as an unselected control group. Estrus was synchronized using fluorogestone acetate-impregnated vaginal pessaries, and ewes were superovulated with FSH. Embryos at the morula or blastocyst stage were surgically recovered from mature ewes at Days 5 to 6 and were frozen following morphological evaluation. The overall average number of freezable embryos per collection was 2.9, and did not differ significantly among years or among lines. Of the embryos collected between 1986 and 1988, 92 were transferred to 53 recipients in 1989, producing 53 lambs. Survival rates were 60.9 and 47.8%, respectively, for embryos evaluated as good and fair after thawing. Good-quality blastocysts yielded the highest survival rate (64.4%). Analyses indicated no significant effects of line, developmental stage or embryo evaluation on the incidence of lambing. It was concluded that embryos of morula or blastocyst stage can be successfully frozen for extended periods. The data on embryo yield and survival following cryopreservation were used to calculate numbers of donors needed to preserve and reconstitute a population of specified size.