The effect of stage of breeding season or pre-mating oestrogen and progestagen therapy on fertility in ewe lambs

Abstract

An investigation was conducted to determine the effect of mating at puberty or third oestrus or after pre-mating oestrogen and progestagen therapy, designed to mimic the changes that occur in plasma oestrogen and progesterone concentrations during the 1st, 2nd or 3rd oestrous cycles, on fertility in ewe lambs. Twenty-five ewe lambs were mated at puberty on mean date of 18 October and 21 were mated at third oestrus on 3 December. Of these, 28% and 81% of animals mated at puberty and third oestrus lambed, respectively (P < 0-05). A similar number of ewe lambs were mated within 24 h of ram introduction in the groups given one, two or three consecutive applications of oestrogen and progestagen (25 fig oestradiol benzoate followed 48 h later by a progestagen pessary left in situ for 12 days). Of the 43 corpora lutea, 21 were represented by viable embryos in the 37 animals holding to service after one application compared with 34 out of 46 corpora lutea in 40 animals given three applications (P < 0-05). Furthermore, embryo crown-rump length, embryo weight and amniotic sac width was greater (P < 0-05), on day 32 of pregnancy, in animals given one compared with three applications of oestrogen and progestagen. These results demonstrate that fertility is improved in ewe lambs mated on third oestrus or after progestagen and oestrogen therapy designed to simulate three oestrous cycles.

A comparison of ovulation rate and late embryonic mortality in ewe lambs and ewes and the rôle of late embryo loss in ewe lamb subfertility

“…Furthermore, they extend these studies by demonstrating that ewe lambs suffer more reproductive losses after day 15 of pregnancy than ewes. Moreover, the results of this experiment confirm reports in which prolonged luteal function and extended inter-oestrus intervals were indicative of losses after the 2nd week of gestation (Keane, 1974;Edey et al, 1978;Langford, 1986;Beck and Davies, 1994). These late losses may have a significant effect on reproductive performance as they can markedly extend interoestrus interval (Moore and Rowson, 1966;Edey, 1967).…”

“…Indeed, two of the animals treated with prostaglandin analogue failed to return to oestrus which suggests that, following late embryonic mortality, some animals become anoestrus. Moreover, it is unlikely that the extended interoestrus intervals observed in experiment 2 were caused by factors other than luteal persistence following embryo death, as ewe lambs mated to vasectomized rams have normal length cycles (Keane, 1974;Hare and Bryant, 1985;Beck and Davies, 1994). Furthermore, the animals used in this experiment exhibited a normal length cycle between the induced oestrus and the subsequent spontaneous oestrus.…”

“…However, none of the above investigations established when, during this time period, these losses occur. Furthermore, there is indirect evidence which suggests that a significant proportion of these losses occur after the 2nd week of pregnancy (Langford, 1986;Beck and Davies, 1994). Moreover, as embryonic losses after day 12 of pregnancy result in a significant extension of the inter-oestrus interval in ewes (Moore and Rowson, 1966;Edey, 1967), it is possible that these late losses may prevent ewe lambs from remating and thereby cause barrenness.…”

Recria de cordeiras em pastagem nativa melhorada submetida à fertilização nitrogenada: 2. Produção animal

“…A subfertilidade em cordeiras é uma das causas de redução da produtividade do rebanho. Embora a taxa de fertilidade sofra influência de vários fatores, 20 a 40% das cordeiras falham em produzir seu primeiro cordeiro (Davies & Beck, 1993;Beck & Davies, 1994;Beck et al, 1996).…”