Oestrous and ovarian responses to exogenous progesterone and oestrogen in prepubertal ewe lambs in relation to the fecundity of their dams

Abstract

Oestrus was consistently induced by 20, 40 or 80 microgram oestradiol benzoate in progesterone-primed lambs aged 23 weeks and 17 weeks respectively in two experiments. The duration of oestrus increased linearly with increasing log dose of oestrogen, and was negatively correlated with the time of its onset. In the absence of progesterone there was a reduced incidence, later onset and shorter duration of oestrus. Progesterone alone did not induce oestrus. In lambs treated during January (Exp. 2), a later date of injection of oestrogen was associated with earlier onset and longer duration of oestrus. The induced oestrus was anovulatory. Oestrogen reduced the proportion of lambs with follicles greater than or equal to 3 mm in diameter whilst progesterone had no effect. Lambs which were the progeny of low- and high-fecundity dams did not differ in their oestrous or ovarian responses. Correlations between the dam's lamb production index and the time to onset and duration of the induced oestrus were also not significant.

Reproductive response of mature and nulliparous yearling ewes to the ram effect during the non-breeding season

“…Similar to these results, it has been reported that the incidence of silent ovulations is also greater in lambs than in mature ewes during the normal onset of the breeding season (Quirke, 1981). It has been proposed that the sensitivity of the hypothalamus-pituitary axis to the increase of estrogens may be lower in lambs that in mature ewes (Whyman, 1978). This lower sensitivity may also explain the delay in oestrous onset in yearling ewes.…”

Oestrous response of lambs to oestrogen in relation to their fertility at two years of age

“…This study confirms previous observations (Whyman 1978) that oestrus can be induced by a small dosage of oestrogen in progesterone-primed prepubertal lambs. In the present trial the induced oestrus had commenced in half of the lambs 27 h after injection at 23 weeks of age.…”

“…Simplification of the treatment by omitting progesterone priming may lead to a practical testing procedure. Oestrus can be induced in lambs without progesterone priming, although a higher dosage of oestrogen is required to compensate for its absence (Whyman 1978). Further study is needed to establish whether this simpler treatment schedule will produce a response related quantitatively to subsequent ewe fertility in flocks differing in fertility genotype, and to obtain estimates of genetic parameters.…”

“…The relationship between the response of the lambs and the lamb production index of their dams was investigated, as in the earlier study (Whyman 1978). This index (Rae 1958) is a measure of the breeding value of the dam based on the number of lambs born over a variable number of matings.…”

“…Oestrus has been induced in progesteroneprimed 14-27-week-old ewe lambs by injection of 20-80 p.g oestradiol benzoate (Whyman 1978). The oestrous response ofthese lambs was not influenced by the fecundity of their dams, but its relationship with the lambs' own fertility was not recorded.…”