Abstract:
Animal Production in Australia Vol. 15 THE 'RAM EFFECT' WILL ADVANCE PUBERTY IN 9 TO 10 MONTH OLD MERINO EWES INDEPENDENT OF THEIR SEASON OF BIRTH C.M. OLDHAM* and S.J. GRAY* Season of birth, nutrition, live weight and social interactions have all been implicated as factors determining the time of puberty in the lamb (reviewed by Dyrmundsson 1981). Normally growing spring-born lambs usually exhibit their first oestrus during the first available breeding season (autumn and winter), but lambs born in autumn do not display their first oestrus until the following autumn. The roles of age and live weight as determinants of puberty are confounded by season of birth. Foster (1981) hypothesised that lambs mature physiologically at the same age and live weight independently of their season of birth but that in autumn-born lambs the unfavourable spring photoperiod causes lambs to pass directly from the prepubertal state to the seasonal anoestrous state. Both conditions, it is hypothesised, are controlled by negative feedback of oestradiol (Ez) on the secretion of luteinising hormone (LH). The 'ram effect' stimulates ovulation within 3-4 days in seasonally anovular adult ewes of many breeds (Oldham 1980). There is no direct evidence that the 'ram effect' can induce ovulation in prepubertal ewes (Dyrmundsson 1981). There are no published data on the critical age or live weight at which Merino ewes in south Western Australia reach puberty but some 10 month old ewes joined in summer It was hypothesised will display oestrus and conceive (K P Croker, pers. comm.). that the 'ram effect' could be used to induce ovulation and therefore advance puberty in Merino ewe lambs independent of their season of birth. The ewes were born and reared on pasture at Shackleton, 200 km east of Perth. Ewes born in April or July/August 1980 were challenged with testosterone-primed wethers (TW) (Fulkerson et al. 1981) at 9 to 10 months of age in January or May 1981 respectively- Each flock (nz300) was isolated from rams for at least one month before the TW were introduced. Fifty ewes drawn at random were joined with 6% of TW (day 0) while the rest of the flock remained isolated from rams. On day 4 the ovaries of 25 ewes with TW and 17 isolated control ewes were examined by laparoscopy after an overnight fast. These ewes weighed 35.4kO.77 and 34.320.59 kg (mean + SEM) in January and May respectively with a range of live weight at both times of 26 to 44 kg- At neither time-were any of the isolated control ewes ovulating spontaneously. The ovarian response of flockmates introduced to TW is shown in Table 1. TABLE 1 The ovulatory response among 9 to 10 month old Merino ewes introduced to TW in Jan or May (live weights (kg)(Z+SEM) The presence of the TW (the 'ram effect') advanced puberty in some ewes both at the beginning (January) or middle (May) of the spontaneous breeding season of In May ewes which adult ewes. More ewes ovulated in January than in May (PcO.02). ovulated were heavier (37 kg range 34-43) than ewes which did not (32 kg range In January weights for the two groups were identical (35 kg range 26-36; P<O.Ol). 26-43) . The 'ram effect' thus advances puberty in ewes which supports Foster's (1981) concept that ewes reach 'physiological maturity' at a fixed age once they have passed the critical live weight for their breed. DYRMUNDSSON, 0-R. (1981). Livestock Production Science 8: 55. = FOSTER, D.L. (1981). Biol. Reprod. 25: 85. FULKERSON, W.J., ADAMS, N.R. and GHEZRDI, P.B. (1981). Appl. Anim. Ethol. 7: 57. = OLDHAM, C.M. (1980). Proc. Aust. Soc. Anim. Prod. 13: 73. *Dept Animal Science & Production, University of Western Australia, Nedlands WA 6009, 727