Induction of LH secretion and ovulation in anoestrous romanov ewes by the introduction of rams

Abstract

Seasonally anoes t rous ewes of many breeds will ovulate in response to the in t roduct ion o f rams. The use of this p h e n o m e n o n in farm pract ice is l imited by the variability in responsiveness of ewes. This may be due to be tween-ewe variation in the dep th of anoest rus (a result of variation in sensit ivi ty to negative feedback on the ton ic secre t ion of luteinising h o r m o n e (LH) by oes t rogen) , or due to diurnal variation in responsiveness to the ram st imulus. We there fore tes ted the e f fec t of the i n t roduc t ion of rams to entire R o m a n o v ewes at two d i f fe ren t t imes of the day (08.00 h, Group A, n = 6; 20.00 h, Group B, n -5), and also tes ted the dose-effects of oes t rogen on the response to the ram ef fec t in the tonic secre t ion of LH in ovar iec tomized R o m a n o v ewes. In the entire ewes, the interval (PI) be tween pulses of LH (observed over 12 h) was shor te r during the day (08 .00--20 .00 h) than during the night (11.2 ± 3.3 h vs. 4.9 ± 0.5 h, P < 0.05). The ram ef fec t decreased the PI (paired compar i son , all ewes, P < 0.001) to 2.9 -+ 0.5 h in Group A and 2.0 -+ 0.1 h in Group B. This last d i f ference be tween groups was no t significant. At laparoscopy 4 days later, young corpora lutea were observed on the ovaries of 3 ewes in Group A (ovulat ion rate 2 ~ 0) and 4 ewes in Group B (ovulat ion rate 3.0 -+ 0.4). None of these be tween-group di f ferences was significant, but the evidence of a circadian r h y t h m in the tonic secre t ion of LH and the cons is ten t t e n d e n c y for greater responsiveness to the ram ef fec t in Group B ewes indicate tha t the ram ef fec t may be more ef f ic ient at cer tain t imes of the day. Fur the r expe r imen ta t i on is war ran ted to tes t t imes o the r than those used here, and wi th more animals so tha t ovula tory responses can be de t e rmined accurately. In the ovar iec tomized ewes, the ram ef fec t decreased the PI f rom 41 ± I min to 29 -+ 1 rain in ewes n o t t rea ted wi th oestradiol (n = 4) and f rom 298 ± 25 min to 186 ± 37 min in ewes bearing a 1 cm oestradiol implant (n = 4). In ewes (n = 4) bearing 2-cm implants , the secre t ion of LH was very s t rongly suppressed (one pulse observed in 6 h in one ewe only) and the ram st imulus had li t t le if any e f fec t (one pulse in 6 h in one ewe and two pulses in another ) . I t is evident tha t the ram ef fec t is unable to s t imulate the secre t ion of LH when the oes t rogen feedback is too strong. This may explain variations be tween breeds, be tween flocks and be tween individuals in response to the ram effect .