Prostaglandin analogue (ICI 8o 99 6) was administered ( 100 tig IM) on day 10 or I of the oestrous cycle in 8 Romanov (R), 10 Romanov X Prealpes (R X P), 8 Prealpes (P), 8 Ile-de-Fvance (IF)ewes. Ovulation rate before treatment was 2 . 9 (R), 2 . 9 (R X P), 1 . 7 (P) and 1 . 5 (IF). The mean ovulation rate in the induced cycle was not significantly different from that in the cycle before treatment (R = 2 . 7 ; R X P = 2 . 4 ; P = z.o ; IF = 1 .6). Delay (hr) between PG injection and onset of

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... trus was significantly (P < o.oi) shorter in the Romanov that in the other genotypes R = 3 6 (range = 34 -40 ) ; R x P = 43 ( 3 8-52 ) ; P = 4 3 (32-50) ; IF = 44 ( 3 6-54 ). Mean duration of oestrus (hr) was longer in the breed of high prolificacy (R = 53 ; 1 R X -P=45;.P=39;7f==3i); this was also the case in the cycle before PG treatment. It appears that luteolysis induced by this dosage of PG analogue is equally effective in animals with 1 , 2 or 3 corpora lutea ; but it is also evident that the delay between injection and onset of oestrus can be breed dependent.