Oestrus synchronisation with a prostaglandin analogue III. Special aspects of synchronisation

Two groups of lactating dairy cows in each of 4 dairy herds were injected with either 0.5 mg (219 cows) or 0.3 mg (118 cows) of Ooprostenol (I.C.I.) The first group of cows were injected from Day 6 to Day 17 of the oestrous cycle (Day 0 — oestrus) and the second group from Day 6 to Day 12. The four herds were selected specifically because oestrus-detection procedures were thorough and reliable. Of the cows receiving the 0.5 mg dose rate, 54.3% were observed in oestrus within 4 days of treatment. The remainder were in groups which were: (i) in oestrus from 5 to 10 days post-treatment (20.5% with prolonged response intervals), and 11 to 20 days post-treatment (15.5% with unaltered cycle lengths); or (ii) experienced a “silent oestrus” in the immediate post-treatment period and either did not show oestrus for a further 3 weeks (6.4% in oestrus from 21 to 29 days post-treatment) or conceived to a single set-time insemination 72 h after treatment (0.9%). Use of the lower dose rate resulted in fewer cows being observed in oestrus within 72 h (43.8% v 31.4%) and a corresponding rise in the percentage experiencing a “silent oestrus” and returning to service (6.4% v 13.6%) or conceiving to set-time insemination (0.9% v 3.4%). Most of the unaltered cycle lengths occurred with cows being treated on Day 6 of the oestrous cycle. The pregnancy rate for the 171 cows observed in oestrus within 4 days of treatment and inseminated at 72 h, or 72h and 96 h, was 57.3%.

Records from this trial, as well as previous trials in which cows were injected with 0.5 mg of Cloprostenol, showed that 446 of 524 cows (85%) injected at from Day 6 to Day 17 of the oestrous cycle were observed in oestrus within 5 days (144 h) of treatment. Amongthese 446 cows, the interval to post-treatment oestrus was influenced by the stage of the cycle at injection. Whereas 89% of the oestrous responses in cows treated on Days 6 to 9 were observed within 72 h of injection, the comparable figures for cows treated on Days 10 to 13 and Days 14 to 17 were 48% and 70% respectively.

Variation in the post-treatment interval to oestrus is greater in lactating dairy cows than in heifers and is not resolved by using double-injection regimes. The reasons for this variation have not been identified. The most effective use of prostaglandins for oestrus synchronisation in lactating dairy cows requires the treatment of groups of animals at similar stages of theiroestrous cycles and their post-treatment insemination at observed oestrus. These requirements may limit the usefulness of this technology in herds with poor breeding management.