SUMMARY Progesterone releasing intravaginal devices (PRIDs) were used in 4 experiments involving 67 cattle to study the effect of the stage of the oestrous cycle and of oestradiol benzoate (ODB) administration on the time interval from PRID removal to oestrus and/or ovulation. Cows in which PRIDs were inserted on days 2 to 4 of the oestrous cycle for 14 days were subsequently observed by endoscopy to ovulate significantly later than cows given identical treatments on days 13 and 14 of the cycle. The concentration of progesterone was higher in the former group at the time PRIDs were removed and remained at a higher level for 3 days thereafter. The length of the oestrous cycle of untreated heifers was significantly longer than in heifers treated for 12 days with PRIDs only inserted on day 3 of cycle (19.7 v 18.2; p <0.01). Treatment with PRIDs inserted for 12 days together with ODB further shortened the cycle length from 18.2 days to 16.9 days (p <0.01). The intervals from PRID removal to onset of standing oestrus in heifers treated with PRID only and those treated with PRID and ODB were 80h and 45h, respectively (p <0.01); the respective variances were 280 and 4; p <0.01). Plasma progesterone concentration, at PRID removal, was 3 times higher in heifers treated with PRID only than in heifers treated with PRID and ODB (p <0.01) and the interval from PRID removal to the lowest progesterone value following it was 3.8 and 1.2 days for the 2 groups, respectively (p <0.05). The study indicated that the stage of oestrous cycle at PRID insertion is an important factor which influences the interval from withdrawal of PRIDs to ovulation. The results suggest closer synchronisation would follow insertion in the mid‐luteal phase (Day 13 to 14) or use of ODB treatment at the time of insertion of PRIDs.
The proportions of cows showing oestrus during the first 5 days were 22%, 78% and 87X, and by 25 days were 85%, 92% and 98% for groups 1, 2 and 3 respectively. In group 3, 83% of animals had exhibited oestrus and 49% subsequently conceived to A.I. at the fixed time of 63h. Pregnancy rates after 25 days of mating as determined by rectal palpation were 46%, 62% and 64% for groups 1, 2 and 3 respectively. These results demonstrate that the CIDR is an effective device for synchronisation of oestrus in dairy cows, and when used in combination with luteolytic prostaglandins this synchronisation is sufficiently precise for successful A.I. at a fixed time.
A study of the survival and growth of lambs was made over a three year period (1968 to 1970) on a flock of 540 ewes grazing annual pasture at Werribee, Victoria. Ewes were stocked at three rates (5, 7 1/2 and 10 ha-1) and lambed between July 6 and August 20 or between September 10 and October 29 each year. Observations were made on the same ewes each year. Rate of stocking had no effect on the birth weight of lambs but single-birth lambs born in September were lighter than those born in July. More twins were born in September than in July and a smaller proportion of lambs born survived from the September than from the July lambing. Of the lambs born to ewes stocked at 10 ha-1 in 1968 and 1969 a smaller proportion survived than of the lambs born to ewes stocked at 5 ha-1. The growth rate of lambs born in July was constant between birth and weaning at 12 weeks of age but declined as rates of stocking increased. When the lambs born in September were about eight weeks old the pastures matured and growth rates of lambs declined and at weaning liveweights of lambs from plots stocked at different rates were similar. Lambs born in July on plots stocked at 5 and 79 ewes ha-1 were as much as eight kilograms heavier at weaning than the lambs born in September at equivalent rates of stocking. Neither rate nor time of onset of milk production was significantly affected by stocking rate.
The effects of different treatments for oestrus synchronisation on the incidence of oestrus and fertility levels in dairy cows were studied in 2 experiments. In Experiment 1, 200 lactating cows were allotted to 5 groups and the treatments imposed were either; 1: Untreated controls, 2: An injection of 0.5 mg of cloprostenol followed 13 days later by a progesterone releasing intravaginal device (PRID) inserted for 12 days, 3: A PRID, with a capsule containing 10 mg of oestradiol benzoate (ODB) attached, inserted for 12 days, 4: A PRID inserted for 12 days with 0.5 mg of cloprostenol administered 24 h before PRID removal or, 5: As for 4 but 14 days after fixed‐time insemination a second PRID was inserted for 12 days. Treated cows were inseminated 56 h after PRID removal and at an observed oestrus during the subsequent 30 days. The control group was inseminated at an observed oestrus during this 30‐day period. For treatments 2, 3, 4 and 5, respectively, the percentage of cows showing oestrus by 60 h after PRID removal was 70, 40, 67 and 43 and conception rates to the fixed time insemination were 34, 33, 49 and 29%. Calving rates of cows inseminated at an observed oestrus during a 30‐day period were 70, 75, 70, 83 and 82% for treatments 1, 2, 3, 4 and 5, respectively. In Experiment 2, 60 lactating cows were divided into 2 groups and the treatments imposed were either 1: An injection of 0.5 mg of cloprostenol followed 13 days later by a PRID inserted for 12 days or 2: As for 1 but 14 days after fixed‐time insemination a second PRID was inserted for 12 days. Treated cows were inseminated 56 h after PRID removal and at an observed oestrus over a period from the first insemination to 6 days after removal of the second PRID. For treatments 1 and 2, respectively, 73 and 71% of cows showed oestrus by 60 h after removal of the first PRID and 40% and 46% conceived to the fixed time insemination. The conception rates to inseminations over the treatment period were 73 and 70% for treatments 1 and 2, respectively. None of the treatments resulted in conception rates which were lower than those of control cows provided that treated cows were reinseminated at observed oestrus. Treatment 4 provided the most practicable technique for oestrus synchronisation.
A flock of 283 Merino ewes, had oestrus synchronized with CIDR devices containing 9% progesterone. At CIDR removal the ewes were divided into four groups. Group 1 had a 2nd CIDR inserted during days 10 to 16 after removal of the first CIDR. Group 2 received 400 iu of PMSG at CIDR removal. Group 3 received both PMSG and a second CIDR. Group 4 had no hormone treatment and served as controls. All ewes underwent intrauterine A.I. at SO-56 h after CIDR removal. PMSG increased the proportion of ewes showing oestrus 36 h after CIDR removal. Exogenous progesterone given during days 10 to 16 increased number of foetuses per ewe joined and number of foetuses per ewe pregnant from 57% and 105% in controls to 83% and 125% respectively. Combined treatment with PMSG and progesterone increased the number of foetuses per ewe pregnant to 128%. The results suggest that progesterone, if given over days lo-16 of pregnancy, can cause a reduction in embryo mortality.
Summary. The 330 Merino ewes used in the study were placed with rams at a synchronized oestrus and, on Days 2–14 after mating, the ewes were placed in a feed lot and fed daily a low, medium or high ration (25%, 100% or 200% of maintenance respectively). Progesterone supplement was given to some ewes on Days 8–14 after mating by using a device containing 340 mg progesterone. Blood samples were taken from all ewes on Day 12 for measurement of plasma progesterone concentrations. On Day 14 after mating all ewes were returned to pasture. Pregnancy rate was determined by returns to oestrus and was later confirmed using ultrasound. There was a decline in the peripheral progesterone concentrations with increasing ration. The pregnancy rate in ewes fed a high ration was significantly reduced when compared with those of ewes fed a medium or low ration (48% vs 68 and 67% respectively; P < 0·05). In ewes fed the high ration exogenous progesterone increased the pregnancy rate from 48 to 76% (P < 0·01). Progesterone treatment did not influence pregnancy rates in ewes fed medium or low rations. The number of fetuses per ewe pregnant was not influenced by level of nutrition or progesterone treatment.
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