SYNCHRONIZATION OF ESTRUS IN GILTS WITH ALTRENOGEST - EFFECTS ON OVULATION RATE AND FETAL SURVIVAL

Previous studies have shown that use of altrenogest resulted in a high rate of fertility and increased litter size compared with controls un der conditions of practical pig production. The present study was desi gned to evaluate whether ovulation rate and/or foetal survival were in creased by altrenogest using crossbred gilts derived from one herd (n = 227) and introduced in the same piggery over 12 months. Each gilt wa s allocated to a treated group (n = 103) receiving an individual daily dose of 20 mg of altrenogest for 18 days in its feed or a control gro up (n = 124) after puberty had been diagnosed, (197 +/- 1 day; mean +/ - SEM). They were inseminated (double At) at the second induced or nat ural oestrus. Pregnancy was diagnosed by ultrasonography at Days 22 an d 42 post-insemination in the absence of return to oestrus. Pregnant g ilts were slaughtered at 48 +/- 3 days of pregnancy following the seco nd examination. The number of living and dead foetuses were recorded b efore uterine contents (foetuses and placentae) were weighed and the n umber of corpora lutea (CL) per ovary counted. Precise synchronization of oestrus was observed after the end of the progestogen administrati on, with 93% of the gilts in oestrus by Days 5 to 7. For the controls, the interval from first to second oestrus ranged from 17 to 25 days i n 93% of the control gilts. The pregnancy rate was 89.3% for treated g ilts and 77.4% for controls(P < 0.05). The ovulation rate was increase d by the treatment (15.4 +/- 0.3 vs 14.6 +/- 0.3; mean +/- SEM, P < 0. 02). Although the altrenogest group had more foetuses (11.1 vs 10.6), this difference was not significant (P > 0.14). The percent of foetal survival was similar in both groups (64.9%; P > 0.27). The foetal and placental weights differed only between dams and increased with stage of gestation. The increase in litter size through feeding altrenogest was associated with an increased ovulation rate.