Increase in ovulation rate by active immunization against bovine inhibin-based synthetic peptides in a non-prolific sheep breed

The objective of this study was to explore the possibility of a recurrent increase in the ovulation rate of Malpura sheep, a non-prolific breed, by immunization against inhibin-based peptide immunogens over a period of 3 years. Adult ewes (4–7 years of age) and weighing between 28 and 38 kg were randomly allocated equally to three treatment groups. The immunization of the ewes was initiated during the autumn breeding season. Ewes were divided into three groups (n = 5 ewes/group) and actively immunized against the synthetic peptides from the α_C [bIα(1–29)-Tyr³⁰] (Group I) or α_N [bI-₄₃-Tyr¹⁵²(153-167)Cys¹⁶⁸] (Group II) area of the bovine inhibin α-subunit, conjugated to ovalbumin or against ovalbumin (control). Each ewe received a primary immunization of 400 μg immunogen and 3 booster injections, 200 μg immunogen each at 4-week intervals. Estrous was synchronized in all the ewes by administering two doses of PGF₂α at 10-day intervals for three consecutive years. Ovaries of ewes were examined each year between days 4 and 6 of the synchronized cycle, with the aid of the laparoscope to determine the ovulation rate. Active immunization significantly (p < 0.05) increased the ovulation rate. The overall ovulation rate, irrespective of the treatment period was 5.2 ± 0.44, 2.3 ± 0.38 and 0.9 ± 0.11 in Group I, Group II and the control, respectively. Although the beneficial effect of immunization on ovulation rate persisted for the entire period of the study, the interaction between immunization treatment and the time period was non-significant. The results clearly indicate that the active immunization against inhibin peptides can induce multiple ovulations in Malpura ewes and its effect on multiple ovulations is sustained for a prolonged period of time after the initial immunization.