122 OVARIAN FOLLICULAR DYNAMICS IN CROSS-BRED EWES DURING THE RAINY SEASON UNDER TROPICAL CONDITIONS

Abstract

Small ruminants are an important livestock species on many island nations of the Eastern Caribbean region and serve as a vital source of food and income. This study was designed to characterise follicular development during the inter-ovulatory interval (interval between 2 consecutive ovulations) under tropical conditions. Non-lactating, 2- to 4-year-old crossbred Barbados-Black Belly × White Virgin Island ewes weighing a mean 30.4 kg with mean body condition score 2.9 (score 1–5) were used during September–November for the study. Ewes were maintained in a paddock of 100 m² at the research farm in St. Kitts and Nevis (17.3°N, 62.7°W). Animals were fed daily with 100 g of concentrate (17% crude protein) and had free access to elephant grass (Pennisetum purpureum), mineral salt, water, and shade. Ovaries of 19 females were examined daily for 46 days by transrectal B-mode ultrasonography using a 7.5-MHz lineal array transducer (Honda HS-2200UV, MA, USA) to detect ovulations and growth and regression of antral follicles. Data were analysed using chi-square, Student t-test, or one-way ANOVA. Proportion of females with 2, 3, and 4 follicular waves was 2/19 (10.5%), 10/19 (52.6%), and 7/19 (36.8%), respectively. Mean (± SEM) length of the inter-ovulatory intervals did not differ (P = 0.9) among females with 2 (16.5 ± 0.5 days), 3 (16.3 ± 0.4 days), and 4 (16.6 ± 0.2 days) follicular waves, respectively. Mean diameter of ovulatory follicles was 4.4 ± 0.1 mm, proportion of multiple ovulations was 11/19 (57.9%), and mean number of follicular waves was 3.3 ± 0.1. Wave emergence was normalized to the first ovulation of the inter-ovulatory interval (ovulation = Day 0). Mean days of wave emergence for 2 follicular waves were Days 1.0 ± 0 and 10.0 ± 0.0; for 3 follicular waves were Days 0.7 ± 0.1, 5.6 ± 0.5, and 10.4 ± 0.3; and 4 follicular waves were Days 0.8 ± 0.3, 4.7 ± 0.6, 8.4 ± 0.5, and 11.7 ± 0.3, respectively. The inter-wave intervals were 9.0 ± 0.0 days for 2 waves; 5.1 ± 0.5 and 4.8 ± 0.3 days for 3 waves; and 3.8 ± 0.5, 3.7 ± 0.3, and 3.3 ± 0.4 days for 4 waves. Growth rate (mm/day) of the largest follicle/wave did not differ (P = 07) among animals with 2 follicular waves (1.2 ± 0.7 v. 1.1 ± 0.0) or 3 follicular waves (0.9 ± 0.1 v. 0.9 ± 0.1 v. 1.2 ± 0.2). However, in animals with 4 follicular waves, the largest follicle grew faster (P < 0.05) for wave 1 (1.3 ± 0.2) than that of waves 2 (0.5 ± 0.2), 3 (0.6 ± 0.1), and 4 (0.6 ± 0.1). In conclusion, length of inter-ovulatory intervals, emergence of antral follicles, growth, and regression follicles in a wave-like pattern, inter-wave intervals, and diameter of ovulatory follicles in cross-bred ewes under tropical conditions are comparable to that described in sheep from temperate climates.