Assessment of follicle population changes in sows from day of weaning and during estrus using real-time ultrasound.

Follicle selection and maturation for determining ovulation rate in the pig appears to occur during much of the follicular phase and up to the time of ovulation. There is evidence to suggest that counts of follicles classified as ovulatory-sized at onset of estrus may not reflect the final corpora lutea counts. Follicle size heterogeneity has been reported at estrus and may be related to increased embryonic asynchrony and mortality (Hunter et al. 1989). It is not dear which follicles ovulate at estrus but reports indicate follicles > 4 mm are LH responsive (Dufour & Mariana 1993; Lucy et al. 2001). Counts and size measures for ovulatory follicles may differ by the follicle size classification system and in response to whether follicles were assessed by physical or ultrasound measurement (Soede et al. 1998; Knox et al. 2002; Bracken et al. 2006). We hypothesized that the numbers of follicles classified as ovulatory at estrus may not reflect expected ovulation rate or litter size. We performed two experiments to characterize the changes in follicle populations from onset of estrus (Experiment 1) and from time of weaning to ovulation (Experiment 2). In experiment 1, our objectives were to measure proportions of weaned sows having large, medium and small follicles on the two days before ovulation. A total of 21 sows that had expressed estrus and ovulated between day 2 and 3 and which had real-time ultrasound digital video recordings for both the left and right ovaries on the first (period 1) and second day (period 2) of estrus were included in this study. The images of the ovaries were obtained transrectally using an Aloka 500V ultrasound with a 7.5 MHz linear transducer. The images were digitally recorded and follicles individually counted and measured using a digital display system that was calibrated to the measures of the ultrasound. The follicles were classified as small (S, <3.5 mm), Medium 1 (M1, 3.5-4.99 mm), Medium 2 (M2, 5.0-6.49 mm), Large 1 (L1, 6.5-7.99 mm), Large 2 (L2, 8.0-9.49 mm), and Large 3 (L3, 9.5-12.0 mm). Data were analyzed using the GLM procedures of SAS for the main effects of sow and period (day 1 and 2 of estrus). The response measures included the proportions of sows having the specified size class, numbers of follicles in class, and the size of the follicles. Period did not affect the percentage (22%), number (<3 follicles), or size of small follicles (3.1 mm). Period during estrus also did not affect the percentage of sows with Mt follicles (90%), or their size (4.4 mm), but numbers were reduced (P< 0.05) in period 2 (6.5 vs. 4.5). Period did not affect the percentage of sows with (100%) or numbers (14.1 follicles) of M2, but size was increased (P< 0.05) in period 2 (5.8 vs. 5.9 mm). Period did not affect the percentage of sows having L1 (100%), but numbers (7.8 vs. 9.1, P—0.01) and size (7.2 vs. 7.3, P<0.05) were both increased in period 2. Period tended (P<0.10) to increase the percentage of sows having L2 in period 2 compared to period 1 (37 vs. 63%) but there was no affect on numbers (2.2) or size of these follicles (8.6 mm). There was no effect of period on the proportion of sows having L3 (140/0),or their number (1.7), or their size (10.8 mm). In assessing the total number of large follicles (all those > 6.49 mm), there were more (P—0.01) large follicles in period