The effect of ovulation synchronization with OvuGel on ovarian follicles and fertility responses to a single fixed time insemination in different parities and seasons

Increasing use of semen from superior sires can accelerate genetic improvement when using a single fixed-time AI (SFTAI) to reduce the number of sperm required to produce a litter. The objectives of this study were to: 1) Assess whether follicle development, ovulation, and insemination was altered by parity and season in OvuGel and Controls; 2) Determine the impact of Day of OvuGel administration and estrus in Controls; and 3) Evaluate OvuGel and Control interactions with parity and season on farrowing rate and litter size. The experiment was performed in replicates in summer and fall with sows assigned from Monday and Thursday wean groups by parity to OvuGel or Control after weaning (Day 0). Sows received OvuGel (n = 1,636) on Days 3, 4 or 5 based on proestrus or estrus symptoms and then a SFTAI 24 h later. Controls with a wean to estrus interval of 3 to 6 d (n = 1,676) received an AI on each day standing. Ovaries of a sub-population of the Thursday weaned sows (n = 445) were scanned by ultrasound. The number (17.5) and size (7.0 mm) of ovulatory follicles on Day 4 did not differ between treatments, but P1 and P2 sows had smaller follicles and sows weaned in Sep-Oct had fewer and smaller sized follicles (<i>P</i> ≤ 0.05). OvuGel increased (<i>P</i> = 0.004) ovulation by Day 6 (92.2%) compared to Control (79.4%). Sows with large follicles on Day 4 were more likely to ovulate (87.8%, <i>P</i> < 0.0001) by Day 6 than those with medium (62.1%). Although there was no interaction with parity, ovulation was lower in the Control for parities 1, 2, and ≥ 7. OvuGel treatment on Days 3 or 5 reduced (<i>P</i> = 0.0003) farrowing rate (FR) by 7 to 11% and were excluded from further analyses. Day 4 OvuGel treatment had no effect on FR (86.7 vs. 87.4%) or total born (TB) compared to Control (13.2 vs. 13.4), respectively. Interactions were due to OvuGel improving FR in P1 sows (87.9 vs. 76.5%) and for sows weaned in Jul-Aug (89.2 vs. 83.4%) compared to Control, respectively. There was an effect of day of weaning that was linked to semen storage (<i>P</i> = 0.02) indicating 4 to 5 d (84.5%) reduced FR compared to ≤ 3 d (91.3%). The effect was evident with the SFTAI but not Control and was thought to originate with changes in fertility with additional days of handling and storage in summer. The results indicate OvuGel treatment on Day 4 at proestrus or estrus and followed by a SFTAI has great potential to mimic the fertility performance of conventionally inseminated sows.