Effect of using frozen-thawed boar sperm differing in post-thaw motility in the first and second inseminations on pregnancy establishment, litter size, and fetal paternity in relation to time of ovulation

Frozen-thawed boar sperm (FTS) has reduced motility and viability compared to cooled semen. Motility of FTS is related to in vitro and in vivo fertility, but this effect has not been determined in relation to the timing of ovulation. To test the effect of variable FTS motility in a multiple-AI system, ejaculates from 38 boars were collected and frozen in 0.5-mL straws. Upon thawing, samples were classified (mean ± SEM) by motility as poor (P, 20.2% ± 1.1%), moderate (M, 31.3% ± 0.9%), or good (G, 43.5% ± 0.8%). In replicates, mature gilts were synchronized and checked for estrus at 12-h intervals and assigned (n = 207) to receive 4.0 billion total sperm in each AI at 24 and 36 h after onset of estrus using the treatments: 1) P and M (P-M), 2) M and P (M-P), 3) G and M (G-M), and 4) M and G (M-G). For each treatment combination, a set of 3 boars was randomly selected within motility class for their allelic distinction with M sperm from a single boar represented across all treatments and sires used in both first and second inseminations. The insemination to ovulation interval (IOI) was determined using ultrasound every 12 h. Reproductive tracts were collected at approximately d 32 after AI. Treatment did not interact with IOI (P > 0.10) and did not affect (P > 0.10) pregnancy rate (57%, 67%, 71%, 76% ± 7.2%, pooled SEM) or total number of fetuses (9.2, 9.1, 9.5, 10.0 ± 0.8) for P-M, M-P, G-M, and M-G treatments, respectively. Treatment did affect (P < 0.05) the number of fetuses sired from the first AI (3.1, 7.2, 6.4, 6.3 ± 1.2) and second AI (5.7, 2.6, 3.0, 3.6 ± 0.9) for the P-M, M-P, G-M, and M-G treatments, respectively. The IOI also influenced (P < 0.05) the proportion of offspring sired by the second AI (30.0%, 57.7%, 51.3%, 18.3% ± 6.5%), as well as the number of fetuses sired by each AI. These results indicate FTS motility had no effect on pregnancy rate or litter size but did affect the number of fetuses sired from the first and second inseminations. The first AI appears to sire most of the litter except when P sperm was used. Number of fetuses sired was reduced when P sperm was used in either insemination compared to M, although no difference was evident between M and G. Fetal paternity appears to be a more sensitive marker for identifying the effects of sperm quality and IOI in a multiple-AI system with use of FTS. These results suggest that use of semen of various qualities can be used in combinations to aid in pregnancy establishment and contribute to litter size.