Use of cooled stored boar semen for artificial insemination accounts for 99% of inseminations in pork production sectors worldwide. The aim of the present study was to examine use of GameteGuard®-CP, a natural plant derived blend of antioxidants, supplemented in one commercially available semen extender on sperm quality, conception rate, farrowing rate, and litter size. Ejaculates from 16 commercial Duroc boars were used in a split-ejaculate single-sire artificial insemination study. Sperm were evaluated for motility, viability, acrosome integrity, membrane stability, mitochondrial membrane potential, DNA intactness, and total antioxidant reactivity on both day 1 and 4 subsequent to ejaculate collection. Sows were inseminated after a treatment regimen was imposed to synchronize time of ovulation (n = 1476) among females using sperm collected 4 days subsequent to ejaculate collection extended in AndroStar Plus® or GameteGuard®-CP supplemented extender. At day 4 post-ejaculate collection, there was maintenance of sperm viability, membrane stability, acrosome intactness, mitochondrial membrane potential, and DNA intactness (P < 0.01) when there was use of GameteGuard®-CP treatments. When control samples were used, there were lesser (P < 0.01) values from day 1-4 for all variables evaluated. Conception rate and farrowing rates were 7.4% and 9.7% greater, respectively (P < 0.05), but there was no difference as a result of GameteGuard®-CP-treatment in mean litter outcomes. The results in the present study indicate antioxidant supplementation using GameteGuard®-CP maintains sperm quality during cool sperm storage resulting in improved conception and farrowing rates when using semen stored in the cooled state for as long as 4 days.