Genetic Inheritance and the Impact of Low Birth Weight on the Incidence of Cryptorchidism in Hyperprolific Sows

Cryptorchidism in piglets, characterized by undescended testicles, causes economic losses and reduces consumer acceptance. Hyperprolific sows (HPS) have been hypothesized to produce a higher incidence of cryptorchid offspring. This study investigated the incidence of cryptorchidism in piglets born to HPS and its association with piglet birth weight and litter size in an observational study. Data from 276 litters (144 Landrace × Yorkshire sows; 4003 piglets) were analyzed. Sows were classified by genetic line (conventional: 68 litters; HPS: 208 litters) and parity (primiparous: 144; second parity: 132). At first parity, all gilts were inseminated with semen from a phenotypically unilateral cryptorchid Duroc boar, whereas at second parity, semen from three normal Duroc boars, which were full siblings, was used. The Landrace × Yorkshire HPS line produced more piglets per litter than the conventional Landrace × Yorkshire line (16.5 ± 0.3 vs. 12.4 ± 0.6; <i>p</i> < 0.001). Cryptorchidism occurred in 25.7% (37/144) of litters inseminated with semen from the cryptorchid boar, compared with 3.8% (5/132) of litters inseminated with semen from normal boars (<i>p</i> < 0.001). In total, 42 sows produced at least one cryptorchid piglet across both parities. Among affected sows (<i>n</i> = 42), the average number of cryptorchid piglets per litter was 1.3 ± 0.6 (range: 1-3). In the HPS line, cryptorchidism was detected in 24.1% (26/108) of litters, compared with 30.6% (11/36) in the conventional line (<i>p</i> = 0.441). HPS piglets had lower birth weights than conventional piglets (1.14 ± 0.01 vs. 1.30 ± 0.02 kg; <i>p</i> < 0.001). In the HPS line, litters with cryptorchid piglets had lower birth weights than those without (1.11 ± 0.02 vs. 1.18 ± 0.01 kg; <i>p</i> = 0.012), whereas no difference was observed in the conventional line (1.31 ± 0.04 vs. 1.28 ± 0.02 kg; <i>p</i> = 0.917). Litter size did not differ between litters with and without cryptorchid piglets in either genetic line. In conclusion, the lower average birth weight in cryptorchid litters of the HPS line, but not in conventional lines, suggests that HPS breeds may influence cryptorchidism incidence. These findings highlight the need to optimize fetal growth especially in the HPS to reduce this risk.