Response to excessive selenium intake in the modern sow; a retrospective analysis of a case review

In June 2023, an incident at a Norwegian feed mill caused pig feed being over-supplemented with sodium selenite (NaSe), resulting in feed selenium concentrations ranging from 2.47 to 4.70 mg Se/kg feed from sodium selenite (NaSe), levels exceeding the European Union (EU) regulatory upper limit of total 0.5 mg Se/kg feed (standardized at 88% DM). The feed containing excessive selenium was unknowingly sent out to production farms. Selenium is an essential trace mineral with a narrow dose range between deficiency and toxicity. The National Research Council (NRC) reports selenium requirements at concentrations of 0.15 mg Se/kg feed for sows and finishing pigs, and 0.30 mg Se/kg feed for weanlings, but studies suggest these levels may not always meet requirements in modern swine production systems. This case study aimed to evaluate the effects of excessive dietary selenium intake on reproductive performance in breeding sows and piglet weaning weight. Data were collected from 516 sows (155 TN70 and 361 Landrace sows) across two Norwegian farms between 2022 and 2024, yielding 895 litter records. Mixed effects models were fitted to evaluate nonpregnant outcomes, stillbirths, litter size, and piglet weaning weight. Despite excessive dietary selenium concentrations, no clinical signs of selenosis or adverse reproductive effects were observed. Excessive selenium intake had no significant effect on nonpregnant outcomes, stillbirths, litter size or piglet weaning weight (all P > 0.05). Parity significantly influenced reproductive parameters: higher parity sows showed reduced conception rates (P = 0.008), larger litters (P < 0.001), more stillborn piglets (P = 0.033), and higher piglet weaning weights (P < 0.001). Litter size also significantly affected outcomes, with larger litters producing more stillborn piglets and lower piglet weaning weights (both P < 0.001). The absence of selenium-related effects suggests that, although dietary selenium concentrations exceeded NRC and EU upper limits, the intake remained within a physiologically tolerable range for high-yielding sows and their offspring. These findings highlight the complexity of selenium tolerance for high-yielding breeding sows under modern production systems.