Objectives: To assess effects of swine respiratory disease (SRD) on nursery pig responses during gas euthanasia and to compare responses to carbon dioxide (CO2) and argon (Ar) gas euthanasia in terms of efficacy and welfare.

Materials and methods: Fifty-four pigs identified for euthanasia were classified as having SRD or euthanized for other reasons (OT). These pigs were distributed among three treatments: prefill CO2 (P-CO2), gradual fill CO2(G-CO2), and prefill Ar (P-Ar). Behavioral and physiological indicators of efficacy and welfare were assessed directly and from video. Modified atmosphere CO2 and O2concentrations (%) were collected throughout the process.

Results: Respiratory disease status did not affect behavioral or physiological responses associated with efficacy or welfare with P-CO2or G-CO2. Conversely, SRD pigs lost consciousness faster than OT pigs with P-Ar (P< .05) and duration of open-mouth breathing was shorter (P < .05), but duration of ataxia tended to be longer (P < .10). Regardless of disease status, P-CO2 was associated with superior animal welfare, with shorter latency to loss of consciousness than P-Ar, and shorter duration of ataxia and duration and intensity of righting responses.

Implications: Standard operating procedures for gas euthanasia utilizing CO2 or Ar do not require adjustment for nursery pigs with respiratory disease. Minimum exposure of 10 minutes at > 70% CO2 concentration is required to reliably produce respiratory arrest in nursery pigs. Argon is not recommended as a euthanizing agent for nursery pigs. Duration of exposure to Ar required to reliably produce respiratory arrest remains unknown.