Risk factors for smothering in three commercial free-range layer poultry farms, Australia 2019-2022

In intensively managed poultry production systems the term 'smothering' refers to deaths from suffocation that occur as a consequence of piling behaviour where birds crowd together into densely packed groups. Smothering is a non-negligible source of loss in free-range layer hens, having both negative welfare and economic effects. Smothering events are rarely observed and are usually detected by the discovery of groups of dead hens. The aim of this study was to identify risk factors for smothering deaths in three commercial free-range layer poultry farms in Australia. This was a prospective cohort study of poultry flocks managed by three commercial free-range layer farms in eastern Australia. Flocks were enrolled into the study from 1 January 2019-29 March 2021 and were followed until the end of lay or until the end of the study on 31 March 2022, whichever occurred first. Throughout the follow-up period of the study, daily production and weather data, details of flock management and details of the place and time of smothering events were recorded. Time to event (survival) analyses were used to quantify the association between hypothesised risk factors and the number of days in lay at the time of smothering. Shed and bird level characteristics associated with time to event were quantified using a stratified Cox proportional hazards model which included a frailty term to account for birds clustered within sheds within farm. Across the three farms, for every 100 birds placed into a shed, there were 12 deaths over the duration of the production period. Of the 12 deaths per 100 birds, 2 were due to smothering. Our Cox proportional hazards regression analyses showed that the daily hazard of smothering was increased for birds housed in aviary sheds compared with flat-deck sheds (HR 4.0, 95 % CI 1.7-9.7). The daily hazard of smothering mortality was increased on warm, humid and rainy days, and in birds with low fear of humans and high fear of novel objects. Rainy days on which outdoor daily average humidity was greater than or equal to 70 % were associated with a 3.7 (95 % CI 3.5-3.9) fold increase in the daily hazard of indoor smothering deaths, compared with days when outdoor daily average humidity was less than 70 % and no rain. This study provides useful insight into the determinants of smothering in Australian free-range layer hens, in particular risk factors that do not change over time (e.g., shed type) and those that change daily (e.g., weather conditions). This information allows flock management strategies to be adapted accordingly.