Antibiotic and medical zinc oxide usage in Danish conventional and welfare-label pig herds in 2016-2018

This register-based study compared the usage of antibiotics and medical zinc oxide (ZnO_med) in three different pig production systems: organic, conventional free-range and conventional indoor in 2016-2018. ZnO_med is used to prevent weaning diarrhoea with a dosage of max 2500 mg zinc/kg feed for 14 days post weaning. The treatment incidence (TI) of antibiotics (injectable and oral) and ZnO_med was calculated as the total number of animal daily doses (ADD) per 100 animals per day at herd level over a calendar year. Zero-inflated negative binomial models were used to compare the antibiotic and ZnO_med usage across the three production systems. The correlation between antibiotic and ZnO_med usage was evaluated using Spearman's correlation coefficient. In all age groups, the antibiotic TI was highest in the conventional indoor system, lower in the conventional free-range system and lowest in the organic system. In weaners, which is the age group with the highest antibiotic usage, conventional indoor, conventional free-range and organic herds had an average TI of 7.20, 3.37 and 0,48 ADD/100 animals/day, respectively. Another important finding in the study was that non-use of antibiotics is more common (>30% of herds) in welfare-label production systems (organic and conventional free-range) than in conventional indoor production (16% of herds). The overall differences in usage of ZnO_med between the three production systems were not statistically significant, but the probability of not using ZnO_med, was significantly higher for organic (36%) and conventional free-range herds (61%) compared to conventional indoor herds (19%). There was a weak to moderate positive correlation between antibiotic and ZnO_med usage at herd level with the weakest correlation in conventional indoor herds (0.31). The results indicate that ZnO_med does not appear to replace or prevent the use of antibiotics and suggest that ZnO_med is used differently in different systems. A weak correlation between the usage of antibiotics and ZnO_med in conventional indoor herds could reflect a frequent use of routine treatments within this system. The lower level of antibiotic usage in welfare-label systems indicates that a significant reduction in antibiotic use in pig production would require housing and management changes or regulatory changes in the conventional indoor system. The large variation in both antibiotic and ZnO_med usage between herds in similar production systems indicates that a further reduction in use of antibiotics and ZnO_med is achievable.