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

•Antibiotic usage in organic weaners was 6.7% of that in conventional indoor weaners.•>30% of Danish welfare-label pig herds did not use medical zinc oxide.•>30% of Danish welfare-label pig herds did not use antibiotics.•Positive correlation between antibiotic and medical zinc oxide usage within herds.•Overall usage of medical zinc oxide is not different between pig production systems. This register-based study compared the usage of antibiotics and medical zinc oxide (ZnOmed) in three different pig production systems: organic, conventional free-range and conventional indoor in 2016–2018. ZnOmed 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 ZnOmed 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 ZnOmed usage across the three production systems. The correlation between antibiotic and ZnOmed 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 ZnOmed between the three production systems were not statistically significant, but the probability of not using ZnOmed, 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 ZnOmed usage at herd level with the weakest correlation in conventional indoor herds (0.31). The results indicate that ZnOmed does not appear to replace or prevent the use of antibiotics and suggest that ZnOmed is used differently in different systems. A weak correlation between the usage of antibiotics and ZnOmed in conventional indoor herds could reflect a frequent use of routine treatments