Monitoring trace minerals and heavy metals in liver of free-living large herbivores in the Netherlands.

Trace minerals are essential for animal health but can also, together with heavy metals, have a negative impact, making their monitoring crucial to assess animal health. These elements were examined through a long-term post-mortem monitoring system based on routine liver sampling for Heck cattle, Konik horses and red deer in place at the Oostvaardersplassen nature reserve in the Netherlands, using data from this system to determine reference intervals and investigate trends in liver trace element concentrations. Throughout the monitoring programme, inductively coupled plasma mass spectrometry was used to measure concentrations of trace minerals and heavy metals, including arsenic, cadmium, chromium, cobalt, copper, iron, lead, manganese, molybdenum, nickel, selenium, vanadium, and zinc. Species-specific patterns in trace element profiles were identified, with red deer showing comparatively higher copper levels and horses elevated iron and lead levels. Temporal declines in certain elements, including iron and lead, were observed across all species. Seasonal and age-related variations were also evident. Importantly, reference intervals estimated in this study differed from livestock standards, in particular for copper and selenium, highlighting the need for species- and context-specific reference intervals when assessing health in free-living herbivores. These findings provide valuable baseline data for ongoing environmental and health monitoring in minimally managed, multi-species populations at the reserve, highlighting the importance of mineral surveillance in free-living animals to enhance wildlife health assessment, track long-term environmental changes, and support management decisions in nature reserves across the Netherlands and more globally.