Equine atypical myopathy: consumption of sycamore maple seedlings (Acer pseudoplatanus) by pastured horses is driven by seedling maturity and might be associated with phenolic compounds

Background Poisoning with Acer pseudoplatanus L. in horses contradicts the hypothesis of coexistence between plants and vertebrate herbivores being mediated through antipastoral traits as toxins. However, incidental observations showed that horses evaded Acer seedlings with primary leaves. The objective of the present cross-discipline study was (i) to analyse whether developmental stages of A. pseudoplatanus L. differed as to phenolics hypothesised as antipastoral traits, and (ii) to observe systematically the selection behaviour of pastured horses towards A. pseudoplatanus seedlings. Methods Phenolic profiles of five developmental stages from fruits to seedlings of progressing age up to adult leaves of A. pseudoplatanus and Acer campestre L. were characterised. Video recordings of grazing behaviour of 29 pastured horses towards seedlings of A. pseudoplatanus resulted into 117 sequences as additional field data. Results The horses ingested 19.1 per cent of juvenile seedlings with cotyledons (1.65 mg total phenolics/g fresh weight (FW), 82 compounds, 0.02 mg total gallic acid/g FW) yet only 5.46 per cent of older seedlings with primary leaves (8.48 mg total phenolics/g FW, 120 compounds, 3.13 mg total gallic acid/g FW). Conclusion Horses distinguished between seedlings in distinct stages that could be chemically distinguished, too. Acer seedlings with primary leaves provide a strong, but not complete antipastoral effect that correlates with dramatic changes in phenolic compounds.