Polystyrene microplastic particle size and concentration effects on in vitro apparent nutrient digestibility and ruminal degradation kinetics

Context. Microplastics are present in ruminant production systems. Aims. This study quantified the effects of polystyrene microplastic particle size and concentration on in vitro feed digestibility and ruminal degradation kinetics. Methods. The Ankom filter bag method was used to investigate the effects of two sizes of microplastic particles (5 and 15 μm) at five concentrations (0.0, 0.5, 1.0, 3.0, and 5.0 g/L), and five incubation times (96, 72, 48, 24, and 6 h) on the in vitro dry matter degradability (IVDMD) of several different silage types. The Tilley and Terry method was used to assess if results were replicable to another common in vitro method. For the Tilley and Terry method, the effects of two sizes of microplastic particles (5 and 15 μm) at five concentrations (0.0, 0.5, 1.0, 3.0, and 5.0 g/L), and two incubation times (96 and 48 h) on the IVDMD of one silage type were investigated. Key results. Microplastic exposure of any combination of particle size and concentration did not affect the IVDMD for any of the silage types or in vitro methods investigated, irrespective to the incubation time (P > 0.05). Conclusions. In isolation of host effects, virgin polystyrene microplastic particles do not impact silage digestibility when 5 or 15 μm in size and at concentrations ≤ 5.0 g/L of ruminal liquor. Implications. Research is necessary to confirm whether in vitro findings can be transferred to in vivo models and to investigate other polymers and weathered plastic.