A meta-analysis of the impact of forage particle size (FPS) on chewing activity, rumen pH and volatile fatty acids (VFA) in dairy cattle was conducted following a search of the literature. A meta-regression was conducted to test the additional effect of forage level, source and preservation method on the heterogeneity of the results of FPS on the same target variables. A total of 42 papers and 86 trials with the change of FPS in the diet of dairy cattle were identified. Decreasing of FPS in the diet from an average of 10.0 +/- 4.9-6.7 +/- 4.11 mm lowered eating (P < 0.01), rumination (P < 0.01), and total chewing time (P < 0.01) by 19, 28, and 44 min/d, respectively. A similar effect and heterogeneity were observed for all these variables when expressed as min per kg of DMI. Meta-regression analysis revealed forage level as an important source for this heterogeneity. Accordingly, this analysis indicated that eating time (P=0.04) and total chewing time (P < 0.01) per kg of DMI decreased with lowering FPS in high level of dietary forage inclusion (>500 g/kg forage). Forage preservation method was another source of variation accounting for high heterogeneity in the results of FPS in which eating (P=0.03), rumination (P=0.01), and total chewing (P=0.02) activity decreased only in silage-based diets but not in hay-based diets. Total VFA concentration (P < 0.01) and butyrate proportion (P=0.03) increased by 25 and 9%, respectively, with decreasing of FPS. Forage level also was an important source of the variability in which rumen pH dropped (P < 0.01) with decreasing of FPS only in low level of forage feeding. In contrast, VFA concentration (P=0.02) and propionate proportion (P < 0.01) increased with decreasing FPS in high level of forage inclusion. As result, the A:P ratio lowered (P = 0.04) with decreasing of FPS in high level of forage feeding. Moreover, rumen pH decreased (P < 0.01) with decreasing of FPS in silage-based diet, but acetate proportion increased (P < 0.01) with decreasing FPS in hay-based diet, resulting in greater (P < 0.01) A:P ratio with decreasing FPS in hay-based diet. In conclusion, the study confirms enhancing effects of feeding long FPS on chewing activity and ruminal fermentation; however, these effects were strongly modulated by forage level and preservation method of forages, with decreasing FPS being more influential in high level of forage inclusion and in hay-based diets. (C) 2016 Elsevier B.V. All rights reserved.