This experiment was conducted to investigate mid-lactating Holstein cows performance and behaviorin response to alfalfa hay particle size in acidogenic based diets. Nine multiparous mid-lactation Holstein dairy cows (653 ± 53BW; 45 ± 4 milk yield; 88 ± 15 DIM; mean ± SD) fed total mixed rations (40:60 forage to concentrate ratio) containing 3 sizes of alfalfa hay (with geometric mean 2 (fine), 3.5 (medium), and 5 mm (long)) were used in a replicated 3 × 3 Latin square designwith 21-d periodsincluding 14 d of adaptation and 7 d of data collection. Cows within a square were assigned randomly to dietary treatments. Cows were blocked into 3 squares of 3 cows each based upon milk production, body condition score, and DIM, and within blocks were assigned to 1 of the 3 experimental diets. Treatment diets were randomly assigned after blocking to minimize carry over effects. Cows were housed individually in box stalls (4 m x 4 m) that were equipped with concrete feed bunk and waterers. Clean wood shavings and sawdust were used for bedding and refreshed once a day.Cowswere fed ad-libitum once a day permitting 10% refusals with free access to water.A second cut alfalfa hay at 50% flowering, provided by Lavark farm waschopped by a harvesting machine with screen size regulator. The geometric means of rations were 3.00, 3.54, and 3.87 mm in fine, medium, and long alfalfa hay treatments, respectively. Reduction of alfalfa hay particle size linearly decreased rations geometric mean, physically effectiveness factor of 2 (pef 8 ) and 3 (pef 1.18 ) sieves, physically effective NDF of 2 (peNDF 8 ) and 3 (peNDF 1.18 ) sieves. Intake of DM, OM, EE, peNDF 1.18 , and yield of raw and 4% fat corrected milk, and also ruminating time changed quadratically to be higher in medium sized alfalfa hay. Furthermore, intake of peNDF 8 , milk fat percentage and fat to protein ratio, eating time and eating time when expressed as kg of DMI, and sorting of particles retained on pan linearly decreased with decreasing alfalfa hay particle size.However, reduction of alfalfa hay particle size linearly increased intake of CP, eating rate and chewing time when expressed as kg of peNDF 8 intake.Also, intake of neutral detergent fiber, milk total solid yield, total chewing time, and rumen fluid pH (4 h post-feeding) changed quadratic ally, however, DMI (6 h post-feeding), milk fat yield, eating time when expressed as kg of NDF intake, eating length (min/meal), and sorting of particles retained on 19 mm sieve tended (P? 0.15) to decrease linearly with decreasing particle size of alfalfa hay. Total tract nutrient digestibility and rumen fluid pH (2 h post-feeding) were not affected by treatment. Results indicate that change of alfalfa hay particle size in an acidogenic based diet can be an effective way to manipulate DMI and chewing activity, whereas further study is warranted to determine whether alfalfa hay particle size and acidogenic diet have interaction. In conclusion, cows fed an acidogenic diet positively responded to medium size alfalfa hay under current feeding conditions. Keywords: Alfalfa hay particle size, physically effective fiber, acidogenic ration, sorting behavior