Effect of Total Mixed Ration Particle Size on Rumen pH, Chewing Activity and Performance in Dairy Cows

Two experiments were conducted to determine effects of particle size in total mixed ration (TMR) on performance of lactating cows. Three rumen cannulated Holstein cows were used in a $3{\times}3$ Latin square design for the metabolic experiment. The particle size of the diets was determined using the Penn State Particle Size Separator (PSPSS) and weighing the proportion of sample remaining on the top screen (19 mm diameter). The 3 treatments were short, medium or long diets (4.9, 24.2 and 27.8% of sample remaining on the top screen of the PSPSS, respectively). Nine farms in the Edmonton area were surveyed and the farms were placed into groups based on the particle size of the ration fed. The groups were short ${\leq}6%$, medium 7-12% and long ${\geq}13%$ of sample weight remaining on the top screen of the PSPSS. Dry matter intake was greater (p=0.07) for the medium diet than the long diet in the metabolic study and resulted in a higher (p=0.07) efficiency of milk production. On the commercial farms, a significantly (p=0.002) lower milk fat percentage was observed for the long diet compared to the short diet. The results of these studies confirm that forage particle size influences milk composition and milk fat was negatively correlated to TMR particle size.