Introduction- Together with chemical composition, also the physical form of the Total Mixed Ration (TMR) has been demonstrated to affect dairy cattle digestive physiology and productive performance. However, few studies have been performed on the hay-based diets typically administered to dairy cattle in specific regions like the Parmigiano-Reggiano cheese production area. Moreover, almost none of these studies focused on the relationship between particle size distribution and diet digestion parameters. This relationship needs to be investigated together with the effect of particle size on sorting, in order to provide recommendation on the particle size distribution to be achieved to improve the efficiency of use of these hay-based diets. Aim- The aim of the present trial was to investigate the relationship between TMR particle size distribution, diet digestibility and sorting of hay-based diets in lactating dairy cows. Materials and methods- Five farms located in the Parmigiano-Reggiano production area were involved in the study. Three sampling procedures were performed in each farm with 15 day intervals at 0, 12 and 24 hours after TMR delivery. Five fecal samples were collected 12 hours after feed distribution from fresh healthy lactating cows (60 to 90 days in milk). Physical, chemical and digestibility analyses were performed on the TMR samples. Particle size distribution was determined using the Penn State Particle Separator (PSPS) and total tract apparent dry matter digestibility (ttaDMDe) and total tract apparent neutral detergent fibre digestibility (ttaNDFDe) were estimated using undigested NDF (uNDF-residual NDF after 240 h of fermentation) as a marker in both diet and feces. Dietary uNDF was calculated as weighted average of the uNDF determined on TMR samples collected at the 3 intervals assuming a 60% TMR intake in the first 12 hours after distribution. The relationship between the dietary residues retained on each sieve of the PSPS and ttaDMDe and ttaNDFDe were studied through a curve fitting procedure. The effect of particle size distribution at feed delivery on sorting was also investigated. Results and discussion- The distribution of the TMR particles, expressed as percentage of the total mass, on the 3 screens and bottom pan was on average 12.1%, 25.2%, 35.1% and 27.4% (Upper-U-, Medium-M-; Lower-L-; and Bottom -B- respectively). The estimated digestibilities were the highest when the U sieve residues ranged between 10 -15% (DMD: 68.35% DM; NDFD: 52.76% NDF); the M sieve residue was around 25% (DMD: 68.73% DM; NDFD: 52.27% NDF), the L sieve ranged from 35 to 40% (DMD: 66.56% DM; NDFD: 50.60% NDF) and the proportion of particles retained in the bottom pan was around 40% (DMD: 68.36% DM; NDFD: 51.57% NDF). Aside from the general sorting against longer particles, the farm with the lowest geometric mean value in the delivered TMR (farm 4: 3.92 mm) shows an increase, after 12 h, in the proportion of particles held in the B. The increased proportion of the biggest particles in the diet (>19 mm) retained on the U sieves was directly related to the variation in the particle size distribution after 12 h. Conclusions- Particle size seems to affect both digestibility and sorting parameters. A careful preparation of the hay-based TMR diet, considering the suggested values of particle size distribution, may improve the efficiency of its degradation and digestion.
