Variation of TMR particle size and physical characteristics in commercial Iranian Holstein dairies and effects on eating behaviour, chewing activity, and milk production

Evidence is limited on the effect of physical form of total mixed rations (TMR) on management of group-housed dairy cows in commercial dairy herds. The objective of this study was to consider the variation of TMR particle size (PS) and physical characteristics in commercial dairies and its effects on eating behaviour, chewing activity, and milk production. Fifteen dairy herds, feeding TMR, were each studied for 7 consecutive days. In each herd, a group of multiparous (3.5±0.8 parity; mean±SD), high producing cows (47±4 kg milk/d) in mid-lactation (109±12 DIM) was studied. Herds were visited for 7 consecutive days for sampling fresh and refusal feed from the test group to assess particle size distribution, dry matter, and chemical composition. Chewing activity and milk production were monitored for the group. The 15 herds were divided to 3 classes according to geometric mean particle size of TMR: short, medium, and coarse, with average lengths of 4.6, 5.4, and 6.7 mm respectively. The classes were significantly different in geometric mean particle length (GMPL) and physically effective neutral detergent fibre1.18 (peNDF1.18) (P<0.01), but physically effective neutral detergent fibre8 (peNDF8) was similar between medium and coarse classes. Amount of TMR retained on the 19-mm sieve of Penn State Particle Separator was highest for dairy farms with medium GMPL (P<0.01). Diurnal changes in particle size of refusals and chewing activity were similar among classes; the only difference observed was that daily rumination activity in the medium class tended to be greater than for the short and coarse classes (P=0.10). Milk fat percentage and yield as well as fat-to-protein ratio (F:P) were higher in the classes with medium and coarse TMR PS (P<0.01). Although GMPL and peNDF1.18 of TMR were correlated (P<0.05) only with milk F:P (r=0.53 and 0.64, respectively), peNDF8 was correlated (P<0.05) with both milk F:P (r=0.56) and chewing activity (r=0.60). The 19-mm TMR fraction was the only variable that correlated (P<0.05) with milk fat percentage (r=0.51) along with a tendency to be correlated (P=0.09) with chewing time. Interestingly, grain percentage of TMR was negatively correlated (P<0.05) with both milk F:P (r=−0.58) and chewing time (r=−0.68). In conclusion, commercial dairy farms considered in this study had a high variation in TMR PS, and peNDF8 and the 19-mm fraction of the Penn State Particle Separator were good indicators for predicting bio-physical responses of cows.