The objective of this study was to determine daily ration and feed bunk variability in physical composition of TMR offered to lactating cows in dairy herds. Four commercial herds (12 pens) in the south of Cordoba province, Argentina, were visited during 3 consecutive days on February. Fresh TMR offered in the morning was sampled to assess particle size distribution with a Penn State Particle Separator. Two samples were taken at the beginning and 2 at the end of the feed bunk to determine TMR variability for each wagon. Daily ration variability was expressed as CV, and calculated by dividing the SD of each particle length % over the 3 d period by the average of them over the same period. The effect of place at the feed bunk on particle size distribution was analyzed with linearized mixed models using the mixed procedure of InfoStat. Place at the feed bunk (beginning vs end) was included as a fix effect. Herd and mixer wagon within herd were included in the model as random effects. The greatest daily CV was found with long and fine particles (17.4 and 17.6% respectively) but all of them where greater than 8%. We observed a great variability and a significant difference between places of TMR samples in the short particle size sieve (Table 1). These results may be indicating heterogeneity on the feed delivered due to procedures of loading and mixing the TMR. It is very important to determine factors of management and infrastructure associated with variability of TMR to improve consistency and then to increase profitability of dairy farms. Table 1. Distribution (mean ± SD) of particles

Particles (%)	Mixer wagon		P-value
Beginning	End
Long	17.0 ± 2.84	17.6 ± 2.85	0.52
Medium	34.8 ± 3.80	34.1 ± 3.80	0.34
Short	35.7 ± 2.01	34.4 ± 2.01	0.05
Fine	12.5 ± 2.16	13.9 ± 2.16	0.11