A recent experiment in the Journal of Dairy Science (Maulfair et al. 2010, J. Dairy Sci. 93: 4791-4803) by Jud Heinrich’s group at Pennsylvania State University lends support to Jaylor’s recommendations for optimal particle size distribution of total mixed rations (TMRs) for dairy cattle.
For field testing of TMR mixer performance and overall TMR particle size distribution, Jaylor recommends removing the third (0.05 inch) sieve of the Penn State Forage Particle Separator, and then targeting about 5% retained on the top (3/4 inch) sieve, 45% on the middle (5/16 inch) sieve, and no more than 50% on the bottom pan (Table 1). A functional generalization is to have at least some material on the top sieve, but no more than 10%, with at least half of the remainder being retained on the middle sieve.
| Table 1. Recommended proportion (%) of TMR sample retained on each section of PSPS | ||
| Penn State† | Jaylor | |
| Top | 2-8 | 5 |
| Middle | 30-50 | 45 |
| Bottom | 42-68 | 50 |
| † Penn State Extension publication DAS 2002-42 | ||
With good quality forage, and a minimum of 25% from alfalfa or grass, achieving such particle size distribution with Jaylor mixers has consistently proven to minimize ingredient sorting and related health problems, while supporting high levels of milk production (>40 kg/h/d) and fat content (>4.0%).
In the Penn State study mentioned above, early lactation cows were fed four TMRs that differed only in particle size distribution, due to differences in the particle size of the grass hay which comprised 20% of the forage DM and 11.8% of the TMR DM (Table 2). Of the four rations, the Short ration appeared closest to the Jaylor recommendations, while the remaining three were progressively longer, and retained more than the recommended amount on the top sieve. However, all four diets had a similar amount, as well as less than 50%, that would pass through the 5/16 inch sieve and be retained on the bottom pan.
| Table 2: Particle size distribution (%, PSPS equivalent) of four TMR rations differing only in particle size of grass hay (11.8% TMR DM).† | ||||
| Short | Medium | Long | X-:Long | |
| Hay: Top | 17.3 | 47.0 | 71.9 | 84.4 |
| Middle | 37.9 | 25.3 | 16.6 | 9.0 |
| Bottom | 44.8 | 27.7 | 11.5 | 6.6 |
| TMR: Top | 6.2 | 11.0 | 12.4 | 14.9 |
| Middle | 46.4 | 43.1 | 41.3 | 39.4 |
| Bottom | 47.4 | 45.8 | 46.3 | 45.7 |
| † Based on data from Maulfair et al. 2010, J. Dairy Sci. 93: 4791-4803: Top = particles retained on 26.9 and 18.0 mm sieves, Middle = 8.98 and 5.61 mm sieves, and Bottom = 1.65 mm sieve and Pan of ASABE (2007) forage particle separator. | ||||
Despite the large differences in the proportions of rations retained on the top sieve, there was no significant difference in milk production or composition, total times sent eating or ruminating, nor ruminal pH among treatments, though there was a trend for dry matter intake to decrease linearly as ration particle size increased, beyond sixteen hours after feeding (Table 3). This caused some differences when chewing behavior was expressed per kg intake, but nevertheless led the researchers to conclude the following:
“Despite large differences in particle size among these diets and certain chewing and ruminating differences, no changes in rumen fermentation, milk production, or milk components were found in this study.”
| Table 3: Milk production and chewing behavior of dairy cattle fed four TMR rations differing only in grass hay (11.8% TMR DM) particle size. † | ||||
| Short | Medium | Long | X-Long | |
| DMI (24 h, kg/d) | 26.9 | 27.0 | 24.1 | 25.1 |
| Milk (4% FCM, kg/d) | 38.7 | 38.6 | 37.2 | 38.2 |
| Total chewing (min/d) | 894 | 924 | 878 | 916 |
| Eating (min/d) | 376 | 400 | 383 | 394 |
| Ruminating (min/d) | 518 | 525 | 495 | 523 |
| † Based on data from Maulfair et al. 2010, J. Dairy Sci. 93: 4791-4803: Top = particles retained on 26.9 and 18.0 mm sieves, Middle = 8.98 and 5.61 mm sieves, and Bottom = 1.65 mm sieve and Pan of ASABE (2007) forage particle separator. | ||||
Further to the above, the Penn State researchers found that sorting against longer particles increased dramatically as TMR particle size increased, as well as with time after feeding. Thus it seems reasonable that increased sorting activity against long forage particles may have contributed to the decrease in intake of the Long and X-Long rations. However, the resulting decrease in nutrient intake appears to have been insufficient to cause a decrease in milk production
In summary, the Penn State study appears to support Jaylor’s philosophy that all particles retained above the 5/16 inch sieve contribute to effective fiber, and that excessive amounts (>10%) retained on the top (3/4 inch) should be avoided as they increase the risk of sorting. The authors of another study by Penn State, summarized in the Penn State Extension Dairy Digest, emphasized that “When adding hay or straw for effective fiber, [it is important to] chop forages enough to minimize sorting”. Clearly, TMRs with less than 50% of the particles retained above a 5/16% sieve are capable of supporting high levels of milk production, especially if they contain highly digestible fiber sources. However, the risk of low milk fat and ruminal acidosis likely increases as the proportion of TMR passing through the 5/16 inch sieve increases above 50%, as forage quality declines. Because Nutrition Matters™
