Particle Length in Dairy Diets

In a past article we looked at different methods of evaluating fibre requirements and briefly touched on forage particle length. In this article, I will take a closer look at how forage length changes with handling and incorporation into a total mixed ration (TMR).

As particle length decreases, cows spend less time chewing and produce less saliva with which to buffer rumen contents. This tends to reduce rumen pH, fibre digestion and buttterfat. On the positive side, shorter particle lengths increase dry matter intake and improve the efficiency of microbial protein synthesis. The actual particle length of silages can vary greatly from the TLC (theoretical length of cut) set on the harvesting equipment. The actual length of cut will depend upon forage type, plant moisture and condition of the knives. Forage going into a vertical silo will need to be finely chopped to allow for movement and better compaction. Forage material going into a horizontal silo can have a longer length of cut. All movements of the silage from harvest to feeding will reduce particle size.

Cows consume particles of many different sizes and this, along with feeding frequency and intake, allows for a more steady rate of digestion in the rumen. The average particle size as well as particle size distribution is important to the cow and therefore to the dairy cow feeder. As mentionned in the previous article, this information is available from several commercial laboratories. A simplied model of the equipment (Penn State University), consisting of two screens and a bottom pan, can be used onfarm to evaluate forage and TMR particles sizes.

The onfarm system for particle size evaluation sorts the forage into three groups:> 19mm, 8 - 19 mm and < 8 mm. The commercial analysis includes several more fractions within the range of 8 and 19 mm. The material remaining on the top screen should have an average particle length of 1.5 inches or more.

A recent article in the Journal of Animal Science by Jud Heinrichs at Penn State looked at the effects that mixing had on 12,920 samples of forages and TMRs. Analysis showed that 16 to 18.4% of particles, by weight, in grasses, legumes and mixtures exceeded 19 mm in length. Small grain silages had a slightly lower average percentage of particles greater than 19 mm, whereas the percentage for corn silages was approximately 50% of that for legume and grass silages. In the TMR samples, typically only 7.1% of the particles were greater than 19 mm. The minimum and maximum values for samples with particles on the top screen ranged from 1.1 to 86.9% showing tremendous variability in this fraction.

The average percentage of particles between 19 and 8 mm in length ranged from 35 to 51% across all samples. There was much less variability within this sample set than was found on the larger screen. The average percentage for particles less than 8 mm was nearly the same for all forage samples - 37 to 47%. The average percentage of TMR particles in this fraction was 58% reflecting the addition of concentrate, protein supplements and other feed additives.

Although the correlation is low there was found to be a relationship between ADF or NDF levels and particle size. Higher quality forages (ie. low ADF/NDF) tended to have lower particle size likely due to their high leaf to stem ratio and low lignin content. Increasing mixing time was also found to decrease the size of particles within a TMR.

What Does This Mean?

  1. Particle size analysis is a key component of ration evaluation.
  2. Samples of TMRs for particle size analysis should be taken from the bunk before cows have an opportunity to eat or sort the feed.
  3. Particle size lengths for TMRs should fall within the following guidelines: 6-10% on the top screen, 30-50% on the middle screen and 40-60% on the bottom pan.
  4. If there is insufficient long particles, it may be necessary to provide 5 to 10 pounds of long hay.
  5. Mixing and distribution equipment can reduce particle size of feeds and forages, and they need to be accounted for when evaluating the diet.
  6. Do not cut silage any finer than what is necessary for proper compaction and ensiling. All movements of the silage or forage material will further reduce particle size.

For further information contact:

Karen Dupchak
Farm Production Extension, Animal Nutritionist
Manitoba Agriculture Food, and Rural Initiatives
204-545 University Crescent
Winnipeg, MB R3T 5S6
Phone: 204-945-7668
Fax: 204-945-4327