Processing maize grain for dairy cows

When unprocessed maize grain is fed to dairy cows 15-30% will pass though the digestive tract as undigested whole grain therefore it must be processed to maximise its feed value. Processing grain disrupts the kernel and exposes the nutrients to rumen fermentation and lower gut digestion. Reducing the particle size or increasing starch solubility (such as in gelatinised or high moisture grain) increases the rate and level of rumen fermentation and digestion.

Methods of processing maize grain for dairy cows

There are a number of different mechanical methods that can be used to reduce the particle size of maize grain.
In New Zealand the most common are the hammer mill and roller mill although other options include disk mills,
steam-flaking and steam-rolling.


Hammer mill advantages and disadvantages19


  • Produce a wide range of particle sizes.
  • Work with any friable material and fibre.
  • Less initial purchase cost compared to roller mills.
  • Offer minimal expense for maintenance.
  • Generally feature uncomplicated operation.


  • Provide less efficient use of energy compared to the roller mill.
  • May generate heat (source of energy loss).
  • May create dust and noise pollution.
  • Produce greater particle size variability
    (less uniform).

Roller mill advantages and disadvantages19


  • Energy efficient.
  • Uniform particle-size distribution.
  • Little noise and dust generation.


  • Little or no effect on fibre.
  • Particles tend to be irregular in shape and dimension.
  • May have high initial cost (depends on system design).
  • When required, maintenance can be expensive.


One of the key benefits of roller mills is that they tend to produce very evenly sized particles with fewer fines (less dust) and large particles which may pass through the cow undigested. Disc mills are a compromise between roller mills and hammer mills. They produce more noise and fines than a roller mill but less noise and fines than a hammer mill.

Figure 13. Maize particle size distribution – roller mill vs. hammer mill20

Steam flaking AND steam rolling

Steam-flaking and steam-rolling gelatinise starch by heat and steam application. However, the degree of "cook" is highly dependent on the amount of moisture, pressure, and heat actually obtained. Steam-rolled grain is usually steamed for 10 - 15 minutes to increase grain moisture. Then, it is rolled into a thick flake with a density around 47.5 kg/hL. Steam-flaked grain is steamed for 30 - 60 minutes in a vertical steam chamber to increase grain moisture to 18 - 20%. Then, it is flaked through rollers to a density of around 30 - 37.5 kg/hL. Lower flake densities indicate more extensive processing and starch gelatinisation. Caution should be used against over-processing. In some studies, feeding low density flaked grains has resulted in lower production because of problems with intake and acidosis21.

Impact of particle size on starch digestion & milk production

Maize that is finer ground has a higher digestibility than coarse ground maize. Fine grinding will increase total tract and rumen starch digestion and reduce small intestine starch digestion.

In a USA feeding trial, ground maize (568 micron particle size) or rolled maize (3458 micron particle size) was fed to lactating cows at 5.6kg maize grain per day22. When compared to the rolled maize, the ground maize had 8% higher total tract digestibility, 16% more starch was digested in the rumen and 8.5% less starch was digested in small intestine (i.e. smaller particles shifted the site of digestion from the intestines to the rumen22 ).

Grain processing targets

A fine line exists between maximum rumen performance and an unhealthy rumen environment particularly where a high amount of starch or sugar is being fed. The ideal amount of grain processing will depend on a number of factors including the amount of grain being fed, the frequency, the grain feeding system and the type and amount of forage in the diet.

Dr Mike Hutjens (University of Illinois) makes the recommendation that maize grain in USA dairy cow rations is ground to 1100 microns. He says, “To determine if your dry maize averages 1100 micron, take a cup of it and sift through a baker or kitchen flour sifter. If 2/3 of the grain passes through the screen, it is about 1100 microns” 23.

Time between processing and feeding

Processed maize grain will hold its quality for long periods when stored in a clean, dry place.

Figure 14. Effects of grinding on the site and extend of maize starch digestion22

19 Koch, K. 2002. Hammer mills and roller mills. Kansas State University, May 2002.

20 Heimann. M. 2008. Advantages and disadvantages in particle size reduction techniques. American Soybean Association International Marketing, Singapore.

21 De Ondarza, M. 2003. Grain processing.

22 Remond, D., Cabrera-Estrada, J.I.; Champion, M.; Chauveau, B.; Coudure, R.; Poncet, C. 2004. Effect of corn particle size on site and extent of starch digestion in lactating dairy cows. J. Dairy Sci. 87:1389-1399.

23 Hutjens, M. 2008. Dairy strategies with expensive corn. The Dairy Site Wed 29th October, 2008.