Particle size reduction

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We monitor feed bunk forage particle length for physically effective fiber, but the cow must reduce those particles to form and swallow a bolus. To what size does she chew those forages, and how does forage dry matter (DM), neutral detergent fiber (NDF) and NDF digestibility (NDFD) affect eating time?

In an attempt to determine how rate of digestion and particle size reduction are related, Miner Institute research has included a look at ruminal digestion and retention time of various-sized forage particles. One of the first steps is to know the forage particle size when it first enters the rumen.

Much of our data has shown reduced chewing time for both eating and ruminating as NDFD increases. Total minutes eating per day tends to be similar, but dry matter intake (DMI) differs, indicating less time chewing per unit of DM and NDF consumed of higher NDFD forage. The faster a cow can grab, chew, salivate a bolus and swallow, the faster she can take another bite and fill her rumen, possibly requiring less bunk space time.

Questions include:

• How long does it take to form and swallow a bolus, and what affects that?

• Do cows chew all forages to a similar particle size before swallowing?

A recent study by Schadt et al (JDS 2011) examined particle size of swallowed boluses of various lengths of rye grass hay and silages. The forages were long ryegrass hay, ryegrass hay separated into fractions using the Penn State Particle Separator of >19 mm, 8- 19 mm, 1.18-8 mm and pan, along with fresh ryegrass cut to 50mm then dried and a grass silage and corn silage. The researchers emptied cows’ rumens, then fed each forage and recovered the swallowed bolus to analyze particle size with a horizontal wet sieving technique.

Table 1 details the length in millimeters (mm) – Average (Ave) and Standard Error of the Mean (SEM) – of the feed and bolus particles, along with chews per gram of ingested DM. Using the forage NDF, I have calculated chews/g NDF consumed.

Take-home points

1. Time required to chew, create a bolus and swallow each mouthful impacts the cow's total DMI and time budget of bunk space availability.

2. Particle length of low NDFD forage impacts time required for eating. With lower NDFD forages, harvesting at 30%-35% DM and proper 0.75” theoretical length of cut (TLC) – rather than longer, drier forage – might improve DMI, shorten time required to chew and swallow a lower digestibility forage.

3. Long ryegrass hay, the 50 mm ryegrass and particles retained on the 19 and 8 mm sieves were all chewed to similar size of 10-11 mm before being swallowed. Initial particles >25 mm in length were all reduced to 10-11 mm. But, one caution is not to assume forages should be chopped to 10mm to facilitate faster DMI.

4. Ryegrass hay retained on the 1.18mm sieve averaged 9.7mm in length, but was only reduced to 8.1mm upon chewing. Chewing is most likely to moisten particles for swallowing. Results were the same for the grass and corn silage, with little particle size reduction at initial ingestion.

5. Dry feeds require more chewing than silage, and longer hays tended to require longer chewing time.

Kurt Cotanch is Forage Lab Director with the William H. Miner Agricultural Research Institute. Contact him via e-mail at cotanch@whminer.com



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