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Theoretical length of cut: Theory and practice

Dan Wiersma Published on 30 September 2013

Corn silage season is here, and one harvest-time adjustment growers must make is the length of cut, or particle size. Most dairy producers I know have set goals to improve milk production and quality; beef producers using high-forage diets want to sustain high growth rates for their cattle.

These goals are greatly affected by animal dry matter intake. Feed intake is influenced by proper diet formulation, mixing and feeding. Particle length plays an important role here since it can impact multiple aspects of rumen function, and ultimately, animal health.

Longer forage particle length is important to promote rumination and salivation activity. A ration with many small particles may result in particles spending less time in the rumen, meaning less microbial digestion and lower fiber digestibility.

Studies also show that finely chopped forages may result in milk with lower butterfat and increased metabolic disorders, such as ruminal acidosis and displaced abomasum, due to their ineffectiveness in maintaining chewing activity and rumen pH.

What is effective fiber?
The term effective fiber means the ability of a fiber source to stimulate rumination. Two key forage characteristics – amount of fiber (cellulose, hemicellulose and lignin) and particle size (when fed to the animal) – determine forage’s ability to contribute to rumen stimulation.

Particle size is often measured and reported as physically effective neutral-detergent fiber. The industry goal for physically effective neutral-detergent fiber is 21 to 24 percent.

A plant’s fiber levels are chiefly determined by plant species (i.e., corn vs. alfalfa), crop growing conditions and stage of maturity at harvest. Particle size is affected by the initial harvest cut length, but can be further reduced in size by silage mixing and feeding activities.

What are the cows saying?
Theories are great – until you have to put them into practice. Ultimately, the judge of particle-size adequacy is the animal. One simple observation guideline is to monitor cud-chewing activity.

Following a meal, at least 40 percent of the cattle should be actively chewing. In herds where effective fiber is optimized, 50 to 70 percent of the animals will be actively chewing.

In addition to cud-chewing activity, there are other animal observations that can be monitored. These include manure consistency, incidence of cattle “off feed” due to indigestion, lame animals experiencing laminitis, milk components, hair coat status, feed sorting in the bunk and consistency of dry matter intake from day to day.

How can length of cut influence ensiling?
In addition to effective particle size for proper rumen function, length of cut has an effect on the ability to efficiently ensile chopped forage. Regardless of storage structure or moisture content, short-chop-length forages are easier to pack, minimizing air infiltration in the silage.

When dealing with long-chop-length forages, it is important to monitor packing activity to ensure proper silage density is achieved. A shorter cut helps in any situation where lower-than-desired crop moisture is an issue.

When ensiling forage, one chief goal is to eliminate air, allowing for efficient fermentation by the anaerobic lactic acid-producing bacteria. In an efficient fermentation, sugars are used as fuel for the lactic acid bacteria supplied by inoculants.

When dealing with drier forages or inadequate silo packing, aerobic bacteria, molds and yeasts have more opportunity to grow, consuming more valuable forage energy shortly after ensiling. Once oxygen is eliminated, the anaerobic lactic acid-producing bacteria can start to produce lactic acid, which lowers silage pH.

What guidelines should I use for adjusting length of cut?
Current recommendations are to harvest grasses and alfalfa at a quarter-inch to half-inch theoretical length of cut (TLC), unprocessed corn at a ⅜-inch to ⅝-inch TLC and processed corn at ⅜-inch to ¾-inch TLC.

Corn harvested for silage should be processed to maximize animal utilization. Use a shorter cut length for unprocessed corn to help break or crack more kernels.

Silages chopped at ⅜- to ½-inch have 15 to 20 percent of particles longer than 1½ inches. This is ideal for rations with high levels of corn and alfalfa silages. No dry hay or straw should be required in these rations to increase effective fiber in the rumen.

While TLC guidelines help producers, there is the possibility within each storage and feeding system to further reduce particle size prior to feeding. In upright silos, particle size can be reduced by the unloader, especially with frozen forages.

In silage bag systems, the bagger itself can reduce the particle length as the crop is fed into and compressed by the bagging machine. Silage stored in bunkers or drive-over piles has the lowest reduction of particle size, especially when a silage facer is not used.

Finally, your TMR mixer on feedout can reduce particle size. It is important to measure particle size in the whole TMR mix fed at the feedbunk so you can determine how particle size is reduced from initial chopping to final feedout.

How can length of cut and particle size be measured?
Chop length can be monitored in the field by using a Penn State Forage Particle Separator. Forage samples are weighed and then shaken in this unit, separating forages into three fractions or screens.

The target range of forage particles for each screen is as follows: Twenty-five to 50 percent remain on the smallest screen (less than 5/16- inch), 40 to 50 percent on the middle screen (5/16-inch to ¾-inch) and 10 to 25 percent of particles on the largest screen (more than ¾-inch).

Because corn silage makes up a greater proportion of the ration, more material should remain in the middle two screens, and less material should remain on the top screen and bottom pan.

If corn silage is the only forage source in the ration, at least 8 percent of the particles should be in the upper screen (more than ¾-inch), compared to a minimum of 3 percent when other forage types are included.

It’s important to monitor chop length frequently to ensure it is not changing from your desired size. Crop conditions change quickly and variations exist from field to field, requiring periodic changes to achieve length-of-cut goals.

To adjust chop length on the forage chopper, operators can change feed roll speed or remove knives from the cutterhead. Newer choppers with kernel processors can create silage with a large percentage of long particles without large pieces of whole cobs or stalks. This forage can be excellent quality because it packs and ferments well in the silo.

Particle size variability exists with haylage due to the machinery type used to cut alfalfa, swath width and density, and the harvested crop’s dry matter. Ten to 20 percent of the crop should be in the upper sieve of the Penn State particle separator.

Particle size may need to be altered based on silo type. Forages stored in upright, sealed silos likely fall at the lower end (10 percent). Bunker silos can handle appreciably longer material, up to 20 percent on the upper sieve.

The middle sieve should contain 45 to 75 percent of the material, and the lower sieve should contain 20 to 30 percent. As with corn silage, no more than 5 percent of the material should remain on the bottom pan.

Field investigations at Penn State found considerable variability in overall rations. Feeding management plays an important role in the particle-length needs of the cow.

Ideally, no more than 8 percent of the material should remain on the upper sieve. Guidelines for TMRs for high-producing dairy cows are 2 to 8 percent of the particles in the upper sieve, 30 to 50 percent in the middle and lower sieves, and no more than 20 percent in the bottom pan.

Beef cattle response to silage cut length is minimal and would be dependent on the class of beef cattle being fed and the kind of ration being fed. In beef rations with very restricted roughage levels, corn silage cut length may become increasingly important to encourage proper rumen function.

How does shredlage influence length of cut?
Recently, a new word – shredlage – has entered the cattle and custom-harvesting vocabulary. It is used to describe a new kind of corn silage produced when corn is processed by a chopper using an aftermarket, cross-grooved, crop-processing roll.

Rather than cut, these rolls have a lengthwise shredding action, resulting in a greater proportion of coarse forage particles in the shredlage. It also has been shown to improve kernel breakage (as compared to conventional crop-processing rolls), resulting in a higher kernel processing score and higher starch availability.

This technology should be helpful for achieving excellence on our effective fiber goals when feeding high-forage diets. But while this technology is promising, additional studies are needed to help determine the effects of shredlage-processed corn silage on neutral-detergent fiber digestibility and animal performance.  FG

Dan Wiersma


Dan Wiersma
Livestock Information Manager
DuPont Pioneer