Much has been said about the importance of maintaining adequate levels of effective fiber in lactating dairy rations to support normal chewing behavior and rumen function and maximize dry matter intake and milk production.
Yet despite our current state of knowledge, two of the most common problems still encountered on-farm are subacute rumen acidosis (SARA) and depressed milkfat.
This situation seems to persist due to a lack of understanding about what actually constitutes effective fiber, and how certain attempts to increase effective fiber in the diet can actually decrease its intake if it facilitates increased sorting.
Effective fiber
Cattle manure rarely contains particles greater than ¼-inch in length, with the majority being less than ⅛-inch in length. Since particle size reduction appears to be minimal beyond the rumen, it is generally accepted that particles greater than ¼-inch in length must be reduced in size by both digestion and chewing during rumination before they can leave the rumen.
Chewing during rumination also produces saliva that helps neutralize volatile fatty acids produced by microbial digestion and thereby helps prevent ruminal acidosis and milkfat depression. It is rumen particles, primarily from forage, of a size capable of stimulating rumination that are normally defined as effective fiber.
Field consultants often recommend that forages should ideally “be about the width of an animal’s muzzle to effectively stimulate rumination yet not inhibit the rate of dry matter intake.” Such recommendations, however, are not generally supported by research.
An older study at the University of British Columbia showed time spent ruminating increased linearly with the logarithm of forage particle length but also increased in efficiency (decreased time per bolus), while time spent eating was not affected.
Researchers at Lethbridge Alberta demonstrated the amount of forage in the diet had about 10 times more effect in increasing time spent ruminating than did increasing forage particle size alone.
Therefore, the threshold for effective fiber in stimulating rumination and normal rumen function appears to be the amount of particles with a length greater than ½-inch.
Interestingly, this is approximately the minimum size of particle retained above the second (9 mm) sieve of the Penn State Particle Separator (PSPS). Excessive particle length (greater than 2 inches) in the diet, while offering potentially effective fiber, may in practice prove detrimental if it permits cows to sort against its consumption.
Practical TMR evaluation
The PSPS was developed as a tool to measure particle size distributions of TMRs, evaluating the particle size characteristics required to maintain normal chewing behavior and rumen function.
The question of how much effective forage or fiber a lactating dairy cow requires in quantitative terms remains unresolved, though a number of approaches have been suggested, such as a minimum level of forage neutral detergent fiber (NDF) as a percent of the diet dry matter. However, if the forage is finely ground, it will likely no longer be “effective” as defined above. Herein lies the challenge.
Researchers have tried various methods to relate the proportion of dry matter or NDF retained on a given PSPS sieve, or combination thereof, to time spent chewing or other indicators of normal rumen function as a measure of physically effective fiber, with inconsistent results.
In field practice, users of the PSPS express their results based on as-is (undried) fraction weights. This means field target values may need to be different than published values if dry matter content varies between the separated fractions, which is likely. (Forages are often wetter than finer grains and supplements.)
Alternatively, effective fiber may be a threshold effect (i.e., a minimum quantitative requirement above which little benefit is realized), or its effect in the diet may be confounded with uncontrolled effects on sorting behavior.
Through personal experience, it appears normal chewing behavior and rumen function are maintained, and sorting is minimized, when TMR rations have the following characteristics:
- They contain some, but not more than 10 percent, of particles retained on the top sieve of the PSPS (plus or minus 5 percent), and few of those particles exceed 2 inches in length.
- Less than half of the particles pass through the second (5/16-inch) sieve (and are retained on the bottom tray of the three-tray PSPS).
- There is adequate surface moisture to bind the finer particles to the longer ones.
Despite containing adequate levels of forage NDF and effective fiber, as well as an appropriate particle size distribution, many farms continue to experience unacceptable incidences of SARA and milkfat depression. More often than not, this is due to sorting of the ration due to a lack of surface moisture in the TMR.
Ration sorting during eating
It is common in practice to find that producers or consultants respond to incidences of SARA or low milkfat situations by routinely adding coarse forage to the ration “to add effective fiber and stimulate rumination,” without first assessing if the diet is actually low in potentially effective fiber or if the ration is being sorted by the cows during eating.
Sorting is an inherent behavior of cattle moderated by how rewarding the practice has been in the past and by level of satiety or hunger. It is rare for all animals in a herd to exhibit a similar level of sorting or desire to sort. In most herds, there will be a large variation in sorting activity exhibited by individual cows, and this can change as stage of lactation and milk production changes.
For example, older, late-lactation and low-production cows will tend to sort rations more extensively than younger, early and peak-lactation and high-production cows.
One of the most effective tools to diagnose sorting is to compare the proportion of cows with milkfat content equal to or less than protein content in the first, second and third-plus hundred days in milk (zero to 100, 101 to 200 and greater than 200 days in milk, respectively).
As sorting becomes an issue, the proportion of greater than 200 days in milk cows with so-called “depressed milkfat” differences will begin to exceed that in either of the two earlier groups.
Normally, the 101 to 200-day group should biologically have a greater predisposition for lower milkfat as well as reduced milkfat-to-protein ratios. The zero to 100-day group may have high fat-to-protein ratios if bodyweight mobilization is high in early lactation. Otherwise, it is not uncommon for minimally sorting herds to have similar proportions (15 to 20 percent) of cows with equal or negative fat-protein differences among the three groups.
When lactating cows sort against the long particles, or for the finer concentrate particles, the amount of effective forage that will be consumed will decrease below the formulated target.
This has two potential effects: The animals successful in sorting (later-lactation and lower-production cows) will consume a greater proportion of concentrates in their diet, have reduced milkfat concentrations and a higher risk for SARA and laminitis, and the remaining animals (early lactation and higher-producing cows) will consume a higher-forage, lower-energy content diet than intended, resulting in lower peak milk production, increased bodyweight loss and delayed conception. This behavior can be minimized by adding water to the TMR.
Addition of water to dairy TMRs
There is no specific TMR moisture content that will minimize sorting; the solution is to add water in increments until the behavior is controlled. The following method works well for lactating dairy cattle:
- Begin by adding about 10 percent water by weight, as-fed (4.5 kilograms or around 9 pounds per head, full feed basis) to the final mix (vertical mixers), slowly, over the duration of the final mixing period. With horizontal mixers, liquids are normally added earlier in the loading sequence.
- Wait four to five days to observe the results; if there are improvements in eating behavior, intake or manure consistency, add another 5 percent water by weight (2.25 kilograms or around 4 pounds per head) and observe again for four to five days.
- Continue adding water in 5 percent increments and observing the results until no subsequent improvement is apparent – and then decrease the water addition by one step.
Lactating dairy cattle appear to have an obligate requirement for a specific amount of effective fiber to maintain normal chewing behavior and rumen function. However, to be effective, forage fiber need not be overly long (i.e., likely just greater than ½-inch, perhaps 1 inch on average), but it must be consumed as intended and not promote ration sorting and, thereby, excessive consumption of concentrates.
In practice, surface moisture content may have a greater effect on TMR sorting than particle size distribution. This may explain why many efforts to add straw or other long forage to correct SARA and low-milkfat problems do not work as anticipated. 
PHOTO 1: Sorting negates effecitve fibre in rations using the Penn State Particle Separator tool.
PHOTO 2: Excessive particle length (greater than 2 inches) in the diet, while offering potentially effective fibre, may in practice prove detrimental if it permits cows to sort against its consumption.
PHOTO 3: The Penn State Particle Separator (PSPS) was developed as a tool for the measurement of particle size distributions of TMRs and thereby evaluate the particle size characteristics required to maintain normal chewing behaviour and rumen function. Photos provided by Jaylor Fabricating Inc.
Alan S. Vaage is a Ruminant Nutritionist with Jaylor Fabricating Inc. Email Alan S. Vaage
