The dairy cow is an amazing animal because of her ability to achieve high levels of feed intake relative to body size while maintaining the ruminal environment within certain physiological limits. These limits are required to be maintained to provide a favorable symbiotic relationship between the ruminant host and ruminal microorganisms. The ruminant should provide the microorganisms an environment limited in oxygen, neutral to slightly acidic pH, constant temperature, periodic influx of water and digestible organic matter, constant removal or absorption of end products and indigestible matter, and an average retention time greater than microbial generation time. The feeding systems necessary in modern dairy cattle production and behavior of the animal have made it increasingly difficult to provide a ruminal environment that stays within all of these narrow constraints. The enormous energy requirements of high producing cattle require dairy farmers to feed rations of increasing dry matter intakes and levels of concentrate feeds. One of the problems associated with this incorporating higher energy feedstuffs is an increased susceptibility to ruminal acidosis.
Ruminal acidosis is a condition where ruminal pH falls below a certain physiological range. There are two distinct types of ruminal acidosis. The first, more severe, condition is referred to as acute ruminal acidosis and it is generally defined as such when ruminal pH drops below 5.0. The second, less severe and more common, condition is referred to as subacute ruminal acidosis (SARA), and it is generally defined as a condition when ruminal pH falls in the range of 5.0 to 5.5 for greater than 3 hours. The decreased ruminal pH that causes acute acidosis is thought to be mainly caused by an increase in ruminal lactate, while the decreased ruminal pH that causes SARA is thought to be mainly caused by an accumulation of volatile fatty acids (VFA). There are many negative side effects associated with SARA including decreased dry matter intake (DMI), decreased milk production and milk fat content, and decreased feed efficiency. The level of these decreases will depend on the cow, the diet, and a variety of other factors. Thus SARA has an economic impact on the dairy industry.
There are three major causes of SARA in dairy herds: excessive intake of rapidly fermentable carbohydrates, inadequate ruminal adaptation to a highly fermentable diet, and inadequate ruminal buffering caused by inadequate dietary fiber or inadequate physical fiber. Dairy cattle can consume excessive amounts of fermentable carbohydrates through high levels of concentrate in the ration or moderate levels of concentrates at high DMI.
The U.S. National Research Council has recommended a minimum neutral detergent fiber (NDF) level of 25% of ration dry matter with a forage NDF level of 19% of ration dry matter for lactating dairy cows. The NRC based its recommendations on NDF as it is the fiber measure that best separates structural from nonstructural carbohydrates and is comprised of most of the compounds that are considered fiber. Forage NDF is included in these recommendations because NDF from non-forage sources is estimated to be about 50% as effective at maintaining chewing activity, milk fat content, and ruminal pH; therefore for every 1 percentage unit decrease in forage NDF, total NDF content should be increased by 2 percentage units. These recommendations are based on cows fed: a TMR, alfalfa or corn silage as the predominant forage, forage with adequate particle size, and dry ground corn as the predominant starch source. Forages are the major supplier of NDF in rations and their slower fermentation and physical characteristics are essential for maintaining ruminal health. The decreased digestibility of forage helps to maintain an optimal ruminal environment by diluting the effects of large amounts of VFA produced by starch fermentation. Fiber (NDF) with adequate length is thought to increase chewing in cattle, which increases salivary secretion of sodium bicarbonate and buffers the rumen digesta. Saliva is an extremely important component to buffering the rumen as lactating dairy cows can produce large volumes (>100 liters) per day. These compounds will associate with free hydrogen ions in the rumen and decrease pH. Bicarbonate and phosphate ions are very strong buffers at higher pH, but when pH drops too low (approximately 5.5), VFA become the primary buffering system in the rumen.
There are a variety of factors that affect saliva production in the dairy cow. Particle size, dry matter, and NDF content of forages are factors affecting rate of eating and time spent eating; chewing rate generally increases and thus saliva secreted per unit of DMI increases when ration particle size, dry matter, and NDF are increased.
Chewing was probably first suggested as a means of estimating a feed's effectiveness at maintaining ruminal health in the 1970s followed by studies developing a roughage value index system to estimate the effectiveness of fiber. Most methods relate a feed's effectiveness to its ability to stimulate chewing activity in the cow. Mertens from the Dairy Forage Lab in Wisconsin first defined the concept of effective NDF (eNDF) as the sum total ability of a feed to replace forage or roughage in a ration so that the percentage of fat in milk produced by cows eating the ration is effectively maintained. While maintaining or improving milk fat is a major impetus for trying to define fiber requirements of dairy cattle there are many factors that influence milk fat, some not related to diet such as stage of lactation, making the eNDF concept broad and hard to measure.
Physically effective NDF (peNDF), a term also defined by Dr. Mertens in the later 1990s, is defined as the physical characteristics of fiber (primarily particle size) that influence chewing activity and the biphasic nature of ruminal contents. This measure combines the physical and chemical properties of a feedstuff to predict chewing and is a product of a feed's physical effectiveness factor (pef) and its NDF content. Physically effective NDF differs from other measures of effective fiber in that it is based on the relative effectiveness of NDF to promote chewing. This eliminates animal variation from being attributed to a feed's effectiveness because chewing per unit of feed varies with animal size, breed, and intake. The more specific concept of peNDF is easier to measure than eNDF since peNDF is only concerned with the effect of a feed on chewing and the ruminal mat, which are mostly influenced by particle size, NDF, and dry matter content. Fiber fragility and specific gravity probably also have a small influence on peNDF, but these factors have yet to be further developed for use.
A system was first developed to calculate peNDF that ranged from 0 for a feedstuff that has no effectiveness in promoting chewing to 1 for a feedstuff that has maximum effectiveness in promoting chewing. A hypothetical long grass hay with 100% NDF was defined as having a physical effectiveness of 1 and an estimated 240 min of chewing per kg of dry matter or NDF for non-lactating cows eating 0.4 to 2.0 times maintenance. Various feedstuffs were classified by types and physical forms and each feedstuff was assigned a pef value that could be multiplied by the NDF content to achieve its peNDF. This peNDF method not only included NDF content and particle size but differences in NDF composition, specific gravity, and fragility would be partially accounted for by classifying different feedstuffs separately. A laboratory assessment of peNDF was developed where feeds are separated via dry vertical shaking (Ro-Tap) and the proportion of the samples retained above a 1.18-mm sieve (1.65-mm sieve diagonal) were multiplied by sample NDF content. This method is based on three assumptions: NDF is uniformly distributed over all particles, chewing activity is equal for all particles retained on a 1.18-mm sieve, and fragility is not different among sources of NDF. Measurement of peNDF has become widely used in dairy cattle nutrition and research. Many farms instead use the Penn State Particle Separator (PSPS) with as-fed feed samples measured by shaking in 2 vs 3 dimensions. More discussion of this topic will follow. Another problem with using particle size data is that the NRC did not publish a requirement for peNDF because of a stated lack of a standard, validated method to measure effective fiber of feeds or to establish requirements for effective fiber. A weakness of using the latter peNDF method is that NDF fractions are not chemically identical for all forages. NDF composition (the ratio of hemicellulose to cellulose to lignin) of forage varies between forage types and is affected by species, maturity, and storage method. This is likely part of the reason for the many contradictions in the literature about effects of peNDF on intake, milk production, milk fat content, and chewing behavior.
Recent studies and forage lab comparisons now show that the PSPS with the 4.0-mm sieve, using as-fed samples, can give very similar values to the Ro-Tap separator values done on dry samples. Studies reported by the Miner Institute confirm these data. This means that we can get a very good value for peNDF simply by measuring the amount (%) of the sample above 4.0 mm on the farm and then multiplying by the diet (or forage) NDF analysis value determined from the forage lab. The Ro-Tap system uses dry feeds and separates them in a 3-dimensional manner, while the PSPS uses as-fed (wet) feeds and separates them in a 2-dimensional manner. However, results from the two methods are amazingly similar for many feeds.
In addition, researchers from Penn State, as well as labs in Canada and Japan, have studied particle size of diets and their impact on rumen metabolism and have clearly shown that the critical threshold for as-fed feed particles escaping the rumen of high producing cows is greater than 1.18 mm and more in the range of 4 mm. While there is no one perfect sieve size to measure particles for all diets and all forages, data from three independent labs show that the 4-mm sieve is more accurate for the high producing dairy cow for estimating peNDF. The newest sieve found in the 2013 PSPS, is a 0.16-inch (4-mm) sieve. Feed particles found on this sieve will primarily be small forage pieces that are often, but not necessarily, high fiber in nature. Initially these particles will likely be trapped in the forage mat of the rumen, but they can be broken down easily with minimal rumination or by rapid microbial action. Typically they will hydrate quite rapidly and will not remain trapped in the fiber mat for a long period of time, yet will spend some time in the upper rumen layer. In either event these feed particles will have a small, yet significant, impact on buffering the rumen. It should be noted however, that many feed ingredients and byproducts may also be trapped on this 4.0-mm sieve. This is obvious and must be handled with the judgment of the operator. In some situations, this fraction must be discounted in its amount when using the PSPS for determining peNDF.
The overall peNDF of a forage or TMR can be estimated by adding the amount of feed on the top three sieves (all ≥ 4 mm) and multiplying by the NDF content of the feedstuff. It should be noted that this is an estimated value, as the NDF content and digestibility of each fraction are unknown and likely slightly different than the NDF value of the whole feed sample. In addition, some portion of the contents on the smallest (0.16-inch; 4-mm) sieve will likely contain grain or rapidly digested carbohydrates; for example when steam-flaked corn is used in a diet. Furthermore, peNDF by itself will not guarantee that the diet is well balanced and that rumen pH will be correct, as diet starch or total carbohydrate levels have been shown to significantly impact rumen pH in high producing cows with high dry matter intakes.