Issues related to sub-acute rumen acidosis and physically effective fiber

Understanding the importance of physically effective fiber and knowing how to measure it accurately can be very helpful in managing high producing cows to avoid sub-acute ruminal acidosis and its negative impact on health and performance.

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 must be maintained to provide a favorable symbiotic relationship between the ruminant host and ruminal microorganisms. The cow provides 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.

Major causes

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 affecting 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 decreases 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.

Link to entire article where you can learn more about how peNDF is measured (click here)

Author:

Jud Heinrichs, [email protected]

Professor of Dairy Science Penn State
 
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