Dietary fiber is a key component of dairy cattle ration. Decreased milk fat percentage, reduced feed conversion ratio and hoof health problems are often related with the low fiber content in the animal diet. The effectiveness of the fiber in the ration depends on the type, quality, particle size as well as the amount of forage and non-forage fibrous sources being fed to the animal. Adequate length of the fiber is necessary in dairy cattle ration to maintain proper rumen function because long forage particles in the diet promote chewing and salivary secretion, thus maintaining the rumen pH. The physical effectiveness of dietary particles can affect feed intake, digestive efficiency, milk production and composition as well as and health of the dairy animals cattle. The concept of physically effective fiber (peNDF) was introduced to relate the physical characteristics of feeds to rumen pH by assessing the effects of feed particle size on chewing activity. The term peNDF combines the physical effectiveness factor (pef) of the feed with its neutral detergent fiber (NDF) content and can be used in diet formulation to ensure adequate particle size.
Importance of fiber animal ration
Long forage particles in the diet promote chewing and salivary secretion which facilitates the buffering of acids resulting from feed digestion. In addition, it creates a floating mat (dense, floating layer located just under the gas in the top portion of the rumen containing the more recently consumed feed to be available for microbial population for fermentation) in the rumen stimulating contractions of the rumen. In absence of fiber in the diet rumen becomes stagnant pool and removal of VFA via absorption and fluid passage from the rumen declines thereby increasing the risk of ruminal acidosis. Thus, particle length of forages and the amount of fiber in the diet can have a significant impact on ruminal pH through the provision of salivary buffers. Inclusion of fibrous diet slows the rate of feed digestion in the rumen as fiber is more slowly digested than starch and sugar. It is established that more VFA are produced after concentrate feeding as compared with forage which causes the depressions in ruminal pH. Therefore, addition of forages to the diet not only increase the rumination time but also balances the VFA production. This may also shift the site of starch digestion from the rumen to the intestine which reduces the potential risk of ruminal acidosis.
Fiber and Non-Fiber Carbohydrates
The reduced level of effective fibre decrease animal performance by lowering chewing activity, leading to less salivary buffer secretion. This may cause reduced ruminal pH and results in altered ruminal fermentation patterns with low ratios of acetate to propionate (A: P) that ultimately result in modified animal metabolism and reduced milk fat synthesis. Nonfibrous carbohydrates (NFC) or nonstructural (NSC) carbohydrates, rapidly fermenting carbohydrates, are used to replace fibre in low fibre rations. Unlike other nutrients where requirements are provided in grams per animal per day for specific body weight and milk production level, fiber requirements are considered as minimum amount for maintaining normal rumen environment and preventing various metabolic disorders such as ruminal acidosis, abomasal displacement and milk fat depression. NRC (2001) guidelines for minimum NDF from forage, minimum total diet NDF, and maximum diet NFC are presented in the Table 1.
Diets with less than 19% NDF from forage should contain high-fiber by-products by replacing grains to increase total diet NDF and reduce diet non-fibrous carbohydrate (NFC). NDF is a measure of cellulose, hemicellulose, and lignin fractions of feeds. NDF is more highly correlated with feed volume and chewing activity than ADF or CF. Although the NDF in high fibrous by-products is not as effective as NDF from forages to maintain normal milk fat percentage, it is effective in high concentrate or low forage diets, as it aids in meeting the total diet NDF and NFC recommendations. Fiber percentage in the dairy cattle ration should not be less than 15% NDF as it would result into the milk fat depression. On DM basis the NDF concentration for the diet containing 42% or 35% forage would be 19% and 16% NDF respectively.
Physically effective NDF is the fraction of fiber that stimulates chewing and contributes to the floating mat of large particles in the rumen. It divides the rumen contents into floating mat of large particles on a pool of liquid and small particles). Earlier the term effective NDF (eNDF) was used to determine the total ability of a feed to replace forage in a diet and maintain milk fat percentage. The terms eNDF and peNDF are often used interchangeably though effective NDF (eNDF) is the overall effectiveness of NDF for maintaining milk fat content and physically-effective NDF (peNDF) is the specific effectiveness of NDF for stimulating chewing activity in relation to particle size of the forage or feed. Recommended level of peNDF to maintain ruminal pH at 6 would be 22% and for maintaining milk fat percentage of 3.4% is 20%. The peNDF (% of DM) of feeds is determined by multiplying NDF concentration by the proportion of particles retained on a 1.18-mm sieve or peNDF effectiveness factor. Percentages of particles retained on a 1.18-mm sieve for some feed components are soybean hulls, brewer grains, corn silage, legume silage-coarse chop, whole cotton seed, legume hay, and grass hay is 3%, 18%, 81%, 82%, 90%, 92% and 98% respectively. But the analysis of individual feed and fodders for the proportion of particles retained on a 1.18-mm sieve is a limiting factor for application of this system in the field. It could be overcome by standardizing the particle size at feed manufacturing organizations such as CLFMA. The actual amounts fed should be determined by formulating diets based on the requirements and limits for nutrients, such as CP, RUP, RDP, NDF, NFC, fat and P, especially when multiple high-fiber by-products are used in the same diet. The peNDF will always be less that NDF, whereas eNDF can be less than or greater than the NDF concentration in a feed.
Need for physically effective fiber??
Neutral detergent fiber (NDF) is the most common method to estimate fiber in the animal feed. The requirement for long coarse fiber in the form of forage has long been recognized in cattle. The deficiency of fiber in the diet results in the syndrome like failure of rumination, difficulty in eructation causing tympany or bloat, reduction in food consumption in cattle and depraved appetite. The concept of physically effective NDF (peNDF) is to estimate the NDF portion of the diet that stimulates chewing activity and possibly the growth and functioning of the rumen microbes. peNDF would accurately predict the cow’s chewing response to forage/feed particle size.The adequate amount of physically effective fibre in high producing dairy cattle is important for maintaining normal rumen functions, decreasing the risk of metabolic disorders and avoiding suppression of fibre digestion, feed intake, milk production as well as alterations in milk composition. On the other hand, feeding excessive amounts of physically effective fibre decreases feed intake and lowers the feed efficiency due to reduced microbial protein synthesis. Thus, it is essential to find out an optimum amount of dietary fibre that is required to decrease the risk of ruminal disorders without impairing production performances in dairy animals. The particle size of the forage is also a critical factor to determine normal rumen fermentation characteristics. Increasing forage particle size generally results in increased rumination time per unit of dry matter consumed and affect the nature of feeding behavior. In normal feeding patterns a consistent supply of nutrients to the rumen leads to a constant environment for bacterial growth. Alternatively, rapid or selective ingestion may result in large diurnal variations in acid production and ruminal pH. The average dairy cattle spends maximum of about 14 h/d in chewing and ruminating depending upon the diet. The peNDF content of the diet can be increased either by: increasing the NDF content i.e., including more forage or byproduct feeds or by increasing the chop length of forages for low fiber diets. This increases the chewing activity resulting in the increase of rate of flow of saliva, thus providing the buffering capacity which may adequately buffer the digestion of the feeds. Fiber digestion may be impeded and milk fat levels may become depressed when rumen pH levels fall below 6.0. Rumen pH is a function of lactic acid and VFA production. The diets with longer particle size and greater amounts of effective fiber stimulate saliva production. The intake of particles greater than 19.0-mm was found to be negatively correlated with the amount of time rumen pH remains below 5.8.
Several methods to measure peNDF have been proposed with each at differing stages of development and validation. The modified Penn State Particle Separator (PSPS) is a widely used tool to quantitatively estimate forage and total mixed ration (TMR) particle size. Until more research is available on peNDF systems, the most practical method to evaluate the effective fiber level in dairy cattle diets is to ensure that level of NDF in ration and forage and TMR particle size are within recommended ranges.
The physically effective fiber aims at balancing diets to promote healthy rumen functions in dairy cattle reducing the risk of acidosis and improving feed conversion efficiency. Other factors such as maintaining the optimum ruminal pH, during the fermentation of diet (mainly starch content and grain processing) and feeding management practices need to be considered in addition to physically effective fiber to prevent ruminal acidosis. A greater proportion of forages can be included in the diet without lowering its digestible energy content. With the aim of maintaining normal ruminal functions, fiber digestion and for preventing milk fat depression syndrome and metabolic disorders in high producing dairy cattle, diets can be formulated or evaluated for chemical fiber and effective fiber (minimum) and non-fibrous carbohydrate (maximum).
by Deepika Tripathi, Srobana Sarkar, Ravi Prakash Pal and Veena Mani, National Dairy Research Institute