BRAHMAN NEWS JUNE 2013 ISSUE #179
In high roughage diets, ruminal pH is relatively stable at 6.2-7.0. However, rumen acidity can quickly change depending on the rations composition and the level of feed management.
Rations rich in highly fermentable carbohydrates (eg starches and sugars) and low in fibre can reduce rumen pH levels to less than 5.5 – 5.8 which increases the risk of acidosis and laminitis.
High grain-low roughage diets lack the fibre necessary to maximise rumination times and production of saliva. Insufficient roughage in the diet reduces chewing and the production of saliva buffers necessary to maintain rumen pH at an acceptable level.
"Ruminants require good quality roughage for a healthy rumen"
Stock in pasture based systems can produce up to 120 to 180 litres of saliva per day depending on the structural characteristics of the roughage. Factors which are important to saliva production are the dry matter content, particle size and the digestibility of the roughage.
Effective fibre must have a high quality
and sufficient particle size to ensure maximum dry matter intake for optimal chewing activity.
The ability of roughages to stimulate chewing is particularly important to the production of salival buffers (bicarbonates) necessary for the maintenance of an optimal ruminal pH.
This article explains the concept of effective fibre (eNDF) and its role and importance in maintaining rumen health.
"Effective fibre increases the secretion of salival buffers"
High intakes of non structural carbohydrates place stock at the risk of acidosis.
High grain–low fibre diets can result in lower rumen pH, reduced feed intake and increased digestive disorders. When ruminal organic acids (ie volatile fatty acids – VFA's) production exceeds the ability of the rumen to neutralise or absorb the acids acidosis occurs.
The severity of acidosis (acute or subacute) depends on the type and level of fermentable carbohydrates and the amount and effectiveness of the roughage in the diet.
"Lower pH decreases fibre intake and reduces fibre digestibility"
The intent of grain processing is to increase starch availability (NSC) and digestion in the rumen. Highly processed – low fibre grains are more digestible than cracked – high fibre grains.
"Maximising starch digestion decreases fibre intake and digestion"
The effect of increased starch digestion on the rumen function depends on the magnitude of the ruminal pH depression (metabolic insult) and reduced fibre intake. Subsequently when formulating rations containing high levels of grain it is essential to supply a suitable source of effective fibre to minimise the incidence of ruminal acidosis.
Depending on type of diet being fed,
there are different populations of bacteria in the rumen.
Cattle on high fibre diets will have a ruminal bacterial population that ferment fibre (cellulose and hemicellulose) resulting in the production of acetic acid. In contrast, digestive sugars and starches yields propionic acid and lactic acid.
Long high quality fibre forms a floating mat or matrix in the rumen. Fibre from this mat is regurgitated and chewed producing
large amounts of saliva that naturally buffers the rumen.
"Low fibre intake and digestion can reduce saliva production by 75 per cent"
The fibre mat also modifies the passage of material through the rumen so that digestion occurs at the proper rate.
Without sufficient saliva production and the correct rate of passage of food particles the rumen can lose mobility increasing the risk of digestive disorders.
Salival buffers contain bicarbonates (sodium and potassium) and adult stock can produce 1000 to 1500 grams of bicarbonates per day when fed high quality roughages and ruminating 10 hours per day.
"Low quality roughage reduces rumen flow rates, saliva production and animal performance"
Since fibre is digested more slowly than starches and sugars (NSC), including fibre in the diet slows down the rate and pattern of fermentation and subsequently the type of acid production in the rumen.
Higher levels of rumen organic acids are produced more quickly with grains compared with fibre. Adding effective fibre (ie high quality-long roughage) to the diet not only increases chewing activity and saliva production it also evens out ruminal acid production.
Feeding effective fibre can also shift site of starch digestion from the rumen to the lower intestine which not only reduces the risk of acidosis, swollen joints and feet disorders, but it can improve the overall performance of the ration due to post ruminal starch digestion.
The amount and size of fibre particles in the diet is extremely important to maintaining optimal rumen function and health.
Abnormal manure (Table 1) and reduced appetite are the first signs to poor rumen health and digestive disorders.
|1||Very liquid with consistency of pea soup. Leaves the rectum in a steady stream. Classic sign of acidosis.||• Increase fibre intake
• Remove grain source
• Seek nutritional and veterinary advice
|2||Runny manure which does not form a distinct pile. Manure splashes on contact with the ground and piles are less than 25mm high. Indication of digestive disorder which may eventuate in acidosis.||As above|
|3||Manure has porridge like consistency. Forms a soft pile 40-50mm high which may have concentric rings and a small depression in the middle indicative of good rumen health||• Maintain good feed management|
|4||Thick manure readily forming pellets||• Consider the level and type of roughage and the energy-protein ratio of the diet
• Check water quality and intake
• Possibly seek nutritional advice
"Manure observation is a practical guide to rumen health"
The structural components of roughage include hemicelluloses, cellulose and lignin. Most feed tables describe fibre as neutral detergent fibre (NDF) and acid detergent fibre (ADF). Since NDF is a measure of total insoluble fibre it is generally the most frequent used to describe fibre.
However, because chemical fibre analysis does not take into account the physical digestive properties important to chewing and saliva production, effective fibre (eNDF) in conjunction with NDF better defines the fibre requirements of stock.
Effective fibre (ie form and particle length of fibre) stimulates rumination, has a significant impact on chewing time and the production of saliva. It also maintains normal rumen pH and a normal rumen mat.
"eNFD is more highly related to the production of saliva"
Effective fibre is a better predictor of chewing activity than NDF and ADF. Table 2 shows the eNDF values and the various feed nutrients (NSC, starches, sugars) that impact on rumen pH.
"Level and source of eNDF needs to consider the level and source of grain"
When roughage particle size is reduced and diets contain low levels of fibre, the intake of effective fibre is frequently less than necessary to prevent digestive disorders. The inclusion of "fast" grains accentuates the potential problems of rations low in effective fibre.
Increased eNDF intakes can be achieved by increasing the particle size (more than 5–10 cm long) improving quality of the roughage and formulating diets with higher eNDF values (Table 2).
|Lucerne (mid fl)||85||8.7||19.5||49.3||49.3||2.7||22.5||1.7||8.0|
|Kikuyu (late )||25||8.5||18.0||52.2||56.0||3.0||19.8||2.6||10.5|
Note 1: In dry cracked "slow" grain and hay diets it is generally recommended that the ration should contain a minimum of 25-28 per cent NDF and 23 per cent eNDF. When more fermentable grains (ie fast grains, eg flaked maize and dry cracked wheat) are fed, the level of eNDF provided by forages needs to increase to offset digestive disorders.
Note 2: The selection of feeds needs to consider both eNDF and ME values and the level of soluble carbohydrates provided in the ration.
Roughages differ in their inherent buffer capacity. For instance, legumes (particularly lucerne) tend to have a greater buffering capacity than tropical forages and grasses.
Total ash and cation (Na, K) content of the forage provide an indication of the buffering capacity of forages. The buffering capacity of the forage increases as the minerals increase.
Stock have minimal requirement for fibre in order to maintain normal rumination, chewing and saliva production. Rumen acidosis occurs more often when there is insufficient amounts of total digestible fibre and effective fibre intake.
The concept of effective fibre (eNDF) offers a feed management tool to promote rumen health, rumen function, improve feed conversion and increase the overall performance of the diet.
Incorporating suitable sources and levels of effective fibre and selecting less fermentable sources of energy supplements can reduce the risk of "off feed" situations. Poor feed management and unsuitable feeding systems increases the risk of feeding problems (eg acidosis).
Feed management factors that reduce ruminal pH and depress appetite and roughage intake need to be fully considered to reduce the risk of digestive disorders.
"Feeding high quality fibre improves total nutrient intake, reduces digestive disorders and improves feed efficiency and production"
Further ABBA Reading: Nutrition - Acidosis & Laminitis -
ABBA Jnl Dec 2012; Fast & Slow Grains –
ABBA Jnl Sept 2007; Walking on Eggshells –
ABBA Jnl June 2001; Supplements for Beef Cattle –
ABBA Jnl Sept 2000; Selection – Structure and Lameness – ABBA Jnl March 2010.