Dietary fibre is the part of plant material consisting mainly of cellulosic and non-cellulosic polysaccharides, and a non-carbohydrate component, lignin. These components are highly resistant to hydrolysis by alimentary enzymes and cannot, therefore, be digested or absorbed in the blood stream. Yet fibre plays an important role in poultry diets, if applied properly.
By Dr Salah H. Esmail , Cairo, Egypt
It is generally believed that fibre content of feedstuffs is associated with their bulkiness. In some cases, however, there is no marked relationship between the two. For example, although wheat bran is one of the bulkiest feedstuffs included in feed mixtures for chickens, it contains only about one third as much fibre as do sunflower seeds which are one of the less bulky feeds (9% crude fibre in wheat bran compared with 26% in sunflower seeds).
Other examples of more bulky feedstuffs given to chickens are soybean hulls, alfalfa meal, dried grains, oats, barley and hominy feeds. A few of the less bulky feeds are rice polishing, wheat, meat and bone meal, corn and fishmeal. Shown in Table 1 is the percent crude fibre in each of those feedstuffs.
Fibre requirements Although the general opinion of poultry producers and feed manufacturers is that the fibre content of the rations should be kept below 7%, production does not appear to be influenced much by some increase to 8-10%. Studies on broilers showed no adverse effects of such a level of fibre from any source on chicken growth, mortality and feed consumption. With layers, there was no effect on egg production but the efficiency of feed utilisation was reduced, particularly with fibre sources such as oats, barley and wheat bran.
Dietary fibre is preferentially utilised by Lactobacillus and Bifidobacteria species which lead to production of lactic acid and short chain fatty acids. This results in a low pH which will maintain the normal microorganism population, thus preventing the establishment of Salmonella and other pathogens in the gastrointestinal tract. On the other hand, the slow rate of passage of fibre feeds through the gastrointestinal tract should allow for better digestion of feed and microbial fermentation and hence limit food borne pathogen colonisation through the action of the fermentation products.
Fibre and cannibalism A study was conducted to compare incidence of cannibalism with rations containing 80% yellow corn, and with 8%, 13%, and 18% fibre rations produced by substituting oat mill feed for corn in the aforementioned ration. The percent body parts pecked, the severity of pecking as determined by the scoring system, and the cannibalism mortality are given in Table 2.
The relationship of fibre content of the ration and prevention of cannibalism is not fully understood. Conceivably, it may be related to the increased consumption of feed and the time occupied in eating, or to the increased utilisation of sodium and potassium with high fibre diets, as will be indicated later.
Fibre and ammonia emission A study was conducted to evaluate the effect of including various fibre ingredients in laying-hen diets on ammonia emission. It was found that the fibre ingredients led in all cases to a decrease in ammonia emission from laying-hen manure (Table 3).
The effect of fibre here could be attributed to two factors:
• Fibre provides energy to bacteria in the lower gastrointestinal tract where the bacteria use nitrogen that would otherwise be excreted as uric acid for bacterial protein synthesis.
• The bacterial metabolism produces short-chain fatty acids that lower manure pH, thereby shifting ammonia (NH3) to ammonium (NH4+), which is less volatile.
The reduced ammonia emission certainly helps alleviate a number of health problems commonly observed in chicken flocks such as ascites, gastrointestinal irritation, respiratory diseases, contact dermatitis, and foot burns. Alleviation of such problems would better be achieved when fibre feeding is considered along with other management practices, such as use of nipple drinkers, balancing bird density with ventilation capacity, and using litter materials with high water-holding capacity, etc.
Yolk and plasma cholesterol Fibre sources such as ground whole oats, alfalfa meal, or wood shavings greatly reduce egg yolk and plasma cholesterol levels and hence minimise the health hazards associated with consumption of poultry eggs and meat (Table 4). This could be related to the high levels of bioactive anti-nutritive factors present in fibre feeds such as saponins which possess hypocholesterolemic properties.
It could also be that fibres bind bile acids in the intestines and cause more of such acids to be excreted in the faeces. This reduces the amount of bile acids returning to the liver and forces the liver to produce more bile acids to replace those lost in the faeces. In order to produce more bile acids, the liver converts more cholesterol into bile acids which lowers egg yolk and plasma cholesterol levels.
Fibre and mineral balance In long-term feeding of rations having moderate levels of fibre, there might be an improved utilisation of minerals. This effect, however, might vary with the source of fibre used. For example, it was found that retention of sodium and potassium was increased by oat hulls, but not affected by fibre sources such as alfalfa meals or soybean hulls.
On the other hand, retention of copper was found to increase with soybean hulls but not with the other two sources of fibre. The three fibre sources equally increased retention of iron, suggesting that the iron contained in either source has a high relative bioavailability. Selection of the fibre source to be incorporated in the ration could, therefore, be an effective means of satisfying requirements for a specific mineral and correcting deficiency.
Fibre and nitrogen balance Nitrogen balance was found to be higher for hens fed corn DDGS and wheat middling compared with hens fed diets devoid of such fibre sources. This result was expected because amino acids in fibrous feed ingredients are typically less digestible than those in low-fibre ingredients, requiring consumption of larger amounts of amino acids to satisfy the requirement for digestible amino acids. The excretion of nitrogen was unaffected by fibre feeding, indicating that nitrogen retention was higher relative to the control diet (Table 5).
Pigmentation of egg and meat Among the different fibre sources, corn DDGS is known to have an effect on yolk colour, imparting reddish appearance thereon, which is preferred by most consumers. This is because corn contains relatively high levels of xanthophylls which are primary contributors of yolk pigmentation. The xanthophylls present in corn DDGS also affect meat colour, as they can effectively be absorbed and deposited in the abdominal fat pad and skin.
This aspect is particularly important in some parts of the world such as Asian countries where the coloured chickens are highly demanded. Studies in Taiwan, for example, have shown that the addition of 20% corn DDGS can decrease the supplementation of artificial pigments by 50% and can thus save up to NT$ 300 (US$ 10) per metric ton of feed due to the reduced artificial pigment supplementation to meet the colour requirement of the Taiwan market.
Effects of excess fibre It is generally accepted that fibres may improve intestinal digestion by reducing the number of goblet cells present on the villous structures in the small intestines, and hence reduce the amount of goblet mucin which acts as a luminal barrier against absorption. This, however, may not always be the case, especially with fibre sources of high molecular weight or those having high methoxyl contents such as citrus pomace, apple pomace, tomato pomace, etc.
Excess feeding of such fibre sources may lead to enlargement of the intestinal villi arising from physical stimulation of villous growth similar to that observed with ruminants fed on high fibre diets, where rumen papillae are also enlarged through the physical action of fibres. The increased size of the villi is often coupled with about two-fold increase in goblet cell numbers which adversely affects absorption. The excessive use of such fibre sources in the diet may also increase viscosity of the intestinal content, with a resulting decrease in bioavailability of vitamin A and utilisation of dietary fats, which adversely affects body weight gain and carcass quality. It is, therefore, recommended to use such sources of fibre in limited amounts when better performance is to be achieved.
* References are available from the author upon request