Chicken are omnivorous animals with simple, single-chambered stomach and are therefore termed monogastrics. Digestion is mostly carried out by enzymes residing in the gastro-intestinal tract. As a result, chicken require high quality feed which will provide readily available nutrients to the enzymes [1]. Besides feed composition as the most important factor that determines the efficiency of feed utilization by animals, feed structure (particle size) and feed form (mash, pellets) are also important for the optimal nutrient utilization [2-5]. The success or failure of any poultry industry depends to a large extent on feeding. The cost of feeding ranges between 60-70% of the total cost of production [6,7]. It is therefore imperative that much emphasis must be laid on the type and form of feed given to chicken with the ultimate aim of boosting production. Numerous researches have been done on the form of diet to be used in feeding broilers; the pelletized form and the mash form. Abdollahi et al. [8], 2014 reports that, because of the high appetite for food by modern meat-type birds, their feed intake (FI) must be tightly monitored in order to achieve their high genetic potential. It is also well documented that feeding pelleted diets results in marked performance improvement in fast growing birds mainly through increased feed consumption. Coffey et al. [9], reports that the pelleting process was introduced to the United States feed industry by Purina in the mid-1920s. The main reason of pelleting at the time was to convert fibrous, bulky, finely-ground and unpalatable blends of feed ingredients into a compact, free-flowing pellet that facilitate easy prehension [9,10]. Pelleting has become the most common and popular hydro-thermal process in broiler diet preparation worldwide. Recent studies on the management, nutrition, genetics and other husbandry practices has led to the shift from fibrous and poorly digestible feedstuffs to the use of low fibre, texture-less and diets enriched in nutrients. This change has affected the development, functionality and health of the gastrointestinal tract (GIT), which in turn affects nutrient digestibility and also limits the advantages generated by steam-pelleting [11,12]. Among the numerous advantages associated with pelleting are enhancement of feed intake and growth of animals as a result of the reduction in feed wastage, decreasing of energy used for consumption, improving palatability, and the reduction of dustiness of feed [8]. Pelleting is a costly process but considering the enhancement of growth performance it is a justified venture. Pelleting diets also eliminate the negative effects of feeding bulky, fibrous feedstuffs creating an avenue for the incorporation of more inexpensive and less palatable feedstuffs into practical broiler diets [13]. Pelleting as a feed form also has its own disadvantages, their physical quality and ability to withstand handling stresses also affects its use [14]. Mash diets, increase starch digestibility [15]. Feeding mashed diet of high density performs the same function like pelleted diets. Feeding mashed diets have the tendency of reducing feed intake, more time spent on feeding and energy lost as a result of that. Ferket and Gernat [16], reported that when viscous grains such as wheat are finely ground and not pelleted feed prehension is compromised and consumption becomes very difficult due to the formation of a viscous sticky saliva-food mass in bird’s mouth and beak pasting caused by wheat gluten. Regarding feed particle size, there is an opinion that the smaller the particle size, the larger the surface area of the feed ingredient which would possibly result in higher digestibility in poultry due to a greater interaction with digestive enzymes in the gastrointestinal tract [17]. In more recent years, however, it is thought that a large particle size aided by some structural components is beneficial to gizzard functions and gut development [15,18,3]. The chicken digestive system is an organ which performs numerous functions with great complexity and dynamics. Among the functions of the digestive system are the digestion of feed into smaller molecules by means of enzymes and microbial fermentation, which can then be absorbed into the body, the creation of barrier against antigens and pathogens as the GIT is the largest interface between the host and the environment and finally, the GIT is considered to be the largest organ of the immune system as more than 70% of the cells of the immune system can call the GIT home [19]. Ingested feed and by extension nutrients play a significant role in the development and functionality of the GIT, diet composition (ingredients, nutrients and additives) can influence the development and function of the digestive system, including the immune system and the microbiota [20]. The aim of the present article is to give an overview of the effects of feed forms on parts of the digestive tract of broiler chicken and ultimately evaluate their effects on broiler performance.

Pellets consist of a mash that has been pelletized, that is, compressed and molded into pellets in a pellet mill. Unlike mash, where the ingredients can separate and the poultry can pick and choose among the ingredients, the ingredients in a single pellet stay together, and the poultry eat the pellets whole. Pellets are often too large for newly hatched chicken. Chicken feed can be pelletized into different sizes and shapes. Basically, the purpose of pelleting is to take a finely divided, sometimes dusty, unpalatable and difficult-to-handle feed material and, by using heat, moisture and pressure, form it into larger particles. These larger particles are easier to handle, more palatable and usually result in improved feeding when compared to the non-pelleted feed. Many researches have been in favor of pelleting the diet of poultry because animals fed pelleted diets make better gains than those fed mash diets [21,18,22]. This is because, in conditioning and pelleting process the heat produced breaks down the starch making the feedstuffs more digestible, the feed is in concentrated form as a result of pelleting and minimization of waste during the eating process [23,24]. Feeding pelleted diet also ensures that every animal receives a well balance diet because it eliminates the process of selective picking and choosing between ingredients [25,26]. Several researches have proven the preference of pelleted diet over mash diet in most animal species when given the choice. When moisture, heat and pressure is applied on feed ingredients, it produces a level of gelatinization which helps animals to utilize the nutrients better [27]. There is also the prevention of individual nutrients being separated during mixing, handling and feeding. Pelleting prevents feed wastage since the animals pick it in whole and there is likelihood that all the animals would receive a balanced ration [28]. There is a better flow and handling of pelleted feed thereby ensuring that feed contamination is minimized. The process of producing feed pellets can roughly be described as assembling of various compounds such as proteins, acids, sugars, fibers, and minerals. In modern feed mills, the ingredients are usually stored in bins above a weighing system composed of one or more scales. Grinding of coarse textured ingredients such as whole grains and other fibrous materials are done to get fine meal which will enhance the mixing and pelleting process. Quantities of each ingredient are weighed and thoroughly mixed and then conveyed to a bin above the pellet mill. These products can be softened (conditioned) by the addition of heat and water. After which the conditioned feed ingredients are compressed to form a dense mass which shape conforms to the die against which they are pressed. It is then allowed to dry and cool to be used in future as a form of animal feed.

Mash refers to a nutritionally complete poultry diet in a ground form. When typical ingredients are ground and mixed into a complete diet and fed, this is mash feed. The quality of the mash is assessed by the size and uniformity of its particles. Several authors have demonstrated that there is a positive correlation between increase in feed particle size and broiler growth. It is therefore very important that uniformity of particle size is ensured in preparing mash diets because birds are known to favor bigger particles and the preferred particle size increases with age [29,30]. This will prevent the dominant birds from eating the bigger particles while the rest of the birds will eat the finer particles and this does not ensure each chicken getting a well balance diet. The uniformity in particle size promotes good performance because it eliminates the differences in energy expended in ingesting feed with lower energy being used to ingest bigger particles and energy used for ingesting smaller particles [21,31]. The number of pecks to eat one given feed amount is reduced when particle size increases. According to Savory [32], particle size and nutrient concentration of the feed are the two main factors determining FI of birds. Portella et al. [33] reported that altering feed particle size without changing diet composition affected FI in layers. Portella et al. [33] demonstrated that feed consumption of broilers was related directly to particle size, with only a weak correlation with nutrient composition. The uniformity affects FI and eventually affects the performance of the birds. The differences in the uniformity may likely be due to factors such as type and variety of cereal, particle characteristics of feed components, grinding method and age of the birds [8,11].

The gizzard, also known as the muscular stomach or ventriculus serves as the grinding mill of birds. The gizzard can grind the food with previously swallowed stones and pass it back to the true stomach, and vice versa. Bird gizzards are lined with a tough layer made of the carbohydrate-protein complex koilin to protect the muscles in the gizzard. Diet form (e.g., structure: pellets vs. mash) can have a significant impact on gizzard functionality. It basically serves as the teeth of chicken. A large and well-developed gizzard is able to grind feed particles more thoroughly [34], to elevate pancreatic enzyme secretion through increased release of cholecystokinin [11], to increase proteolysis by pepsin, trypsin and other endogenous proteases in the small intestine, to improve gastrointestinal tract motility and to improve nutrient digestibility [35]. Thus, gut development and health can be enhanced. It is therefore very important that the promotion of the health and development of the gizzard should be of importance in the broiler industry. Nir et al. [36] reports of a positive relationship between gizzard weight and particle size which can be achieved by manipulating feed particle size. Svihus et al. [18] reported that feed form had a great effect on gizzard development emphasizing that there was a remarkable gizzard relative weight reduction when broiler mash diets were replaced by whole wheat diets or pelleted diets. This reduction of gizzard weight could be attributable to the lack of mechanical stimulation by the feed. Pelleting reduced feed particle size, and small particles are retained in the gizzard for less time than coarse particles, resulting in less mechanical stimulation [12] and reduced organ size [11]. There have been various studies on the effect of particle size of feed on the gizzard. Chewning et al. [29] reported that feeding smaller particle size (1.6-mm screen) of maize compared to coarse particles (7.9 mm screen) in mash form, reduced feed per unit gain by 31 and 17 points during 0 to 21 and 0 to 44 day, respectively. Charbeneau and Roberson [37] reported that in turkey poults, there was a linear decrease in weight gain when the size of maize grain was increased, due mainly to decrease FI. Jacobs et al. [38] however reported that in broilers, when the particle size of maize was increased it resulted in a linear decrease in weight gain and feed efficiency. Numerous researchers have reported that in young birds fed large particle size of maize, when fed mash diets there is a negative effect because of the poorly developed gizzard which cannot grind coarse particles [30,39-41]. When fed pelleted diets, replacing half of finelyground maize with coarse maize, from day 14 to 49, increased body weight (day 35) and feed efficiency at 35 and 49 day of age but at day 1 when the same feed was fed FI and weight gain decreased [42]. It was therefore concluded that young broilers are not able to utilize larger particles of feedstuffs efficiently because of the poorly developed nature of the gizzard. This situation exerts much pressure on the gizzard which will have to use more energy to perform its grinding function. The healthy growth and development of the gizzard to a greater extent affects the performance of the birds [30,43]. According to Abdollahi and Ravindran [44], when the gizzard is underdeveloped, there is a short retention time of digesta and an elevated pH and this condition leads to a possible minimization of the digestion process in boiler chicken. These conditions (pH and retention time) becomes more limiting when birds are fed diets in pelleted form [8]. Engberg et al. [22], Huang et al. [45], Frikha et al. [46] reported that when birds are fed pelleted diets, the pH is higher than those fed mash diets and this was assigned mainly to the high intake of feed by those fed pelleted diets and possibly a lower secretion of hydrochloric acid, [11]. Available literature favors the use of mash diets for proper gizzard growth than pelleted diets.

Effects on intestinal morphology

Feeding plays very important role in the life of every organism and has direct relationship with the gastro-intestinal tract. The growth and development of the gastrointestinal tract is vital for broiler chickens as it helps to effectively utilize nutrients in their diet [47]. The size, morphology and how the digestive system functions is very important in helping broiler chicken to adapt to the various conditions it will be subjected to. Therefore measures that would help in the post hatch period are necessary to ensure the proper growth of the chicken [48,47]. Lim and Low [49] reports that the gastrointestinal tract of chicks develops at the embryonic stage but undergoes rapid morphological changes at the onset of feeding. Researches have shown that within 48 h post hatch, the weight of the small intestine, which is the primary site of digestion and absorption, doubles, reiterating the presence of specific components in the diet affecting intestinal morphology [50]. When pelleted diets are fed to chicken, there is increased excreta score and reduced excreta dry matter (DM). The site for the reabsorption and recycling of water in the GIT is the caeca, studies have shown that the length and weight of the caeca is lower in birds fed pelleted diets than those fed mash diets and this might be the reason for the greater loss of water through the excreta in pellet-fed birds [8]. Several study have also reported of lower relative length of the different segments of the digestive tract of birds fed pelleted diets compared to those fed mash diets [8,22,36,51,52]. Although several studies have reported that the length and weight of the segments of the GIT are lowered in pellet-fed birds, Amerah et al. [51], Zang et al. [53] reported of an increase in the villus height and crypt depth of birds fed pelleted diets compared with mash diet. The villi serve as the direct surface for absorption. For absorption to occur, the nutrient must come into contact with the epithelial cells lining the small intestine which consists of circular folds known as mucosal folds and tiny finger-like projections known as villi lining the walls of the small intestine. The villi increase the surface area of the small intestine which greatly increases the amount of nutrients to be absorbed. The increased villus height may increase total luminal villus absorptive area and subsequently result in greater digestive enzyme action and enhanced transport of nutrients at the villus surface [54,55]. Study shows that when there is over feeding of wheat-based pelleted diet, digestibility of starch is impaired [56]. Diet form, type and particle size also has effect on ileal digestibility of nutrients [57]. These researchers reported of lower N digestibility in wheat-based pellets compared to mash diets. Abdollahi et al. [8] showed that feeding pelleted diets either had no effect on N and starch or improve fat, Ca and P ileal digestibility in maize-based diets, whereas pellet feeding reduced the digestibility of all nutrients in wheat-based diets. Selle et al. [58] reported that feed form had no influence on ileal starch digestibility while Abdollahi et al. [59] showed that the effect of feed form on starch digestibility depends on the grain type; while pelleting had no effect on ileal digestibility of starch in maize-based diets it decreased starch digestibility in wheatbased diets. To this, de Vries et al. [60] reported that the rise in concentration of high molecular weight of soluble NSP and digesta viscosity in the small intestine may be responsible for the poor digestibility of starch in pelleted wheat based diet. Svihus and Hetland [56], also reported that there is an overload of wheat starch in the small intestine and this may account for the poor digestibility of pelleted wheat based diet. The endosperm of sorghum grains contain kafirin proteins which surround starch granules with both embedded in the glutelin protein matrix [61]. There is the possibility that during steampelleting, an interactions occurs between kafirin-glutelin-starch that promote polymerisation of the kafirin protein to high molecular weight polymers that may eventually impede starch digestion. Abdollahi et al. [62], reported of a reduction of the CAID of Ca, P and fat of wheat- based diets when they are pelleted whiles the pelleting of maize-based diets improved digestibility of Ca, P and fat compared to mash diet. Intact pellets when fed led to a reduction in the relative length of small intestine and caeca, [52,36]. The ileum functions mainly in the absorption of vitamin B12 and bile salts and any other products of digestion which were not absorbed by the jejunum. The wall is made up of folds, each of which has many tiny finger-like projections known as villi on its surface. The jejunum, along with the other areas of the small intestine, is responsible for absorbing nutrients from digested food into the bloodstream. The duodenum is the section of the intestines that connects to the stomach and pancreas and it is the site for digestion. It accepts chyme from the stomach and completes the digestion of food [63-67]. Feed form and its characteristics also influence the microbiota composition and function in the GIT. The presence of these microbiota play very crucial role by stimulating the development of the digestive tract to enable it perform functions of nutrient digestion, gut mucosal proliferations, growth of digestive enzymes, synthesis and utilization of vitamins and fermentation processes [68,69]. Short-chain fatty acids (SCFA) are produced through fermentation and this stimulates the development of the digestive system through the increase in size and tissue components of the guts (Table 1). Fermentation in the hindgut by microbes yields mainly lactic acid like acetate, propionate and butyrate which are produced in the cecal digesta [69]. These appear to accelerate gut epithelial cell proliferation, thereby increasing intestinal tissue weight [70-74]. Mucosa development is retarded in chicks if feeding is delayed therefore the form and particle size must be considered in relation to the age of the broiler [16,75-77]. The compositions of the diet and the microbiota and their interactions also affect the intestinal development, mucosal architecture and the mucus composition of the lower digestive tract [69]. The microbiota also help in the building of a strong immune system and since there is a strong relationship between diet form and microbiota development, the health of the broilers are also improved [65,69].

Table 1: Effect of feed form and particle size on gizzard, Intestinal morphology and microbiota composition of broiler chicken.

It can therefore be concluded that diet form (mash or pelleted), type of grain and level of non-starch polysaccharides have various impacts on the function, shape and overall morphology of the gizzard and the intestines with mash diets being recommended for proper gizzard function and weight. Using less expensive, bulky, fibrous feed ingredients and by-products to prepare feed in pellet form is also a good way to maximize productivity in a broiler enterprise.