In poultry nutrition, most attention is given to protein products, due to the importance of protein as a major constituent of the biologically active compounds in the body. It also assists in the synthesis of body tissue, for that renovation and growth of the body. Furthermore, protein exists in form of enzymes and hormones which play important roles in the physiology of any living organism. Broilers have high dietary protein requirements, so identification of the optimum protein concentration in broiler diets, for either maximizing broiler performance or profit, requires more knowledge about birds' requirements for protein and amino acids and their effects on the birds' growth performance and development. It also requires knowledge about the protein sources available that can be used in poultry diets. The broad aim of this review is to highlight the importance of some of the available high-quality specialized protein products of both animal and plant origins which can be explored for feeding broiler chickens. Minimization of the concentration of anti-nutritional factors (ANFs) and supplementation with immunologically active compounds are the main focus of gut health-promoting broiler diets. These diet characteristics are influenced by feed ingredient composition and feed processing. The general hypothesis is that these protein products are highly digestible and devoid of or contain less ANFs. Feeding these products to broiler chicks, especially at an earlier age, can assist early gut development and digestive physiology, and improve broiler growth performance and immunity.

1. The aim of this study was to examine the effect of yeast cell wall (YCW) on performance and physiological responses of broiler chickens under subclinical necrotic enteritis challenge.

2. Six treatments in a 2 × 3 factorial arrangement (non-challenged or challenged plus no supplement, YCW or antibiotics (AB)) was used. Each treatment was replicated eight times with 12 birds per replicate. The treatments included: (1) Positive control (PC; no additive, not challenged); (2) Negative control (NC; no additive, with challenge); (3) YCWN = yeast cell wall (2.0 g/kg diet, not challenged; (4) YCWC = yeast cell wall (2.0 g/kg diet, challenged); (5) ABN = zinc bacitracin 50 ppm + Salinomycin 60 ppm, not challenged); (6) ABC = zinc bacitracin 50 ppm + Salinomycin 60 ppm, challenged).

3. Eimeria challenge at 9 d of age did not affect feed intake (FI), body weight gain (BWG), FCR or liveability at 10 d. The BWG and FCR at 10 d were greater (P < 0.05) in birds fed YCW or AB (AB) diets relative to the PC or NC groups. On 24 and 35 d, FI, BWG, FCR and flock uniformity (28 d) were greater (P < 0.05) in the challenged groups fed YCW or AB diets compared to NC group.

4. Supplementation with YCW ameliorated the negative effects of NE on liver, spleen and bursa weight of birds.

5. Necrotic enteritis challenge decreased (P < 0.05) caecal Lactobacillus and Bifidobacterium spp. counts, and increased ileum lesion score and caecal Clostridium perfirngens counts. This was reversed by the addition of either YCW or AB.

6. Supplementation with YCW and AB resulted to a greater (P < 0.05) dressing percentage and meat yield (35 d).

7. The results indicated that YCW plays a vital role in improving the physiological response and performance of broiler chickens under subclinical necrotic enteritis challenge.

The commercial poultry industry is considered the most rapidly growing of all the agricultural sectors. Feed costs constitute around 70% of the total cost of poultry production. The most important feed ingredients for poultry production are energy and protein sources. The poultry industry mainly relies on a limited number of animal and vegetable protein ingredients, such as oilseed meals, legumes and animal by-products (Broomhead, 2013; FAO, 2013). The commonly used animal protein sources such as blood, meat, meat and bone and fish meals are recognized as high- quality protein, with excellent nutritive value and balanced amino acids. Furthermore, chickens tend to prefer animal by-products to vegetable proteins (Hossain et al., 2013). On the other hand, there are some constraints to the use of animal by-products as feed ingredients for animals; high prices, restricted hygienic conditions and the risk that birds may suffer from zoonotic diseases if the animal by-products are processed under sub-optimal conditions. Therefore, for ethical and/or health reasons, animal proteins are excluded from production systems in some parts of the world such as the European Union (Hamilton, 2002).
There are numerous vegetable protein sources of local importance around the world, such as soybean, canola, cottonseed, sunflower seed, peanut, and sesame meals, but these have less nutritive value than animal protein sources (Hamilton, 2002; Aftab, 2009). Most of the vegetable protein sources contain one or more anti-nutritive factors, which can limit the digestion of their nutrients and eventually affect overall animal health, for instance trypsin inhibitors, glucosinolates, and gossypol in soybean meal, canola and cottonseed meals, respectively (Akande et al., 2010). The average crude protein content of different vegetable protein sources ranges between 235 g/kg in peas and 480 g/kg in soybean meal (SBM). Soybean meal is the primary plant protein source used by the poultry industry around the world. However, canola meal (CM) is increasing in importance (Hamilton, 2002; Nagalakshmi et al., 2007). The price of both soybean and canola meals do fluctuate but are generally high, particularly in importing countries. Besides CM, there are other vegetable protein sources close to SBM in nutritive value, low in prices and locally produced such as cottonseed meal (CSM) and sunflower seed meal (SFM).
Cotton (Gossypium), a genus of the Malvaceae family, covers approximately 2.5 % of the agricultural land around the world. Cotton production worldwide is estimated as 23013 thousand tonnes. The highest cotton producing countries in 2015/2016 were India, China, United States, Pakistan, Brazil, Uzbekistan, Turkey and Australia with 5748, 4790, 2806, 1524, 1285, 827, 577 and 566 thousand tonnes, respectively (USDA, 2017).
Cotton yields a number of by-products which are of great value to humans and domesticated livestock. Cottonseed is one of the most valuable by-products produced after the fine cotton fibres are harvested. Jones (1985) reported that for each kg of fibre produced there is 1.5‐1.7 kg of cottonseed separated out in the ginning process. Cottonseed meal or cake is a by-product of oil extraction from cottonseed. It has been reported that crushing one tonne of cottonseed produces around 200 kg of oil, almost 500 kg of cottonseed cake and 300 kg of cottonseed hulls or exteriors (Campbell et al., 2009). Several factors affect the quality of cottonseed obtained, including genetic differences, environmental conditions and harvesting techniques, which indirectly affect the composition of the resulting cottonseed meal. In addition to the genetic differences and environmental effect, the differences in the produced cottonseed meal arise from the residual oil content due to the method of extraction. For this reason there are different types of cottonseed meals, in terms of their protein, fibre and oil contents. The three main methods used by the oil industry to extract oil from oilseeds are: mechanical, solvent and pre-press solvent extraction. Mechanical extraction is the traditional method; it uses a circular motor and hydraulic press or expeller. In this method the seed may need to be decorticated, dried and/or heated before extraction. Besides the cakes produced by this method being tough and large, another important disadvantage is that around 20% of oil remains inside the meal. This high amount of oil, although it considered as valuable energy source, but it may increase the cost and reduce the palatability and storage period of diets. The difference between the mechanical method and the direct solvent extraction method is that in the latter method the oil is extracted by solvents (hexane or ethanol) alone without mechanical pressing and the meal produced has lower oil content. The third method, the pre-press solvent extraction, was developed from a combination of the preceding two methods. This method is considered an integrated method because screw-pressing is followed by solvent extraction, resulting in the extraction of almost 97 % of the oil content of oilseeds (Morgan, 1989; Ash, 1992; O'Brien et al., 2005).
Cottonseed meal is a palatable and excellent source of protein for ruminants. Although it's nutritive value is less than SBM, but its low cost in some regions makes it the main source of protein for cattle especially in parts of India, Australia and United States. Furthermore, CSM can replace all other oilseed meals in dairy cow feeds without affecting milk production (McGregor, 2000). Using whole cottonseed as a major source of protein has been tested to some extent in large animals, but its use in poultry diets as such results in decreased feed consumption and conversion, reduced nutrient digestibility, and poor growth (Devanaboyina et al., 2007). Furthermore, incidence of lameness and a high mortality rate are also associated with feeding entirely CSM as a source of protein to birds (Kakani et al., 2010). The presence of anti-nutritional factors such as gossypol and cyclopropenoid fatty acid, high fibre content and a deficiency in lysine are the well-known factors that limit the use of CSM in poultry diets (Swiatkiewicz et al., 2016). Cottonseed meal has a high crude protein content that ranges between 220 g kg-1 in the in the non-decorticated and 560.2 g kg^-1 in the completely decorticated seed, with metabolizable energy in the range of 7.4 to 11.99 MJ kg^-1. Furthermore, the fibre content of CSM exceeds that of SBM by 25% in the non- decorticated to 5% in the fully decorticated seed (Nagalakshmi et al., 2007). This promising nutrient profile of CSM, along with the fluctuation in the price of SBM around the world encourages poultry nutritionists and producers to trial CSM as a cost-effective and best nutritional alternative to SBM (Aftab, 2009).
Numerous ways have been reported that help in alleviating the limitations associated with the inclusion of CSM in poultry diets and raise its nutritive value. These include genetic manipulation of Gossypium through conventional breeding approaches and/or modern biotechnology, ingredient processing, using effective feed processing techniques, to decrease and inhibit anti-nutrients, and supplementation with nutrients such as synthetic amino acids, fat and vegetable oils. However, microbial enzymes appear to be the most effective solution to overcoming the limitations of the high-fibre and the non-starch polysaccharide (NSP) contents of alternative vegetable proteins that limit their inclusion at high levels in poultry diets (Scott et al., 1998: Leeson and Summers, 2001). All the above-mentioned techniques have helped to increase the CSM inclusion rate from 5% to around 30% of complete formulated diets for broiler chickens without compromising birds' performance (Watkins et al., 1995). Poultry lack specific enzyme systems to target NSP. For this reason, researchers are concentrating on developing single and composite microbial enzyme products that target NSP and enhance the nutritive value and nutrient digestibility of diets containing fibrous vegetable protein meals (Scott et al., 1998).
The poultry industry has employed microbial enzymes to improve the quality of temperate cereals and oilseed cakes. Therefore, inclusion of appropriate exogenous microbial enzymes in poultry feeds has clearly been demonstrated to increase the bio-availability of poorly digested diets, promote utilization of fibrous diets and improve the feed conversion ratio. These positive effects of the usage of exogenous enzymes have been frequently reported in recent studies as a result of the use of newly developed products for specific ingredients (Creswell, 1994; Slominski et al., 2006; Raza et al., 2009).
Much research and many industry field studies have been conducted to investigate the possibilities of replacing more expensive plant protein sources, like SBM, with alternatives with a similar nutritive value but lower prices such as CM, CSM and SFM. The present study is one of these investigations, and, hence, the main objectives of this study are to:

• Test the response of broiler chickens to CSM-containing diets supplemented with new microbial enzyme products (Avizyme 1502 and Axtra XB).
• Assess the potential of microbial enzymes in improving the nutritive value of CSM in diets for broiler chickens, especialy NSP-targeting enzymes.
• Evaluate CSM as a cost-effective alternative protein ingredient to SBM without compromising broiler performances.
• The study is intended to, among other things, determine the optimum levels of CSM and the test microbial enzymes in diets for broiler chickens and establish CSM as a competitive alternative to SBM.

1. The present study examined the potential of new-generation microbial enzymes to improve the utilisation of energy and protein of cottonseed meal (CSM)-containing diets, with the aim of increasing its inclusion level in broiler chickens diets.

2. Four hundred and eighty, one-day-old Ross 308 male broilers were used to assess the utilisation of energy and protein by broiler chickens fed diets containing four graded levels of CSM – none, low (4, 8, 12%), medium (5, 10, 15%) or high (6, 12, 18%) in the starter, grower, and finisher phases, respectively, supplemented with 100 mg/kg of a composite enzyme product (xylanase and β-glucanase).

3. Inclusion of CSM improved (P < 0.01) apparent metabolisable energy (AME), with further improvement (P < 0.001) seen in the enzyme-supplemented diets. Inclusion of CSM reduced (P = 0.002) the metabolisable energy intake (MEI), but this was increased (P < 0.05) with enzyme supplementation.

4. Enzyme addition increased (P < 0.001) the net energy of production (NEp), while heat production (HP) decreased (P < 0.001) with CSM inclusion. More energy was retained as fat (P < 0.05) and protein in birds fed diets with the enzyme, but this was reduced (P < 0.029) by CSM.

5. There was an increase (P < 0.05) in efficiencies of ME use for energy, lipid and protein retention, with higher CSM levels. The enzyme improved (P < 0.013) efficiency of ME use for lipid retention.

6. Feeding diets containing CSM to the broilers enhanced (P < 0.05) protein intake (PI) and protein efficiency ratio (PER). Positive effects (P < 0.05) of enzyme were observed on protein gain (PG) and net protein utilisation (NPU).

7. Results obtained from this study suggested that nutrient utilisation of diets containing CSM by broiler chickens can be improved by enzyme supplementation.