1. Introduction

The livestock and poultry industries are vital to global industries that recorded consistent growth over the last 30 years. The poultry industry has relative advantages of being simpler in management, higher productivity, and faster return on investment than other livestock production [1]. The worldwide consumption of poultry products such as meat plus eggs is currently high and tends to grow continuously compared to other livestock products [2]. Poultry meat is an essential source of animal protein in human diets. Besides being moderately cheap and readily available, its composition makes it a necessary part of healthy and balanced diets [3]. The poultry sector is an integral part of Malaysia’s livestock industry. Hence, Malaysia has been one of the world’s highest poultry meat-consuming countries, with an annual consumption of poultry meat reaching up to 50 kg/person [2].

Costs of feeds have always been a significant concern in local poultry production [4]. Poultry production costs have continually increased because of the fluctuating prices of high-quality raw materials such as soybean, corn, and others [5]. Several initiatives have been put in place, such as finding cheaper and locally available materials as partial substitutable protein and energy sources instead of SBM and corn in poultry feed formulations. Various locally available and low-priced feedstuffs have been proposed but have not been satisfactorily acceptable for poultry and consequently used in small proportions [6]. Fluctuations in price and comparatively inadequate SBM supply, particularly during the Covid-19 pandemic, lead to city lockdown, and logistics limitation ^[7][8][7,8]. As a result, the discovery of alternative protein sources for poultry feeding diet has become a particular focus in the current scenario to decrease reliance on SBM as the main ingredient of protein in poultry feeds [9].

Generally, most agricultural industries produce large amounts of residue every year. If waste is not effectively managed, it could contribute to the environment and harm both human and animal health. Waste products from the palm oil extraction industry constitute a huge problem, and any initiative towards economic utilization is a positive step towards solving environmental pollution [10]. Some countries around the world produce plenty of local alternative feedstuffs which are by-products of the agricultural industry. More often, these materials such as wheat bran, rice bran, cottonseed meal, copra meal, PKC, palm kernel meal (PKM) wheat pollard, cocoa pods, oil palm fronds (OPF), palm oil mill effluent (POME), sugarcane bagasse, and cassava waste are widely used as livestock feeds ^[11][12][11,12]. Most of these by-products contain significant amounts of anti-nutritional factors (ANF) which are also considered as non-starch polysaccharides (NSPs) [11]. Locally, large quantities of PKC have been produced from the oil palm industry, and comprehensive studies are needed to assess its potential as alternative protein sources for poultry feeds [2].

Being the primary source of vegetable oil production, the oil palm tree (Elaeis guineensis) is one of the most critical trees in many tropical countries such as Brazil, Indonesia, Malaysia, Thailand, Columbia, and Ecuador [13]. PKC is a by-product after oil extraction, which is generally a good source of fat, protein, minerals, and carbohydrates [14]. Many studies have been conducted to assess PKC’s feeding value and determine its effects on feeding diets in the broiler chicken industry. Malaysia is one of the world’s largest palm oil producers with a large amount of readily available PKC. There is a need to use PKC in the poultry industry as a source of protein and energy [15]. The main objective of the palm industry is to generate palm oil. PKC derives from the nuts of palm fruits is generated as by-products [16].

PKC usage is common in ruminant diets but limited in non-ruminant, especially in poultry diets due to its high fiber content [6]. PKC can be a promising feedstuff for poultry feed because of its moderate CP (16.43%) content and energy [17]. Cellulolytic bacteria can significantly improve the nutrient quality of PKC through SSF ^[18][19][18,19].

2. Composition and Nutritional Value of Palm Kernel Cake (PKC)

PKC has been accepted as one of the components in animal feeds. Its nutritional values, attractive prices compared to other meals, and long-term availability make PKC more competitive in the international meal market [20].

There are two methods of palm oil extraction: expeller or screw press, and solvent extraction. PKC is the result of the expeller oil extraction procedure, while the solvent extraction technique yields PKM. Extraction with solvent generally produces less residual oil than the expeller process, whereas crude protein and crude fiber are higher in solvent-extracted PKM ^[21][22][21,22]. Therefore, the nutritional values of PKC and PKM differ depending on their method of extraction [10].

More than 75% of PKC are made from cell-wall components, which made up of 35.2% mannose, 2.6% xylose, 1.1% arabinose, 1.9% galactose, 15.1% lignin, and 5.0% ash [23]. β–mannan is the main component of palm kernel by-products NSPs which is regarded as a prebiotic and is known to enhance birds’ immune system and reduce pathogenic bacteria in the small intestines [10].

The nutritional profile of PKC is shown in Table 1. It has a low nutritional value; however, processing and conversion through SSF could significantly increase its nutritional values and make it useful for poultry ^{[10][22][23][24][25][26]}[10,22,23,24,25,26].