Project Topic On DETERMINATION OF OPTIMUM TRYPTOPHAN AND CRUDE PROTEIN REQUIREMENTS OF BROILER CHICKENS REARED IN THE TROPICAL ENVIRONMENT.

INTRODUCTION

Poultry production represents a large section in animal production. The world poultry production has increased significantly over the past sixty years with broiler production making up a large part of the industry. The chicken meat production account for 88% of the world poultry meat output (Economic Research Service/ USDA, 2001). There are many facets of production, and hence many areas that are of potential concern for the welfare of animals. These areas may include nutrition, genetics and breeding, pre- slaughter, slaughter practices among others (Mench and Siegel, 1997).

Feed cost is a major factor in animal production, representing 50-70 percent of the total cost of animal production (Bawa et al., 2003; Ogundipe et al., 2003), as   it is well understood that a large portion of costs associated with poultry production involves meeting the protein or amino acids requirements of birds (Corzo et al., 2005a). Protein and amino acids are generally considered as one of the major cost components of poultry diets and the trend of researches among broiler nutritionist is to formulate diets for maximum broiler performance. Therefore, providing balanced dietary crude protein and essential amino acids according to the requirements is very important (Ogundipe et al., 2003: Eits et al., 2004). All stakeholders in poultry production and business are interested in the appropriate quantity and quality of a formulated feed that results in an optimum economic return (Onimisi et al., 2008). Feed intake is one of the important factors  that  may influence  animal  live  performance  as  well  as  health  and  carcass characteristics. The nutrient content of a diet may affect voluntary feed intake and growth performance of broiler chickens. Feed intake depends largely on energy density (Noblet and Van Milgen, 2004).  On the other hand, feed intake can be depressed both by a severe deficiency in the limiting amino acids or an excessive supply of some essential amino acids.

Protein synthesis requires that all necessary amino acids are available to the tissues (Boisen et al, 2000). Therefore the dietary intake of essential and the non-essential amino acids needs to meet the requirement. Requirements of dietary amino acid can be classified as qualitative and quantitative. Qualitative requirements are related to the question of “what amino acids are required for maintenance, optimum performance (e.g.,  growth,  lactation, reproduction, and sports  competition), and optimum  health (e.g., prevention of chronic metabolic disorders, resistance to infectious disease, and recovery from illness)?” Quantitative requirements refer to the question of “how much of an amino acid (AA) is required for maintenance, optimum growth, and optimum health?”  Feeding  experiments  have  traditionally  been  employed  to  determine  both qualitative and quantitative requirements of dietary AA by animals. The ideal amino acid is useful in several ways; it results to more efficient use of protein, reduction in overfeeding and reduction in nitrogen excretion (NRC, 1994).  There is still a common belief  that  whenever  crude  protein  levels  are  lowered,  performance  is  negatively affected. This is a result of a number of studies where researchers have lowered crude protein levels beyond practical formulation levels and then failed to supplement all limiting amino acids to sufficient levels (Abebe and Morris 1990; Khaled, 2001). In some cases crude protein levels were lowered without supplementing amino acids other than  lysine  and  methionine.  Although  several  theories  for  why  performance  is negatively affected have been proposed including amino acids imbalance, increase in blood ammonia level and change in the ratio of net energy to metabolizable energy (Aftab et al., 2006). Results from different studies about performance of broilers fed low crude protein diets are contradictory.  This may be due to differences based on the basal  diets,  assay  length,  age  of  chicks,  environment,  genotype  or  other  factors. Furthermore results pooled from researches carried out on the manipulation of dietary protein showed that protein can have various effects on broiler performance, body weights, feed intake and especially carcass composition (Waldroup et al., 2005). The increased concentration of animals together with environment constraints (i.e. reduction of land and water pollution by nitrogen and phosphorus from manure spread) have forced the nutritionist to propose new feeding strategies for animals. The most efficient technique involves reducing the amount of nitrogen intake and keeping the amino acid supplies adequate for meeting the animal‟s requirements (Dean et al., 2006).

Tryptophan is an essential amino acid in poultry and is required for a wide variety of metabolic activities, because its concentration in organisms is among the lowest of all amino acids, it can easily play a rate-limiting role in protein synthesis. Tryptophan is a well known protein- forming (proteogenic) amino acid. Tryptophan which is the third or fourth limiting amino acid in poultry is the precursor of serotonin, which plays an important role in feed intake, weight gain, animal performance and feed conversion ratio. Its levels influence behaviour and carcass composition (MacKenzie et al., 2004; Corzo et al., 2005b). Tryptophan was found to have a sedative effect because of its influence  on  levels  of  the  neurotransmitter  serotonin  (Newberry  and  Blair,  1993). Several studies have indicated that an increase in tryptophan levels causes a decrease in the total liver lipids of broiler and layer chickens. According to Peganova and Eder (2003), tryptophan is significant in the formulation of feed with reduced content of protein in the nutrition of laying hens. NRC (1994) pointed out the requirement of tryptophan for broilers for the age of 0-3week as 0.20%, 3-6 week as 0.18% and 6 – 8weeks  as  0.16%.  Increased  dietary  tryptophan  resulted  in  improvement  of  blood plasma parameters such as glucose, serum concentration of total protein and albumin (Corzo et al., 2005b). Amino acids like lysine, methionine, threonine and tryptophan are indispensable  amino  acids for  body  protein  deposition  and  growth  in  broiler production. Thus, deficiency in tryptophan affects the utilization of dietary lysine and threonine, reduction in feed intake and consequently animal growth (Peganova and Eder, 2003).  L-Tryptophan supplementation also contributes to a better utilization of the other amino acids and of the feed in general. Besides its utilization for protein synthesis,  tryptophan  is  involved  in  other  biological  functions  such  as  appetite regulation,  immune  response  and  health  maintenance.  The  depressive  effect  of tryptophan deficiency (Uttrecht et al., 1991) is aggravated in the presence of excess protein in the form of large neutral amino acids.

There is little information in the literature about tryptophan requirements for broilers and egg laying hens and some of the results are controversial. Reasons for this may be due to genotype, seasons of rearing, protein levels used in the diets and tryptophan digestibility of the feedstuffs used in chickens. Uttrecht et al. (1991) estimated that the requirement of tryptophan for male broilers, from 1 to 20 d of age, is not greater than 0.16%. The level of tryptophan recommended for broiler chickens by the NRC (1994), from 0 to 3 wk of age, is 0.20% and from 4-8 weeks is 0.18% of the diet. Larbier and Leclercq (1994) reported that the tryptophan requirement, expressed in relation to the lysine level of the diet is 19.2% and 18.0%, respectively.

In a tropical climate such as in Nigeria, there are four distinct seasons in a year; the early dry season which covers the period between October and January, the late dry season which covers the period between February and March, the early wet season which covers the period between April- July and the late wet season which covers the period between August - September, (Institute for Agricultural Research (2011).