The feeding value of proteins depends not only on amino acid composition, but also on the effective utilization of the amino acids by the animals. Although a large volume of published amino acid digestibility values for poultry feedstuffs is available, there are still many gaps in our knowledge on digestibility and utilisation of amino acids. The major focus of this thesis is to explore selected aspects of amino acid digestibility and utilisation in broiler chickens. Initial studies examined the influence of age and physiological state of the bird on amino acid digestibility. Studies were also initiated to improve the utilisation of dietary protein by the broiler chicken through the application of the ileal digestible amino acid system in feed formulations. In particular, the use of digestible amino acid data throughout the broiler growing cycle and the effects of varying dietary concentrations of digestible lysine and methionine on performance and lean carcass yield were studied. Furthermore, an evaluation of deposition of protein and amino acids during starter and finisher phases of broilers was undertaken to provide empirical evidence for the refinement of protein nutrition of broilers.
The results show that the concerns about the effects of age and physiological status of birds on amino acid digestibility are justified. The study reported in Chapter 4 was conducted to examine the effect of the age of broiler chickens (14, 28 and 42 days post-hatching) on the apparent ileal digestibility of protein and amino acids in feedstuffs (wheat, sorghum, maize, millrun, soybean meal, canola meal, cottonseed meal and, meat and bone meal). Mill run is a by-product from the wheat flour milling industry and consists of coarse bran, fine bran, shorts, germ, flour and the offals from the "tails of the mills" (Patrick and Schaible, 1980). In wheat, the digestibility of most amino acids were higher (P<0.05) in 14-day than in 28-day and 42-day old broilers. In maize, the digestibility coefficients of amino acids were higher (P<0.05) at 28 and 42 days compared to those at 14 days. The digestibility coefficients in maize and wheat at 28 and 42 days were similar (P>0.05).The digestibility of amino acids in sorghum at 42 days was higher (P<0.05) than those at 28 days, but similar (P>0.05) to those at 14 days except for histidine, lysine, serine and glycine, which were higher (P<0.05) at 42 days.
Digestibility of amino acids in sorghum was similar (P>0.05) between 14 and 28 days except for isoleucine, leucine, phenylalanine, glutamic acid and alanine. The digestibility of amino acids in millrun at 42 days was significantly (P<0.05) higher than those at 14 and 28 days. There were no differences (P>0.05) in the digestibility between 14 and 28 days. In general, the digestibility of amino acids in canola meal, soybean meal and, meat and bone meal were higher (P<0.05) at 28 and 42 days compared to those at 14 days, and similar (P>0.05) between 28 and 42 days of age. In cottonseed meal, age had no effect (P<0.05) on the digestibility of amino acids, except for lysine and arginine, which increased with age.
The study reported in Chapter 5 was conducted to examine the influence of the physiological state (broilers, layers and roosters) on the apparent ileal digestibility of protein and amino acids in seven feedstuffs (wheat, sorghum, maize, soybean meal, canola meal, cottonseed meal and, meat and bone meal). The digestibility of protein for wheat was higher (P<0.05) in broilers than in layers and roosters, whereas the digestibility for maize and sorghum were similar (P>0.05). The digestibility coefficients of individual amino acids for wheat, maize and sorghum, were higher (P<0.05) in broilers than in layers and roosters. The digestibility of most essential amino acids for maize and sorghum were higher (P<0.05) in roosters compared to layers, while the digestibility for wheat in layers and roosters were similar (P>0.05). In general, the digestibility of protein and amino acids for canola meal, cottonseed meal and, meat and bone meal were similar (F>0.05) for broiler layers and roosters. The digestibility of protein and amino acids for soybean meal were higher (P<0.05) in layers compared to those in broilers and roosters, and similar (P>0.05) between broilers and layers.
The results presented in Chapters 4 and 5 suggest the age and physiological state of chickens significantly influence the apparent ileal digestibility of protein and amino acids. The effects, however, varied among amino acids and ingredients. The observed differences may be due, in part, to the structure and characteristics of ingredients and their interactions with the age or physiological status of birds.
Digestible amino acid systems offer nutritionists an opportunity to formulate more precise diets. In Chapter 6, diets formulated using apparent ileal digestible amino acids were shown to improve (P<0.05) performance and breast meat yield, compared to those formulated using total amino acids. The benefits were observed in all phases of the broiler growth cycle from hatch to processing (1-42 days), in diets containing substantial amounts of canola meal, cottonseed meal and meat and bone meal. Compared to diets based on total amino acids, broilers fed those based on ileal digestible amino acids increased weight gains by 14%, reduced feed per gain by 6 points, increased breast meat yield by 21% and reduced abdominal fat by 18%.
The positive results in Chapter 6 were extended in Chapter 7 to examine the effects of graded dietary concentrations of digestible lysine and digestible methionine (Table 7-4) on performance and carcass yield of broilers. Digestible lysine concentrations did not affect (P>0.05) growth performance and carcass yield during the 40-day trial period. In contrast, weight gain, feed intake and feed to gain ratio were significantly improved (P<0.05) by increasing concentrations of digestible methionine. Also, breast meat yield was increased (P<0.05) and abdominal fat reduced (P<0.05) by additions of digestible methionine. Increasing concentrations of digestible methionine during 1-40 days showed a significant (P<0.05) linear response in terms of performance and breast meat yield.
The current emphasis of broiler nutrition is not only on improving the efficiency of nutrient utilization, but also on maximizing lean meat production. The deposition of protein and amino acids during starter (1-18 days) and finisher (18-42 days) phases of broiler growth, with varying digestible lysine concentrations, was investigated in Chapter 8. The optimum requirement of digestible lysine during the starter phase was estimated to be 12.1 g/kg for both weight gain and feed to gain. During the starter phase, the deposition of protein, fat, ash, energy and amino acids to increasing digestible lysine concentrations showed a quadratic (P<0.05) relationship. During the finisher phase, the deposition of protein, fat, ash, energy and amino acids to digestible lysine concentration resulted in linear responses. Significant (P<0.05) quadratic relationships were observed only for glutamic acid and lysine. In general, the utilization (%) of protein and aminoacid decreased with increasing digestible lysine concentrations. Age of broilers had significant (P<0.05) effects on protein and amino acid utilization, which decreased with advancing age.
Overall, the results presented in this thesis demonstrate that the apparent ileal digestibility of protein and amino acids are influenced by the age and physiological state of birds. Ileal digestible amino acid values were shown to offer advantages in feed formulation compared to total amino acid values, particularly when ingredients of low digestibility are incorporated into diet formulations. The deposition of protein and amino acids in broilers was significantly affected by dietary amino acid levels. These data could be used to manipulate lean meat output and to improve the economic returns of broiler production.