SUMMARY. The use of exogenous feed enzymes in poultry diets is becoming a norm to overcome the adverse effects of antinutritional factors and improve digestion

Despite having huge responsibilities in avian species, published reports on the influence of dietary factors and other possible constraints on the size, development and function of liver are limited. Consideration of the factors that could influence and alter liver function is therefore of critical relevance. In the current review, aspects of liver structure and function, and the influence of feed restriction, anti-nutritional factors, structural components and feed additives on liver are discussed. Effects of feed technology techniques such as thermal treatment and pelleting, feed particle size and whole grain feeding on the liver are also reviewed. A discussion of lipogenesis and lipid storage in poultry is presented to provide a better understanding and to differentiate the normal pathways of lipid metabolism from abnormal (i.e., disordered) pathways. The liver is the main site of fat synthesis in poultry, but under certain conditions, excessive fat can accumulate in the liver and cause problems. Factors contributing to the fatty liver syndrome are also examined.

This review highlights the limited amount of research conducted regarding the optimum particle size of different feeds for efficient poultry production. The current industry practice of using highly processed, pelleted diets masks the influence of particle size, but some reports suggest that the effects of feed particle size on performance may be maintained even after pelleting. There appears to be a general consensus that particle sizes of broiler diets based on maize or sorghum, optimum particle size should be between 600 and 900 μm. Available data clearly show that grain particle size is more critical in mash diets than in pelleted or crumble diets. Although it has been postulated that finer grinding increases substrate availability for enzymatic digestion, there is evidence that coarser grinding to a more uniform particle size improves the performance of birds maintained on mash diets. This counter-intuitive effect may result from the positive effect of feed particle size on gizzard development. A more developed gizzard is associated with increased grinding activity, resulting in increased gut motility and greater digestion of nutrients. Although grinding to fine particle size is thought to improve pellet quality, it will markedly increase energy consumption during milling. Systematic investigations on the relationships of feed particle size and diet uniformity with bird performance, gut health and pellet quality are warranted if efficiency is to be optimised in respect of the energy expenditure of grinding.

The apparent ileal digestibility coefficients of amino acids in 107 samples representing 22 food ingredients were determined using 6-week-old broiler chickens. The ingredients assayed included five cereals (barley, maize, sorghum, triticale and wheat), two cereal by-products (rice polishings and wheat middlings), four oilseed meals (canola, cottonseed, soya-bean and sunflower meals), full-fat canola, maize gluten meal, four grain legumes (chickpeas, faba beans, field peas and lupins) and five animal protein sources (blood, feather, fish, meat and meat and bone meals). The mean ileal digestibility coefficients of amino acids in wheat and maize were higher than those in sorghum, triticale and barley. However, variations observed in individual amino acid digestibilities among samples within cereal type were greater than those determined between cereals. Threonine and lysine were the least digestible indispensable amino acids in the five cereals evaluated. The most digestible indispensable amino acid was phenylalanine in wheat and, leucine in maize and sorghum. In the case of the wheat middlings and rice polishings, threonine was the least digestible indispensable amino acid and arginine was the best digested.In the oilseed meals assayed, amino acid digestibility was highest for soya-bean and sunflower meals, intermediate for canola meal and lowest for cottonseed meal. Ileal digestibility coefficients of amino acids in lupins were found to be slightly lower than those in soya-bean meal. The amino acid digestibilities of field peas, faba beans and chickpeas were considerably lower than those of lupins. Digestibility of arginine was the highest and that of threonine was the lowest of the indispensable amino acids in oilseed meals and grain legumes, except in cottonseed meal. Lysine was the least digestible amino acid in cottonseed meal.In the animal protein sources assayed, digestibility coefficients of amino acids in blood meal were high, intermediate in fish meal, and low in meat meal, meat and bone meal and feather meal. Variation in amino acid digestibility coefficients determined for blood meal samples was small. However, wide variations in amino acid digestibilities were observed for other animal protein sources, highlighting significant batch-to-batch differences. In particular, marked variations were determined for meat meal and meat and bone meal samples. Cystine was the least digested amino acid in animal protein meals, with the exception of blood meal in which isoleucine had the lowest digestibility. The limitations of using apparent digestibility values in diet formulations and the concept of the standardized digestibility system to overcome these limitations are discussed.

Because the intestine is the primary nutrient supply organ, early development of digestive function in newly hatched chick will enable it to better utilize nutrients, grow efficiently, and achieve the genetic potential of contemporary broilers. Published data on the growth and digestive function of the gastrointestinal tract in neonatal poultry were reviewed. Several potential strategies to improve digestive tract growth and function in newly hatched chick are available and the options include breeder nutrition, in ovo feeding, early access to feed and water, special pre-starter diets, judicious use of feed additives, and early programming.

The influence of microbial phytase on the ileal amino acid digestibilities in three cereals (corn, sorghum, and wheat), four oilseed meals (soybean meal, canola meal, cottonseed meal, and sunflower meal) and two cereal by-products (wheat middlings and rice polishings) was determined using 5-wk-old broilers. Supplementation of microbial phytase (1,200 FTU/kg) improved (P < 0.001 to 0.10) the digestibilities of protein and amino acids in all feedstuffs, but the magnitude of response varied depending on the feedstuff and the amino acid considered. Mean digestibility of the 15 amino acids in the feedstuffs without and with phytase were: corn, 78.0 and 80.4%; sorghum, 74.7 and 79.4%; wheat, 77.7 and 84.6%; soybean meal, 82.2 and 85.5%; canola meal, 78.7 and 80.7%; cottonseed meal, 70.8 and 74.2%; sunflower meal, 76.7 and 80.2%; wheat middlings, 70.8 and 73.4%; and rice polishings 62.1 and 66.9%, respectively. When individual amino acids were considered, the increments in digestibility were relatively higher for threonine and valine. This effect was consistent across all feedstuffs. The observed variations in response among feedstuffs were influenced by the inherent protein digestibility, but not by dietary phytic acid concentration. No correlations were determined between the dietary concentrations of phytic acid and phytase responses in terms of protein digestibility (r = 0.20; P > 0.31) and mean amino acid digestibility (r = 0.12; P > 0.51); however a significant negative correlation was observed between inherent protein digestibility and phytase responses in protein digestibility (r = -0.42; P < 0.03). It appears that solubilities of phytate salts and protein, and their influence on the degree of phytate-protein complexing in different feedstuffs, may be more relevant than total phytic acid concentrations. Interestingly, dietary phytic acid concentrations were negatively correlated with inherent protein (r = -0.81; P < 0.001) and mean amino acid (r = -0.85; P < 0.001) digestibilities of the feedstuffs evaluated in this study.