Nitrogen metabolism in genetically fat and lean chickens

CircDOCK7 facilitates the proliferation and adipogenic differentiation of chicken abdominal preadipocytes through the gga-miR-301b-3p/ACSL1 axis

BACKGROUND

Abdominal fat deposition depends on both the proliferation of preadipocytes and their maturation into adipocytes, which is a well-orchestrated multistep process involving many regulatory molecules. Circular RNAs (circRNAs) have emergingly been implicated in mammalian adipogenesis. However, circRNA-mediated regulation in chicken adipogenesis remains unclear. Our previous circRNA sequencing data identified a differentially expressed novel circRNA, 8:27,886,180|27,889,657, during the adipogenic differentiation of chicken abdominal preadipocytes. This study aimed to investigate the regulatory role of circDOCK7 in the proliferation and adipogenic differentiation of chicken abdominal preadipocytes, and explore its molecular mechanisms of competing endogenous RNA underlying chicken adipogenesis.

RESULTS

Our results showed that 8:27,886,180|27,889,657 is an exonic circRNA derived from the head-to-tail splicing of exons 19-22 of the dedicator of cytokinesis 7 (DOCK7) gene, abbreviated as circDOCK7. CircDOCK7 is mainly distributed in the cytoplasm of chicken abdominal preadipocytes and is stable because of its RNase R resistance and longer half-life. CircDOCK7 is significantly upregulated in the abdominal fat tissues of fat chickens compared to lean chickens, and its expression gradually increases during the proliferation and adipogenic differentiation of chicken abdominal preadipocytes. Functionally, the gain- and loss-of-function experiments showed that circDOCK7 promoted proliferation, G0/G1- to S-phase progression, and glucose uptake capacity of chicken abdominal preadipocytes, in parallel with adipogenic differentiation characterized by remarkably increased intracellular lipid droplet accumulation and triglyceride and acetyl coenzyme A content in differentiated chicken abdominal preadipocytes. Mechanistically, a pull-down assay and a dual-luciferase reporter assay confirmed that circDOCK7 interacted with gga-miR-301b-3p, which was identified as an inhibitor of chicken abdominal adipogenesis. Moreover, the ACSL1 gene was demonstrated to be a direct target of gga-miR-301b-3p. Chicken ACSL1 protein is localized in the endoplasmic reticulum and mitochondria of chicken abdominal preadipocytes and acts as an adipogenesis accelerator. Rescue experiments showed that circDOCK7 could counteract the inhibitory effects of gga-miR-301b-3p on ACSL1 mRNA abundance as well as the proliferation and adipogenic differentiation of chicken abdominal preadipocytes.

CONCLUSIONS

CircDOCK7 serves as a miRNA sponge that directly sequesters gga-miR-301b-3p away from the ACSL1 gene, thus augmenting adipogenesis in chickens. These findings may elucidate a new regulatory mechanism underlying abdominal fat deposition in chickens.

Integrative analysis of miRNA and mRNA profiles reveals that gga-miR-106-5p inhibits adipogenesis by targeting the KLF15 gene in chickens

Background

Excessive abdominal fat deposition in commercial broilers presents an obstacle to profitable meat quality, feed utilization, and reproduction. Abdominal fat deposition depends on the proliferation of preadipocytes and their maturation into adipocytes, which involves a cascade of regulatory molecules. Accumulating evidence has shown that microRNAs (miRNAs) serve as post-transcriptional regulators of adipogenic differentiation in mammals. However, the miRNA-mediated molecular mechanisms underlying abdominal fat deposition in chickens are still poorly understood. This study aimed to investigate the biological functions and regulatory mechanism of miRNAs in chicken abdominal adipogenesis.

Results

We established a chicken model of abdominal adipocyte differentiation and analyzed miRNA and mRNA expression in abdominal adipocytes at different stages of differentiation (0, 12, 48, 72, and 120 h). A total of 217 differentially expressed miRNAs (DE-miRNAs) and 3520 differentially expressed genes were identified. Target prediction of DE-miRNAs and functional enrichment analysis revealed that the differentially expressed targets were significantly enriched in lipid metabolism-related signaling pathways, including the PPAR signaling and MAPK signaling pathways. A candidate miRNA, gga-miR-106-5p, exhibited decreased expression during the proliferation and differentiation of abdominal preadipocytes and was downregulated in the abdominal adipose tissues of fat chickens compared to that of lean chickens. gga-miR-106-5p was found to inhibit the proliferation and adipogenic differentiation of chicken abdominal preadipocytes. A dual-luciferase reporter assay suggested that the KLF15 gene, which encodes a transcriptional factor, is a direct target of gga-miR-106-5p. gga-miR-106-5p suppressed the post-transcriptional activity of KLF15, which is an activator of abdominal preadipocyte proliferation and differentiation, as determined with gain- and loss-of-function experiments.

Conclusions

gga-miR-106-5p functions as an inhibitor of abdominal adipogenesis by targeting the KLF15 gene in chickens. These findings not only improve our understanding of the specific functions of miRNAs in avian adipogenesis but also provide potential targets for the genetic improvement of excessive abdominal fat deposition in poultry.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40104-022-00727-x.

Dynamic Expression and Regulatory Network of Circular RNA for Abdominal Preadipocytes Differentiation in Chicken (Gallus gallus)

Circular RNA (circRNA), as a novel endogenous biomolecule, has been emergingly demonstrated to play crucial roles in mammalian lipid metabolism and obesity. However, little is known about their genome-wide identification, expression profile, and function in chicken adipogenesis. In present study, the adipogenic differentiation of chicken abdominal preadipocyte was successfully induced, and the regulatory functional circRNAs in chicken adipogenesis were identified from abdominal adipocytes at different differentiation stages using Ribo-Zero RNA-seq. A total of 1,068 circRNA candidates were identified and mostly derived from exons. Of these, 111 differentially expressed circRNAs (DE-circRNAs) were detected, characterized by stage-specific expression, and enriched in several lipid-related pathways, such as Hippo signaling pathway, mTOR signaling pathway. Through weighted gene co-expression network analyses (WGCNA) and K-means clustering analyses, two DE-circRNAs, Z:35565770|35568133 and Z:54674624|54755962, were identified as candidate regulatory circRNAs in chicken adipogenic differentiation. Z:35565770|35568133 might compete splicing with its parental gene, ABHD17B, owing to its strictly negative co-expression. We also constructed competing endogenous RNA (ceRNA) network based on DE-circRNA, DE-miRNA, DE-mRNAs, revealing that Z:54674624|54755962 might function as a ceRNA to regulate chicken adipogenic differentiation through the gga-miR-1635-AHR2/IRF1/MGAT3/ABCA1/AADAC and/or the novel_miR_232-STAT5A axis. Translation activity analysis showed that Z:35565770|35568133 and Z:54674624|54755962 have no protein-coding potential. These findings provide valuable evidence for a better understanding of the specific functions and molecular mechanisms of circRNAs underlying avian adipogenesis.

Growth performance and accretion of selected amino acids in response to three levels of dietary lysine fed to fast- and slow-growing broilers

Literature data indicate that feed intake is sensitive to the dietary Lys content particularly in fast-growing birds. From a conceptual and a practical viewpoint, an interaction between genotype (i.e., fast-growing vs. slow-growing birds) and dietary Lys content is of interest, but it needs confirmation owing to a dearth of studies addressing this issue. A study was conducted with 266 Cobb 500 birds and 266 Thai native crossbreed birds serving as models for fast-growing broilers (FGB) and slow-growing broilers (SGB), respectively. Within genotype, chicks were randomly allocated to diets containing either a high (H-LYS = 1.36%), medium (1.17%), or low Lys (1.01%) content. Growth performance and the accretion of protein and selected amino acids were determined in birds from 1 to 21 d of age. Treatments were arranged in a factorial design with 6 replications/treatment. Low Lys vs. H-LYS caused a 42.1% lower feed intake in FGB (P < 0.001), but not in SGB (P = 0.596). The feed conversion ratio (FCR (g feed/g BW gain)) was lowest in FGB (P < 0.001) and increased with decreasing dietary Lys contents (P < 0.001). The Lys induced increase in FCR, however, was more pronounced in SGB (P = 0.025). The absolute protein gain (g/bird) was influenced by the Lys content of feed and decreased by ∼54% and ∼23% in FGB and SGB, respectively (P < 0.001). The efficiency (% of intake) of protein accretion was found to be greater in FGB (P ≤ 0.001) and decreased with decreasing dietary Lys (P ≤ 0.001). The efficiency of Lys accretion was found to be negatively affected by the dietary Lys content in FGB (P < 0.001) but not SGB (P_{genotype × dietary Lys} = 0.008). It can be concluded that a dietary Lys content of 1.01% does not safeguard both growth performance and body protein accretion efficiency in both FGB and SGB. The suboptimal growth performance in FGB, but not SGB, is partially counteracted by a Lys-induced reduction in feed intake.

Comparison of mathematical and comparative slaughter methodologies for determination of heat production and energy retention in broilers

Understanding factors affecting ME availability for productive processes is an important step in optimal feed formulation. This study compared a modelling methodology with the comparative slaughter technique (CST) to estimate energy partitioning to heat production and energy retention (RE) and to investigate differences in heat dissipation. At hatch, 50 broilers were randomly allocated in one of 4 pens equipped with a precision feeding station. From day 14 to day 45, they were either fed with a low-ME (3,111 kcal/kg ME) or a high-ME (3,383 kcal/kg ME) diet. At day 19, birds were assigned to pair-feeding in groups of 6 with lead birds eating ad libitum (100%) and follow birds eating at either 50, 60, 70, 80, or 90% of the paired lead's cumulative feed intake. Heat production and RE were estimated by CST and with a nonlinear mixed model explaining daily ME intake (MEI) as a function of metabolic BW and average daily gain (ADG). The energy partitioning model predicted MEI = (145.10 + u) BW^0.83 + 1.09 × BW^−0.18 × ADG^1.19 + ε. The model underestimated heat production by 13.4% and overestimated RE by 22.8% compared with the CST. The model was not able to distinguish between net energy for gain values of the diets (1,448 ± 18.5 kcal/kg vs. 1,493 ± 18.0 kcal/kg for the low-ME and high-ME diet, respectively), whereas the CST found a 148 kcal/kg difference between the low-ME and high-ME diets (1,101 ± 22.5 kcal/kg vs. 1,249 ± 22.0 kcal/kg, respectively). The estimates of the net energy for gain values of the 2 diets decreased with increasing feed restriction. The heat increment of feeding did not differ between birds fed with the low- or high-ME diet (26% of MEI). Additional measurements on heat dissipation, physical activity, and immune status indicated that the energetic content of the diet and feed restriction affect some parameters (shank temperature, feeding station visits) but not others (leukocyte counts, heterophil to lymphocyte ratio, and immune cell function).

Factors affecting energy metabolism and evaluating net energy of poultry feed

Different energy evaluating systems have been used to formulate poultry diets including digestible energy, total digestible nutrients, true metabolizable energy, apparent metabolizable energy (AME), and effective energy. The AME values of raw materials are most commonly used to formulate poultry diets. The net energy (NE) system is currently used for pig and cattle diet formulation and there is interest for its application in poultry formulation. Each energy evaluating system has some limitations. The AME system, for example, is dependent on age, species, and feed intake level. The NE system takes AME a step further and incorporates the energy lost as heat when calculating the available energy for the production of meat and eggs. The NE system is, therefore, the most accurate representation of energy available for productive purposes. The NE prediction requires the accurate measurement of the AME value of feed and also an accurate measurement of total and fasting heat production using nutritionally balanced diets. At present, there is limited information on NE values of various ingredients for poultry feed formulation. The aim of this review is to examine poultry feed energy systems with the focus on the NE system and its development for chickens.

Potential environmental benefits of prospective genetic changes in broiler traits

A system approach-based Life Cycle Assessment (LCA) framework, combined with a simple mechanistic model of bird energy balance was used to predict the potential effects of 15 y prospective broiler breeding on the environmental impacts of the standard UK broiler production system. The year 2014 Ross 308 genotype was used as a baseline, and a future scenario was specified from rates of genetic improvement predicted by the industry. The scenario included changes in the traits of growth rate (reducing the time to reach a target weight 2.05 kg from 34 d to 27 d), body lipid content, carcass yield, mortality and the number of chicks produced by a breeder hen. Diet composition was adjusted in order to accommodate the future nutrient requirements of the birds following the genetic change. The results showed that predicted changes in biological performance due to selective breeding could lead to reduced environmental impacts of the broiler production chain, most notably in the Eutrophication Potential (by 12%), Acidification Potential (by 10%) and Abiotic Resource Use (by 9%) and Global Warming Potential (by 9%). These reductions were mainly caused by the reduced maintenance energy requirement and thus lower feed intake, resulting from the shorter production cycle, together with the increased carcass yield. However, some environmental benefits were limited by the required changes in feed composition (e.g., increased inclusion of soy meal and vegetable oil) as a result of the changes in bird nutrient requirements. This study is the first one aiming to link the mechanistic animal modeling approach to predicted genetic changes in order to produce quantitative estimates of the future environmental impacts of broiler production. Although a more detailed understanding on the mechanisms of the potential changes in bird performance and their consequences on feeding and husbandry would be still be needed, the modeling framework produced in this study provides a starting point for predictions of the effects of prospective genetic progress.

Prediction of the net energy value of broiler diets

Thirty pelleted diets were given to broiler chickens (eight birds per diet; 21 to 35 days of age) for individual in vivo measurements of dietary net energy (NE) value, using three trials with 10 diets/trial. Amino acid formulation of diets was done on the basis of ratios to CP. NE was measured according to the body analysis method. The basal metabolism component of NE values was calculated on the basis of mean metabolic weight using a coefficient obtained in a previous experiment. Information about apparent metabolisable energy (AME) value of diets, AME corrected to zero nitrogen retention (AMEn) and digestibilities of proteins, lipids, starch and sugars was available from a previous publication. In each trial, mean NE/AME ratios of diets varied by about 6%. From the multiple regressions (n=30) expressing NE and AMEn values as functions of digestible component contents, it was deduced that the NE/AMEn ratios assigned to dietary components were 0.760, 0.862, 0.806, 0.690 and 0.602 for CP, lipids, starch, (sucrose+glucose) and fermentable sugars (α-galacto-oligosaccharides and lactose), respectively. The NE/AME ratio of CP was 0.680. Regression calculations showed that the NE values assigned to individual birds (n=240) could also be predicted with diet AMEn values (NE=0.80 AMEn; R 2=0.770) or with an equation combining AMEn value and CP/AMEn ratio (R 2=0.773). The latter ratio was found to be the only additional parameter that was significant when added in the NE regression scheme based on AMEn.

Partition of metabolizable energy, and prediction of growth performance and lipid deposition in broiler chickens

The study presented here consisted of the calculation of cross relationships between growth performance parameters, body growth composition, and feed characteristics, using data from an experiment reported in 2 previous publications. In the previous experiment, 30 pelleted diets were given to broiler chickens (8/diet) (21 to 35 d) for in vivo measurement and prediction of AMEn and net energy (NE) values of diets, using 3 trials with 10 diets/trial. In the course of NE determination, individual values for growth, feed intake, and deposition of lipid and protein were measured. Average energy deposited as lipid and protein represented 25.4 and 19.1% AME intake, respectively. Using a multiple regression predicting AME intake, the partial efficiencies of AME for energy deposition as lipid and protein were calculated to be 91.6 and 67.3%, respectively, and the daily amount of AME required for maintenance was evaluated at 0.683 MJ/kg BW0.7. The mean diet NE/AMEn ratios were predicted by an equation combining the lipid content of body growth (positive coefficient) and the apparent digestible protein (ADP) to AMEn ratio (ADP/AMEn), with a quadratic expression for the latter variable. This quadratic response expressed a positive asymptotic relationship, with a plateau for ADP/AMEn values above 1.45 [%/(MJ/kg)]. The equations predicting growth always included either the dietary percentage of water-insoluble cell wall or the AMEn value. The other major parameters predicting growth were either the lipid content of body growth or the CP/AMEn ratio. In many cases, quadratic responses were observed in growth prediction equations. Regressions predicting feed efficiency showed only linear responses. Feed efficiency was predicted precisely by multiple linear regressions based only on AMEn and a dietary protein parameter. According to these regressions, 1% CP was equivalent to 0.247 MJ/kg AMEn in terms of feed efficiency. The most efficient regression predicting the individual lipid content of body growth combined the protein efficiency value (negative coefficient), the CP/AMEn ratio (negative coefficient), AMEn (positive coefficient), and the feather content of body growth (positive coefficient).

Genome-wide interval mapping using SNPs identifies new QTL for growth, body composition and several physiological variables in an F2 intercross between fat and lean chicken lines

BACKGROUND

For decades, genetic improvement based on measuring growth and body composition traits has been successfully applied in the production of meat-type chickens. However, this conventional approach is hindered by antagonistic genetic correlations between some traits and the high cost of measuring body composition traits. Marker-assisted selection should overcome these problems by selecting loci that have effects on either one trait only or on more than one trait but with a favorable genetic correlation. In the present study, identification of such loci was done by genotyping an F2 intercross between fat and lean lines divergently selected for abdominal fatness genotyped with a medium-density genetic map (120 microsatellites and 1302 single nucleotide polymorphisms). Genome scan linkage analyses were performed for growth (body weight at 1, 3, 5, and 7 weeks, and shank length and diameter at 9 weeks), body composition at 9 weeks (abdominal fat weight and percentage, breast muscle weight and percentage, and thigh weight and percentage), and for several physiological measurements at 7 weeks in the fasting state, i.e. body temperature and plasma levels of IGF-I, NEFA and glucose. Interval mapping analyses were performed with the QTLMap software, including single-trait analyses with single and multiple QTL on the same chromosome.

RESULTS

Sixty-seven QTL were detected, most of which had never been described before. Of these 67 QTL, 47 were detected by single-QTL analyses and 20 by multiple-QTL analyses, which underlines the importance of using different statistical models. Close analysis of the genes located in the defined intervals identified several relevant functional candidates, such as ACACA for abdominal fatness, GHSR and GAS1 for breast muscle weight, DCRX and ASPSCR1 for plasma glucose content, and ChEBP for shank diameter.

CONCLUSIONS

The medium-density genetic map enabled us to genotype new regions of the chicken genome (including micro-chromosomes) that influenced the traits investigated. With this marker density, confidence intervals were sufficiently small (14 cM on average) to search for candidate genes. Altogether, this new information provides a valuable starting point for the identification of causative genes responsible for important QTL controlling growth, body composition and metabolic traits in the broiler chicken.

Transcriptional profiling of hypothalamus during development of adiposity in genetically selected fat and lean chickens

The hypothalamus integrates peripheral signals to regulate food intake, energy metabolism, and ultimately growth rate and body composition in vertebrates. Deviations in hypothalamic regulatory controls can lead to accumulation of excess body fat. Many regulatory genes involved in this process remain unidentified, and comparative studies may be helpful to unravel evolutionarily conserved mechanisms controlling body weight and food intake. In the present study, divergently selected fat (FL) and lean (LL) lines of chickens were used to characterize differences in hypothalamic gene expression in these unique genetic lines that develop differences in adiposity without differences in food intake or body weight. Hypothalamic transcriptional profiles were defined with cDNA microarrays before and during divergence of adiposity between the two lines. Six differentially expressed genes identified in chickens are related to genes associated with control of body fat in transgenic or knockout mice, supporting the importance of these genes across species. We identified differences in expression of nine genes involved in glucose metabolism, suggesting that alterations in hypothalamic glycolysis might contribute to differences in levels of body fat between genotypes. Expression of the sweet taste receptor (TAS1R1), which in mammals is involved in glucose sensing and energy uptake, was also higher in FL chickens, suggesting that early differences in glucose sensing might alter the set point for subsequent body composition. Differences in expression of genes associated with tumor necrosis factor (TNF) signaling were also noted. In summary, we identified alterations in transcriptional and metabolic processes within the hypothalamus that could contribute to excessive accumulation of body fat in FL chickens in the absence of differences in food intake, thereby contributing to the genetic basis for obesity in this avian model.

Divergent selection for shape of growth curve in Japanese quail. 5. Growth pattern and low protein level in starter diet

1. The effect of crude protein (CP) concentration in starter diet (259 or 216 g CP and 11.7 MJ ME/kg, fed from 0 to 21 d of age) on postnatal growth pattern (from hatching to 70 d of age) was analysed in Japanese quail lines divergently selected for high (HG) and low (LG) relative gain of body weight (BW) between 11 and 28 d of age, and constant BW at 49 d of age. 2. Males and females of both lines fed on the low CP diet showed a transient BW retardation between 7 and 28 d of age, and 7 and 35 d of age, respectively, when compared with their counterparts receiving the standard CP diet. 3. Although the negative effect of low CP concentration on growth rate was observed in both lines, a lower tolerance of young HG vs. LG quail to the reduction of CP level in food was evident from their (i) stronger BW retardation at 14 d of age (16 vs. 7%), (ii) more delayed onset of compensatory growth (21 vs. 7 d of age) and (iii) greater prolongation of the acceleration growth phase (3 vs. 1 d of age) following insufficient dietary CP. 4. The line differences in early growth rate were accompanied by significant differences in food intake. The LG line consumed more food than the HG line on both CP diets and consumption was not influenced by food quality. In contrast, HG quail reduced food intake with the decrease of dietary CP concentration. On both CP diets, this was associated with a higher body fatness of LG vs. HG quail. 5. The protein-deficient food could thus represent an important factor contributing to the selection advantage of developmentally accelerated genotypes during the selection for high BW in young age categories.

A comprehensive analysis of QTL for abdominal fat and breast muscle weights on chicken chromosome 5 using a multivariate approach

Quantitative trait loci (QTL) influencing the weight of abdominal fat (AF) and of breast muscle (BM) were detected on chicken chromosome 5 (GGA5) using two successive F(2) crosses between two divergently selected 'Fat' and 'Lean' INRA broiler lines. Based on these results, the aim of the present study was to identify the number, location and effects of these putative QTL by performing multitrait and multi-QTL analyses of the whole available data set. Data concerned 1186 F(2) offspring produced by 10 F(1) sires and 85 F(1) dams. AF and BM traits were measured on F(2) animals at slaughter, at 8 (first cross) or 9 (second cross) weeks of age. The F(0), F(1) and F(2) birds were genotyped for 11 microsatellite markers evenly spaced along GGA5. Before QTL detection, phenotypes were adjusted for the fixed effects of sex, F(2) design, hatching group within the design, and for body weight as a covariable. Univariate analyses confirmed the QTL segregation for AF and BM on GGA5 in male offspring, but not in female offspring. Analyses of male offspring data using multitrait and linked-QTL models led us to conclude the presence of two QTL on the distal part of GGA5, each controlling one trait. Linked QTL models were applied after correction of phenotypic values for the effects of these distal QTL. Several QTL for AF and BM were then discovered in the central region of GGA5, splitting one large QTL region for AF into several distinct QTL. Neither the 'Fat' nor the 'Lean' line appeared to be fixed for any QTL genotype. These results have important implications for prospective fine mapping studies and for the identification of underlying genes and causal mutations.

Diet-induced thermogenesis and glucose oxidation in broiler chickens: influence of genotype and diet composition

The main objectives of this study were to explore the role of diet-induced thermogenesis in the regulation of voluntary feed intake and to determine the glucose oxidation of broiler chicken strains, known to differ in glucose-insulin balance. From 2 to 7 wk of age, male broiler chickens of a fat and a lean line were reared on 1 of 2 isoenergetic diets with constant gross energy and carbohydrate levels but with substitutions between fat and protein. The low protein (LP/HF) diet contained 126 g of protein/kg and 106 g of fat/kg, whereas the low fat (LF/HP) diet contained 242 g of protein/kg and 43 g of fat/kg. There was no significant effect of the genetic background of the broilers on the glucose oxidation rate (as measured by stable isotope breath test) or protein oxidation (as measured by plasma uric acid levels). Considering the difference in carcass composition (fat content) of both lines, this leads to the hypothesis that the lines differ predominantly in fat metabolism. Although there was no line effect on plasma triglyceride and free fatty acid concentrations, it was hypothesized that there might be differences in fat oxidation or de novo lipogenesis, or both, between the genotypes. Diet-induced thermogenesis per metabolic body weight (kg of BW0.75) per 24 h, expressed per gram of feed intake, was not significantly influenced by genetic background or by diet composition. Therefore, a model linking feed intake to diet-induced thermogenesis, as postulated for adult mammals, could not be corroborated for growing broiler chickens.

Nutrient and fatty acid deposition in broilers fed different dietary fatty acid profiles

The aim of this study was to determine the effect of different dietary fatty acid profiles on efficiency of energy, fat, nitrogen, and fatty acid deposition in broiler chickens. Sixty female broiler chickens were fed a basal diet without additional fat or with 4 other diets with different fats (tallow, olive, sunflower, and linseed oils) at 10% from 28 to 48 d of age. Among broilers fed diets with added fat, those fed linseed oil had less abdominal fat (in grams and percentage) than those fed tallow (P < 0.05). Absorbed fat losses were slightly higher for birds fed linseed oil, and nitrogen efficiency was lower in those fed tallow (P < 0.05). However, there were not significant differences in energy deposition among broilers fed diets with added fat. Fatty acid balance showed the highest values of fatty acid oxidation during the experimental period in broilers fed linseed oil (48.2 g), followed by those fed sunflower oil (23.2 g). Contribution of endogenous fat synthesis to total body fat deposition was minimal in birds fed diets with added fat accounting for 3, 1.2, 8.5, and 7.5 g for broilers fed tallow, olive, sunflower, and linseed oils, respectively. This reflects lipogenesis inhibition by dietary fat addition. Interestingly, between broilers fed diets with added fat, higher values of fatty acids from endogenous synthesis were found in broilers fed diets rich in polyunsaturated fatty acids (PUFA). Results suggest that reduction of abdominal fat in broilers fed linseed oil seems to be a consequence of higher lipid oxidation despite the higher synthesis of endogenous fatty acids.

The effects of dietary protein independent of essential amino acids on growth and body composition in genetically lean and fat chickens

1. Growth performance between 28 and 49 d of age and carcase composition at 49 d in genetically lean (LL) and fat (FL) broilers fed on diets varying in non-essential amino acid (NEAA) concentrations were compared in 2 experiments. In experiment 1, 3 crude protein (CP) contents (133, 155, and 178 g/kg) were compared. In experiment 2, 4 CP levels (131, 150, 170 and 189 g/kg) were compared. All diets were supplemented with synthetic amino acids to cover the EAA requirement of the LL birds. 2. Weight gains of FL chickens were not affected by dietary treatments, while those of LL increased when CP level increased. 3. Reducing CP content always increased body lipids, abdominal fat and food conversion ratio in both lines in both experiments; however, the effect on abdominal fat was more pronounced in the FL birds. 4. Reducing CP concentration always decreased breast muscle proportion in both lines in both experiments, even when growth rate was not affected by CP. 5. It is concluded that LL chickens require diets more concentrated in NEAA than fat chickens and that there seems to be an effect of NEAA on breast muscle development.

Comparative responses of genetically lean and fat broiler chickens to dietary threonine concentration

Genetically lean (LL) or fat (FL) male broiler chicken's were fed on 5 diets containing either 3.80, 4.27, 4.75, 5.22 or 5.70 g true digestible threonine per kg. Threonine deficiency induced a more pronounced reduction in growth in the LL than in the FL but did not influence abdominal fat and breast muscle proportions in either line. Plotting weight gain or protein gain against threonine intake suggests that the requirement of both lines is very similar in terms of mg per g of gain. Thus food intake or appetite should account for differences between genotypes. Requirement for true digestible threonine was estimated as 10.70 mg per g of weight gain or 63.8 mg per g of protein gain, using a linear regression approach. The quadratic polynomial equations suggest that the requirements are 13.9 and 12.4 mg digestible threonine per g of gain for LL and FL respectively.

Response of chick lines selected on carcass quality to dietary lysine supply: live performance and muscle development

Broiler carcass quality can be improved by conventional selection techniques. In this regard, an experimental "quality" line (QL) was selected for high breast meat yield. We analyzed the effects of this selection on the dietary lysine requirement in chicks from 0 to 3 wk. Control (CL) and QL chicks were provided ad libitum access to isoenergetic diets containing 20% crude protein but differing in their lysine content (0.75, 0.88, 1.01, and 1.13%). Two-way ANOVA showed a significant effect (P < 0.01) of genotype on body weight, growth rate, feed intake, and weight of Pectoralis major and Gastrocnemius muscles. Conversely, the Sartorius muscle weight was not modified (P = 0.21) by genotype. Lysine deficiency markedly reduced body weight, growth rate, and feed intake, and increased feed conversion ratio (P < 0.001). Low dietary levels of lysine also depressed the weight of Gastrocnemius, Sartorius, and P. major (P < 0.001). The body or muscle weight response to diet lysine concentration depended on the line, with QL chicks appearing less sensitive to lysine deficiency. Consequently, their dietary requirements could be lower. Finally, when weight gain and P. major muscle protein deposition were plotted against lysine intake, QL chicks appeared to be more efficient than CL chicks. The underlying mechanisms responsible for this await clarification.

Virginiamycin and caloric density effects on live performance, blood serum metabolite concentration, and carcass composition of broilers reared in thermoneutral and cycling ambient temperatures

One experiment utilizing Cobb x Cobb male broilers was conducted to evaluate virginiamycin (VM; 0, 15, 20 ppm) and diet caloric density (CD; 2,945, 3,200 AMEn/kg) effects on broiler live performance, blood serum metabolites, and carcass composition. The starter period exposed birds to recommended brooding conditions, whereas from 3 to 7 wk birds were exposed to thermoneutral (TN, 24 C) or cycling temperature (CT, 24 to 35 C) environments (E). During the 21-d starter period, VM levels and high CD increased (P < 0.05) BW gain (G) and gain:feed (G:F) improved (P < 0.05) with 20 ppm VM and high CD. During 3 to 7 wk, CT reduced (P < 0.05) most live performance and carcass variables as well as heat production (HP) and energetic efficiency whereas energy content per gram of tissue increased. The main effect of VM and CD on blood serum constituents was not significant; however, CT decreased (P < 0.05) serum Na, K, Ca, Mg, Fe, albumin, and total protein. Within CT, G increased (P < 0.05) with high CD and with the 15 ppm VM combination, whereas within TN, G was unaffected by CD, but increased (P < 0.05) with the 20 ppm VM and low CD combination compared with the control. High CD increased (P < 0.05) BW, G, carcass weight, dressing percentage, carcass percentage fat, carcass dry matter, carcass energy content per bird, HP, fat, and protein gain but reduced (P < 0.05) carcass percentage protein and energetic efficiency. Carcass weight, breast yield, fat, and protein gains as well as dry matter carcass energy content increased (P < 0.05) with VM compared with controls. The reduced (P < 0.05) calorie intake and HP with concomitant increase (P < 0.05) in calorie gain with 20 ppm VM increased (P < 0.05) caloric efficiency. In summary, the results suggest that VM improves bird performance by reducing HP and that reduced HP during high CT improves body temperature homeostasis.

Effects of dietary protein under high ambient temperature on body weight, breast meat yield, and abdominal fat deposition of broiler stocks differing in growth rate and fatness

The effect of dietary protein on growth, feed intake and efficiency, abdominal fat deposition, and breast meat yield was investigated in broiler males from a commercial stock (WI) and from experimental stocks selected for low (LF) or high (HF) abdominal fat. All birds were kept at constant high ambient temperature (32 C) and were provided with low- (LP) or high-protein (HP) diets from hatch until 8 wk of age (Experiment 1) or from 4 to 8 wk of age (Experiment 2). In both experiments, HP diet significantly increased 4- to 8-wk BW gain in the LF and HF stocks but reduced it in the WI stock as compared with the LP diet. Abdominal fat, as percentage of BW, was almost twofold higher in the HF birds than in the LF ones, with WI mean being intermediate. In contrast to the HF and WI birds, in which abdominal fat decreased with increased protein intake, abdominal fat was not affected by dietary protein in the LF stock. The HP diet substantially increased breast meat yield in LF birds but not in the WI birds, with HF birds exhibiting intermediate increase in breast meat weight. It was concluded that birds of varied inherent growth rate and tendencies toward protein and fat deposition respond differently to dietary protein level under heat stress.

Are genetically lean broilers more resistant to hot climate?

1. Genetically lean (LL) or fat (FL) male chickens were exposed to either high (32 degrees C) or control (22 degrees C) ambient temperature up to 9 weeks of age. They were fed on one of two isoenergetic diets differing in protein content: 190 or 230 g/kg. 2. At 22 degrees C, weight gain of LL broilers was the same as in FL chickens, but at the high temperature LL birds grew to a greater weight than FL ones. 3. Food conversion efficiency was not affected by ambient temperature in LL chickens but was depressed in FL ones at 32 degrees C. 4. Increasing dietary protein content did not alleviate heat-induced growth depression irrespective of the genotype. 5. Gross protein efficiency was higher in LL chickens and was less depressed at 32 degrees C than in FL birds. 6. Fat deposition decreased with increasing protein concentration at normal temperature in both genotypes; at high temperature, high protein content enhanced fatness, particularly in LL chickens. 7. Thus, genetically lean broilers demonstrated a greater resistance to hot conditions: this was indicated by enhanced weight gain and improved food and protein conversion efficiencies.

Comparative utilisation of sulphur-containing amino acids by genetically lean or fat chickens

1. Genetically lean (LL) or fat (FL) male chickens were fed from 28 to 47 days of age on 5 experimental diets differing by their methionine+cystine content (5.4, 5.8, 6.2, 6.6 and 7.0 g/kg, respectively). 2. Growth rate of LL chickens was reduced by the lower sulphur-containing amino acid (SAA) concentrations whereas that of FL was not modified. 3. LL chickens exhibited a larger feather protein gain than FL, which was stimulated by SAA intake. 4. SAA retention, when plotted against SAA consumption, was always greater in LL than in FL. 5. Large differences were observed between genotypes for plasma-free amino acids. Lysine, glutamic acid, histidine and serine were found at significantly higher concentrations in LL birds. Branched amino acids, aromatic amino acids, SAA and arginine were found at higher concentrations in FL. No differences were observed for aspartic acid, glycine, alanine and total amino acids. Methionine supplementation decreased free amino acid concentrations, with the exceptions of arginine and leucine. 6. It is concluded that lean chickens require a higher dietary concentration of SAA than FL. This is mainly caused by their lower food consumption and their greater feather synthesis. However, LL use SAA more efficiently than FL.

Research note: effect of high ambient temperature on dietary metabolizable energy values in genetically lean and fat chickens

Effect of high ambient temperature (32 versus 22 C) on dietary ME value was investigated in 32 genetically lean and fat 8-wk-old male chickens. Lean broilers exhibited higher AME and TME values than fat chickens. Hot climatic conditions significantly increased AME and TME values, particularly in leaner birds. Protein retention efficiency was enhanced by selection for leanness and increased with ambient temperature. Correction for nitrogen balance (AME(n) and TMEn) reduced the effect of temperature but lean genotypes still revealed higher TMEn values than fatter ones.

Further investigations on protein requirement of genetically lean and fat chickens

1. Genetically lean (LL) or fat (FL) chickens were fed from 28 to 42 d of age on one of 6 diets with different protein contents (from 73 to 208 g/kg). In order to keep a constant amino acid balance the experimental diets were made by diluting a well-balanced protein-rich diet with a protein-free diet. 2. Dietary protein influenced the growth rate of both genotypes similarly. However, maximum weight gain was reached in LL at a lower protein intake than in FL. 3. Regression between total protein gain (body protein + feather protein) or body protein gain and protein intake exhibited significantly different slopes, that of LL being superior to that of FL. 4. At a given protein intake, feather protein gain was also superior in LL to FL. Moreover feather protein, as a percentage of total protein gain, was superior in LL to FL. When the dietary protein fell below 126 g/kg, feather protein represented a higher proportion of total protein gain. 5. Multiple linear regressions of protein intake (as the dependent variable), and body weight and protein gain or weight gain (as the independent variables) suggest that the maintenance requirement for protein is similar in both lines but that the protein efficiency for growth is significantly superior in LL. 6. In a second experiment both genotypes were offered either a single high protein diet (232 g/kg) or a single medium protein diet (186 g/kg) or had free-choice between a high (269 g/kg) and a low protein (145 g/kg) diet. In free-choice feeding, FL chickens selected an overall dietary protein content which was significantly lower (179 v. 200 g/kg) to that of LL. In both genotypes, free-choice feeding led to fatter and less efficient chickens than predicted by the linear regression between adiposity or food conversion and protein content.