Enzyme activity and organ development in newly hatched chicks selected for high or low eight-week body weight

Nutrition and Digestive Physiology of the Broiler Chick: State of the Art and Outlook

Simple Summary

The first week after hatch is the most challenging period in the life of broilers. The digestive tract of the newly hatched chick is immature and must undergo dramatic changes before it can efficiently digest and absorb nutrients. The gut is the vital organ where nutrient digestion and absorption take place. Ontogenic changes that accompany improved digestion and absorption include increased secretion of digestive enzymes, increase in the gut absorptive surface area, and enhanced nutrient transporters. The obvious limiting factors are the secretion and activities of digestive enzymes, and the surface area for absorption. These limitations are overcome as the birds grow older, with concurrent improvements in nutrient utilization. In addition, substantial changes also take place in the physical and functional development of the immune system and intestinal microbial ecology. However, the focus of the current review was on nutrition-related challenges and nutritional approaches to assist the chick during this highly demanding period.

Abstract

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.

Supplementation of Bile Acids and Lipase in Broiler Diets for Better Nutrient Utilization and Performance: Potential Effects and Future Implications – A Review

Bile acids are used for better emulsification, digestion and absorption of dietary fat in chicken, especially in early life. Similarly, exogenous lipases have also been used for the improvement of physiological limitation of the chicken digestive system. Owing to potential of both bile acids and lipases, their use has been increased in recent years, for better emulsification of dietary fat and improvement of growth performance in broilers. In the past, pancreatic lipases were used for supplementation, but recently, microbial lipase is getting attention in poultry industry as a hydrolysis catalyst. Bile acids strengthen the defence mechanism of body against bacterial endotoxins and also play a key role in lipid regulation and sugar metabolism as signaling molecules. It has been demonstrated that bile acids and lipases may improve feed efficiency by enhancing digestive enzyme activity and ultimately leading to better fat digestion and absorption. Wide supplemental range of bile acids (0.004% to 0.25%) and lipases (0.01% to 0.1%) has been used in broiler diets for improvement of fat digestibility and their performance. Combinations of different bile acids have shown more potential to improve feed efficiency (by 7.14%) even at low (0.008%) levels as compared to any individual bile acid. Lipases at a lower level of 0.03% have exhibited more promising potential to improve fat digestibility and feed efficiency. However, contradicting results have been published in literature, which needs further investigations to elucidate various nutritional aspects of bile acids and lipase supplementation in broiler diet. This review focuses on providing insight on the mechanism of action and potential application of bile acids and lipases in broiler diets. Moreover, future implications of these additives in poultry nutrition for enhancing nutrient utilization and absorption are also discussed.

Effects of High-Dose of Copper Amino Acid Complex on Laying Performance, Hematological and Biochemical Parameters, Organ Index, and Histopathology in Laying Hens

The objective of the study was to evaluate the maximum tolerance limit of amino acid copper complex (Cu-Lys-Glu) for laying hens by measuring their laying performance, hematological and serum biochemical parameters, organ index, and histopathology. A total of 450 18-week-old Beijing White layers were randomly allocated to 5 groups (90 birds per group) with 6 replicates of 15 birds each. After a 2-week acclimation on a basal diet (analyzed copper content 8.63 mg/kg), the birds were fed diets supplemented with 0 (control), 15, 75, 150, and 300 mg Cu/kg as Cu-Lys-Glu for 10 weeks. Results showed that, compared with the control group, dietary supplementation with 15, 75, and 150 mg Cu/kg as Cu-Lys-Glu did not affect (P > 0.05) laying performance, whereas hens receiving with 300 mg Cu/kg significantly decreased (P < 0.001) the laying rate as compared with the control. No significant differences (P > 0.05) were observed among the hens receiving 0, 15, 75, and 150 mg Cu/kg as Cu-Lys-Glu in hematological and serum biochemical parameters, organ indexes, and histopathological changes. However, hens receiving 300 mg Cu/kg significantly increased (P < 0.05) the concentrations of mean corpuscular volume (MCV), albumin (ALB), total bilirubin (TBILI), alkaline phosphatase (ALP), alanine aminotransferase (ALT), aspartate aminotransferase (AST), urea nitrogen (UN), and creatinine (CRE), as well as caused severe microscopic histopathological changes in the liver and kidney. In conclusion, 150 mg Cu/kg as Cu-Lys-Glu is identified as no-side-effect supplementation level in laying hens after daily administration for 70 days.

Safety evaluation of zinc methionine in laying hens: Effects on laying performance, clinical blood parameters, organ development, and histopathology

The study was conducted to investigate whether high-dose zinc methionine (Zn-Met) affected the safety of laying hens, including laying performance, hematological parameters, serum chemical parameters, organ index, and histopathology. A total of 540 20-week-old Hy-Line White laying hens was randomly allocated to 6 groups with 6 replicates of 15 birds each. Birds were fed diets supplemented with 0 (control), 70, 140, 350, 700, or 1,400 mg Zn/kg diet as Zn-Met. The experiment lasted for 8 wk after a 2-week acclimation period. Results showed that dietary supplementation with 70 or 140 mg Zn/kg diet as Zn-Met significantly increased average daily egg mass (ADEM), laying rate (LR), and feed conversion ratio (FCR) (P < 0.05) and lowered broken and soft-shelled egg ratio (BSER) (P < 0.05) in comparison with the control group; no significant differences were detected among hens fed with 0, 350, or 700 mg Zn/kg as Zn-Met (P > 0.05); hens administered 1,400 mg Zn/kg showed a significant increase in BSER and remarkable decreases in ADEM, LR, and FCR (P < 0.001). There were no significant differences among hens receiving 0, 70, 140, 350, or 700 mg Zn/kg as Zn-Met in serum chemical parameters (P > 0.05); supplementation with 1,400 mg Zn/kg as Zn-Met remarkably elevated the concentrations of serum total bilirubin (TBILI), glucose (GLU), uric acid (UA), and creatinine (CRE) (P < 0.001), and enhanced activities of serum glutamic oxalacetic transaminase (GOP) and alkaline phosphatase (AKP) (P < 0.001) compared with the control group. No significant histopathological changes were found in hens administered 0, 70, 140, 350, or 700 mg Zn/kg as Zn-Met, while significant histological lesions were observed in the heart, liver, lung, and kidney tissues of hens receiving 1,400 mg Zn/kg as Zn-Met. No significant differences were detected in hematological parameters or organ index (P > 0.05). In conclusion, a nominal Zn concentration of 700 mg/kg as Zn-Met is considered to be no-observed-adverse-effect level following daily administration to hens for 56 days.

Speeding up Growth: Selection for Mass-Independent Maximal Metabolic Rate Alters Growth Rates

Investigations into relationships between life-history traits, such as growth rate and energy metabolism, typically focus on basal metabolic rate (BMR). In contrast, investigators rarely examine maximal metabolic rate (MMR) as a relevant metric of energy metabolism, even though it indicates the maximal capacity to metabolize energy aerobically, and hence it might also be important in trade-offs. We studied the relationship between energy metabolism and growth in mice (Mus musculus domesticus Linnaeus) selected for high mass-independent metabolic rates. Selection for high mass-independent MMR increased maximal growth rate, increased body mass at 20 weeks of age, and generally altered growth patterns in both male and female mice. In contrast, there was little evidence that the correlated response in mass-adjusted BMR altered growth patterns. The relationship between mass-adjusted MMR and growth rate indicates that MMR is an important mediator of life histories. Studies investigating associations between energy metabolism and life histories should consider MMR because it is potentially as important in understanding life history as BMR.

Messenger RNA sequencing and pathway analysis provide novel insights into the biological basis of chickens' feed efficiency

Background

Advanced selection technologies have been developed and continually optimized to improve traits of agricultural importance; however, these methods have been primarily applied without knowledge of underlying biological changes that may be induced by selection. This study aims to characterize the biological basis of differences between chickens with low and high feed efficiency (FE) with a long-term goal of improving the ability to select for FE.

Results

High-throughput RNA sequencing was performed on 23 breast muscle samples from commercial broiler chickens with extremely high (n = 10) and low (n = 13) FE. An average of 34 million paired-end reads (75 bp) were produced for each sample, 80% of which were properly mapped to the chicken reference genome (Ensembl Galgal4). Differential expression analysis identified 1,059 genes (FDR < 0.05) that significantly divergently expressed in breast muscle between the high- and low-FE chickens. Gene function analysis revealed that genes involved in muscle remodeling, inflammatory response and free radical scavenging were mostly up-regulated in the high-FE birds. Additionally, growth hormone and IGFs/PI3K/Akt signaling pathways were enriched in differentially expressed genes, which might contribute to the high breast muscle yield in high-FE birds and partly explain the FE advantage of high-FE chickens.

Conclusions

This study provides novel insights into transcriptional differences in breast muscle between high- and low-FE broiler chickens. Our results show that feed efficiency is associated with breast muscle growth in these birds; furthermore, some physiological changes, e.g., inflammatory response and oxidative stress, may occur in the breast muscle of the high-FE chickens, which may be of concern for continued selection for both of these traits together in modern broiler chickens.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1364-0) contains supplementary material, which is available to authorized users.

Physiological status of broiler chicks at pulling time and the relationship to duration of holding period

Newly hatched chicks may be held longer than 48 h and experience long periods of fasting in commercial hatcheries. Limited information is known about the physiological status of chicks in such situations, due to the difficulty of precisely recording time of hatch. This study investigated the effect of the time from hatch to pulling (holding period) on physiological measures/parameters in 109 broiler chicks. Fertile Ross 308 eggs were incubated in a custom built small-scale incubator. The individual hatching time of each focal chick was determined using eggshell temperature monitoring. At 'pulling' (512 h of incubation time), the quality of focal chicks was assessed using the chick scoring method and physiological parameters were measured including BW, organ (heart, liver and stomach) weights, blood values and plasma corticosterone level. The time from hatch to pulling varied from 7.58 to 44.97 h. Egg weight at setting was significantly correlated with chick BW and weight of organs at pulling, but had no effect on chick quality, blood values and plasma corticosterone. Relative BW at pulling was negatively associated with the duration of holding period (P=0.002). However, there was a positive correlation between relative stomach weight and the duration of the holding period (P<0.001). As the holding period duration increased, there was a trend that blood partial pressure of oxygen, haematocrit and haemoglobin also increased, and blood partial pressure of carbon dioxide, total carbon dioxide and bicarbonate decreased (P<0.05). A wide range of plasma corticosterone was observed from chicks that had experienced different durations of holding period. We conclude that shortening the hatch window and minimising the number of chicks that experience a long holding period before pulling may improve chick quality and physiological status, which may be due to unfavourable environmental conditions that include feed and water deprivation.

Proteína bruta e energia metabolizável para codornas de corte de um a 14 dias de idade

Com o objetivo de avaliar diferentes níveis de proteína bruta e de energia metabolizável para codornas de corte de um a 14 dias de idade, foi realizado um estudo utilizando 840 codornas de corte, mistas. As aves foram distribuídas em arranjo fatorial (2x5) constituído por 10 tratamentos (dois níveis de energia metabolizável - 2800 e 3000kcal/kg e cinco níveis de proteína bruta - 22,0; 24,0; 26,0; 28,0 e 30,0%), sete repetições com 12 aves cada. Foram avaliados os seguintes parâmetros: consumo de ração, ganho em peso aos sete e 14 dias de idade, conversão alimentar aos 14 dias e viabilidade. Verificou-se que codornas alimentadas com dietas com o nível energético mais baixo, 2800kcal/kg, apresentaram maior consumo. As aves que consumiram ração com menor nível de energia apresentaram maior peso aos sete dias de idade independentemente do nível de proteína bruta utilizado. O peso aos 14 dias de idade apresentou resposta quadrática com o acréscimo de proteína bruta na ração, sendo que dietas contendo 28,86% de PB proporcionaram melhor ganho independentemente do nível de energia utilizado. A conversão alimentar apresentou melhores resultados com o aumento do nível de proteína bruta da ração, independentemente do nível de energia metabolizável utilizado. Não foram observados efeitos significativos dos níveis de proteína e energia ou de sua interação sobre a viabilidade das aves. Rações contendo 30,0% de proteína bruta e 2800 ou 3000kcal/kg de energia metabolizável proporcionam satisfatórios resultados de desempenho para codornas de corte de um a 14 dias de idade.

Effect of age and diet composition on activity of pancreatic enzymes in birds

Digestive enzymes produced by the pancreas and intestinal epithelium cooperate closely during food hydrolysis. Therefore, activities of pancreatic and intestinal enzymes processing the same substrate can be hypothesized to change together in unison, as well as to be adjusted to the concentration of their substrate in the diet. However, our knowledge of ontogenetic and diet-related changes in the digestive enzymes of birds is limited mainly to intestinal enzymes; it is largely unknown whether they are accompanied by changes in activities of enzymes produced by the pancreas. Here, we analyzed age- and diet-related changes in activities of pancreatic enzymes in five passerine and galloanserine species, and compared them with simultaneous changes in activities of intestinal enzymes. Mass-specific activity of pancreatic amylase increased with age in young house sparrows but not in zebra finches, in agreement with changes in typical dietary starch content and activity of intestinal maltase. However, we found little evidence for the presence of adaptive, diet-related modulation of pancreatic enzymes in both passerine and galloanserine species, even though in several cases the same birds adaptively modulated activities of their intestinal enzymes. In general, diet-related changes in mass-specific activities of pancreatic and intestinal enzymes were not correlated. We conclude that activity of pancreatic enzymes in birds is under strong genetic control, which enables evolutionary adjustment to typical diet composition but is less adept for short term, diet-related flexibility.

Nutritive value of high-oleic acid sunflower seed for broiler chickens

Two experiments were conducted to evaluate the use of high-oleic acid sunflower seed (HOASS) in broiler diets. In the first experiment, HOASS was included in a basal diet at 80, 160, 240, and 320 g/kg at the expense of the energy-yielding ingredients, and the AMEn values of the experimental diets were determined. The linear regression equation of AMEn values on rate of inclusion was calculated. Extrapolation value for the AMEn of HOASS at 100% inclusion was 4224 +/- 77 kcal/kg. In the second experiment, diets containing up to 200 g of HOASS/kg were given to broilers (Cobb) from 0 to 42 d, and performance parameters, nutrient digestibility, and AMEn value were determined at 12 and 42 d of age. Birds fed the diets containing 100 and 200 g HOASS/kg gained less weight (P < 0.001) than those fed the diet containing no HOASS at both ages. Differences in feed-to-gain ratio were only significant for the diet with the highest concentration of HOASS. Apparent digestibility of nutrients and dietary AMEn contents of diets increased with age; thus, the mean digestibility of diets for amino acid N and for total fatty acids increased from 82.1 and 68.0% at 12 d to 86.7 and 84.7% at 42 d, respectively, and AMEn content was improved by 6.5%. Inclusion of HOASS in the diet decreased the digestibilities of fat, oleic acid, and total fatty acids. A decrease in the digestibility of aspartic acid, threonine, tyrosine, valine, isoleucine, and AMEn with increasing inclusion level was also observed at 12 d of age.

Selection for growth in brown trout increases feed intake capacity without affecting maintenance and growth requirements1

The correlated responses in feed intake and G:F ratio with selection for increased growth rate were evaluated by comparing selected (S) and control (C) brown trout (Salmo trutta) reared under conditions known to affect feed efficiency: feed restriction and periods of compensatory growth. Nitrogen and energy requirements for maintenance and growth were also measured. Trout were allotted at comparable BW (3.7+/-0.06 and 3.8+/-0.04 g, for C and S respectively) to triplicate groups per treatment. The experiment lasted a total of 198 d, during which animals were successively submitted to a 116-d feeding phase and fed 10, 30, 50, 70, 100, and 140% of their usual daily ration (UDR), a 35-d phase of food deprivation, and a 47-d refeeding phase. The G:F of C and S were comparable in all experimental conditions tested. During the feeding phase, S grew better than C only when fed 100 and 140% UDR (P < 0.001). This was explained by a higher feed intake capacity. The requirements for growth and maintenance were similar among the lines, which is in agreement with their comparable loss of weight (mean energy loss of -53 and -55 kJ/(kg x d) for C and S, respectively; P > 0.38) observed during the feed deprivation phase and the lack of differences in carcass composition (fat, P > 0.35; protein, P > 0.54). During the refeeding phase, growth performance and G:F were high in all groups. The daily growth coefficient was higher in S than in C (P < 0.001) because of a higher feed intake (P < 0.001). An increase in absolute individual variability in final BW and length was associated with the level of food restriction in both lines; however, it always remained lower in S than in C. In conclusion, fish selected for growth under ad libitum conditions will only exhibit growth superiority when fed diets close to ad libitum, and there was no evidence that selection was associated with an improvement in efficiency of maintenance nor in retention of body tissues.

Effects of delayed feed intake on body, intestine, and muscle development in neonate broilers

The short-term effects of delayed feeding (DF) for 2 d posthatching were measured in neonate chicks and compared to early feeding (EF). Chicks from 10 independent families were used in this study to determine whether genetic background control of growth may be influenced by EF and DF. Early feeding maintained significant interfamily body weight variations from hatch to 4 d of age, whereas there were no significant differences from 1 d of age when feeding was delayed to 48 h posthatching. These results suggest that posthatching feeding delay may distort genetic selection by masking the expression of genetic potential and disturbing the estimation of chick breeder value. In DF chicks, overall body growth was delayed until the beginning of feeding and body weight at 6 d of age was 25% lower than EF chicks. Availability of feed after the fasting period was not sufficient to compensate for the retardation of weight gain in either body weight or in intestine and breast muscle weight. However, initiation of intestine growth in DF chicks occurred from 1 d of age despite the lack of feeding, whereas feed intake was essential to enhance muscle growth. The potential for protein synthesis was lower in DF than in EF chicks during the first 2 d posthatching (P < 0.001) and then reached similar values after feed intake. These results confirm that initiation of growth in neonate chicks is improved by earlier feeding after hatching. Awareness of changes in overall body weight caused by posthatching food deprivation, especially in the intestine and muscle might help in the development of new diets which could minimize retardation of body weight gain in chicks.

Alpha-amylase supplementation of broiler diets based on corn

A 42-d trial was conducted to study the influence of exogenous alpha-amylase on digestive and performance traits in broilers fed a corn-soybean meal diet. There were two treatments (control and alpha-amylase supplemented diet) and six replicates (14 Cobb male chicks caged together) per treatment. At 7 d of age, alpha-amylase supplementation improved daily gain by 9.4% (P < or = 0.05) and feed conversion by 4.2% (P < or = 0.01). At the end of the trial, birds fed the alpha-amylase-supplemented diet ate more and grew faster (P < or = 0.05) and hadbetter feed conversion (P < or = 0.10) than broilers fed the control diet. Also, alpha-amylase supplementation improved apparent fecal digestibility of organic matter and starch (P < or = 0.01) and AMEn of the diet (P < or = 0.001). However, no effects were detected for CP or fat digestibility. Nutrient digestibility and AMEn of the diet increased with age (P < or = 0.001); however, no interactions of alpha-amylase x age were observed for any trait. Coefficients of apparent ileal and fecal digestibility of starch at 28 d of age were similar, which indicated that most of the undigested starch was not fermented in the hindgut of the chick. alpha-Amylase supplementation reduced relative pancreas weight (P < or = 0.001) but did not affect the weight of the remaining organs. Age consistently reduced intestinal viscosity and relative weights of all the organs (P < or = 0.001). The data indicated that alpha-amylase supplementation of a corn-soybean meal diet improved digestibility of nutrients and performance of broilers.

Atividade enzimática pancreática de frangos de corte alimentados com dietas contendo enzima ou probiótico

Este trabalho foi realizado para avaliar o efeito da adição de enzima e/ou probiótico, bem como do estresse calórico sobre a atividade de enzimas digestivas de frangos de corte aos 7, 14, 28 e 42 dias de idade. O delineamento experimental utilizado foi o inteiramente casualizado em parcelas subdivididas com 6 tratamentos primários, constituindo um esquema fatorial 6x2 (rações x temperatura) e 4 tratamentos secundários (idades de amostragem), com 2 repetições. Não foi verificado efeito significativo dos tratamentos sobre a atividade específica das enzimas, exceto para as atividades de amilase aos 14 dias e tripsina aos 28 dias de idade, nas quais a adição de enzima proporcionou maiores atividades. Já a idade de criação afetou significativamente todas as enzimas quantificadas, sendo que a atividade específica de lipase diminuiu e as atividades específicas de amilase, tripsina e quimotripsina aumentaram com a idade das aves. A temperatura ambiente (calor) também afetou a produção enzimática de acordo com a idade dos frangos de corte, com um aumento na atividade de lipase e redução na tripsina e amilase.

Intestinal dimensions of mice divergently selected for body weight

BACKGROUND

Selection for body weight provides important animal models for studying mechanisms of growth regulation. This study evaluated growth responses of the gastrointestinal tract (GIT) to long-term selection of mice for high (H line) or low (L line) 8-week body weight as compared with random-bred controls (C line).

METHODS

Weights and dimensions of the various parts of the GIT were recorded from 8-month-old mice. For light microscopic stereological analyses, samples were taken from eight equidistant locations covering the whole jejunum/ileum. Vertical sections were used for estimation of fractional volumes of mucosa, submucosa, and muscularis and of villous surface area density and for measurement of villus length.

RESULTS

Differences between groups in weights and dimensions of the various parts of the GIT were more pronounced in the proximal than in the distal segments, with greatest values in H, followed by C and L mice. Relative to body weight, intestinal growth was similar in the three lines, except for significantly (P < 0.001) increased relative weights of jejunum/ ileum, caecum, and colon in L mice. The fractional volume of mucosa and villus length decreased, whereas the fractional volumes of submucosa and muscularis increased from the proximal to the distal locations. The absolute volume of mucosa was greatest in H mice, followed by C and L mice. Relative to body weight, the volume of mucosa was significantly (P < 0.01) greater in L mice than in the two other lines. The mean total villous surface area of jejunum/ileum was significantly (P < 0.01) different among the three lines (L line: 144 cm2; C line: 227 cm2; H line: 304 cm2) but proportionate to differences in metabolic body weight.

CONCLUSIONS

Selection for body weight affected various parts of the GIT to a different extent. The parameters investigated changed markedly along the small intestine, demonstrating the need for systematic sampling. Vertical section stereology provides unbiased estimates of total villous surface area, which is a parameter of major biological significance.

Evaluation of the non invasive TOBEC (total body electrical conductivity) procedure for prediction of chemical components of male broilers with special consideration of dietary protein level

TOBEC (total body electrical conductivity) measurement as a non invasive procedure for the estimation of body chemical composition was used to calculate calibration curves for the prediction of crude protein mass (CPM), crude water mass (CWM), crude ash mass (CAM) and fat-free mass (FFM) of male broiler chickens. A growth experiment with 3 protein levels (130, 230 and 330 g CP/kg diet, isoenergetic with 13.3 MJ AMEN/kg) was combined with TOBEC measurements and body chemical analysis in order to obtain the values necessary for calibration. A total of 196 TOBEC measurements and body chemical analysis were undertaken in time intervals of two days beginning with batch until day 17 of age. Different dietary protein levels resulted in marked differences in body weights and body chemical compositions but in similar TOBEC-responses for a given mass of FFM, CPM, CAM or CWM. Values for birds fed a diet with 130 g CP/kg diet tended to be more variable. Linear broken relationships were found between FFM, CPM, CAM and CWM, respectively and TOBEC values (E#). A set of different regression equations is given and yielded high proportions of variance accounted for the piece wise regression model (R2 ranged from 0.83 to 0.99). In spite of these high determinations the prediction of crude fat mass (CFM) by subtracting the FFM from the body weight resulted in most cases in weak determinations between observed and predicted CFM (R2 ranged from 0.38 to 0.86). The highest R2 was observed when the E# was expressed per unit metabolic body weight to the power of 0.67 and regressed on FFM expressed to the same power. In conclusion, FFM, CPM and CWM may be predicted reasonably well by TOBEC. However, these high determinations are not high enough to predict CFM accurately. In addition, the application of such regressions to an individual bird seems to be impossible. Assessment for groups of animals should be possible if errors of estimation, standard deviations and differences to be detected are taken into account in the calculation of the number of birds necessary for the TOBEC measurements.

Long-term divergent selection for eight-week body weight in white Plymouth rock chickens

This paper provides an overview of results from a long-term (38 generations) selection experiment. Lines were developed from individual phenotypic selection for high or low body weight at 8 wk of age. Included are data for the selected lines, sublines in which selection was relaxed, crosses of the selected lines, and sublines in which the sex-linked dw gene was introduced. Periodically (and in some cases every generation) data were obtained for unselected traits. These unselected traits included feed consumption and intake behavior, reproduction, allomorphic relationships, and metabolic, immunological, endocrine, and molecular factors. These responses have been integrated into a resource allocation paradigm.