Growth curve analyses in selected duck lines.

1. Growth patterns of male ducks from 4 lines (lines A, B, C and D) selected for market weight were analysed and compared to growth patterns of ducks in the respective line 7 generations earlier. Growth curves were analysed using procedures derived from the Weibull sigmoidal function and the linear-linear relative growth rate model and simple allometry. 2. The ducks were fed ad libitum under 24-h lighting throughout the experiment. At weekly intervals from the time of hatch through 70 d of age, 16 ducks from each line were killed to determine body, carcase, breast-muscle, leg and thigh-muscle, and abdominal fat weights. 3. Line A was the heaviest line, followed by line B, line C and line D. However, body weight, carcase weight and breast-muscle weight at 49 d of age were not significantly different between lines A and B. After 7 generations of selection, the breast-muscle yield was increased to >19% and the abdominal fat percent was reduced to <1.4% in all lines. 4. The Weibull growth curve analysis of body weight showed an increase in the asymptotes during selection, while the age of the inflection point remained constant in all lines (21.3 to 26.0 d). For breast-muscle growth, ducks reached the inflection point 12.8 to 14.3 d later than for body weight. Between line A and line B, asymptotes for body weight, asymptotes for breast-muscle weight and allometric growth coefficients of breast muscle and leg and thigh muscles from 14 to 49 d were not significantly different. 5. The relative growth rate model discriminated body and breast-muscle growth patterns of line A and line B. The initial decline in the relative body growth rate was less and the time to reach the transition was longer in line A than line B. On the other hand, the initial decline in the relative breast-muscle growth rate was greater in line A than line B.     

Divergent selection for shape of growth curve in Japanese quail. 4. Carcase composition and thyroid hormones

1. Changes in the relative weights of carcase, abdominal fat, breast and leg muscles, and plasma thyroid hormone concentrations occurring during the first 6 weeks of postnatal growth were analysed in males of HG and LG lines divergently selected for high and low relative body weight (BW) gain between 11 and 28 d of age, respectively, and constant adult BW. 2. The second week of postnatal life was a critical age at which the HG males exhibited a relatively faster growth in comparison to their LG counterparts and permanently exceeded LG males in the percentage by weight of carcase, breast and leg muscle. A higher production of muscle tissues was associated with lower accumulation of abdominal fat before sexual maturity. 3. In general, the plasma T(3) level of HG quail exceeded that of LG quail. Nevertheless, significant differences were found only at 14, 21 and 28 d of age, that is, in the period during which the highest inter-line differences in relative growth rate were noted. Also the T(3)/T(4) ratio followed a similar trend while plasma T(4) level showed no clear and consistent inter-line differences. 4. The results suggest that the selection for the shape of the growth curve, like the selection for body fat, modifies the carcase quality owing to shortening/prolongation of the acceleration growth phase. Individuals with a short acceleration phase of the growth curve are characterised by low carcase quality during the fattening period.     

Dietary L-arginine supplementation beneficially regulates body fat deposition of meat-type ducks

1. Unlike mammals, uricotelic species, such as the duck, cannot synthesise endogenous arginine (Arg). This study was conducted to test the hypothesis that dietary L-Arg supplementation might regulate body fat deposition in ducks without affecting their fast growth rate. 2. A total of 160 21-d-old male and female White Pekin ducks were assigned randomly to two treatments: a non-supplemented control and supplementation at 10 g/kg L-Arg of a maize and soybean meal-based diet. 3. The 3-week feeding trial showed that the addition of L-Arg had no significant effect on feed intake, but significantly increased body weight gain by 5·2 %, breast muscle weight relative to live body weight by 9·9%, carcase crude protein content by 9·2%, ether extract content in breast muscle by 11·9%. Arg supplementation significantly decreased skin with fat and abdominal fat pad contents by 7·6% and 4·9% respectively and the ether extract content of carcase by 7·2%. 4. The results of this study indicate that a diet with 10 g/kg supplemental L-Arg could reduce the fat deposition of carcase and abdominal adipose cell size (diameter and volume), enhance intramuscular fat in breast muscles, as well as increase muscle and protein gain. The decreased fat depot in the carcase may be attributed to a reduction of hepatic lipogenic enzyme activity.     

复合酶制剂对北京填鸭胴体性状、体脂沉积和营养物质表观消化率的影响

本试验旨在研究饲粮复合酶制剂水平对北京填鸭胴体性状、体脂沉积和营养物质表观消化率的影响。选取35日龄健康、大小均匀的雄性北京鸭96只,随机分成4个组,每个组6个重复,每个重复4只鸭。对照组饲喂玉米-豆粕型基础饲粮,试验1组、试验2组和试验3组饲粮分别在基础饲粮中添加100、200和300 mg/kg的复合酶制剂,试验鸭在35和36日龄饲粮填饲量分别为260和300 g,之后5 d各组饲粮填饲量均为400 g/d。试验期为7 d。结果表明:1)饲粮复合酶制剂水平对北京填鸭屠体重、屠体率、全净膛重、全净膛率、腿肌重、腿肌率、龙骨长和胸宽均无显著影响(P0.05),而对北京填鸭增重、胸肌重、胸肌率、胸肌厚、肌胃重和肌胃率有显著影响(P0.05)。试验2组和试验3组北京填鸭胸肌重和肌胃重分别比对照组显著提高了7.88%和7.60%(P0.05)。2)饲粮复合酶制剂水平对北京填鸭皮脂重、皮脂率、腹脂重、腹脂率、肝脏重、肝重率和肝脂率均无显著影响(P0.05)。试验1组皮脂厚与对照组相比显著提高了16.2%(P0.05)。3)饲粮复合酶制剂水平对北京填鸭干物质和总能表观消化率无显著影响(P0.05),而对北京填鸭粗蛋白质表观消化率有显著影响(P0.05)。由此可见,饲粮中添加复合酶制剂对北京填鸭体脂沉积无显著影响,但能够提高饲粮中粗蛋白质表观消化率,并促进胸肌和肌胃的生长发育。     

Abdominal fat weight and thickness as predictors of total body fat in broilers

In two experiments broilers of both sexes from two strains were reared to 16 weeks of age to determine how total body fat could be estimated and predicted from some carcase traits. In the first experiment, age, carcase weight and abdominal fat thickness were found to be significant factors in the prediction of total body fat weight estimated from abdominal fat weight. In the second experiment abdominal fat weight was the best predictor of total body fat weight, obtained by ether extraction of the minced whole carcase. Inclusion of abdominal fat weight in the prediction equation after carcase weight and abdominal fat thickness significantly improved the regression. If the carcase cannot be weighed and abdominal fat weight is not available, because of the more laborious nature of its determination, then the measurement of abdominal fat thickness can be useful in predicting the total body fat content.     

Effects of alfalfa meal on carcase quality and fat metabolism of Muscovy ducks

1. The effects of alfalfa meal on carcase quality and fat metabolism of Muscovy duck were evaluated. The objective of this research was to establish whether alfalfa meal can reduce fat content and improve carcase quality of Muscovy duck. Animal products with a high fat content present a risk factor for many diseases. Reducing fat content in poultry products is an important goal for the poultry industry.

2. A total of 240 14-d-old white Muscovy ducks were selected and randomly allocated to 1 of 4 dietary treatments containing 0, 3, 6, and 9% of alfalfa meal for 5 weeks. Growth performances were recorded and carcase characteristics and lipid parameters were analysed.

3. Results showed that 3, 6, and 9% alfalfa meal in diet had no significant effects on growth performance of Muscovy ducks from 14 to 49?d of age. Ducks given 3, 6, and 9% alfalfa meal had significantly higher dressing percentage and lower abdominal fat percentage compared with those given no alfalfa meal. Ducks given 9% alfalfa meal had higher breast meat percentage compared with those given no alfalfa meal. The concentrations of triglyceride, total cholesterol, low density lipoprotein (LDL), very low density lipoprotein (VLDL) and free fatty acid in serum of ducks fed on alfalfa meal decreased. Alfalfa meal in the diet decreased abdominal fat percentage and improved carcase traits of Muscovy duck.

4. The study showed that dietary alfalfa meal decreased abdominal fat percentage and improved carcase traits, without an adverse effect on performance.     

多元回归方程估测北京鸭屠体性状的研究

试验测定了202只6周龄北京鸭公鸭的体重X1、体尺性状(体斜长X2、龙骨长X3、胸宽X4)和屠体性状(胸肉重Y1、腿肉重Y2、皮脂重Y3、腹脂重Y4、瘦肉重Y5和脂肪重Y6)共10项指标,在分析体重、体尺性状与屠体性状的相关关系的基础上,以体重、体尺性状为自变量,屠体性状为依变量,构建了估测北京鸭活体屠体性状的回归方程。结果表明:6周龄北京鸭胸肉重、腿肉重、皮脂重、瘦肉重、脂肪重分别与体重、体尺性状呈极显著正相关(P<0.01),腹脂重与体重呈极显著正相关(P<0.01),与体尺性状呈显著正相关(P<0.05);建立的6个最优多元回归方程,经F检验回归关系均达极显著水平(P<0.01),各回归系数也均达到极显著水平(P<0.01),表明各方程的自变量与依变量之间真实存在着多元回归关系,可为估测北京鸭活体胸肉重、腿肉重、皮脂重、腹脂重、瘦肉重和脂肪重提供理论依据,具有一定的参考使用价值。     

Effects of ambient temperature on growth performance and carcass traits of male growing White Pekin ducks

ABSTRACT

1. An experiment was conducted to investigate the effects of ambient temperature on growth performance and carcass traits in male growing Pekin ducks from 14 to 42 d of age in order to establish their optimal temperature requirements.

2. A total of 216 14 d old male White Pekin ducks were allocated randomly to six environmentally controlled chambers with ambient temperature set at 20°C, 22°C, 24°C, 26°C, 28°C, and 30°C from 14 to 42 d of age, respectively.

3. As ambient temperature increased from 20°C to 30°C, the body weight and weight gain decreased linearly or quadratically (P < 0.05) and was accompanied by linearly decreasing feed intake (P < 0.05). According to broken-line regression, the upper critical level of ambient temperature during the growing period for body weight, weight gain, and feed conversion ratio were 27.4°C, 27.4°C, and 26.0°C, respectively.

4. The weight of breast meat, leg meat, and abdominal fat decreased linearly or quadratically as ambient temperature increased and declined to a minimum when the temperature increased to 30°C (P < 0.05). The percentage of breast meat and abdominal fat showed a linear or quadratic decreasing response to increasing temperature, but leg meat percentage increased as temperature increased and reached maximum at 30°C (P < 0.05). According to broken-line regression, the upper critical ambient temperatures during the growing period for breast meat weight and percentage were 25.5°C and 25.6°C, respectively.

5. It was concluded that both growth performance and breast meat of growing ducks were sensitive to increasing ambient temperature and this should be kept below the upper critical temperature during the growing period in order to optimise growth performance and carcass traits at market age.     

Carcase characteristics of a heavy Japanese quail line under introgression with the roux gene

1. The roux plumage sex-linked recessive gene may be used for early sexing of Japanese quail in crossbreeding production systems with wild-type and roux lines. However, associated effects of the gene on carcase and meat composition need to be assessed. 2. Quail carcases from pure Line K males and females (100% K), a heavy meat line which was used as the recipient line for the introgression of the roux gene, and from same-age roux or wild-type females from the second generation of introgression (75% K) were dissected. The effects of sex, line and plumage colour on carcase components and on protein and lipid contents of various tissues were estimated. 3. Expected sex differences in carcase weights were obtained, with marginally higher values for females. However, weights of parts and carcase yield (ratio of empty carcase weight without head, neck and feet over live body weight) were similar for both sexes in pure Line K which had a 68% carcase yield. Lipid contents in leg meat were higher in males (3.1%) than in females (2.7%). 4. The roux gene did not seem to have any major impact on carcase parts or composition. However, in roux birds, leg skin was marginally higher in lipids and pectoralis major lower in proteins than in wild-type ones.     

北京鸭胸肌厚度与屠体性能指标的相关关系研究

先后进行两次饲养试验和屠宰试验研究肉鸭胸肌厚度和胸肌质量、胸肌率的相关关系。试验1饲养大体型A品系和小体型B、C品系的北京鸭各150只至7周龄,每个品系随机选取8只公鸭和8只母鸭进行屠宰试验。探索超声波扫描技术活体测定北京鸭胸肌厚度的方法,建立北京鸭胸肌厚度与胸肌质量、胸肌率等指标的相关关系。试验2饲养B系肉鸭1900只至7周龄,随机选取100只公鸭和100只母鸭进行屠宰试验,测定胸肌厚度和主要屠体指标,进行相关分系。结果:超声波测定北京鸭的胸肌厚度,图象清晰;大体型品系北京鸭7周龄的胸肌厚度达到22.3mm,小体型为16.2和16.3mm。胸肌厚度与胸肌质量、胸肌率的相关系数分别为0.6328(P<0.01)、0.4721(P<0.01),均达到极显著水平。结果表明,超声波扫描技术能够准确地测定北京鸭的胸肌厚度;北京鸭的胸肌厚度与胸肌产量、胸肌率存在显著的正相关关系。     

不同填饲量对北京鸭胴体品质、体脂沉积和营养物质表观消化率的影响

旨在研究不同填饲量对北京鸭胴体品质、体脂沉积及营养物质表观消化率的影响。选取35日龄健康、大小均匀的W系雄性北京鸭96只,随机分成8个处理,每处理6个重复,每个重复2只鸭。基础日粮中加入0.5%的Cr2O3作为外源指示剂测定填鸭营养物质表观消化率,对照组自由采食,试验组在35和36日龄时分别填饲260和300g.d-1的基础日粮,之后5d各处理填饲量保持不变,分别为300、330、360、390、420、450、480g.d-1,试验期为7d。结果表明:(1)不同填饲量对北京鸭屠体率、全净膛率、胸肌率、腿肌质量和腿肌率影响不大(P>0.05),而对北京鸭胸肌质量影响显著(P<0.05),并随填饲量的增加呈现先升高后平稳的趋势,以胸肌质量为评定指标,采用直线折线模型进行估算的最适填饲量为390.7g.d-1。不同填饲量对北京鸭皮脂质量、皮脂率、腹脂质量、腹脂率、肝脏质量、肝重率和肝脂率均有显著的影响(P<0.05),但填饲量增加到一定值后,体脂沉积量保持稳定(P>0.05);(2)在填饲期,不同填饲量对北京鸭干物质和粗蛋白表观消化率影响显著(P<0.05),而对能量表观消化率影响不大(P>0.05)。以上结果表明,填饲能够快速增加北京鸭的体脂沉积,对胸肌生长发育影响显著,但随填饲量的增加,营养物质表观消化率下降,填鸭体脂沉积量不再增加。     

Influence of food intake and sex on the growth and carcase composition of Pekin ducks

1. The effects of food intake on growth and carcase composition of male and female meat‐type Pekin ducks were investigated between 14 and 56 d of age and between 0.416 kg (14 d) and 2.5 kg live body weight.

2. Males grew faster between 14 and 56 d of age and contained less fat and more protein and water in the eviscerated carcase at 56 d than females.

3. For both sexes, food : gain ratio deteriorated as food intake increased from 0.49 ad libitum intake to ad libitum intake. Growth rate, carcase weight and carcase fat content increased linearly with increase in food intake. Carcase fat content at 56 d was more sensitive to change in the amount of food consumed than either growth rate or carcase weight.

4. Between 0.42 kg (14 d) and 2.50 kg live body weight, growth rate and food :gain ratio improved as food intake was increased from 0.61 ad libitum to ad libitum. Males grew faster and produced leaner carcases than females.

5. Carcase fat content at 2.50 kg live body weight (1.60 kg carcase) increased in a curvilinear fashion with increase in food intake, and the magnitude of the response was lower for females than for males. A similar interaction existed between the effects of sex and the amount of food consumed for the proportions of protein and water in the eviscerated carcase at 2.50 kg live body weight.     

Effect of dietary concentrations of fat and energy on fat deposition in broilers divergently selected for high or low abdominal adipose tissue

1. Fat deposition in abdominal, mesenterial, sartorial and gizzard adipose tissues (AT), liver, breast muscle, skin and carcase was studied in male broilers, selected for high (HF) and low (LF) abdominal fat and fed on diets differing in energy density and total fat content. 2. There were no significant differences in body weight in the experimental groups. The relative weight (g/kg body weight) of the dissected adipose tissues was higher in HF than in LF birds. Fat concentration in the AT (sartorial excepted), skin and body was higher in the HF compared with the LF birds. The lines did not differ significantly in liver and breast muscle fat content. 3. Abdominal AT was affected by selection or dietary fat more than other AT and total body fat. 4. In the HF birds increasing energy density from 12.3 to 13.4 MJ/kg (dietary fat kept constant: 5.46 g/MJ) significantly increased the weight of the abdominal, mesenterial and sartorial AT. Increasing dietary fat (at both energy densities) decreased the weight of the AT, whereas increasing both energy and fat did not affect it. In the LF birds, similar but milder and insignificant trends were observed. It is suggested that this interaction has biological significance.     

Influence of dietary energy on bird performance,carcase parts yields and nutrient composition of breast meat of heterozygous naked neck broilers reared at natural optimum and summer temperatures

1. Heterozygous naked neck birds were raised under natural spring (average 21.2°C) and summer temperatures (average 27.1°C) to investigate the influence of dietary energy on broiler performance, carcase yield and nutrient composition of breast meat. 2. Birds were fed on a low energy diet of 12.12 MJ ME/kg, a medium energy diet of 12.96 MJ ME/kg and a high energy diet of 13.79 MJ ME/kg with 2 protein concentrations per energy treatment, 230 and 200 g/kg, from 0 to 3 and 3 to 7 weeks of age, respectively. 3. Summer rearing resulted in a decrease in body weight, body weight gain, carcase weight and carcase part yields of birds. 4. Increasing dietary energy from 12.12 to 13.79 MJ ME/kg increased body weight at 3 and 7 weeks, body weight gains from 0 to 3 and 3 to 7 weeks, carcase weights and relative abdominal fat weights of birds in a linear manner. There was no effect of dietary energy on the nutrient composition of breast meat. 5. It was concluded that there was no differences in dietary energy requirements of heterozygous naked neck birds when grown under natural optimum (21.2°C) and summer temperatures (27.1°C).     

Body weights,carcase characteristics,organ weights,abdominal fat,and lipid content of liver and carcase in two lines of Japanese quail (Coturnix coturnix japonica), unselected and selected for four week body weight

1. The present study was conducted on quails to evaluate the effects of line and sex on body weight, carcase characteristics and organ weights and also to evaluate the effect of line on sex organs, and on carcase and liver lipid content of male quails.

2. Chicks of both sexes were weighed weekly until 6 weeks when they were slaughtered. Carcase yield, parts yield and measurement, weight of liver, heart, gizzard, abdominal fat and sex organs were obtained. Carcase and liver lipid content of male quails were determined.

3. Weights of body, carcase, breast, thigh, liver, heart, gizzard, abdominal fat, testes and ovary, yield of testes were affected by line. Line had no significant effect on the yields of carcase, breast, thigh, liver, heart, gizzard plus heart plus liver, abdominal fat and ovary. Carcase and liver lipid contents of male quails were affected by line.     

Genetic analysis of growth curve parameters for male and female chickens resulting from selection on shape of growth curve

The objectives of this research were to evaluate gender differences and selection on body weight as they affect growth curves of chickens. Marginal posterior densities of growth curve parameters were studied by Gibbs sampling on 10,671 male and female chickens originating from five lines. Line X-+ was selected on low body weight at 8 wk (BW8) and high body weight at 36 wk (BW36), line X+- on high BW8 and low BW36, X++ on high BW8 and BW36, X-- on low BW8, and BW36, and X00 was an unselected control line. Growth was modeled by a Gompertz function. Heritabilities and genetic correlations among parameters of the Gompertz curve were estimated. Marginal posterior densities were drawn for parameters of the growth curve and for sexual dimorphism at ages ranging from hatching to 1 yr. Lines selected for a higher BW8 had higher initial specific growth rates (L), higher maturation rates (K), and lower ages at inflection (T(I)). Lines selected for a higher BW36 had higher asymptotic body weights (A). Estimates of A, L, and T(I) were higher in males and K was higher in females. Difference between sexes for A was greater in the line selected for a lower BW8 and a higher BW36. Dimorphism for L and K was the lowest in lines X++ and X--, respectively. The greatest difference in T(I) was observed in the line selected for lower BW8 and BW36. Sexual dimorphism of body weight was lower at most ages in the lightest line. Before 15 wk, sexual dimorphism in X++ line was lower than in the line selected for higher BW8 and lower BW36. The increase in sexual dimorphism with body weight could be reduced by selecting animals on body weight at two ages instead of one, as is usually done in commercial lines.     

不同饲养方式对优质肉鸭屠宰性能和肌肉营养成分的影响

为制定优质肉鸭最优饲养技术方案,提高优质肉鸭饲养的经济效益和品质,本研究选择900只1日龄优质肉鸭(益客集团E品系),随机分为3组,每组3个重复,分别采用地面平养、笼养、网上平养,在70日龄进行屠宰性能和肌肉主要营养成分测定。结果表明:网上平养肉鸭的屠体重较其他2种饲养方式有显著提升(P<0.05);网上平养的全净膛重与半净膛重、头脚重、胸肌重与笼养无显著差异,但其半净膛重、头脚重、胸肌重、腹脂重高于地面平养(P<0.05);网上平养的腿肌重与其他2种饲养方式无显著性差异;网上平养肉鸭的胸肌中蛋白质含量较其他2种方式显著提升(P<0.05),在腿肌蛋白质含量上三者差异不显著;网上平养肉鸭的腿肌脂肪含量高于地面平养(P<0.05),而3种饲养方式的胸肌脂肪含量无显著差异;胶原及水分含量与笼养相比在胸腿肌中差异不显著。可见,优质肉鸭采用网上平养较为适合。     

Effects of sex and inclusion of dried distillers grains with solubles on slaughter yield and meat characteristics of Pekin ducks

1. The effects of dried distillers grains with solubles (DDGS) dietary inclusion concentration, and sex, on body weight, slaughter efficiency and meat characteristics of Pekin ducks (Anas platyrhynchos f. domestica, strain P55) were studied.

2. Sexed ducklings (n?=?160) were divided randomly into 4 groups (each with 4 replicates). From d 1 to d 21, all the birds received the same commercial feed, then from 22 to 49 d of age the ducks were fed in the following groups: control (commercial feed) and three experimental groups (15%, 25% and 30% inclusion of DDGS). All ducks were weighed individually at d 1, 21 and 49. On the day of slaughter, 5 males and 5 females, of body weight close to the average weight for group and sex, were selected from each group, slaughtered and the following parameters were evaluated: slaughter yield, weight, and percentage of particular elements of carcase, physical and chemical characteristics of meat.

3. The results obtained showed that DDGS included from 22 to 49 d of rearing, at concentrations up to 30%, in a commercial Pekin duck diet did not affect the live body weight, slaughter yield, weight, and percentage of breast and leg muscle, skin with subcutaneous fat, and abdominal fat. There were no differences in physical characteristics (pH₁₅, pH_24, meat colour values L*, a*and b*, and hygroscopicity) of breast muscle, as well as in cholesterol content. DDGS addition at 30% significantly increased fat content in male, and crude protein in female, breast meat. Sex effect was observed only in a few traits and was diet dependent. Final body weight of females fed 30% DDGS was significantly lower than males; in the control and 15% DDGS group females had higher percentage of skin with subcutaneous fat.

4. The results obtained, and relative costs of feeds produced, allows the recommendation of DDGS addition at up to 30% to commercial Pekin duck diets from 22 d of age.     

High dietary niacinamide and performance of male poults at 16 weeks of age

1. Male poults were fed diets with 50 and 100 mg supplemental niacinamide/kg and grown at stocking densities of 3.2 and 4.3 birds/m2 from 56 to 112 d. 2. At 50 mg/kg niacinamide high stocking density (HSD) depressed 112 d body weight and gain from 56-112d. High dietary niacinamide (100 mg/kg alleviated the growth depression under HSD but decreased body weight gain under low stocking density (LSD). 3. Dressed yield and hock width were not affected by dietary niacinamide or stocking density. 4. Dressed carcase composition was altered by stocking density. The dressed carcase of HSD birds contained 3.7% more protein and 7% less fat. 5. High dietary niacinamide tended to increase carcase protein (+2.4%) and decrease carcase fat (-4.0%).     

Rates of muscle protein breakdown in chickens selected for increased growth rate, food consumption or efficiency of food utilisation as assessed by N tau-methylhistidine excretion

1. N tau-methylhistidine excretion, growth rate, food consumption and body composition was determined in 12 4 to 5 week old chickens sampled from each of 4 lines selected for increased body-weight gain (line W), for increased food consumption (line F), for improved efficiency of food utilisation (line E) or at random (line C), after 12 generations of selection. 2. The use of N tau-methylhistidine as an index of myofibrillar protein breakdown was validated in male and female chickens of lines E and F by following the fate of injected N tau-(14CH3)methylhistidine. Most of the radioactivity (79.3 +/- 1.1%) was excreted in 4 d, with the remainder retained in the carcase. In excreta, 94 +/- 2% of the radioactivity was associated with free N tau-methylhistidine and for the carcase, this value was 88 +/- 3%. 3. In the main experiment, final body weights averaged 497, 651, 588 and 537 g and food: gain ratio averaged 2.47, 2.21, 3.14 and 2.06 for lines C, W, F and E respectively, Carcase protein content (g/100 g body weight) was not different between the lines. 4. N tau-methylhistidine excretion was 5.86, 5.48, 6.43 and 4.99 mumoles/mole carcase-N/d for lines C, W, F and E, respectively. The rate for line F was significantly higher than for lines W and C and that for line E was significantly less than for the control line. 5. The N tau-methylhistidine excretion rate was positively correlated with food: gain ratio. 6. Selection for rapid growth, high food consumption or improved food utilisation results in changes in N tau-methylhistidine excretion which suggest proportionate changes in muscle protein turnover.