In order to achieve improvements in production efficiency in livestock, herds of high sexual precocity and good fertility are needed. These traits increase the availability of animals in herd, either for sale or selection, allowing both greater selective intensity and greater genetic progress. This study aimed at estimating genetic parameters for reproductive traits measured directly in females in order to verify whether they could be used as selection criteria for genetic improvement in Nellore cows, as well as estimating the genetic relationship among these traits and scrotal circumference (SC), the traditional selection criterion for sexual precocity in cattle. In addition to SC, stayability (STAY), number of calvings at 53 months (NC53) and heifers rebreeding (HR) were studied. The (co)variances and genetic parameters were estimated using Bayesian inference. STAY, NC53 and HR were analyzed assuming a threshold model, whereas SC was analyzed with a linear model. Heritability estimated for NC53 was 0.22, and this trait was strongly and positively correlated with STAY, meaning selection for NC53 would improve productive longevity of Nellore cows. Correlations estimated between HR and STAY (≈0.97) and between HR and NC53 (≈0.99) allow an improvement on HR rates if selection was applied to traits related to longevity. Genetic correlations among SC and female reproductive traits were positive but weak, suggesting the need to use reproductive traits directly measured in females in order to obtain greater improvements in sexual precocity and longevity.

A biological growth model was developed to study economic values for average ad libitum food intake capacity (FIC) in growing pigs. The model was based on the linear/plateau relationship between protein deposition and food intake. Input variables were: minimum fat to protein deposition ratio (R), maximum protein deposition rate (Pdmax)and food intake (FI). Output variables were production traits and production costs.

Economic values (under commercial conditions with ad libitum feeding) were derived with the growth model for each of the traits FIC, R, and Pdmax keeping the other two traits constant, for three alternative levels of FIC. If FIC was too low to realize Pdmax, FIC had a positive economic value, R had a negative economic value and the value of Pdmax was zero. If FIC was higher than necessary to realize Pdmax, economic values were negative, zero and positive for FIC, R, and Pdmax respectively. If FIC was just sufficient to realise Pdmax, the lowest production costs occurred. Now, R had a negative economic value and Pdmax had a positive economic value.

With a restricted feeding regimen under commercial conditions a daily food supply just sufficient to realize Pdmax should be pursued. It was concluded that use of a biological growth model to estimate economic values for FIC would give more insight into correct selection strategies than would the use of an economic model.

The main object of the artificial insemination service is herd improvement. This is facilitated by the larger selection differential obtained when only a small number of bulls are used. Robertson and Rendel (1954) have shown that artificially bred daughters by bulls used at centres outwith the Milk Marketing Board system did not outyield naturally bred daughters milking in the same herds and these results have been substantiated by data from the Board’s centres. This failure to select bulls better than those used in the average herd has probably been due to the multiple aims of selection necessary to satisfy all those interested in the artificial breeding movement and to the methods which have been used to evaluate bulls. A method has been designed aimed at improving this evaluation.

Three experiments, involving a total of 84 pigs of approximately 90 kg live weight, were undertaken to determine the effects of feeding sugar solutions immediately prior to slaughter on carcass yield, liver weights, ultimate pH of the musculature and some chemical characteristics of muscle and liver. In the first, where water-only (14 h) prior to slaughter, or access to a solution of either glucose syrup (12 h) or sucrose (12 h) followed by water-only (2 h) were the treatments, sugar feeding did not influence carcass yield. Liver weight was increased with glucose (39%) and markedly so with sucrose (77%). Livers from sugar-fed pigs had higher glycogen and lower protein concentrations. Glucose reduced muscle ultimate pH (0·2 to 0·4 units) but sucrose had little effect. In the second study various sugars were tested (pure glucose, glucose syrup, sucrose and fructose) and pigs had access to solutions of them for 4 h and water-only thereafter for 10 h prior to slaughter. All sugar treatments increased liver weight and reduced muscle ultimate pH relative to water-only before slaughter. The final experiment involved access to varying amounts of a glucose syrup solution for 4 h followed by water-only (10 h). In comparison with ‘water-only’ prior to slaughter, an intake of some 250 g hexose equivalent reduced muscle ultimate pH (0·2 to 0·3 units) and increased liver weight but a response in carcass yield required in excess of 1000 g hexose equivalent.

A multi-trait selection index designed to improve the overall economic performance of hill sheep, including both maternal and lamb traits, has been developed and tested in a selection experiment over 7 years. Two versions of the index were tested, with different economic weights applied to the traits, on two different hill farms: one version including maternal and growth traits; the other version with additional breeding goals of carcass weight, fatness and conformation scores. Responses to selection, using both versions of the index, suggest that improvements are being made in overall index score and lamb growth. This study investigated the indirect effects of these selection indices on lamb carcass composition and muscularity traits, as measured using X-ray computed tomography (CT) scanning. A total of 499 lambs from the two hill farms were CT scanned at weaning (approximately 120 days of age). Approximately half of the lambs from each farm were from the selection line (S, animals with highest index scores selected for breeding), while the other half were from a control line (C, animals with average index scores selected). Composition and muscularity traits were estimated on each lamb from CT data and differences between genetic lines investigated, within farm, using restricted maximum likelihood analyses, adjusting for either live weight or age. Results showed that the selection index without carcass traits produced lambs with carcass composition that was not significantly different to control lambs at a given live weight or age. Including carcass traits in the index resulted in lambs with no compositional differences (except for a slight increase in bone) at a set age compared with controls. At a given live weight however, selection lambs had less fat and lower carcass weights and killing-out percentage. Muscularity (3-D muscle shape) and muscle area shape (2-D) were not improved as a result of selection on either version of the index (including carcass weight and grades in the breeding goals or not) and, at a fixed live weight, muscularity in hind leg and lumbar regions tended to be higher in the C line. To accelerate changes in carcass composition and muscularity within the context of a multi-trait selection index for hill sheep, consideration should therefore be given to including objective CT-derived carcass traits in the index in addition to the Meat and Livestock Commission (MLC) carcass grades or ultrasound measurements.

Gilts were allocated at 55 kg live weight to one of three treatments: 1. 1·4 kg of food daily to service, served at puberty, and 1·4 kg of food daily throughout pregnancy.

2. 1·4 kg of food daily to service, served at the third heat, and 1·4 kg of food daily during pregnancy.

3. 2·3 kg of food daily to service, served at the third heat, and 1·4 kg of food daily during pregnancy.

An attempt was made to encourage early puberty by stimulation using the presence of a boar. Natural service with boars was used on all gilts.

There were no significant differences between the treatments in time taken to reach puberty and the gilts were mated at 77, 97 and 116 kg live weight on Treatments 1, 2, and 3 respectively. The gilts on Treatment 1 had a significantly lower conception rate and a significantly smaller litter size than those on Treatments 2 and 3. However, the average birth weight did not vary significantly among treatments.

Adipose tissue cell volume, lipolytic rates, Iipoprotein lipase (LPL) and acetyl CoA carboxylase (ACC) activities were determined in biopsy samples taken from the back, shoulder, rump and groin subcutaneous fat depots of 17-month-old female sheep (no. = 24). Half of the sheep had received a copper (Cu) supplement at 6 weeks of age. Biopsy samples were taken after fasting overnight.

Differences in cell volume and ACC activity were apparent between sites. Supplementation with Cu at 6 weeks of age led to a proportional increase of between 0·24 and 0·45 in adipose cell volume and a proportional increase in lipolysis which varied from 0·70 to 2·27 depending on site and hormone addition. Correlations between sites for cell volume and lipolytic activity were not high in general but those for ACC and LPL activities were higher (for ACC r = 0·40 to 0·79). Overall the best correlations were between the back and rump sites. Correlations between the various metabolic measurements and cell volume were not apparent within treatment groups.

To investigate the effect of dietary allicin on health and growth performance of weanling piglets, at 21 days of age. Two hundred and twenty-five piglets were weaned and randomly allocated into five groups. Piglets in the control group were fed diets supplemented with antibiotics. Those in the treatment groups were fed diets without antibiotics, but supplemented with allicin product (25% pure allicin oil) in the proportion of 0.10 g/kg, 0.15 g/kg, 0.20 g/kg and 0.25 g/kg in the diet, respectively. During the 28 days of the experiment, average daily weight gain increased linearly (P < 0.0001) and quadratically (P = 0.0014) as the level of dietary allicin increased. The feed gain ratio decreased linearly (P < 0.0001) and quadratically (P < 0.0001). As the dietary allicin level increased, the incidence of diarrhoea in the treatment piglets, especially female piglets decreased linearly (P = 0.0003) and tended to decrease quadratically (P = 0.0716). The number of flies alighting on the surface of the faeces of the piglets at each counting time point decreased linearly (P < 0.0001), quadratically (P < 0.0001) and cubically (P < 0.0001) as the dietary allicin level increased. In conclusion, supplementation of the diet with allicin may improve growth performance, reduce the incidence of diarrhoea and possibly improve their local environmental conditions by reducing the attractiveness of faeces to flies.

There is a paucity of data on the beef production potential of Norwegian Red (NOR) compared with ‘modern’ Holstein-Friesian (HF) cattle. The present study used a total of 64 bulls in a 2 × 2 factorial design study encompassing two breeds (HF and NOR) and two slaughter ages (485; E, and 610; L, days). The mean initial age and live weight of the HF bulls were 179 (s.d. 47.1) days and 203 (s.d. 64.0) kg, while the corresponding data for the NOR bulls were 176 (s.d. 39.7) days and 185 (s.d. 63.6) kg, respectively. Bulls were offered a 50 : 50 mixture (dry matter (DM) basis) of grass silage and concentrates. No breed × slaughter group interactions were recorded for any parameters evaluated (P > 0.05). HF bulls had higher (P < 0.001) DM intake and poorer (P < 0.01) efficiency of conversion of food to carcass gain than NOR bulls. HF bulls tended (P = 0.07) to have a higher rate of live-weight gain and were heavier (P < 0.001) at slaughter than NOR bulls, though both carcass weight and rate of carcass gain did not differ between the breeds (P > 0.05). NOR bulls had higher (P < 0.001) dressing proportion and carcass conformation score than HF bulls, while breed of bull had no influence (P > 0.05) on carcass fat classification, depth of subcutaneous fat, marbling score or on the weight of fat in the internal depots. Daily food intakes did not differ (P > 0.05) across the two slaughter age groups, though efficiency of conversion of food to carcass gain was poorer (P < 0.05) in the L compared with E bulls. Rate of live-weight gain was lower (P < 0.01) for L bulls, although rate of carcass gain did not differ (P > 0.05) between the E and L bulls. Increasing age at slaughter increased (P < 0.01 or greater) dressing proportion, carcass fat class, depth of subcutaneous fat, marbling score and internal fat depots, but had no effect (P > 0.05) on the carcass conformation score. Instrumental measures of meat quality indicated that meat from NOR bulls was tougher (P < 0.01) than meat from HF bulls, while delaying slaughter increased (P < 0.001) a* and C*ab, and decreased (P < 0.01) h0, indicating improved redness. It is concluded that NOR bulls have higher food efficiency and produce more highly conformed carcasses than HF bulls, but HF bulls produce more tender meat.

Two hundred and twenty-four pig carcasses were used to compare the Hennessy Grading Probe and the Fat-O-Meater in their accuracy for measuring fat and muscle thickness (m. longissimus). Accuracy was defined in terms of precision and bias. The precision (relationship between repeated measurements) of the two probes in measuring fat and muscle thickness varied with the probing locations. When differences were detected, the higher precision was found with the Fat-O-Meater. The mean bias in measuring fat thickness was negative for the Hennessy Grading Probe (P < 0·01); however, for the Fat-O-Meater, no mean bias was detected. Furthermore, the extent and the direction of the bias in measuring fat over the range of fat thickness studied varied with the probe and the probing location.

Two surveys were conducted on commercial farms in England, Wales and Scotland to ascertain whether breeding contributed to variation in the number of lambs born per ewe, in lamb mortality and in the number of lambs remaining to be weaned per ewe. Each flock comprised two or more breed groups, but within flocks differences in breed were often quite small.

In one survey of 74 flocks (average size 198 ewes, comprising 175 breed groups) the lambing percentage averaged around 150% and the lamb deaths to weaning (including stillbirths) around 9%. Within flocks there were statistically significant differences in the number of lambs born and weaned per ewe attributable to the breed of lamb and of its dam and sire. Crossbreeding was associated with a higher weaning percentage. Lamb mortality differed significantly among breeds and crosses of lamb. Lambing performance varied with type of farm.

The second survey was a comparison of different crossbred types of dam, themselves the offspring of three breeds of sire and seven of ewe. Thirty flocks participated, providing 29 sets of records for 1-yearold females producing on average 120% lambs and losing through death (including stillbirths) 25 % of lambs born, and 10 sets at 2 years old, with 176 lambs per 100 ewes lambing and 11 % of the lambs born dying before 8 weeks old. Variation attributable to the crossbred type of the dam and to the breed of the dam's sire was significant for lambing and weaning percentage and the incidence of lamb mortality.

The rationale for a method of computing selection indices for dairy cows is outlined. It uses predicted transmitting abilities of sires from a best linear unbiased prediction or similar method of evaluation together with mean transmitting abilities (cow indices) of dams to estimate herd genetic levels. Indices for individual cows are computed using deviations from those of contemporaries in the herd of her sire's transmitting ability, her dam's index and her own production in each lactation.

The method is being used in the UK, cow indices being expressed as transmitting abilities relative to the same fixed genetic base as for sire evaluation.

The effect of active immunization against somatostatin and β-agonist administration alone and in combination on growth, carcass composition and endocrine responses was investigated in young steers given a pelleted diet of dried grass and barley (0·7: 0·3) ad libitum for 16 weeks. Immunization alone increased growth rate by proportionately 0·10 but had no effect on plasma growth hormone (GH) or insulin-like growth factor-1 (IGF-1) concentrations (P > 0·10). Carcass protein content was unchanged but fat content tended to be increased. This was possibly associated with increased (P < 0·10) plasma insulin concentrations observed in these animals. Cimaterol administration significantly increased growth rate (P < 0·05) and carcass weight (P < 0·01). Carcass protein content was significantly (P < 0·01) increased and carcass fat was significantly (P < 0·001) reduced. Administration of cimaterol to immunized animals resulted in an enhancement of the repartitioning effect of the pagonist although growth rates were intermediate between the separate treatments. The mechanism by which this was achieved is unclear. Plasma IGF-1 concentrations tended (P < 0·10) to be lower in these animals compared with controls. Plasma insulin concentrations were also reduced (P < 0·05) in these animals. Both immunized animals and those given cimaterol alone showed an increased GH response to a single intravenous injection of growth hormone releasing factor (GRF) (P < 0·05). Animals given the combined treatment however showed a reduced response suggesting that the effects were not mediated through direct changes in GH secretion. The improved lean:fat ratio in the carcass was observed to be mediated through an enhancement of the lipolytic response to the /J-agonist with only small effects on protein accretion. It is suggested that immunoneutralization of somatostatin may increase the sensitivity of adipose tissue to lipolytic stimuli. This could occur through changes in the secretion of gastrointestinal or pancreatic hormones as a result of neutralization of the inhibitory influences of somatostatin. Further work is required to clarify these effects.

For energy feeding systems for ruminants to evolve towards a nutrient-based system, dietary energy supply has to be determined in terms of amount and nature of nutrients. The objective of this study was to establish response equations of the net hepatic flux and net splanchnic release of acetate, butyrate and β-hydroxybutyrate to changes in diet and animal profiles. A meta-analysis was applied on published data compiled from the FLuxes of nutrients across Organs and tissues in Ruminant Animals database, which pools the results from international publications on net splanchnic nutrient fluxes measured in multi-catheterized ruminants. Prediction variables were identified from current knowledge on digestion, hepatic and other tissue metabolism. Subsequently, physiological and other, more integrative, predictors were obtained. Models were established for intakes up to 41 g dry matter per kg BW per day and diets containing up to 70 g concentrate per 100 g dry matter. Models predicted the net hepatic fluxes or net splanchnic release of each nutrient from its net portal appearance and the animal profile. Corrections were applied to account for incomplete hepatic recovery of the blood flow marker, para-aminohippuric acid. Changes in net splanchnic release (mmol/kg BW per hour) could then be predicted by combining the previously published net portal appearance models and the present net hepatic fluxes models. The net splanchnic release of acetate and butyrate were thus predicted from the intake of ruminally fermented organic matter (RfOM) and the nature of RfOM (acetate: residual mean square error (RMSE)=0.18; butyrate: RMSE=0.01). The net splanchnic release of β-hydroxybutyrate was predicted from RfOM intake and the energy balance of the animals (RMSE=0.035), or from the net portal appearance of butyrate and the energy balance of the animals (RMSE=0.050). Models obtained were independent of ruminant species, and presented low interfering factors on the residuals, least square means or individual slopes. The model equations highlighted the importance of considering the physiological state of animals when predicting splanchnic metabolism. This work showed that it is possible to use simple predictors to accurately predict the amount and nature of ketogenic nutrients released towards peripheral tissues in both sheep and cattle at different physiological status. These results provide deeper insight into biological processes and will contribute to the development of improved tools for dietary formulation.

Thirty-two Border Leicester ♂ × Scottish Blackface ♀ wether lambs, aged about 5 months, were divided into two groups on the basis of live weight, such that group G1 contained the 16 lightest lambs and group G2 the 16 heaviest. Lambs in group G1 were subdivided equally at random either to be sham-implanted controls (Group C1) or to be implanted with 35 mg trenbolone acetate (TBA) + 5 mg oestradiol-17β (OE) (group T1) at 24 kg initial live weight. The lambs in group G2 were also subdivided into two groups (groups C2 and T2), and similarly treated approximately 1 month later at 37 kg initial live weight. The lambs were offered ad libitum a diet containing an estimated 12·5 MJ metabolizable energy (ME) per kg dry matter (DM) and 140 g/kg DM crude protein. Comparisons were made for the main effects of hormonal treatment and initial live weight. Both hormonal treatment and initial live weight gave increases for DM intake, gut fill, empty body weight, carcass weight and, in the half carcass side weight, weights of dissected lean tissue, bone and intermuscular fat and chemically determined DM, crude protein and lipid. Weights of mm. semitendinosus, longissimus dorsi, supraspinatus and gastrocnemius were also increased due to hormonal treatment and in group G2 lambs compared with those in group G1. When expressed as a proportion of carcass side weight, hormonal treatment effects were not significant for individual muscles and dissected carcass lean, bone and fat and chemically determined lipid and ash. Variable effects on other body components were recorded for both treatments in the absence of any significant interactions. The responses to hormonal treatment were essentially similar in groups G1 and G2. The question is raised as to the contribution of the greater food intake in implanted lambs to the maintenance of fatness in these animals.

Food intake and body weight of 119 mice was measured from 3 to 18 weeks of age. Residual food intake was calculated for each week as the variation in food intake independent of variation in weight gain, weight maintained and sex. Growth efficiency and maintenance requirement were calculated by fitting curves to data from 3 to 18 weeks. The repeatability of residual food intake was low in young mice, but increased as they matured. Growth efficiency was correlated with residual food intake in very young mice. Residual food intake was not correlated with maintenance requirement in young mice, but as mice matured the correlation of residual food intake with maintenance requirement increased to 0·6. Body composition at maturity was correlated with residual food intake and maintenance requirement of mature mice, but a large proportion of the variation in residual food intake and maintenance requirement was independent of body composition. The results suggest that the age at which residual food intake is measured is important if it is to be used as a criterion for selection for efficiency.

Twenty Friesian (group 1) and 20 buffalo (group 2) calves and 14 of each of lactating Friesians (group 3) and lactating buffaloes (group 4) were maintained in middle of winter conditions of Egypt for 2 months. The calves (groups 1 and 2) were of similar age (6 to 7 months) and average body weight (120 kg). The lactating animals were in the fifth season of lactation, non-pregnant, in mid lactation and yielded 12 to 14 kg milk on average daily. Similar groups of animals of the same types (groups 5, 6, 7 and 8, respectively), numbers, live body weights and physiological status were maintained under hot summer conditions for 2 months. The effect of hyperthermia during summer season on Friesians and buffaloes compared with winter was also studied.

The results showed that buffaloes were more tolerant than Friesians to the environmental conditions of Egypt. In winter, buffalo calves had poorer (P < 0·05) food efficiencies (kg gain per MJ net energy), higher dry matter intakes (DMI, P < 0·05) and total lipids in the plasma (P < 0·01) than Friesians, while the contrary occurred in total proteins, albumin, phospholipids and cholesterol (P < 0·05). In summer, growth rate and the concentrations of total lipids, phospholipids and cholesterol in the plasma were higher (P < 0·01), and total proteins and albumin were lower (P< 0·01) in buffalo than in Friesian calves. In summer, in Friesians, both DMI and growth rate decreased (P < 0·05) and food efficiency as kg weight gain per MJ net energy was poorer (P < 0·05). In the plasma total proteins and albumin increased (P < 0·01) but haematocrit %, phospholipids and cholesterol decreased (P < 0·05). In buffalo calves, only total proteins and albumin increased (P < 0·01) due to heat stress.

In lactating buffaloes, food efficiency was poorer (P < 0·01) and DMI and plasma haematocrit % were significantly higher, while the concentrations of total proteins, globulin, phospholipids and cholesterol in the plasma, and milk yield were significantly lower than in lactating Friesians in the winter season. Lactating buffaloes were poorer (P < 0·01) in food efficiency and had lower concentrations of total proteins, globulin and cholesterol than lactating Friesians in the summer (P < 0·01).

Net energy efficiencies were calculated from data of an experimental herd with respect to type of cow, lactation number, stage of lactation and diet. The trial consisted of 71 Holstein-Friesians, 71 Jerseys and 71 Holstein-Jersey F1-crosses in 1st, 2nd and > 2nd lactation. Data were collected during 210 days of lactation, from calving to week 30 and included total dry matter intake, energy content of foods, milk yield, milk solids, body weight, body condition scores and several body measurements. The cows were divided into four feeding groups : high and low energy content of roughage as well as high and low proportion of concentrates. Net energy efficiency was calculated as the ratio of milk energy to total net energy intake after subtracting energy utilized for maintenance and body tissue change. Body tissue change was calculated either via body-weight changes or body condition-score changes. Due to the distribution of the efficiencies being skewed, efficiencies were transformed applying a Box-Cox transformation. Transformed net energy efficiencies were analysed using a repeated measurements design considering the sequential nature of the observations. Mixed models with a compound symmetry structure for the variance components were applied. Final models contained the fixed effects of type, lactation number, feeding group and the covariates week of lactation and its square. Holstein-Jersey crosses were more efficient than purebreds, second lactation cows were least efficient, cows given low energy roughage and a lower proportion of concentrates were more efficient than cows on the other diets. Least efficient were the cows belonging to the high energy roughage and higher proportion of concentrates group. The coefficients of determination of the final models were between 0·357 and 0·492.

Forty pregnant F1 crossbred dairy cows (20 Friesian × Boran and 20 Simmental × Boran) were stratified in a 2 × 2 diet × work factorial experiment (not working-not supplemented, NWNS; not working-supplemented, NWS; working-not supplemented, WNS; and working-supplemented, WS). Working cows pulled sledges 100 days/year (pull = 350 to 450 N, 4 h/day, 4 days/week). Work output of supplemented and non-supplemented cows was similar over 1 and 2 years. Over all 3 years, dry-matter intake relative to metabolic body size (g/kg M0.75 was greater for working, compared with non-working cows. Body-weight changes and body condition score were similar for working and non-working cows. Non-supplemented cows lost weight throughout the first 2-year period, while supplemented cows tended to maintain or gain body weight over 1 and 3 years. Over 2 years, supplementation of working cows proportionately reduced live-weight loss by 0.73 and doubled the number of conceptions and parturitions. Days in milk, milk, milk fat and protein yields were similar for working and non-working cows, but were greater for supplemented, compared with non-supplemented, cows. Total conceptions and calves born in all 3 years tended to be greater for supplemented and non-working compared with non-supplemented and working cows. A productivity index (PI) that took into account food intake was calculated. The PI for supplemented cows over 2 years was greater than that for non-supplemented cows. Meanwhile the PI was similar for working and non-working cows over all periods considered. A similar PI for working and non-working cows under supplementation indicates potential of on-farm adoption of a cow traction technology that includes improved food production and ng strategies.

Although the tambaqui (Colossoma macropomum) is the most cultivated native fish species in Brazil, estimated breeding values for growth traits are rarely used for selection of superior individuals in commercial fingerling production. This study aimed to estimate the (co)variance components of growth traits. Body weight, length and width of 2500 tambaqui were determined at tagging and at 6 and 12 months after tagging in a commercial breeding programme in Brazil. Heritability estimates were low for traits measured at tagging (0.10 to 0.19) and moderate to high for traits measured at 6 and 12 months (0.23 to 0.81). Common full-sib effects were high at tagging (>73%), low at 6 months and negligible at 12 months. Positive genetic correlations were found among growth traits at 12 months (0.84 to 0.99) and between growth traits at 6 and 12 months (0.80 to 0.92). These results show that animal selection can be performed at 6 months after tagging. Expected genetic gains for growth traits ranged from 8% to 31%. A simulation of the sex ratio was performed, as individuals did not reach sexual maturity during the experimental period. Because of the sexual dimorphism, more accurate heritability estimates were obtained when considering the female proportion to be 90% in the high-weight group. The findings indicate that it is possible to obtain considerable genetic gains in growth by selecting for growth traits. The development of a tool to determine the sex of animals at early stages can improve the response to selection in tambaqui.