I. McMillan

Determination of amino acid requirements of young pigs using an indicator amino acid.

Mixtures of skim milk and free amino acids were compared as diets for pigs which would allow manipulation of dietary amino acid levels. Piglets gained 208 g/d between 3 and 14 d of age on the skim-milk diet, but replacement of 600 g/kg of the dietary nitrogen with free amino acids reduced growth rate to 148 g/d. Supplementation of a lysine-deficient diet with lysine reduced the catabolism of [14C]phenylalanine showing that phenylalanine catabolism could be used as an indicator of the adequacy of diet with respect to another essential amino acid. The dietary level of phenylalanine which would provide an excess for catabolism by the piglet was estimated directly by measuring the influence of dietary phenylalanine level on [14C]phenylalanine oxidation. Reduction of the dietary phenylalanine level below 7 g/kg had no effect on phenylalanine oxidation, whereas increasing the dietary phenylalanine level above 7 g/kg resulted in a linear increase in phenylalanine oxidation. An indirect estimate of histidine requirement was made by examining the influence of dietary histidine level on [14C]phenylalanine oxidation. In diets containing more than 4 g histidine/kg, phenylalanine oxidation was minimal. In diets containing less than 4 g histidine/kg, [14C]phenylalanine oxidation increased as the level of dietary histidine was reduced. This showed that the utilization of the essential amino acid phenylalanine, for protein synthesis, was not limited by histidine supply in diets containing more than 4 g histidine/kg. A direct estimate of histidine requirement was made by examining the influence of dietary histidine level on [14C]histidine oxidation. Diets with more than 4 g histidine/kg contained an excess which was catabolized: there was a linear increase in histidine oxidation in response to dietary histidine levels greater than 4 g/kg. This confirmed the previous indirect estimate of histidine requirement.

Estimation of additive and dominance genetic variances for body weight at harvest in rainbow trout, Oncorhynchus mykiss

Additive genetic, dominance genetic, and common environmental variance components were estimated for body weight at harvest for three populations of rainbow trout under selection for six generations in Norway. Six different models were studied by including all or different subsets of the following effects in an animal model: additive genetic (A), parental dominance (D), common environmental due to full-sibs (CE) and inbreeding coefficient (F) as a covariate. Variance components were estimated using the Average Information Restricted Maximum Likelihood (AIREML) method. In general, the results showed that estimates of additive genetic variance were inflated under the simple models (A) and (A + F), and decreased remarkably under the more complex models such as (A + CE), (A + CE + F), (A + D + F), and (A + D + CE + F). The magnitude of the dominance variance ranged from 0 to 22% and the common environmental variance from 0% to 6% of the total phenotypic variance. The study confirmed the presence of a significant dominance genetic variance for harvest body weight in rainbow trout. However, the results indicate that the dominance and common environmental effects may be confounded in the present data.

Effect of inbreeding on body weight at harvest in rainbow trout, Oncorhynchus mykiss

Abstract Three nucleus breeding populations of rainbow trout (Oncorhynchus mykiss) under selection for six generations were investigated for the effect of inbreeding on body weight at harvest. The coefficients of inbreeding for each individual were estimated from complete pedigree information starting at base generation zero. For each population within generation, inbreeding effect was estimated using a fixed linear model and a sire–dam mixed additive genetic model. Estimates of inbreeding effect per generation vary due to the different distributions of inbreeding coefficients in each generation. The magnitude of inbreeding effect for each population across generations were estimated using a fixed linear model and three sire–dam mixed models: additive (A), additive plus dominance (A+D), and additive plus dominance plus additive by additive epistasis (A+D+A×A) with the inbreeding coefficient, F as covariate. The estimates of inbreeding effect from the fixed linear model were substantially lower than those from the sire–dam mixed models for all populations. The models (A+D) and (A+D+A×A) exhibited quite similar estimates of inbreeding effect, which were substantially higher than the (A) model. The results suggest that addition of the additive by additive genetic effect in the (A+D) model does not have a significant outcome on the estimation of inbreeding effect because additive by additive genetic interaction may not be important for growth. For unbiased estimation of inbreeding effect, the (A+D) model with F as a covariate may be sufficient. The inbreeding effect on body weight at harvest calculated as the average of the estimates obtained from models A+D and A+D+A×A, were −1.6, −5.0, and –4.5% per 10% unit increase in inbreeding coefficients for populations 1, 2, and 3, respectively. These values meant body weight at harvest was moderately affected by inbreeding but were not high enough to cause any serious impact in the selective breeding program for rainbow trout in Norway. This implies that maintenance of moderate effective population size per generation and following a strict mating policy for breeders selected as parents for future generations could delay the accumulation of inbreeding and minimize its effect on growth.

Family differences in relative growth of diploid and triploid Atlantic salmon (Salmo salar L.)

Comparisons of growth for diploid and triploid full sibs from 20 families of Atlantic salmon (Salmo salar L.), ranging from 4 to 24 progeny/family, involved 628 individuals. Length and weight data, after approximately 75 weeks in seawater, indicated higher within and between family variances in triploids than in diploids. Regression analyses revealed that improvement of triploid populations might require measurements on triploid sibs of the diploid to be involved in reproduction.

Inbreeding levels in selected populations of rainbow trout, Oncorhynchus mykiss

Abstract Inbreeding levels in three nucleus breeding populations of rainbow trout ( Oncorhynchus mykiss ) under selection for six generations were investigated. The inbreeding levels across generations as estimated from pedigree information ( P i ) and effective population size ( N e ) with the assumption of no selection and random mating, among the three populations were all at 11.3% or less. The levels of inbreeding estimated from N e were generally lower than those calculated from P i , except in the initial generations. The average rates of inbreeding (Δ F ) calculated from N e and P i were 0.99% and 2.00%, respectively, for population 1; 0.90% and 0.53%, for population 2; and 0.72% and 1.38%, for population 3. These rates of inbreeding correspond to N e of 50 and 25, for population 1; 55 and 94, for population 2; and 70 and 36, for population 3. These rates were within the acceptable range 2% to 0.2%, corresponding to N e of 31 to 250 for avoiding loss of fitness as suggested by Meuwissen and Woolliams [Meuwissen, T.H.E., Woolliams, J.A., 1994. Effective sizes of livestock populations to prevent a decline in fitness. Theor. Appl. Genet. 89, 1019–1026.]. A large proportion of individuals had inbreeding coefficients ( F ) greater than zero across generations for each population but the individual F were generally below 12.5%. The occurrence of inbreeding can be detrimental to populations undergoing artificial selection. Therefore, the F levels reported here should be followed-up by a study on the impact of inbreeding on important performance traits in rainbow trout.

Comparison of three mathematical models of egg production

1. Egg production of four‐way crosses of seven commercial stocks of White Leghorns was summarised for each hen into 28‐d periods starting from the day of first egg. 2. The exponential models of McMillan and of Wood and a linear regression were fitted to the results for individual hens, as well as to the mean results for periods of groups of hens. 3. Overall, the McMillan model gave the best results with mean R2 of 0.71 from fitting it to the results for 223 individuals with sixteen 28‐d periods each and R2 of 0.97 from fitting it to the period means for groups. The respective R2 values were 0.68 and 0.95 for the Wood model and 0.60 and 0.90 for the linear regression.

Heritability of sudden death syndrome and its associated correlations to ascites and body weight in broilers

1. Genetic parameters for the sudden death syndrome (SDS) were estimated in meat-type chickens. Data were collected over 11 generations of selection for body weight within two distinct breeds (Cornish and White Rock). 2. The animal model was used exclusively with linear methods (LM) to estimate genetic parameters. Heritability (h 2) of SDS on the liability scale was 0·30 ± 0·002 and 0·25 ± 0·002 in the Cornish and White Rock breeds, respectively. 3. A positive genetic correlation (r g) with ascites (AS) was determined (∼0·3 ± 0·006). However, it was not possible to estimate the r g of SDS with body weight because of the low prevalence of the defect trait studied (1·8% in the Cornish and 1·5% in the White Rock). 4. Heritability of SDS calculated using male records only was 0·45 ± 0·009 and 0·35 ± 0·009, and r g with body weight was 0·30 ± 0·010 and 0·27 ± 0·009, in the Cornish and White Rock breeds, respectively. 5. In conclusion, the heart defect investigated was heritable with a positive genetic correlation with AS and body weight.

Growth and mortality of diploid and triploid tiger trout (Salmo trutta ×Salvelinus fontinalis)

Abstract Size, growth rate and fry mortality were compared for diploids and triploids of three genetic types, brook trout and the two reciprocal tiger trout hybrids (Salmo trutta × Salvelinus fontinalis). Fry mortality was higher for triploids of all three genetic types compared with the corresponding diploids. Weights and fork lengths were recorded for three year classes at various times from 18 weeks to 38 months after fertilization. Triploids were approximately equal to diploids in size and growth rates at most ages for each genetic type, although triploid tiger trout tended to be slightly larger than their diploid sibs. Total biomass expected from 1000 fertilized eggs was higher for diploids compared with triploids for brook trout and the cross of brown trout females × brook trout males, but the reverse was true for the reciprocal cross. Overall, the brown trout female × brook trout male cross was superior to the reciprocal cross. Production of diploid tiger trout using brown females and brook males is worthwhile when sterile fish are desired.

The effect of simultaneous selection on the genetic correlation

The theoretical effect of simultaneous selection on the genetic correlations between two traits over 20 generations was examined using simulation. For each generation, a population of 50 male and 50 female diploid gen otypes with 15 loci, each with two alleles, was synthesized. None of the loci exhibited dominance. Five loci affected only trait 1, 5 loci only trait 2 and 5 were pleiotropic (affected both traits). Initial allelic frequencies were equal at each locus. Phenotypes were created by adding a random normal deviation for each trait to the genotype. The size of this deviation for each trait determined its heritability (h2). Index selection with h2 combinations of (0.15, 0.15), (0.15,0.45) and (0.45,0.45) and relative economic weights of (1, 1) and (1, 3) for each h2 combination was employed. In each generation, the highest ranking 25 genotypes of each sex were used to generate the next generation with single-pair matings, each producing two male and two female offspring. One hundred replicates were run for both negative and positive correlations. With a positive initial value, the genetic correlation tended to decline (toward zero). The rates of change were moderately affected by index weights and h2. With a negative initial value, the genetic correlation tended to decrease (towards -1). However, unequal heritabilities and unequal relative economic weights slowed the rate of change with the greatest imbalances tending to hold the correlation constant or move it toward zero. These simulations illustrate that changes in parameters over time can affect the selection practiced. Under some of the conditions simulated, the use of initial genetic parameter values without change could have potentially negative effects on overall genetic gain.

Early mortality of tiger trout (Salvelinus fontinalis × Salmo trutta) and the effects of triploidy

Abstract Uneyed-egg, eyed-egg and alevin mortalities were compared for 750 diploid families of brook trout ( Salvelinus fontinalis ), brown trout ( Salmo trutta ) and their reciprocal hybrids, known as tiger trout, produced over 4 years. The reciprocal tiger trout crosses experienced higher overall mortality than either pure species. The brook female × brown male cross experienced high mortality at all three stages examined, while the reciprocal cross experienced heavy losses only at the alevin stage, resulting in higher overall survival rates. Within the brook female × brown male cross, females were the most important source of variation for all types of mortality, but brown males were also an important source of variation for uneyed-egg mortality. In the reciprocal cross, male and female parents were both important sources of variation for alevin mortality, the major source of early mortality in this cross. For brook trout female parents, hybrid offspring mortality was not significantly correlated with mortality among pure brook trout offspring. Similarly, mortality of hybrid offspring of brown trout dams was not correlated with mortality of brown trout offspring except at the eyed-egg stage, a rather unimportant source of mortality in this cross. The influence of triploid induction on survival rates among brook trout and tiger trout families was studied by comparing diploid and triploid full sibs over 87 pairs of families. In general, triploid induction improved the survival rates of tiger trout at the alevin stage, but reduced survival at the uneyed-egg stage, so that overall survival was not greatly improved. Thus, early survival of tiger trout can be maximized by the use of the brown female × brook male cross, and by selection of the females and the brook males based on the survival rates among their hybrid offspring. The usefulness of triploid induction in the production of tiger trout will depend on modification of the induction techniques so that mortality from the induction is minimized.