Genetics of changes in economic traits during the laying year

Body weights, egg weights and egg numbers were measured at different ages in two distinct, fully pedigreed, random bred strains of White Leghorns over a period of 3 years. The heritabilities of the separate measurements and the genetic correlations between them were obtained by an analysis of variance and covariance. The principal findings were:

(i) The heritability of weight gain declined sharply with age over consecutive periods after 12 weeks. The genetic correlation of body weight at 12 weeks with gain in subsequent periods was low. There appeared to be sufficient genetic variance in growth rate in these later periods to alter the growth curve substantially by selection. For example, it should be possible to breed strains with a rapid early growth and low mature body size.

(ii) There was evidence of genetic variation in the change of egg weight with time. The genetic variation was, however, so low that in any selection programme to improve 28‐week egg weight without changing 44‐week egg weight the progress would be very slow.

(iii) Body weight at 12 weeks was genetically negatively correlated with sexual maturity. Its correlation with production in the first 44 weeks was positive in both strains, but not significantly different from zero.

(iv) Body weights after 28 weeks were apparently uncorrelated with egg production or sexual maturity.

(v) Egg weight and production traits show considerable heterogeneity of variance between years, probably partly due to their distribution not being normal.

(vi) The pattern of genetic variation appeared to differ significantly between the two strains so that correlated responses would not necessarily be the same.

(vii) There was a suggestion of maternal effects acting upon body weight but the evidence was somewhat contradictory and inconclusive due to the high standard error of the estimates.

(viii) None of the 15 traits measured regressed over the 3 year period of relaxation of selection.     

Genetics of hatchability and its components and some production traits of chickens 1

Twenty‐egg samples from individual pullets belonging to four closed pedigreed populations were incubated to obtain hatchability and embryo mortality data. Egg production, egg weight and body weight data were obtained from the same pullets and comprehensive variance and covariance analyses were performed on the entire body of data. A slight but consistent genetic relationship was noted between hatch‐ability and egg production, mainly due to the influence of embryo viability during the final 12 d of incubation.

Limited evidence existed suggesting negative genetic correlations between egg size and hatchability. It is suggested that this relationship may be of the curvilinear type and this is substantiated by some data from a population selected for increased egg size.

The results are discussed from the viewpoint of their possible significance in practical breeding programmes.     

Long‐term selection for increased egg production 1

1. Results are presented of a closed White Leghorn flock selected for increased egg production over a period of 17 years.

2. During the last 7 years a random breeding sample of the production flock served as a genetic control.

3. Regressions of deviations of the production flock from the control flock on years, were highly significant for age at sexual maturity (‐2.18 d), egg production up to 275 d old (3.04 eggs), egg production to 500 d old (5.04 eggs) and egg weight at 14 months old (October) ( — 0.79 g).

4. The heritability of egg production up to 275 d old declined significantly from 0.35 to 0.19.

5. The realised heritability of egg production up to 275 d old was estimated to be 0.14.

6. The genetic correlation between egg production up to 275 d old and production in the residual part of the year was 0.48.

7. The rate of inbreeding in the production flock was 0.515% per generation and the expected rate of inbreeding in the control flock 0.21% per generation.     

Sire x location interactions from progeny tests in different countries

1. A total of 529 progeny arising from 19 sires, which had been selected on the basis of their daughters’ body weight (heavy, medium or light) were reared in Berlin, West Germany and Kuala Lumpur, Malaysia to test for sire x location interactions for production and egg composition traits.

2. Highly significant sire x location interactions were found for sexual maturity, egg production, egg weight, food consumption and egg mass.

3. For egg composition traits, significant interactions were found in some but not all periods for shell breaking strength and albumen : yolk ratio.

4. Highly significant location effects were found for all traits studied except sexual maturity, whereas significant sire effects were found mainly for the productive traits, shell breaking strength and albumen : yolk ratio.     

Genetic parameters in two populations of chickens under reciprocal recurrent selection

Genetic information on two traits, age at sexual maturity and survivors’ per cent production, was obtained from two strains of the White Leghorn breed under a reciprocal recurrent selection scheme where the reciprocal crosses were raised at three locations representing diversified environments.

Genotype by environment interactions were detected by a number of methods; (1) from analyses of variance of sire means within years and over locations where sires, locations, and sire by location interactions were important sources of variation; (2) from genetic correlations of the same trait at different locations; (3) from comparisons of the components that constitute the genetic correlations between the two traits at the same versus different locations.

The sire by location interaction variance components when taken as a percentage of the total variation, although not large, 10 per cent for sexual maturity and 8 per cent for egg production, were found to be significant sources of variation.

The genetic correlations between the age at sexual maturity and per cent production had a negative relationship when studied at the same or at different locations. Positive relationships, with one exception, of small magnitude were observed from genetic correlations of the sire's purebred and crossbred progeny.     

Changes in fitness traits associated with selection for divergence in yolk cholesterol concentration

1. Correlated changes in egg production, hatchability, egg weight, age at sexual maturation and body weight of lines selected for divergence in yolk cholesterol were studied in two different populations.

2. Divergence in yolk cholesterol resulted in a highly significant difference in percentage egg production with a negative correlation between yolk cholesterol and egg numbers. However, the calculated total daily mass of cholesterol excreted into the egg was similar in both lines.

3. These results indicate that the physiological mechanism by which these populations responded to selection pressure on the concentration of cholesterol in the yolk was by changing egg numbers rather than total amount of cholesterol excreted.

4. Body weights of the high and low yolk cholesterol lines of the meat type, but not of the layer‐type populations, were significantly different. After four generations the line selected for low yolk cholesterol from the meat‐type population weighed 80% of the line selected for high yolk cholesterol levels.

5. Sexual maturity and egg weight were also significantly different between the high and low yolk cholesterol lines, but these differences could not be related to selection response. No changes in hatchability between the cholesterol lines were observed.     

Genetic parameters of cross‐ and purebred poultry

Heritabilities were estimated from sire and dam components for egg production traits in four generations of two pure‐lines and their reciprocal crosses. Heritabilities in cross‐breds, as estimated from the sire components of variance, were larger than in pure‐breds for production and sexual maturity. Heritabilities estimated from dam components were larger than the sire estimates, thus indicating the presence of maternal and/or dominance effects. Again, heritabilities in cross‐breds were larger than in pure‐breds for production and sexual maturity as well as for specific gravity. Heritabilities were also estimated for disease traits in material from one generation. Cross‐line values were as large or larger than those estimated from pure‐lines but the standard errors of the estimates were quite large.

Genetic correlations were estimated between pure‐line and cross‐line performance using both sire and dam covariances. These approached unity for body weight, egg quality traits and adult mortality but were considerably less than unity for production, sexual maturity, blood spots and mortality during growth and mortality due to Marek's disease.

The possible bias due to genotype x environment interaction is discussed and it is concluded that such a bias, if present, is small, and that in spite of possible G × E interaction effect the genetic correlations between pure‐line and cross‐line performance for production and sexual maturity are less than one. The same holds true of mortality during growth and to Marek's disease although these estimates are much less precise.

The genetic correlation between pure‐breds and cross‐breds appears to decrease over the span of the five generations.

Use of cross‐line performance will permit more efficient selection for traits when the genetic correlations are much less than one.     

The economic value of restricted and full‐feeding during confinement rearing on two‐year egg production1

A pure strain of S.C. White Leghorns was restricted‐fed from 22 to 147 days of age. They were then placed on full‐feeding and put through a first year production test, force moulted and kept through a second year production test.

The food restriction reduced body size, delayed sexual maturity, reduced mortality, reduced food consumption, increased egg production and increased efficiency of egg production. The delay in sexual maturity associated with a greater increase in body weight in the laying‐house tended to reduce the advantageous effects of the biological response during the first production year. The absence of delayed sexual maturity and need for weight gain during the second production year revealed the full effect of the biological response in the restricted group.

The restricted birds had an increased economic return of 44 cents per bird in the first production year and 89 cents per bird in the second production year compared with full‐fed birds.     

Egg production in Turkeys

Full‐ and part‐time records of production were examined in order that the most efficient selection procedure could be established.

Body weight at different ages or changes in weight between these ages has little effect on egg production. Phenotypic correlations are of the order of — 0.1 and genetic correlations, generally insignificant, appear to be slightly negative. It is concluded that high body weight per se is not necessarily incompatible with high egg number and it should be possible to improve both traits by selection.

Genetic variance in sexual maturity is greatly reduced if not eliminated when expression of the trait is delayed by means of light control. Under these circumstances genetic variance in egg number in the first 6–12 weeks of production appears to be much reduced.

Egg production is adversely affected if commenced below a minimum age which, in the case of this strain, appears to be about 32 to 33 weeks.

The first 6 weeks of egg production is not significantly genetically correlated with total production. The first 12 weeks of production is more satisfactory in this respect.

A form of selection based on part record at 12 weeks followed by retrospective culling at 18 weeks is suggested which combines the advantage of extended, hence accurate, measurement and overcomes most of the difficulties of the 2‐year cycle. The scheme carries the disadvantage of a 57‐week cycle which may occasionally enforce reversion to a 2‐year cycle because of the difiìiculties associated with later summer production.     

Food consumption records for the genetic improvement of income over food costs in laying flocks of white leghorns

1. A study of genetic parameters of egg mass based on the weights of all eggs laid, individual food consumption, body weight and age at first egg was made. Data from three populations differing in average egg weight were available over two production periods, from 20 to 40 and from 40 to 60 weeks of age, respectively.

2. Selection indexes designed to maximise income over food costs (IOFC) were used to evaluate the usefulness of food consumption data for the genetic improvement of laying flocks.

3. Given reliable estimates correlating food consumption to egg mass, body weights and age at first egg it is possible to construct highly efficient selection indexes for genetic improvement of IOFC without the direct use of food consumption data.

4. Selection for genetic gains in egg mass appears to be the most important factor for genetic improvement of IOFC. Correlated increases in egg weight were observed for two of the populations.

5. Body weights showed very small and inconsistent genetic responses under index selection suggesting that a reduction in body weight need not be necessary for more efficient egg production.

6. Indexes which showed greatest genetic improvement also resulted in an increase of food consumption. Thus genetic efficiency in egg laying flocks would be achieved by increasing egg mass through earlier sexual maturity, larger eggs and higher production rates with increased food consumption and only slight, if any, reduction in body size.

7. Estimates of heritabilities and genetic correlations indicate that food consumption measurements can be based on partial records; high heritabilities were found for egg mass and food consumption measurements in both early (20 to 40 weeks) as well as late (40 to 60 weeks) production periods.     

The use of selection and strain crossing to reduce egg shell breakage

1. Selection for low egg shell deformation and high egg production was carried out in two White Leghorn strains, M and H, in order to test the feasibility of reducing the incidence of egg breakage without loss in egg production.

2. Selected and control lines were maintained within each strain.

3. After three generations of selection, egg shell deformation was reduced in the M selected line as compared to its control counterpart but this change in deformation was not accompanied by a reduction in egg breakage; however, no reduction in deformation or egg breakage was achieved in the H selected line.

4. Crossing the M and H strains resulted in a reduction of egg breakage in the crosses as compared to the average breakage of the parental strains, suggesting the potential of utilising strain crosses to reduce the incidence of egg shell breakage.     

Biochemical,physiological and production indices related to fat metabolism in the laying fowl at various stages of physiological development

1. The significance of fat accumulation during physiological development of layers was studied using White Leghorn x Australorp pullets from 12 weeks of age.

2. Liver fat, carcass fat, liver weight and body weight increased markedly at the onset of sexual maturity.

3. Within the liver, the protein to DNA ratio did not alter significantly during development; glycogen was not consistently depleted by egg production.

4. Liver fat content was significantly correlated with carcass fat, liver weight and DNA concentration, and plasma total lipid and cholesterol, independent of stage of development; with body weight, liver protein, moisture, and protein to DNA ratio, excluding the period prior to commencement of sexual maturation; and with plasma phospholipid during lay.

5. Obesity was a feature of faster growing fowls which matured earlier, consumed more, utilised food less efficiently for egg production and produced fewer saleable eggs.     

Effects of restricted feeding in the growing and laying periods on the performance of White Leghorn by Australorp crossbred and White Leghorn strain cross chickens

1. The effects of restricted feeding in the rearing and laying periods on the performance of three White Leghorn cockerel by Australorp pullet crossbred strains and one White Leghorn strain cross were studied in two experiments.

2. After feeding ad libitum in the rearing period, mean body weights of the three crossbred strains at 20 weeks of age were 1.75, 1.66 and 1.55 kg and that of the White Leghorn strain cross was 140 kg. The differing restrictions during rearing reduced mean body weight at 20 weeks by 14 to 33% and increased the time to sexual maturity by 8 to 23 d.

3. All restriction regimes during rearing increased mean egg weight and tended to reduce mortality in the laying period. Optimum hen‐day egg number over 64 weeks of lay was obtained following mild restriction (14% body weight reduction) of the two heavier strains or ad libitum feeding of the lighter strains. Over 48 or 64 weeks of lay, egg number per hen housed (at 18 to 20 weeks of age) was optimised after moderately restricting (14 to 21% body weight reduction) the crossbred strains or feeding the White Leghorn strain cross ad libitum.

4. Over 64 weeks of lay, mortality was reduced from 19.3% to 10.5% in the lightest crossbred strain, in which a high proportion of deaths were associated with Marek's disease and lymphoid leucosis, by restriction during rearing.

5. Restricting food intake by 7 or 8% throughout the laying period reduced hen‐day and hen‐housed egg number and mean egg weight of all four strains. The reduction in hen‐day egg number associated with food restriction during lay was less for the heavier strains that had also been restricted during rearing.

6. Metabolisable energy intakes required to support maximum production over 64 weeks of lay were 1.23 to 1.36 MJ/d.     

Normality of egg production in poultry

1. The distribution of survivor egg production has been examined in four control strains of White Leghorn egg laying chickens, a strain of turkeys and a strain of ducks.

2. The most pronounced trend was a persistent negative skewness (an excess of poor producers) associated with a positive kurtosis (an excess of individuals about the mean) and a marked deficiency of high producers.

3. Truncation of these populations at two or one phenotypic standard deviation below the mean tended to reduce the asymmetry but at the expense of the kurtosis which frequently became significantly negative.

4. The greatest departures from normality were observed in the earlier parts of the recording period.

5. A considerable degree of heterogeneity of variance among hatches within strain/years was observed and it is considered that this is largely due to the asymmetry.

6. Negative skewness in the distribution of egg production appears to be a widespread phenomenon and is considered to be due more likely to non‐random environmental factors than fixation of recessive genes.

7. Predictions of progress based on the normal curve must tend to overestimate what is realised and the abnormality observed, which appears to be general, must have negative implications for the accuracy of estimation of biological parameters.     

Genetic analyses of partial egg production in Japanese quail using multi-trait random regression models

1. The aim of the present study was to estimate genetic parameters for average egg weight (EW) and egg number (EN) at different ages in Japanese quail using multi-trait random regression (MTRR) models.

2. A total of 8534 records from 900 quail, hatched between 2014 and 2015, were used in the study. Average weekly egg weights and egg numbers were measured from second until sixth week of egg production.

3. Nine random regression models were compared to identify the best order of the Legendre polynomials (LP). The most optimal model was identified by the Bayesian Information Criterion. A model with second order of LP for fixed effects, second order of LP for additive genetic effects and third order of LP for permanent environmental effects (MTRR23) was found to be the best.

4. According to the MTRR23 model, direct heritability for EW increased from 0.26 in the second week to 0.53 in the sixth week of egg production, whereas the ratio of permanent environment to phenotypic variance decreased from 0.48 to 0.1. Direct heritability for EN was low, whereas the ratio of permanent environment to phenotypic variance decreased from 0.57 to 0.15 during the production period.

5. For each trait, estimated genetic correlations among weeks of egg production were high (from 0.85 to 0.98). Genetic correlations between EW and EN were low and negative for the first two weeks, but they were low and positive for the rest of the egg production period.

6. In conclusion, random regression models can be used effectively for analysing egg production traits in Japanese quail. Response to selection for increased egg weight would be higher at older ages because of its higher heritability and such a breeding program would have no negative genetic impact on egg production.     

Responses of modern hybrid laying stocks to changes in photoperiod

Two replicates of each of 4 light patterns were imposed on 4 breeds of laying hen. The light patterns were designed to induce sexual maturity, defined as 50% production, at a range of ages. Within‐room treatments were ad libitum or rationed feeding during rearing and 4 breeds, namely ISA Brown, Hisex Brown, Ross G‐Link Brown and Shaver Brown. A total of 6144 birds were used to test the 32 treatments.

2. Age at 50% production was significantly affected by both lighting treatment and rearing regimen.

3. The light pattern treatments had no significant effect on any cumulative performance variable measured from 20 to 80 weeks of age.

4. Breed had a significant effect on eggs/hen housed, rate of lay, egg weight, egg output and food intake.

5. Rationing feeding during rearing, by holding intake at its 6‐week level from 6 to 14 weeks of age, was associated with significantly lower egg production, but had no effect on egg weight or food intake during lay.

6. Although both rearing and light treatments influenced age at 50% production, this character was not correlated with egg weight or egg production.

7. It is suggested that modern hybrid laying stocks are so genetically predisposed to ovulate that they are becoming refractory towards treatments hitherto regarded as influential.     

Effects of timing and size of daylength change on brown egg laying domestic hens: Plasma luteinising hormone concentration and sexual maturity

1. Brown egg laying pullets were transferred from an 8‐h photoperiod to an 8‐, 10‐, 13‐ or 16‐h photoperiod at 6, 9, 12, 15, 18 or 20–3 weeks of age. Plasma luteinising hormone (LH) concentrations were measured at transfer and 7 and 14 d afterwards.

2. Significant increases in plasma LH occurred following light stimulations at 6, 9 and 12 weeks of age.

3. Changes in LH concentration 7 d after a light increase from 8 h to 8, 10, 13, 16 h were highly correlated with photoperiod length at 9 and 12 weeks of age.

4. Changes in LH were generally poorly correlated with age at sexual maturity, although the reduced influence on age at first egg of a light increase given close to sexual maturity was reflected in minimal LH responses at 18 and 20.3 weeks.     

On estimating components of genetic variance in diallel matings 1

1. The relative importance of additive and non‐additive genetic effects on body weight, egg weight, maturity and rate of egg production were studied from diallel matings in a Leghorn population.

2. From analyses of variance, heritability estimates of the additive fractions, based on half‐sib variances, and the non‐additive or dominance fractions, based on the sire x dam interaction component were obtained.

3. Non‐additive genetic effects were not statistically significant for any of the traits, though for rate of egg production at 32 and 62 weeks, the non‐additive effects as proportion of total variances were 0.29 (P<0.10) and 0.20 (P<0.16), respectively, compared additive effects of 0.08 (NS) and 0.11 (P<0.05).

4. The ratios of non‐additive to additive variances, 1.89 and 3.62 respectively, give support to inbreeding and hybridisation or reciprocal recurrent selection as methods of genetic improvement of egg production.     

Responses of immature pullets to repeated cycles of gradual increases and abrupt decreases in photoperiod

1. Growing pullets were reared on constant 8, 11 or 14 h photoperiods or given 12 daily increments of 30 min followed by an abrupt 6 h decrease in photoperiod in 14 d cycles from 2 d of age to sexual maturity.

2. Birds on the experimental lighting programme matured earlier than constant 8‐h controls, later than 11‐h controls but at the same age and body weight as constant 14‐h controls.

3. Weight of the first egg was correlated with age at first egg.

4. It is assumed that potential advances in maturity for the experimental birds from the 30 min increments in photoperiod were cancelled by the retarding influences of 6 h decreases in photoperiod, resulting in their maturity being similar to that of birds reared on a constant daylength equal to the longest photoperiod reached during the cycle.     

A diallel crossing experiment with two breeds of laying fowl

Light Sussex and Brown Leghorn flocks were concurrently bred pure and crossed in both directions for 6 years in an experiment to test for genetic interactions (“nicking”) between individual sires and dams. The traits measured in survivors to 450 days of age were (i) egg production, 126–250 days, (ii) egg production, 250–450 days, (iii) egg production, 126–450 days, (iv) mature egg weight, (v) age at sexual maturity and (vi) pullet weight at 18 weeks.

There was little response to selection in the primary trait, egg production to 450 days. Crossbreds showed a degree of heterosis in averages of egg production and there were consistent reciprocal differences between crossbreds in egg weight and pullet weight. It was impossible to distinguish between sex linkage and maternal effects in the determination of these differences.

Analyses of variance were complicated by the breed difference and by heterogeneity of variances, but yielded little evidence of interaction in egg production either between sires and breed of dams or between dams and sires. Some significant interactions were noted in egg weight and pullet weight. A weighted covariance technique yielded high positive correlations between purebred and crossbred sire family means, and confirmed that genetic interactions were unimportant. Thus reciprocal recurrent selection would not have been advantageous for improving egg production of crossbreds from the two flocks.