Comparative performance of two commercial egg strains, the indigenous chickens and their random bred progenies

C C Ogbu, I Udeh* and P C Nwakpu**

Department of Animal Science, University of Nigeria, Nsukka
coschi07@yahoo.com
* Department of Animal Science, Delta State University, Asaba Campus
** Department of Animal Science, Ebonyi State University, Abakaliki

Abstract

The performance of two commercial egg type chickens, the indigenous chickens, and their random bred progenies in body weight (BWT), body weight gain (BWG), feed conversion ratio (FCR), body weight at first egg (BWFE), weight of first egg (WTFE) and age at sexual maturity (ASM) were compared. The objectives were to evaluate the effect of within strain mating on the performance traits and to compare the cost of raising parents and progenies to point of lay.

Results indicate significant (p ≤ 0.05) reduction in all the traits studied in the progenies. Percentages of inbreeding depression (ID %) were significant (p ≤ 0.05) for BWG1 in strains 1, 2 and 3 at tcal 3.57, 9.37 and 10.00, respectively; BWG2 in strains 1 and 3 at tcal 3.42 and 10.44, respectively; BWG3 in strains 1 and 2 at tcal 12.80 and 16.46, respectively; FCR1 and  FCR2  at tcal -3.89 and -5.50, respectively in strain 1, -7.41 and -7.68, respectively in strain 2, and -4.93 and -9.12, respectively in strain 3. Ttab, 0.95 for error df6 for all values was 2.447. These changes resulted in decreases in feed efficiency, reduced BWFE and WTFE, later age at sexual maturity and increase in cost of production to point of lay of the progenies. There was greater loss of performance in progenies of the commercial hybrids than for those of the local strain. Therefore, exotic commercial hybrid chickens should not be used as breeders for the production of replacement day-old chicks for commercial egg production.

Key words: Additive gene effect, dominance, epistasis, heterozygosity, hybrid, inbreeding depression

Introduction

Commercial table egg production in Nigeria is hampered by a number of factors which include high cost of importing parent stocks and seasonal scarcity of replacement hybrid day old chicks. Day old chicks become so scarce and costly at peak seasons that desperate poultry farmers/ ‘hatcheries’ resort to the use of commercial hybrid stocks as breeders to generate day – old chicks. 

 Hybrids are products of crosses between/among highly inbred lines/strains or genetically distinct populations or races (Gregory et al 1992). Most commercial hybrids are terminal crosses of parental lines (male and female lines) synthesised from multiple breeds or specialised grand parent lines (Gura 2007). Hybrids are therefore, heterozygous at all loci for which the constituent lines are homozygous for different alleles. Such materials exhibit improved performance in productive/fitness traits such as growth, egg production, fertility and hatchability as a result of complimentarity between composite lines, as well as heterozygosity at most loci (Deeb and Lamont 2002).  The superior performance by hybrids compared to the mean of the contemporarily reared parental lines (heterosis) makes them economically viable for commercial production. Thus hybrids are widely adopted for the production of animal products (meat, milk, egg, fibre, wool, etc). However, most commercial hybrids are disposable or terminal materials since a substantial proportion of their superiority is lost on further hybridization (a phenomenon known as hybrid breakdown). For this reason, progenies of commercial hybrids are not as good as their parents and replacement stock must be produced each time from parental lines (pure stocks) which are proprietarily controlled (Deeb and Lamont 2002; Gura 2007).  

Heterosis results from an increase in heterozygosity which is essentially a recovery from accumulated inbreeding depression in the composite lines (Gregory et al 1991; Arthur et al 1999; Fries et al 2000; Roso et al 2005). Heterosis results from non linear genetic (non-additive or interaction) effects (dominance, overdominance, epistasis etc) (Abou EL-Ghar and Abdou 2004; Carlborg et al 2004; Hill et al 2008; Johnson et al 2010; Subramanya and Bishop 2011). Heterotic response in a trait is generally taken as evidence that genetic interaction (allelic and/or non-allelic) influences the manifestation of the trait (Kerje et al 2003; Hannan et al 2007). The gene complexes responsible for favourable nicking, and heterosis in hybrids dissociate during gamete formation and when hybrid individuals are mated, new genetic combinations with different values form in the progenies. The principal effects of interbreeding hybrid animals is a reduction in heterozygosity, loss of complementarity and break down of favourable epistatic gene combinations (break down of coadapted gene complexes) as well as recombination loss probably due to the recombination of parental genes and re-establishment of parental genotypes at some loci. These genetic modifications result in reduced performance of the resulting progenies (Cassady et al 2002) and loss of genetic uniformity characteristic of hybrids (Deeb and Lamont 2002). Studies have shown that the percentage reduction in performance on further hybridization is proportional to the loss in heterozygosity (Gregory et al 1991; Arthur et al 1999; Fries et al 2000; Roso et al 2005).  

It appears that the economic consequences of utilizing commercial hybrids as breeders to produce replacement chicks for commercial egg production could be grave given the marginal profit level of poultry production in Nigeria. However no study has substantiated this empirically. The present study was therefore undertaken to evaluate and compare the performances of two commercial egg strains, the Nigerian indigenous chickens and their random bred progenies in other to investigate the impacts on performance and economics of utilizing commercial hybrid chickens as breeders for the production of replacement day old chicks. 

Materials and methods

Two hundred and fifty (250) day-old female chicks and fifty (50) day-old male chicks belonging to two commercial egg strains: H and N brown nick (strain 1) and black Olympia (strain 2) were obtained from two hatcheries: one in Sapele, Edo state, Nigeria (strain 1) and the other in Ibadan, Oyo state, Nigeria (strain 2). In addition, 250 native chicks (strain 3) were hatched from a population of random breeding Nigerian indigenous chickens (NIC) to correspond with the arrival of the exotic chicks. The exotic chicks were commercial hybrids commonly used for table egg production in Nigeria. The hybrid chicks were brooded and reared according to strain and sex from day-old to maturity while the native chicks were separated according to sex at 4 weeks of age. In each generation all strains were reared in two replicates. After two (2) months of egg production on deep litter (about 32 weeks of age), the surviving layers (210 for strain 1, 204 for strain 2 and 115 for strain 3) were randomly mated to their male counterparts in the ratio of one cock to ≤ ten hens (1 cock : ≤ 10 hens). For each strain, the cocks were allowed to run with the hens for two weeks before egg collection and hatching. Two hundred and forty (240) chicks (progenies) were produced from each strain. The chicks were brooded according to strain from 0 - 4 weeks and according to strain and sex thereafter to maturity. As much as was possible similar management conditions were provided for parents and progenies. 

Data collection and analysis

The experimental birds (parents and progenies) were weighed at 4, 8, 12, 16, and 20 weeks of age, respectively to obtain their individual body weight (BWT) at these ages. The difference in body weight values between two consecutive measurements was divided by the number of days in the interval to obtain the daily body weight gain (BWG). Pullets were additionally weighed on the day of their first egg to obtain body weight at first egg (BWFE). The feed conversion ratio (FCR) was calculated as the ratio of daily weight gain to daily feed intake within each measurement period. The age in days from hatch up to first egg of each pullet was recorded as the age at sexual maturity (ASM). The first egg laid by each pullet was weighed and recorded as weight of first egg (WTFE). The cost in Naira for feed consumption from day old up to point of egg production (point of lay) was assumed as the cost of production in each generation. Data on BWT, BWG, BWFE, WTFE, FCR and ASM for parents and their progenies (females only) in the three strains were subjected to analysis of variance (ANOVA) using the ANOVA option of SPSS computer package (SPSS, 2001) version 17. The statistical model was:  

             Χ_ijk = µ + G_i + S_ij + е_ijk

Where, 

            Χ_ijk =   An observation on the kth individual belonging to the jth strain in the ith generation;

            µ = overall mean;

            G_i = effect of the ith generation;

            S_ij = effect of the jth strain in the ith generation;

           е_ijk = residual effects assumed to be independent and normally distributed with zero mean and variance of error.                            

Comparison between generations [parent (P₀) vs progeny (P₁)] for each strain was done using the independent t-test option of SPSS (2001). The coefficients of inbreeding in the parental populations were taken to be zero while the percentage inbreeding depression (ID %) of the progeny generation was calculated using the expression:                                  

            ID % = [(P₀ – P₁)/P₀] x 100 (Talebi et al 2010)

Where, 

             ID % = percentage inbreeding depression.

            P₀= mean of parents for a trait.

            P₁ = mean of progeny for the same trait. 

The significance of inbreeding depression was tested by calculating the t – statistic using the expression:  

            t_cal for ID % = estimated value of ID/SEM (Talebi et al 2010) 

Where, 

            SEM (Standard error of mean) = √σ²P₀ + σ²P₁

Where, 

             σ²P₀ = variance of parental mean;

            σ²P₁ = variance of progeny mean. 

The calculated t - statistic was compared to the tabulated value (t - tabulated) at degrees of freedom (df) for error (within group). 

Results

Table 1 presents the mean performance of the three strains (parent and progeny) in the traits studied. 

The table shows that the exotic chickens (P₀ and P₁ populations) significantly (P ≤ 0.032) surpassed the unimproved local strain in growth performance (BWT) across the entire age periods and in BWFE and WTFE. Between the two exotic strains, strain 2 (Black Olympia) significantly (P ≤ 0.032) exceeded strain 1 (H and N Brown Nick) in BWT performance across the age periods and in BWFE and WTFE (P ≤ 0.024). For BWG, the exotic strains significantly (P ≤ 0.051) surpassed the local strain at two age periods (8-12 and 12-16 wks) in the parental (P₀) generation. Significant differences (P ≤ 0.052) occurred between the exotic strains but none was consistently superior over the other. In the progeny (P₁) generation, the exotic strains generally decreased in BWG and hence significantly (P ≤ 0.052) surpassed the local strain only at the early periods of growth: 4-8 and 8-12 wks. The exotic hybrids differed significantly (P ≤ 0.052) in BWG within the ages of 12-16 and 16-20 wks with strain 2 being superior to strain 1. The two exotic parent strains were significantly (P < 0.048) better than the local chicken (strain 3) in feed conversion ratio at the age periods of 4-8 and 8-12 weeks. The local chicken and strain 1 parents were similar in FCR at the age periods of 12-16 and 16-20 weeks. However, strain 2 was better than strain 1 and strain 3 in FCR at 12-16 weeks but inferior to them at 16-20 weeks of age. Feed conversion ratio decreased for all strains in the progeny generation especially for the exotic strains which were superior to the local strain only at the earliest age period (4-8 wks). The local strain also attained sexual maturity significantly (P ≤ 0.046) earlier than their exotic counterparts (strains 1 and 2) at the parent and progeny generations. 

The highly significant differences among the three strains in BWT, BWFE, FCR, WTFE and ASM for parent (P₀) and progeny (P₁) generations were expected. These differences reflect the genetic variation among the three strains for these performance traits. Strain 2 is a dual purpose chickens improved for meat yield and egg production while strain 1 is a light to medium breed reputed for high egg production. The local chickens are unimproved and expectedly performed least in the above traits. These findings are in line with reports in literature (Roso et al 2005; Abou EL-Ghar et al 2010; Razuki and AL-Shaheen 2011) comparing exotic strains to unimproved indigenous chickens.  

Table 2 presents the generation comparison (parent vs progeny) for performance traits (BWT, BWFE, BWG, FCR, WTFE and ASM) for the three strains. The table shows significant (P < 0.00) superiority of parents over the progenies in BWT across the entire age periods for strains 1 and 2 and at 4 wks of age for the local chickens. BWFE and WTFE were significantly (P < 0.00) lower in the progeny compared to the parental generation of the three strains of chicken while ASM increased significantly (P < 0.00) from parent to progeny generation. Significant (P ≤ 0.02) generational differences in BWG were observed for strain 1 across the entire age periods, strain 2 at 4-8 and 12-20 wks of age and from 4-12 wks of age for the local. FCR also varied significantly (P ≤ 0.04) between parents and progenies especially for the exotic stains. FCR was inferior in the P₁ population of strain 1 at weeks 4-8 and 8-12 wks of age, strain 2 across the entire age periods and strain 3 from 4-12 wks of age. 

The superiority of the exotic hybrids (P₀ generation) over their progenies (P₁ generation) for all the traits studied and across most age periods indicate reduction in performance in the progenies. Such reduction in performance has been reported by previous studies in various species: in chickens (Kerje et al 2003); in cattle   (Syrstad 1989; Sacco et al 1990; Olson et al 1993); in sheep (Boujenano et al 1999); in pig (Cassady et al 2002); in silk moth (Subramanya and Bishop 2011) and in salamander (Johnson et al 2010) and has been attributed to loss of heterozygosity in the progeny generation, loss of complementary gene effects, recombination loss and break up of favourable epistatic gene combinations (Syrstad, 1989; Gregory et al 1991;  Arthur et al 1999; Fries et al 2000; Kerje et al 2003; Subramanya and Bishop 2011). For the exotic strains, the reduction in performance is termed loss of heterosis which is the reduced superiority of progenies of hybrids (compared to their hybrid parents) over their purebred grand parents. The relative importance of these genetic factors to the reduction in performance of progenies of hybrids varies with the trait and the genetic effect(s) controlling its expression as well as the mating design. Traits significantly controlled by genetic interaction: dominance, overdominance and/or epistasis manifest greater heterotic response, recombination loss and inbreeding depression, than traits under additive genetic effects (Talebi et al 2010). When recombination loss is coupled with inbreeding effect, the reduction in performance is accentuated. This could explain the significant reduction in the growth traits and FCR across the entire age periods in the exotic progenies. For the local chickens, the few cases of significant reduction in performance in the progenies could be attributed to inbreeding effect. Table 2 shows that traits were mostly depressed during the early periods of growth in the local chickens. Different stages of growth in the chicken has been shown to be (to some extent) under different genetic regulation (Carlborg et al 2003; Kerje et al 2003; Carlborg et al 2004, 2006; LeRouzic et al 2008). Carlborg et al (2003) and Carlborg and Haley (2004) estimated the relative contributions of additive, dominance, and epistatic effects on growth in the chicken and reported greater contribution of epistatic effect prior to 46 days (6 – 7 wks) of age, whereas additive genetic effects explained the greater portion of the genetic variance in growth performance later in life.  Thus BWT, BWG and FCR were significantly (P≤ 0.05) depressed within the early periods of growth (4-12 wks) in the P₁ generation of the local chickens than in later periods. Ogbu and Omeje (2010) had reported similar significant reduction in performance traits (BWT, BWG and FCR) following within flock mating in a closed population of Nigerian indigenous chickens. 

Table 3 presents the estimate of percentage inbreeding depression (% ID) and t-statistic (t_cal) for BWT at various age periods in the three strains. The table shows that the progeny population (P₁) of the local strain was less severely depressed than their exotic counterparts. Inbreeding depression coefficient ranged from 0.37 - 4.20 % for P₁ of the local strain as against 9.63 – 19.68 % and 6.65 – 22.2 % for P₁ of strains 1 and 2, respectively. BWT at 4 wks had the highest inbreeding depression coefficient of 4.20 % for the local chickens. For all strains, percentage inbreeding depression was insignificant (t_cal ˂ t_{tab 0.95 = 2.447 at d.f. = 6}) across the entire age periods. The highest percentage of inbreeding depression in BWT at 4 wks observed for the local strain followed from the greater reduction in BWT at the early periods of growth (table 2) and indicates that in the indigenous chickens, growth at or around this period may be under greater genetic interaction effect than for later age periods (Carlborg et al 2003, 2004; Kerje et al 2003). The greater reduction in body weight observed for the exotic strains compared to the local strain could be as a result of the combined effects of loss of heterozygosity (inbreeding depression) and hybrid breakdown (loss of favourable epistatic gene combinations as well as recombination loss). However, inbreeding depression coefficient for body weight was not significant (P ˃ 0.05) for all strains across the entire age periods indicating that loss due to hybrid breakdown is low and that dominance may be more important than non-allelic interaction (but less important than additive gene effect) in the expression of growth traits in chickens  (Podisi et al 2011). 

Table 4 presents the percentage inbreeding depression and t-statistic (t_cal) for BWFE, WTFE and ASM. Percentage inbreeding depression for BWFE was 1.06 % for strain 1, 0.64 % for strain 2 and 2.41 % for the local chickens. For WTFE, 4.60 %, 5.79 % and 5.92 % were obtained for strain 1, 2 and 3, respectively while for ASM, -8.29 %, -7.48 % and -3.43 % were obtained, respectively for the three strains. 

WTFE and ASM had higher percentage inbreeding depression and by implication greater loss of heterozygosity than BWFE indicating greater involvement of dominance and/or epistatic gene effects in the control of these traits compared to BWFE. Also WTFE and ASM probably have greater implications for fitness (reproductive capability or physiological efficiency) in the chicken than BWFE even though there is a critical BWT below which sexual maturity will not be attained in a species. The reduced BWFE in the three strains could be a carry over of the effect of reduced growth rate in earlier age periods or the effect of overlapping (common) genetic effects on growth during early and late growth periods. Podisi et al (2011) carried out a genome scan for quantitative trait loci (QTL) for BWT, growth rate, BWFE and age at first egg (age at sexual maturity, ASM) using F2 progenies of a broiler-layer intercross and reported co-localization and overlapping of QTL for BWT, BWFE, growth rate (BWG) and age at first egg (AFE or ASM) suggesting that BWT and growth rate are closely related to the attainment of sexual maturity and that the genetic determination of growth rate has correlated effect on puberty. The reduced growth rate over the age periods delayed attainment of the critical (minimum) body weight for sexual maturity thus elongating the age at sexual maturity while the lower body weight at sexual maturity (lower BWFE) reduced the weight of initial egg. 

Table 5 presents the percentage inbreeding depression and t-statistic (t_cal) for daily gain (BWG) and feed conversion ratio (FCR) for the three strains at various age periods.  

There were significant (P ≤ 0.05) effects of inbreeding depression for the three strains. For the local chickens, traits of BWG₁, BWG₂, FCR₁ and FCR₂ were highly significantly (P ˂ 0.00) depressed. For the exotic hybrids, BWG₁, BWG₂ and as well as FCR₁ and FCR₂ were significantly (P ≤ 0.05) depressed  for strain 1 while BWG₁, BWG₃ and BWG₄ and FCR₁ and FCR₂ were highly significantly (P ˂ 0.01) depressed  for strain 2. 

From the results presented, it does appear that BWG and FCR are more susceptible to inbreeding depression during early age periods (4-12 wks) in egg type chickens. These periods also corresponded with the periods of greatest feed efficiency in the present study. Perhaps dominance gene effect as well as maternal effects may be important in the expression of these traits at these age periods. Growth performance during the prenatal and immediate post natal periods in animals has been shown to be considerably influenced by maternal effects. A number of studies (Jung et al 1981; Bishop et al 1991; Fan et al 1995; Khan and Singh 1995; Duangjinda et al 2001; Deep and Lemont 2002; Snowder and Van Vleck 2003; Hansen et al 2007) had demonstrated high influence of maternal and genetic interaction effects (and low involvement of additive genetic effects) on preweaning traits in various domestic animal species. For the intensively reared chicken, maternal effects are mostly communicated through differences in egg size (or egg weight) and/or composition (Hartmann et al 2003) which in turn influences hatch weight and subsequently the chick^’s immediate post hatch performance: growth, feed intake and feed efficiency (Barbato 1994; Deep and Lemont 2002). Additive genetic heritability of quantitative traits within the first few weeks post hatch has been reported to be low. Hartmann et al­ (2003) reported that maternal heritability for chick weight was 0.5, whereas the direct (additive) heritability was close to zero. Barbato (1994) had shown that feed intake immediately post-hatch is inherited in a non-additive manner. Maternal effects include maternal genetic (additive, dominance and epistatic) effects and maternal permanent environmental effects (Thompson 1976). The significant inbreeding depression observed for BWG and FCR for the three strains within the early growth periods suggest significant effect of maternal dominance. Progenies of the exotic hybrids were on the average more severely affected probably as a result of the additional effects of recombination loss.  

The estimates of loss in performance and the Naira cost of production (parent and progeny) for the three strains are presented in table 6. 

The table shows that cumulative body weight gain (CumBWG) from hatch up to point of egg production was lower for P₁ generation of the three strains. Mean feed conversion ratio (MFCR) and cumulative feed intake (CumFI) were also higher for progenies. Cost of production to point of lay increased by ₦145 per bird for strain 1; ₦280 for strain 2 and ₦51.7 for strain 3. The progeny (P₁) populations were also older than parents (P₀s) at sexual maturity for the three strains: 174 vs 161 days for strain 1; 178 vs 165 days for strain 2 and 160 vs 154 days for the local chickens. 

The lower CumBWG from hatch up to point of egg production observed for the P₁ generation of each strain was sequel to the reduced growth rate across the age periods. The elongated ASM coupled with reduced feed efficiency (higher feed conversion ratio) in the progeny generation meant higher quantity of feed consumed and increased cost of production to point of lay. These alterations in performance indices were more severe for the exotic hybrids than for the indigenous chickens hence the greater cost implications of raising progenies of the exotic strains compared to those of the local strain. It has been shown that ASM influence egg size and egg production (egg number) to a particular age in chickens (Deep and Lamont 2002) hence the overall economic outcome of the egg production enterprise. 

Conclusions

• The most significant findings of the current research are the significantly depressed growth rate and feed efficiency at the early periods of growth affecting the progeny (P₁) generation. Consequently, mature body weight was reduced, sexual maturity was delayed which resulted in increased cost of production to point of lay. These effects have negative implications for the profitability of a layer enterprise given the marginal profit nature of commercial table egg production in Nigeria. The results confirm that progenies of terminal commercial hybrid strains are unviable for economic production. We hence recommend that commercial exotic hybrid chickens reared in Nigeria should not be used as breeders for the production of replacement day-old chicks for commercial (table) egg production.

References

Abou EL-Ghar A, Ghanem H H and Aly O M 2010 Genetic improvement of egg production from crossing two developed strains with a commercial laying hens. Egyptian Poultry Science 30 (11): 457-472. 

Abou EL-Ghar R and Abdou F H 2004 Evaluation of genetic variance components based on the concept of generation means for some economic traits in chicken. Egyptian Poultry Science 24: 687-699. 

Arthur P F, Hearshaw H and Stephenson P D 1999 Direct and maternal additive and heterosis effect from crossing Bos indicus and Bos Taurus cattle: cow and calf performance in two environments. Livestock Production Science 57: 231-241. 

Barbato G F 1994 Genetic control of food intake in chickens In: Annual poultry nutrition conference: food intake regulation, Neuropeptides, circulating factors and genetics. Journal of Nutrition 124: 1341s-1348s. 

Bishop M D, Davis M E, Harvey W R, Wilson G R and Vanstaven B D 1991 Divergent selection for postweaning feed conversion in Angus beef cattle: 1. mean comparisons. Journal of Animal Science 69: 4348-4359. 

Boujenano I, Chafik A and Benbihi M 1999 Heterosis retained in different generations of inter se mating between D’man and Sardi sheep. Journal of Animal Breeding and Genetics 116 (2): 151-159. 

Carlborg O and Haley C S 2004 Epistasis: too often neglected in complex trait studies? Nature Reviews Genetics 5: 618-625. 

Carlborg O, Hocking P M, Burt D W and Haley C S 2004 Simultaneous mapping of epistatic QTL in chickens reveals clusters of QTL pairs with similar genetic effects on growth. Genetics Research Cambridge 83: 199-209. 

Carlborg O, Jacobsson L, Ahgren P, Siegel P and Andersson L 2006 Epistasis and the release of cryptic variation during long-term selection. Nature Genetics 38: 418-420. 

Carlborg O, Kerje S, Schutz K, Jacobsson L, Jensen P and Andersson L 2003 A global search reveals epistatic interaction between QTL for early growth in the chicken. Genome Research 13: 413-421. 

Cassady J P, Young L D and Leymaster K A 2002 Heterosis and recombination effects on pig growth and cacass traits. Journal of Animal Science 80: 2286-2302. 

Deep N and Lamont S J 2002 Genetic architecture of growth and body composition in unique chicken populations. The American Genetic Association 93: 107-118. 

Duangjinda M, Bertrand J K, Misztal I and Druet T 2001 Estimation of additive and non-additive genetic variances in Hereford, Gelbvieh, and Charolais by method R. Journal of Animal Science 79: 2997-3001. 

Fan L Q, Bailey D R C and Shannon N H 1995 Genetic parameter estimation of preweaning gain, feed intake and feed efficiency for Hereford and Angus bulls fed two different diets. Journal of Animal Science 73: 365-372. 

Fries L A, Johnson D J, Hearnshaw H and Graser H U 2000 Evidence of epistasic effects on weaning weight in crossbreed beef cattle. Asian-Australian Journal of Animal Science 13 (supplement B),  242pp. 

Gregory K E, Cundiff L V and Koch R M 1991 Breed effects and heterosis in advanced generations of composite populations for preweaning traits of beef cattle. Journal of Animal Science 69: 947-960. 

Gregory K E, Cundiff L V and Koch R M 1992 Effect of breed and retained heterosis on milk yield and 200-day weight in advanced generations of composite populations of beef cattle. Journal of Animal Science 70: 2366-2372. 

Gura S 2007 Livestock Genetics Companies. Concentration and Proprietary strategies of an emerging power in the global food economy. League for pastoral peoples and Endogenous livestock Development, Ober-Ramstadt, Germany. Retrieved from http://www.pastoralpeoples.org/docs/livestock­_genetics_en.pdf 

Hannan M M, Ahmed M D, Roy U K, Razvy M A, Haydar A, Rahman M A, Islam M A and Islam R 2007 Heterosis, combining ability and Genetics for Brix %, days to first fruit ripening and yield in tomato (Lycopersicon esculentum mill.). Middle-East Journal of Scientific Research 2 (3-4): 128-131.

Hansen G R, Elzo M A, Diley D G, Coleman S W and Paschal J C 2007 Growth and feed efficiency: Utilization of Bos indicus cattle in Florida beef enterprise. Florida beef cattle short course, pp. 47-69. 

Hartmann C, Johanson K, Strandberg E and Rydhmer L 2003 Genetic correlations between the maternal genetic effect on chick weight and the direct genetic effects on egg composition traits in a white leghorn line. Poultry Science 82: 1-8. 

Hill W G, Goddard M E and Visscher P M 2008 Data and theory point to mainly additive genetic variance for complex traits. PLoS Genetics 4 (2): 1-10. 

Johnson J R, Fitzpatrick B M and Shaffer H B 2010 Retention of low-fitness genotypes over six decades of admixture between native and introduced tiger salamanders. BMC Evolutionary Biology 10 (147): 1-14. 

Jung S B, Christain L L and Kuhler D L 1981 Response to selection for feed efficiency in individually fed Yorkshire boars. Journal of Animal Science 53: 323-331. 

Kerje S, Carlborg O, Jecobsson L, Schute K, Hartmann C, Jensen P and Andersson L 2003 The twofold difference in adult size between the red jungle fowl and white leghorn chickens is largely explained by a limited number of QTL. Animal Genetics 34: 264-274. 

Khan B U and Singh S K 1995 Genetics of feed conversion in Babari goats. Small Ruminant Research 15: 283-285. 

LeRouzic A, Alvarez-castro J M and Carlborg O 2008 Dissection of the genetic architecture of body weight in chicken reveals the impact of epistasis on domestication traits. Genetics 179: 1591-1599. 

Ogbu C C and Omeje S S I  2010 Comparison of the growth performance, feed intake and feed:gain ration of two generations of purebred Nigerian indigenous chicken. Proceedings 15^th Annual Conference of Animal Science Association of Nigeria, 13-15 Sept., 2010, University of Uyo, Nigeria, pp. 508-512.  

Olson J A, Peacock F M and Koger M 1993 Reproductive and maternal performance of rotational, three-breed and inter se crossbred cows in Florida. Journal of Animal Science 71: 2322-2329. 

Podisi B K, Knott S A, Dunn I C, Law A S, Burt D W and Hocking P M 2011 Overlap of quantitative trait loci for early growth rate, and for body weight and age at onset of sexual maturity in chickens. Reproduction 141: 381- 389. 

Razuki W M and AL-Shaheen S A 2011 Use of full diallel cross to estimate crossbreeding effects in laying chickens. International Journal of poultry Science 10 (3): 197-204. 

Roso V M, Schenkel F S, Miller S P and Wilton J W 2005 Additive, dominance and epistatic loss effects on preweaning weight gain of crossbred beef cattle from different Bos Taurus breeds. Journal of Animal Science 83: 1789-1787. 

Sacco R E, Baker J F, Cartwright T C, Long C R and Sanders J O 1990 Measurements at calving for straightbred and crossbred cows of diverse types. Journal of Animal Science 68: 3103-3108.

Snowder G D and Van Vleck L D 2003 Estimate of genetic parameters and selection strategies to improve the economic efficiency of postweaning growth in lambs. Journal of Animal Science 81: 2704-2713. 

SPSS 2001 Statistical Package for social sciences (version 13.0). SPSS Inc. Chicago U.S.A.

Subramanya G and Bishop S C 2011 Analysis of heterosis and recombination loss for fitness and production traits in different hybrids of Mulbery silk moth: Bombyx mori. Entomology Research 41: 18-23. 

Syrstad O 1989 Diary cattle cross-breeding in the tropics: performance of secondary crossbred populations. Livestock Production Science 23 (1-2): 97-106. 

Talebi E, Subramanya G and Bakkappa S 2010 An investigation of heterosis and inbreeding depression in the silkworm (Bombyx mori L.). ARPN Journal of Agricultural and Biological Sciences 5 (3): 52-55. 

Thompson R 1976 The estimation of maternal genetic variances. Biometrics 32: 903-917.

Received 20 December 2011; Accepted 9 February 2012; Published 4 March 2012

Go to top