LESSONS FROM SELECTION STUDIES IN POULTRY FOR ANIMAL BREEDERS

Transcription

1 LESSONS FROM SELECTION STUDIES IN POULTRY FOR ANIMAL BREEDERS R.S. Gowe Animal Research Centre Research Branch Agriculture Canada Ottawa. Ontario Canada. KIA 0C6-22-

2 LESSONS FROM SELECTION STUDIES IN POULTRY FOR ANIMAL BREEDERS INTRODUCTION On the invitation of the organizers of the 1980 World Congress on Sheep and Beef Cattle Breeding. I presented a paper co-authored by my colleague Dr. Fairfull entitled "Some lessons from selection studies in poultry". (Gowe and Fairfull. 1982). The organizers of this meeting thought that an update of that paper might make a useful contribution to your meeting. Since some two years have passed, it was interesting to review the ideas and data presented and to update the diagrams and one of the tables with further data. There have been no startling advances made in the "art" of animal or poultry breeding since then that would make Dr. Fairfull or i change our general theses. Two generations more data from our long-term selection study, in our opinion, only confirms the general results presented and continues to support the general interpretation. In updating the graphs. I was pleased to see there were no unexpected results. The "lessons" in this paper are directed at animal and poultry breeders in general but more particularly at those involved with meat species such as sheep and beef and possibly broiler and turkey breeders where the emphasis for too long has been on selecting for a limited number of traits only. In my opinion, the academics and experiment station researchers still do not put sufficient research emphasis on the problems of multi-trait selection, and therefore do not often come to grips with the many problems inherent in this field. The lessons to be learned from poultry egg stock breeding. of course, apply most directly to poultry egg stocks, but poultry have long been a very useful model for breeding in other species such as cattle, sheep and swine. Not only is breeding research with poultry egg Stocks relevant to that important industry, but it is a useful model because the traits have their own economic value and it is easier to relate the results to the real world, something not as easily done with Tribolium or mice studies. Poultry's value as a model has been demonstrated in a wide range of practical and theoretical studies on breeding and selection. No attempt will be made to review this extensive literature and to give credit where it should be. Many of the contributors are in the audience. Only a few of the many lessons that could be considered of significance to animal breeders are presented here. The (over) -23-

3 examples in this report come from a long-term selection program in egg-type poultry that has been underway at the Animal Research Centre (ARC) of the Research Branch of Agriculture Canada. Ottawa. since More detailed information can be found in the papers by Gowe (1977). Gowe et al. ( a. b). Gowe and Fairfull. ( a. b). Fairfull (1982). Fairfull and Gowe (1979). Fairfull et al. (1983). Gavora and Spencer (1979). Gavora et al. ( ) and McAllister (1977). The topics to be discussed are: multi-trait selection: inbreeding, selection and population size: disease and selection for improvement in performance: heterosis and crossing: and environment and management. No attempt will be made to treat these complex subject areas exhaustively, rather some specific results will form the basis of the "lessons" DISCUSSION Multi-trait selection Poultry egg-stock breeders who sell their stocks to skeptical commercial producers are well aware of the need for a multi-trait selection program. There is no way they can sell an egg-type bird that does not have high performance for the complete array of essential traits: they are. high egg production over a long laying period, large but not too large egg size. early sexual maturity, high fertility and hatchability, excellent viability, small body size. strong egg shells, superior interior egg quality and good feed conversion. The need to consider a balance of the complete array of traits necessary is well established, and need not be emphasized for those of you in the egg-stock business. In poultry and meat animals it used to be considered valid to select for growth rate only. ignoring such critical traits as fertility, embryonic mortality, calving or lambing ease. lean meat yield and feed efficiency as well as other useful but less important traits. Single trait selection is no longer valid for these meat species. Note that "type" deliberately has not been included as an important trait. In my opinion. type in the traditional sense should not be considered. Research results will no doubt eventually show that it is desirable to include very specific objective physical measurements in some animal breeding programs. For example, in dairy cattle the size and shape of cows' teats, and testes size of bulls are known to be impoctant economic traits - the one for ease of milking and the other for semen production. In ouc -24-

4 poultry selection study now in its 31st generation for some strains (that is equivalent to i00 years or more of breeding in cattlet) and where we have been selecting for multiple econonic traits. "type" has been totally ignored. Lo and behold the chicken strains resulting still look like chickens and quite good looking chickens, but none would meet the "type" standards for the breeds that were in vogue when the study started. It would have been impossible from any objective data that we are aware of to have predicted 31 years ago the "type" of chicken needed to sustain the very high performance levels achieved. Nevertheless the bird that resulted from this lack of selection for type is not grossly different from the original chicken. If the conventional wisdom of 30 years ago had been followed, a great deal of selection pressure would have been wasted in selecting for the so-called preferred "type" This no doubt would have slowed or stopped improvement in the traits of economic significance, while changing the look of the birds in very minor and unimportant ways. Some large animal breeders are still very reluctant to concede that arbitrary idealized types may not be a valid selection goal. It is frequently critical to improve two traits that are genetically correlated in a negative way. As long as the correlation is not perfect (ie. minus one) it is possible to make progress in both traits over a period of time. Egg size and egg production are such traits. Their genetic correlation is negative, but the correlation is not perfect, in fact it is around in our control strain 5. For the first three years of our long-term selection study egg weight was not considered, and it dropped rapidly as should have been foreseen due to this negative correlation (Figure I). Then selection for egg size utilizing only pedigree information was introduced to attempt to bring this trait up to a reasonable level. This was partially effective, but the selection pressure was low and the rate of change slow with this procedure. Later (1961) both individual and family selection were introduced and egg weight gradually improved to an optimal level. For the last few years selection has been directed at maintaining this optimal size since increasing egg size would be economically disadvantageous. Our current practice is to use family selection to select males, while the females are selected on their individual performance only. As you can see it has been necessary to shift the emphasis placed on egg weight at different times in the selection program depending on the level of the trait and the need for changing it or _ust maintaining it. The point here is that selection should not always be applied in the same way or with the same emphasis at different stages of an improvement (over) -25-

5 program, but it is not good practice to make large changes in the direction or emphasis of the program from year to year. Changes should be made gradually since a balance in the traits is required. Over 30 years ago a well-known New York State poultry breeder made the decision to correct an egg trait deficiency in his strain of chickens quickly and he put all his selection pressure on that trait for two generations or so. The results were disastrous and severely affected the over-all value of this stock. One has to carefully analyze what changes are needed in a stock or strain then plan to achieve this as quickly as possible while still maintaining reasonable performance levels for the whole array of economically important traits. I would like to illustrate the results of our multi-trait selection program by showing some of the genetic improvements that have taken place over the last 9 generations in two of our selected strains being selected for an array of essential economic traits. These traits are all plotted as a deviation from the mean of two of our unselected control populations. therefore the trends shown can be considered the genetic trends since the environmental fluctuations and any general environmental trends were removed. The genetic changes for the following traits are illustrated in six figures: hen-housed egg production to 497 days of age (Figure 2). rate of egg production from age at first egg to 497 days for survivors (Figure 3). age at first egg or sexual maturity (Figure 4). mature body size at 365 days of age (Figure 5). Haugh units - a measure of egg albumen quality (Figure 6). and specific gravity of eggs - a measure of egg shell quality (Figure 7). In addition to these traits I have already illustrated the changes in egg size in Figure I. These changes were also positive although plateauing now as the optimal size has been reached and selection is aimed at maintaining egg size rather than further increases in egg size. Fertility and hatchability (although-not illustrated) have also beenmaihtained at_he[r previous high level. Generally. there has been an improvement or a maintenance of all traits of economic significance to the producer. Negative responses for sexual maturity and body size are improvements: in general, the smaller the bird the more efficient she is if she lays the same number and size of eggs. and the earlier a bird starts to lay the more profitable she is all other factors being equal. There has been a small decline in hen-housed egg production and also in rate of egg production from first egg in strain 8. This strain has been selected for high hen-housed egg production on part-record to 273 days since -26-

6 it was started in Strain 1 has been selected on rate of egg production from first egg since 1971 when it was started. Space and time do not permit me to show all the results in this study. However. selecting on part-year records for rate after age at first egg has been definitely more successful in increasing rate in the latter part of the production test. and therefore more effective generally in raising full-year hen-housed egg production. This is particularly true for strains that have relatively early sexual maturity. The slight drop in hen-housed egg production is not due to any significant change in viability. How did we manage to make improvement in all the necessary economic traits? First we had a pretty good idea of the heritability of the traits of concern from the literature and from analyses conducted during the course of the study (Table i). Hen-housed egg production is really an index of viability. sexual maturity and rate of egg production. We select strain 8 on this trait and strain 1 is selected on rate of egg production after sexual maturity. Both strains are selected on part-year performance and are being selected for viability, but neither strain is directly selected for sexual maturity. Previously. there was indirect selection for early sexual maturity in strain 1 and therefore, early sexual maturity was partially fixed as a trait. We also have had to take into consideration the genetic correlations among traits. For example, some of the important genetic correlations are egg weight with 273 day rate of egg production and also with body weight, body weight with egg production rate over the year. and 273 day rate of egg production with egg production rate over the full year which are approximately and 0.75 respectively. We don't use a formal selection index, but we do use a combination of full-sib and half-sib family selection, and individual selection. Pedigree selection is used only for the reproductive traits fertility and hatchability, since these latter traits with our selection system are really an expression of their dams' and sires' phenotypes. For traits with low heritability and high economic importance such as egg production, most emphasis is placed on family selection - that is sire family or half-sib family means and dam family or full-sib family means - and less emphasis on the individual's own production (Table i). For traits with a higher heritability like egg weight, more emphasis is placed on the individual's own record. McAllister (1977) using a retrospective index technique has confirmed in general quantitative terms that the emphasis we place on the source of information for these traits is not far from what is indicated -27- (over)

7 in this table. The deviations reported can likely be attributed to the simplifying assumptions required for such an analysis. The far right hand column in Table 1 is our estimate of the approximate emphasis put on the various traits. Egg production, egg size (an intermediate optimum trait), viability and specific gravity receive the most emphasis. The emphasis put on different traits depends partially on the economic value of each trait, as well as on its heritability and its genetic correlation with other desirable traits. Traits such as egg production rate and egg size are negatively correlated and generally one desires to increase both. therefore selection emphasis must be high for these traits, particularly egg number as its heritability is low. Body size and 497 day production rate on the other hand are also negatively correlated, but since one wants to increase one (egg production) while decreasing the other (body size) this negative correlation helps rather than hinders the selection process and less emphasis needs to be put on reducing body size than would be necessary otherwise. You might ask why we do not use a full l_near selection index since theoretically all of our goals might have been achieved in this way. My colleague Dr. Fairfull and I think that selection indices as they are presently constructed offer several problems in terms of the assumptions of linearity. This is particularly true for the terms associated with economic values and the underlying assumption of the additivity of the genetic effects. Egg size is an example of a trait that has a non-linear economic value in the markets of all western countries. Small eggs are heavily penalised through lower egg prices _nd as egg size increases the premium paid for larger eggs decreases to a limit above which no increases in price are paid. Yet each gram of egg requires over 2.5 g of feed to produce it. also. large eggs are more frequently broken by automatic egg gathering equipment, so that. at or above a ceiling it would be economically detrimental to increase egg size. Now that our populations are at or near the optimum size we emphasize individual selection (culling levels - both high and low) for females and for families from which males are selected. Fertility is a trait that has low heritability. Experience has shown that high selection pressures to improve it or maintain it. if it is already satisfactory will be generally wasteful of the selection pressure available. Yet the selection of dams and especially sires from full or half sib families with very low fertility (2 or 3 standard deviation below the mean) will lead to a drop in fertility in their offspring. This lower level of fertility will be refractory to improvement. Therefore. we are careful to eliminate progeny from very poor families but pay no attention to minor -28-

8 variations in fertility. As studies of index parameters have shown, in a linear selection index, fertility would be dominated by other traits with higher heritabilities and economic weights. We believe that fertility may be an example of a trait where the underlying genetic values may not be linear or continuous. Further. we believe that fertility and egg size are examples of traits that are not controlled properly in a linear selection index. This is not to say that there are not other problems or undesirable properties of linear selection indices because other problems exist, but these are two major reasons why we have not used a linear selection index up to the present. Inbreeding, Selection and Population Size Population size and inbreeding are two related topics. To improve a finite population some degree of inbreeding is automatic. However, too rapid a rise in inbreeding leads to gene loss and gene fixation, and tends to limit ultimate progress. Due to limited space for our experimental test populations, we are able to use only twenty-eight sires each generation for each of the six selected strains in our overall selection study. Of these 28 sires at least the top 12 contribute to the next breeding population and of these the better balanced sire families contribute more sires to the next breeding population than those that rank lower in overall performance. Of 224 selected females used as breeders each generation for each strain (out of about i000 alive at breeding time) about 20 contribute sires and 120 contribute dams to the next breeding population. The point I am making here is that we attempt to apply reasonable selection pressure while at the same time prevent too rapid a rise in inbreeding, thereby permitting time for the desirable genes in lower frequency and with small effects to be selected and increased in frequency. We could increase the selection pressure for the principal traits, particularly in selecting breeding males, but deliberately refrain from doing so. The progress shown in the previous graphs has been achieved despite a slow. steady increase in the level of inbreeding. As can be seen in Figure 8. inbreeding in our two oldest selected strains is now over 20% and it is rising at about 0.7% a year. Strain 8 which was started only i0 generations ago has a coefficient of inbreeding of about 7% and it is rising at about the same rate. The point is that progress can be made despite a continuous rise in the level of inbreeding as long as this inbreeding does not go up too quickly or too erratically, and it is attained within a multi-trait selection program that encompasses all essential traits. (over) -29-

9 There has to be a balance between population size and selection pressure. Within limits the larger the population size. the larger the selection pressure possible without increasing the rate of inbreeding too quickly. I don't know the answer as to the optimal size of a population. If it is a practical breeding program involving numerous traits several of which are genetically correlated in a negative way and if it includes fitness traits, small populations will either cause too rapid a rise in the rate of inbreeding, or cause too slow or erratic an improvement in the complex of traits for the program to be successful The literature on selection studies is full of examples where the rapid rise of inbreeding has clouded the results. Disease and Selection for Improvement in Performance The need for a high level of genetic resistance to disease in poultry breeding populations has been appreciated for a long time. Prior to the development of a suitable vaccine, the genetic resistance of poultry stocks to Marek's Disease was the only protection a producer had. and successful breeding concerns were vigorously selecting for increased genetic resistance in their stocks. Marek's disease is caused by a virus, and results in neoplastic or tumorous growths and a high level of mortality in affected birds. It is now quite clear from work conducted at ARC that commercial performance is better if birds are both vaccinated for Marek's as well as carrying a high level of genetic resistance to this disease. For a disease such as Marek's. the vaccines available to date only give partial protection and the performance of stocks that are both genetically resistant and vaccinated is generally superior to those also vaccinated but with a lower level of genetic resistance to this disease. More recently, it has been clearly shown by research originally carried out at ARC and later confirmed by others that the disease lymphoid leukosis of poultry can have serious subclinical effects on performance. Lymphoid leukosis is caused by a virus induced lymphoblastic malignancy originating in the_ursa of Fabricius. Even though losses directly attributable to this disease are usually quite low (only 1 to 3% mortality in a laying year), the effects on birds carrying the virus and not showing any clinical symptoms are large and affect nearly all the traits of economic significance from egg production to shell strength. Some of the negative effects associated with this viral infection are given in Table 2. Poultry breeders are eliminating this virus from their primary breeding populations, since this improves the performance of their strain-cross stocks when used commercially. This -30-

10 improvement is possible because the virus is transmitted vertically, while its horizontal spread is limited. A by-product of the elimination of the virus will be an improvement in the within strain selection process, since the presence or absence of the virus will no longer be a factor in the performance record of an individual or family. There is also growing evidence to show that the detrimental effects associated with congenital and horizontal lymphoid leukosis virus infections are unequal. Further. when selection is relaxed such as when numbers are being increased to expand parent lines or in some crossing schemes, the frequency of affected birds can increase dramatically, if the virus has not been eliminated in the parent stocks. It has also been clearly demonstrated by several institutions including ARC that some genotypes of the B or major histocompatability complex in chickens are associated with resistance to Marek's disease. Birds with the _B21 allele have been shown to have a higher level of resistance to Marek's disease than those with other alleles at this locus. The association of disease resistance and susceptibility with major histocompatibility loci of other species such as mice and man is well known. In chickens the B genotype is detectable by blood typing and some major poultry breeding companies are using it in their breeding programs. Although genetic resistance to disease has been demonstrated for specific diseases in the breeding of economic species other than poultry, it remains yet to be formally incorporated into long-term breeding programs. The possibility of the practical use of direct selection for resistance to specific diseases or general disease resistance should be kept in mind by large animal breeders, as these techniques have been clearly shown to be very effective practical tools for the poultry breeder. Heterosis and Crossing Commercial poultry breeders have been utilizing heterosis very effectively for many years, in fact they pointed the way for other domestic species such as swine, sheep and cattle. However. the enthusiasm for utilizing heterosis in large animal breeding sometimes leads to a neglect of the importance of continuous improvement of the strains or breeds to be used in crossing. We have good evidence now in poultry to show that to maximize the performance of crosses the pure strains or pure breeds used in the crosses must be continually improved. I will illustrate this point with data from our selection study. We crossed selected strains 1 and 8 in 1972 and again in (over)

11 The results for these pure strains and their crosses for three traits: hen housed egg production, egg weight and body size, are shown to illustrate the principle (Table 3). Our selection program has resulted in an increase in egg production of approximately 22 eggs in 8 generations. The egg production of the unselected control strain 7 went down slightly in 1980 compared to 1972 (Table 3) which indicates the environment was not quite as good as Note that average heterosis for egg number has increased from 5 to Ii eggs, a substantial increase. The overall increase in the strain-crosses comes both from an increase in the performance of the pure strains 1 and 8 and also from an increase in the heterosis when these two strains are crossed. However, most of the increase you should note comes from the improvement of the two pure strains. Note for body weight we have managed to decrease mature body size in the tw0pure strains. Some of this is lost in the crosses where there is heterosis in the wrong directionl This is probably partly associated with the increase in egg size of the crosses. Another important thing to note in Table 3 is the magnitude of the difference between reciprocal crosses." This is particularly noticeable for egg production, but it is also clear for egg weight. The sex chromosome of the chicken is large and it seems that it carries genes concerned with egg size and production. While reciprocal effects may not be as important in other domestic species, the only way to be certain is to test the reciprocal crosses. Environment and Manaqement In our long-term study we have the usual environmental fluctuations caused by a wide variety of factors from heat and cold waves to feed compounding errors and disease outbreaks. These types of changes have been more or less at random and they lead to the usual fluctuations in any performance curve plotted over generations. We_-have also had-some other deliberate management changes where the effects continue as long as the practice continues. I'll illustrate this by three such changes: i) in 1970 we introduced a change in the lighting program for the test birds and this program has had no major changes since that time: 2) from 1965 on we housed all birds in individual cages whereas they had previously been housed in pens in which the birds were free to move about on a floor covered with litter: 3) in 1969 and 1970 we had a severe Marek's disease outbreak and heavy bird losses, and we subsequently (1971 to the present) vaccinated our stocks for Marek's disease. -32-

12 The lighting change during the brooding and rearing period had most of its effect in the early part of the production year (up to 273 days). The effect was dramatic for rate of egg production of survivors (Figure 9). Rate of egg production of survivors from age at first egg was chosen to illustrate the effect of the lighting change rather than hen-housed egg production in order to remove as much as possible the effect of Marek's disease in 1969 and and changes in sexual maturity due to selection. All strains went up dramatically in part-year rate of egg production after the introduction of the new lighting scheme in This effect has been consistent since Note both control strains 5 and 7 went up as well as the selected strains. Without these control strains the genetic change that has taken place under the new lighting program would have been grossly overestimated. In Figure i0 you can see the full test year hen-housed egg production results for three of our strains and two control strains. Our strain 5 control population did not adapt very well to the laying cages in 1965 and the strain dropped severely in production (most of this drop occurred in the latter half of the test year). This resulted' in a classical case of a genotype-environment interaction, that is. all strains did not react to the change in environment in the same way. Data we have obtained recently suggest that the strain 5 birds get too fat on a full-feed regime in individual cages. since they do not have the genetic ability to lay at a high enough rate to utilize the feed consumed. This obesity reduces their production in the latter part of the test year. Without correcting for this effect genetic gain would be over-estimated. The point we want you to note is that permanent changes in the environment sometimes give rise to large effects which must be accounted for in the selection scheme and not incorrectly attributed to a genetic change, when it is an environmental change. The effects of Marek's disease in 1969 and 1970 were dramatic (Figure i0) and affected all strains, but those highly susceptible to Marek's were affected the most. Again there were genotype-environment interactions under this severe disease exposure. Vaccinations for Marek's disease started in 1971 and restored the general overall performance of all strains (Figure i0). The original Marek's vaccine was cumbersome to use and costly (a cell-associated vaccine), and when a new simpler vaccine (lyophilized) came on the market a test was run in 1974 by splitting the test population in this experiment. This showed that both vaccines were generally effective and the differences averaged over all strains between the vaccines were not large this year. The new lyophilized (over) -33-

13 vaccine was adopted in 1975 even though there were small strain-treatment differences in performance, which at that time we thought were only chance differences. In particular strain 7 was not protected as well with the new vaccine as with the cell-associated vaccine. This can be seen in the rapid decline in the hen-housed egg production of strain 7 from 1974 to 1978 (Figure i0). Laying house mortality for all strains increased slowly from 1974 to 1978 resulting in the decision in 1979 to return to the cell-associated vaccine. The purpose of this discussion is to point out that the change to the use of the lyophilized vaccine for 4 years complicated the evaluation of genetic progress in the selected strains because of the genotype-environment interaction that involved both selected and control strains. The fact we had unselected control strains made it possible to appreciate this situation. Although I am not going to discuss the efficacy of different vaccines, I think this subject needs further investigation. It may be that higher doses are needed when the lyophilized type of vaccine is used. Changes in environment should not be introduced without careful study. If haphazardly introduced they will lead to a dissipation and loss of selection effort. However, if there are sound reasons to make management or diet changes they should be implemented to permit the test populations to perform under the improved nutritional or management regimes, that exist or soon will exist in industry. This may also lead to uncovering further useful genetic variation, as we susp'ect was the case when we cage housed all our birds in There is now good evidence available from poultry breeding studies on the value of developing procedures for separating genetic and environment trends, to make it possible to evaluate the effectiveness of the particular selection program being used and to know when corrective action is required. In practical programs with large animals and also with poultry to a lesser degree, it is rarely possible to maintain a constant environment nor would it be desirable. Selection is best conducted within the environment in which the animals are expected to perform. It should be expected that there will be significant improvements made in knowledge as to the best management or nutritional regimes for the species. Changes to these new regimes should be introduced carefully to keep the environment as close as possible to that under which the species will be expected to perform in industry. However, I want to make it abundantly clear that it is futile to select stocks to perform under bad management whatever the qgneral management or nutritional regime. To make reasonable genetic progress, the environments used to test animals must be -34-

14 representative of those of the good managers and operators. It is futile to use "random environments" that include those of the poor or careless managers. A breeder can be led seriously astray and think that progress is being made. when in effect it may not be. or vice versa, when the breeding program is unable to measure the envrionmental trends and to take these into consideration in evaluating genetic trends and the progress being made. Our selection study clearly shows the need for some technique to separate genetic and environmental trends. It doesn't of course give the answer as to which of the several techniques available is the most practical for different species and particular populations. That topic has been reviewed in detail by Hill in two papers (1972a. b). (over) -35-

15 TABLE I. Traits under selection in the long-term selected strains with their approximate heritabilities and approximate selection emphases. Approx- Approximate Approximate imate selection selection Herita- emphasis emphases Traits bilities* within traits*** among traits*'* Fam. Fam. Ind. igree H.H. Prod. to 273 days-*.i AFE HD% to 273 days** Egg Weight Livability ,. Specific Gravity d. Body Wt Haugh Units Blood Spots.I Fertility Hatchability * Based on sire components of variance ** Each strain is selected for either hen housed egg production to 273 days or for hen day percent egg production from age at first egg to 273 days. The remaining traits are selected for in all strains. *** The number of plus signs indicates the relative emphasis, the greater number of pluses the more emphasis. From Gowe and Fairfull (1982) Sire Dam Ped- -36-

16 TABLE 2. The difference between shedders and non-shedders of the lymphoid leucosis virus in several important traits in the years pooled over several strains Trait Age at first egg (days) Hen-housed egg production to 497 days Survivor egg production to 497 days Egg weight at 240 days (g) Shell strength at 240 days Mortality from all causes (%) (over) -37-

17 TABLE 3. Performance means, mean change and approximate heterosis for selected strains 1 and 8 and their crosses tested in 1972 and Strain Hen-Housed Egg 240 day Egg 365 day Body or Production Weight (g) Weight (g) Stcain- from Housing to Cross 497 days minus minus minus I* " Mean x x I Mean Heterosis 5 II " Strains 1 and 8 were housed in a separate poultry house from the strain crosses and control strain here. -38-

18 -39- (over)

19 _I381_IAN_)E)3-40-

20 -41- (over)

21 -42-

22

23 S.I.I NI'I -44-

24 -45- (over)

25 'l I I I t 0.LN3OI:I3d -46-

26 IN30_3d (over) -47-

27 -48-

28 REFERENCES Fairfull. R.W Combining ability, heterosis and reciprocal effects for first and second year performance in six selected Leghorn strains crossed in a complete diallel. In: Proc. 31st National Breeders' Roundtable. St. Louis, Missouri, pp Fairfull, R.W., and R.S. Gowe Feed consumption and feed efficiency in selected and control strains of egg stocks under long term selection for a complex of economic traits. In: Selection Experiments in Laboratory and Domestic Animals: The Proceedinqs of a Symposium, pp Edited by A. Robertson, Commonwealth Agricultural Bureaux, Farnham Royal, Slough, U.K. Fairfull, R.W., R.S. Gowe, and J.A.B. Emsley Diallel cross of six long-term selected Leghorn strains with emphasis on heterosis and reciprocal effects. Br. Poultry Sci. 24:(in press). Gavora, J.S., and J.L. Spencer Studies on genetic resistance of chickens to Marek's disease - a review. Comp. Immun. Microbiol. Infect. Dis. 2: Gavora, J.S., R.S. Gowe, and A.J. McAllister Vaccination against Marek's disease: Efficacy of cell-associated and lyophilized Herepesvirus of Turkeys in nine strains of Leghorns. Poultry Sci. 56: Gavora, J.S., B.M. Longenecker, J.L. Spencer, and A.A. Grunder New histocompatibility haplotypes and Marek's disease in chickens. 2nd World Cong. Genet. Appl. Livestock Production, Madrid, Spain, 7: Gavora, J.S., J.L. Spencer, R.S. Gowe, and D.L. Harris Lymphoid leukosis virus infection: effects on production and mortality and consequences in selection for high egg production. Poultry Sci. 59: Gowe, R.S Multiple-trait selection in egg stocks, i. Performance of six selected lines derived from three base populations. 2. Changes in genetic parameters over time in the six selected strains. In: Proc. 26th National Breeders' Roundtable, Kansas City, Kansas, pp Gowe, R.S., and R.W. Fairfull Performance of six long-term multi-trait selected Leghorn strains and three control strains, and a strain cross evaluation of the selected strains. In: Proc. South Pacific Poultry Sci. Cony., Auckland, N.Z., pp Gowe, R.S., and R.W. Fairfull. 1982a. Some lessons from selection studies in poultry. In: Proc. World Conq. on Sheep and Beef Cattle Breedinq, Palmerston North, N.Z., i: (over) -49-

29 Gowe, R.S., and R.W. Fairfull. 1982b. Heterosis in egg-type chickens. 2nd World Congr. Genet. AppI. Livestock Production, Madrid, Spain, 6: Gowe, R.S., W.E. Lentz, and J.H. Strain Long-term selection for egg production in several of White Leghorns: performance of selected and control strains including genetic parameters of two control strains. 4th European Poultry Conf., London, U.K., pp Gowe, R:S., A. Robertson, and B.D.H. Latter. 1959a. Environment and poultry breeding problems. 5. The design of poultry control strains. Poultry Sci. 38: Gowe, R.S., A.S. Johnson, J.H. Downs, R. Gibson, W.F. Mountain, J.H. Strain, and B.F. Tinney. 1959b. Environment and poultry breeding problems. 4. The value of a random-bred control strain in a selection study. Poultry Sci. 38: Hill, W.G. 1972a. Estimation of genetic change. I. General theory and design of control populations. Animal Breeding Abst. 40: Hill, W.G. 1972b. Estimation of genetic change. II. Experimental evaluation of control populations. Animal Breeding Abst. 40: McAllister, A.J Multiple-trait selection in egg stocks. 3. Retrospective evaluation involving individual and family selection. In: Proc. 26th National Breeders' Roundtable, Kansas City, Kansas, pp

30 1983 NATIONAL BREEDERS ROUNDTABLE Questions to Dr. Robert Gowe Title: "Lessons from Selection Studies in Poultry for Animal Breeders" Question from Ramakrishna Reddy: Your Strain 1 seemed to be significantly superior to your Strain 8 for egg production. The performance of reciprocal crosses seem to indicate the strong sex-linked effects for egg production. How much of the difference for egg production between reciprocal crosses be due to the effect of Z-chromosome from sire strain? Answer: Strain I is superior to Strain 8 particularly for rate of egg production for the full test year from sexual maturity (to 497 d.) as can be seen in Fig. 3. It was also superior for hen-housed egg production but not as consistently (Fig. 2). The reciprocal crosses of ix8 and 8xl differed by i0 eggs in This difference is not large but some of it could be due to the large sex chromosome (Z) contributed by the sire to each of his daughters. This large sex chromosome is known to carry a great deal of active genetic material. Data from Table 3 also suggests this chromosome affects egg size with Strain 8 having better sex-linked egg size genes than Strain i. Question from Kenneth Goodwin: Why do you not put more emphasis on sire and dam family averages when selecting for livability? Answer: I guess our thinking was that individual selection had been rather complete since we could not breed from dead birds. For live birds, we consider only sire and dam family averages. We did use males and females from families that had mortality, but avoided those with mortality levels considerably higher than average. We treat this trait something like fertility and hatchability in that we don't pay too much attention to minor variation in this trait, treating it as environment. However, when there are large deviations in family means, it is considered that it may have a genetic basis, and males and females from such families are not selected. Question from James Arthur: Did you have a particular reason for avoiding the use of the "Osborne Index" in your selection experiment? Answer: We didn't have any strong reason for avoiding the "Osborne Index" - that is the weighting of the full-sib, half-sib and individual information for a single trait by their respective heritabilities for the trait. It may have been helpful, if one has good estimates of the different heritabilities. It would add an element of complexity when considering numerous traits that I didn't think would be cost-effective. Also, we would have to change our method of selection. One of the things we pay attention to is the variation within families as well as the mean, particularly when selecting males. -51-