Five Years of Selection for Viability in White Leghorn Chickens* PAUL
Alabama Agricultural Experiment Station,
(Received for publication September 23, 1942)
NE method of decreasing mortality in adult chickens is by breeding strains which are resistant to diseases and disorders. An experiment to determine the feasibility of this method was started at the' Alabama Experiment Station in 1935^ The results of the first 6 years of work are reported in this paper.
The objective of the experiment, as outlined in 193S, was the development of strains of fowls resistant primarily to fowl paralysis, since at that time paralysis accounted for a large part of the adult mortality. Later, when mortality from paralysis was reduced considerably, the objective was expanded to include resistance to other diseases and disorders. Selection oj breeders for resistant strain. The breeding females for the 1935 season consisted of hens two years of age or older and pullets. None of these were progeny tested and most of them were not pedigreed. They were mated to old cocks and cockerels. In 1936, the old hens were used again in the breeding pens and likewise pullets which were sib1tested for a period of four to six months. Adult mortality for those hatched in 1935 and 1936 was very high and consequently few desirable families were developed. In succeeding generations, however, an increasing number of progeny-tested and sib-tested birds was available and only those with better than *Data from 1935 to 1939 collected by C. D. Gordon.
average records of livability and egg production were used as breeders. Criteria of selection, other than viability, were egg production, egg size, and body size. In most cases, hens which laid less than 200 eggs during the first laying year and whose egg weights were below standard were not used in the breeding pens. The breeding birds in 1935 were from the general population and, since no selection for viability other than natural selection had occurred among them, their progeny are not considered the first selected generation. Thus the report is concerned with the unselected generation, hatched in 1935, and the five selected generations hatched in the succeeding years 1936-1940, inclusive. Management. Approximately 4,000 chicks were hatched at the experiment station farm each season. The chicks were allowed access to range from six weeks of age until the time when the pullets were removed to the laying houses. An area of 20 acres was available for ranging the birds; however, only one-third of this was used each year, since a three-year rotation of the land was practiced. Thus the same land has been used for ranging chickens at least every third year for a number of years. At about five months of age, all pullets were placed in the laying houses, and in three of the six years of the experiment, none were culled at this time nor at any time in the course of the laying year. The diet of the birds was essentially the same
PAUL D. STURKIE
for all years of the experiment. All birds were subjected to artificial light in the mornings of the fall and winter months. Test period for mortality and egg production. Mortality and egg production for most pullets tested were calculated from the time of first egg laid until completion of the laying year. Each year a few individuals were very late coming into production and could not complete the laying year within a reasonable length of time; therefore an arbitrary period of one year from time they entered the laying house was considered their test period. Autopsy records. All pullets which died during the test period were autopsied. If, after observation of the gross pathological findings and after consideration of case history, a reasonably accurate diagonsis could not be made, such cases were recorded as not diagnosed. RESULTS Effects of Selection on Mortality and Egg Production The results of five years of selection for resistance to disease and the effects of such selection on egg production are shown in Table 1. Mortality. It will be noted from line 2 of Table 1 that the percentage of pullets dying before completing the laying year has decreased from 89 percent in 1935 to 27 percent in 1940. This decrease through
the years was consistent except for 1939. The apparent increase in mortality in 1939 over 1938 is due, in part, to the fact that no culling was practiced in 1939, whereas in 1938, 19 percent were culled. If those which were culled in 1938 had been left in the laying houses, a high percentage of them likely would have died before completing the laying year, thus increasing the mortality figure for 1938, and for other years where culling was practiced. These figures show clearly that the number of deaths recorded varies with the number of birds culled, and that a mortality figure, unless it is related to number culled, is unreliable. Since in some of the years of the experiment culling was practiced and in others not practiced, a more reliable measure of the progress made in lowering mortality is the actual number of pullets alive at the end of the test period. Thus, it is observed from Table 1 (line 4) that the number of pullets which completed the laying year has increased from 11 percent in 1935 to 73 percent in 1940. Furthermore, the increase from year to year was consistent, except for one year, 1936, which showed a decrease of 2 percent over the preceding one. This difference is likely more apparent than real because a large number was culled in 1936 and none in 1935. There has been, also, a consistent increase from year to year in the average
TABLE 1.—Mortality and egg production of White Leghorn pullets housed from 1935-1940, inclusive Resistant strain
1. 2. 3. 4. 5. 6. 7. 8.
Number of pullets in laying houses Percent of pullets dying before completing year Percent culled Percent finishing laying year Survival age of those dying (in days) Eggs laid per bird before death Eggs laid per bird completing year Eggs laid per bird of all housed (6 and 7) * Unselected generation.
454 89 0 11 299 47 213 64
856 46. 38 16 312 74 221 90
872 35 19 46 327 92 220 151
446 27 0 73 363 92 205 171
54 37 9 305 62 238 72
48 0 52 346 81 206 119
FIVE YEARS OF SELECTION FOR VIABILITY IN WHITE LEGHORNS
number of days lived by those which died (line 5, Table 1), from 299 days in 1935 to 363 days in 1940. This increase means that the pullets which died in 1940 lived approximately seven months of the laying year as compared to five months for those in 1935, and that the actual increase in resistance of the stock is greater than the decrease in number of deaths for the test period indicates. It is not clear from the above report to what extent inherent resistance contributed to the decrease in mortality, since exposure to disease agents may have decreased also during the years of the experiment. One means of ascertaining the degree of exposure is by developing a susceptible strain concurrently with a resistant one (Hutt, 1938; Hutt et al., 1941). In this experiment selection for susceptibility was not practiced until 1940, and then selection of breeders was made from the survivors of 1938 and 1939; therefore a certain measure of resistance had been developed in the stock. The mortality for the susceptible. strain in 1940 (34 percent) was 7 percent higher than that for the resistant strain. The slight difference in mortality for the two strains in 1940 offers little information as to the adequacy of exposure to pathogenic agents over the years. The fact, however, that mortality in a Red breed (after three years of selection for resistance) in 1940 was 45 percent as compared to 27 percent for the resistant Leghorns (both under same conditions of management) indicates that an appreciable decrease in exposure to pathogenic agents had not occurred. Egg production. The effect of the five years' selection upon egg production is difficult to evaluate because of unusually high mortality and culling in some of the years. It is obvious that when rigid culling is practiced the average production of those
remaining is higher than would be the case if culling were not practiced. For instance, in line 7, Table 1, it is noted that the number of eggs laid per bird for those completing the year in 1936 was 238 as compared to 205 in 1940, yet production in 1936 was based upon the 9 percent which finished the year (37 percent culled, 54 percent died) while that in 1940 was based upon the 73 percent which completed the year, with none culled. Moreover, in 1935, with an 89 percent mortality, the 11 percent which finished the year averaged 213 eggs per bird. The higher average egg production observed in most years where mortality was high and culling was practiced is due, it appears, to the fact that such culling and mortality have eliminated the low egg producing and less vigorous pullets. When average egg production is figured on the basis of those birds completing the year, considerable variation from year to year is observed, and no trend toward higher or lower production is evident. When it is computed on the basis of all birds entered (which takes into account the number culled and died, line 8, Table 1) then an average increase in egg production per bird over the years is observed. This increase was consistent except for one year, 1939. The production per bird put in laying houses in 1935 was 64 eggs as compared to 171 eggs in 1940. Likewise, the number of eggs laid per bird before death (line 6, Table 1) has increased, in most cases, with the increase in survival age, as might be expected. Thus, it appears from these data that the increase in average annual egg production for all pullets entered from 1935 through 1940 has been effected mainly, if not wholly, by increasing the number of birds which survive the laying year, rather than by increasing production per individual surviving.
Egg size. The average egg weight of the mature pullets of the resistant strain for 1940 was 59.4 grams per egg, or roughly 25 ounces per dozen. These weights were taken in the latter part of the laying year. Diseases Causing the Deaths Leucosis. In the preceding section, it was shown how mortality has decreased as a result of selection for disease resistance. A large part of this decrease has been due to a decline in number of deaths resulting from one disease or disease complex, leucosis. In 1935 and 1936 paralysis and TABLE 2.—Cause of death of 533 pullets in 1939, and 123 pullets in 1940 Diseases and Disorders
Visceral leucosis Neural leucosis Respiratory diseases Peritonitis Hepatitis Kidney disorders Reproductive disorders Internal hemorrhages Cholera and cholera-like PuUorum Miscellaneous No diagnosis
12.91 2.61 13.19 13.60 11.74 8.45 6.18 2.88 1.85 1.04 18.75 15.85
12.10 1.62 15.44 8.94 7.31 3.25 3.25 13.80 7.31 0.00 8.13 25.20
* Total mortality in percent. The proportion of the total mortality due to each disease is given in percentage.
"gray eyes" accounted for nearly twothirds of the total mortality. By 1938, deaths due to leucosis had dropped to 17 percent of the total deaths. In 1939 and 1940, leucosis accounted for only 15.5 and 13.7 percent, respectively, of the total mortality of these years (Table 2). The greater portion of these deaths resulted from the visceral, type of the disease, rather than from paralysis or "gray eyes." The mortality from leucosis has decreased from approximately 60 percent of all pullets entered in 1935 to approximately 4 percent for all entered in 1940.
Other diseases. In Table 2 are shown the autopsy records of all pullets of the resistant strain dying in 1939 and 1940. The proportion of the total mortality caused by each disease is given in percentage. Since the deaths of some of the pullets were due to more than one disorder and consequently such deaths were recorded more than once, the total percentage for all disorders exceeds 100. Among the diseases and disorders listed in the table, leucosis, cholera, pullorum, and respiratory diseases (mainly colds) are considered specific ones, and they accounted for 31.6 percent of the total mortality in 1939, and 36.5 percent in 1940. These results, like those reported by Hutt (1938) and Lubbehusen and Beach (1939), show that a major portion of the adult mortality is caused by diseases and disorders which cannot be controlled by the commonly recommended measures. Development of Superior Families It was shown in Table 1 that 446 pullets of the resistant strain were placed in the laying houses for the test in 1940. Of this number, 418 were descendants of four dams, B 733, C 1299 and C 1133 (full sisters), and C 251, while the remaining 28 represent progeny from other dams not related to the four mentioned above. The pullets tested in 1940 consisted of daughters, granddaughters, or great-granddaughters of the above dams. Two hundred and seventy-two of the descendants of B 733, C 1299, and C 1133 were sired by two full brothers, and these brothers were progeny resulting from a mating of individuals of the two families. One of the sires which was mated to 10 dams of full and half sister relationship produced 141 pullets which were placed in the laying houses. Of this number, 108 (76.6 percent) finished the laying year (none culled) with an average production
FIVE YEARS OF SELECTION FOR VIABILITY IN WHITE LEGHORNS
of 206 eggs per bird, and an average egg weight of 57.5 grams. The other sire was mated to 11 dams, also of full and half sister relationship. From this mating 131 pullets were housed, 97 of which (74.0 percent) completed the laying year with an average production of 203 eggs per bird, and with an egg weight of 60.5 grams. The range in number of daughters tested per dam of the above matings was 1 to 26, with an average of 13 per dam. Of the 21 dams which were progeny tested, only three had a mortality in their daughters of as much as 50 percent. Nine of the dams had as many as 16 to 27 daughters each tested with the highest mortality in the progeny of any one dam not exceeding 33 percent. The mortality in the daughters of six of these dams ranged from 12.5 to 16.6 percent, with the former figure applying to four of them. Two of the dams had 27 and 24 daughters tested with 23 and 21, respectively, completing the laying year with an average production of 200 eggs or better.
the decrease in mortality, from year to year, reported by these workers was, in general, of the same magnitude as reported by those who maintained a susceptible line for all years, suggests that exposure to agents causing diseases was likely adequate on their premises. Although it is probable that exposure to disease agents is adequate on most experiment station farms and large commercial farms, such may not be the case on the average poultry farm, where usually a smaller number of birds is kept per given area of land and where, in some cases, poultry has been kept for only a short time. An example of how conditions of management may affect the degree of exposure and consequently mortality is afforded by the work of Kennard (1938), who showed that by allowing ample and clean range for the growing stock, subsequent mortality in the laying birds could be reduced. Likewise, Barber (1942) demonstrated that by rearing chicks under isolated conditions, pullet mortality could be lowered considerably.
DISCUSSION The foregoing results show that adult mortality can be reduced by breeding for resistance to diseases, and are in agreement with the results of Taylor and Lerner (1938), Bearse et al. (1939), Marble (1939), Gildow et al. (1940), and Hutt et al. (1941). Bearse et al., Hutt et al., and Taylor and Lerner also developed strains which were susceptible to disease, particularly leucosis. The maintenance of a susceptible line is desirable from an experimental standpoint, but this practice is too expensive and therefore impractical for the commercial poultry breeder. Moreover, the results reported herein and by Marble indicate that considerable progress can be made by selection for resistance alone. The fact that
SUMMARY AND CONCLUSIONS
As a result of five years' selection for disease resistance, adult mortality decreased from 89 percent in 1935 to 27 percent in 1940, with none culled in these years. Moreover, there has been a consistent increase in the survival age of those which died during the test period from 299 days in 1935 to 363 days in 1940. Approximately two-thirds of the deaths in 1935 and 1936 were due to leucosis, while in 1940 less than 14 percent of the deaths resulted from this cause. A majority of the deaths in 1939 and 1940 was due to non-specific disorders. The egg production for those completing the laying year in 1940 was 205 eggs per bird and the average egg weight was 59.4 grams, or above standard size. The de-
PAUL D. STURKIE
crease in mortality over the years has had the effect of increasing average annual egg production per bird for all entered in the houses from 64 eggs in 1935 to 171 in 1940. It was demonstrated that by the development of superior inbred families, uniformity of performance, with respect to high livability, satisfactory egg production and egg size can be attained. REFERENCES
Barber, C. W., 1942. The incidence of avian leukosis complex lesions as influenced by rearing environment. Poultry, Current Science and Practice. Number 4:9-10. Published by Poultry Department, Cornell University. Bearse, G. E., C. F. McClary, and M. W. Miller, 1939. The results of eight years' selection for disease resistance and susceptibility in White
Leghorns. Poultry Sci. (abstract) 18:400-401. Gildow, E. M., J. K. Williams, and C. E. Lampman, 1940. The transmission of and resistance to fowl paralysis (Lymphomatosis) Idaho Agr. Exp. Sta. Bull. 235. Hutt, F. B., 1938. The geneticist's objectives in poultry improvement. Am. Nat. 72 :268-284. Hutt, F. B., R. K. Cole, and J. H. Buckner, 1941. Four generations of fowls bred for resistance to neoplasms. Poultry Sci. 20:514-526. Kennard, D. C , 1938. Livability of pullet layers as affected by their previous management. Poultry Sci. (abstract) 17:441. Lubbehusen, R. E., and J. R. Beach, 1939. Adult poultry mortality of a non-infectious origin. J.A.V.M.A. 47:209-222. Marble, D. R., 1939. Breeding poultry for viability. Penn. Agr. Exp. Sta. Bull. 377. Taylor, L. W., and I. M. Lerner, 1938. Breeding for egg production. California Agr. Exp. Sta. Bull. 626.