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Roberts UNIVERSITY OF ILLINOIS AGRICULTURAL EXPERIMENT STATION Bulletin 489 CONTENTS PACE SCOPE OF EXPERIMENTS 123 STANDARD FOR MEASURING RESULTS 124 ANALYSIS OF EXPERIMENTAL DATA 124 Number of Pigs per Litter 124 Birth Weight 126 Survival Ability 126 Weaning Weight 127 Average Daily Gain 128 Economy of Feed Utilization 129 Danish Experiments 129 CORN HYBRIDIZING METHODS NOT APPLICABLE. . . 133 SUMMARY AND CONCLUSIONS 134 BIBLIOGRAPHY.. . 136 Urbana, Illinois July, 1942 Publications in the Bulletin series report the results of investigations made or sponsored by the Experiment Station Crossbreeding in Swine: Does It Offer an Effective Method for the Improve- ment of Market Hogs ? By W. E. CARROLL and E. ROBERTS' lORK PRODUCERS are seeking a market hog that is capable of converting feed into pork more rapidly and economically than is done by present types of swine. The success of hybrid corn has led many to believe that similar beneficial results might be expected to follow the crossbreeding of swine, and many practical swine growers have adopted this method of producing market hogs. Research on the question is not lacking, but the considerable num- ber of investigations that have so far been reported have not clearly established the value of crossbreeding. The uncertainty of its worth in practice and the conflicting reports of investigators appear to justify a careful study of all available data bearing on crossbreeding as it may influence the productivity of swine. Such a study seems to be further justified by the method used in this investigation to compare the crossbred animals with the purebreds (see page 124). SCOPE OF EXPERIMENTS Over 50,000 animals were involved in the experiments reviewed, in which the performance of purebred and of crossbred pigs was studied. The reports have come from the U. S. Department of Agri- culture, nine state agricultural experiment stations, and six foreign countries. The experiments differed widely in extent and duration. Some involved only one or two litters ; others included hundreds of litters and continued for several years. The methods of experimenta- tion also varied ; and several different measures for judging the effect of crossbreeding were used. The differences in experimental procedure made the summarizing of the results a difficult task. No single experiment was found in which an attempt was made to measure the worth of crossbreeding at all the points mentioned in the first paragraph below; but all experiments reporting data on some one or more of them were utilized in this analysis. J W. E. CARROLL, Chief in Swine Husbandry; and E. ROBERTS, Chief in Animal Genetics. 123 124 BULLETIN No. 489 [July, STANDARD FOR MEASURING RESULTS The advantages of crossbreeding, if any exist, might conceivably be expressed in any of several different ways: more pigs farrowed per litter; heavier birth weights; more vigorous pigs as shown by their ability to survive ; heavier weaning weights ; more rapid growth ; and greater economy of feed for gains made. Before examining the results of the experiments, a standard of measurement should be determined. The following standard appears to be exacting and logical: For crossbreeding to be judged beneficial, the performance of the crossbreds must excel the performance of the better of the two parental strains of purebreds. If, for example, the cross is between purebred Duroc Jersey and purebred Poland China swine, the crossbreds should gain more rapidly and economically than do either the Duroc Jerseys or the Poland Chinas of the same parental stocks. If the crossbreds excel only one of the purebreds and are inferior to the other, then they must be considered only a blend of the two purebred lines with no evidence of hybrid vigor. Unfortunately the necessity for comparing the crossbreds with both parental strains in order to prove the value of crossbreeding was not considered in a majority of the experiments. In some experiments the pigs of only one of the two pure breeds were tested with the crossbreds, whereas in other experiments the average performance of the two pure breeds was compared with the performance of the cross- breds. Results from such experiments give no opportunity to judge whether a given cross excels the better purebred parent stock or is only a blend of the two purebred lines. Adherence to the above standard meant the exclusion from this study of all experiments in which separate data for each of the two pure breeds were not given. Data gathered at certain Danish pig-testing stations over a period of years are of somewhat different nature and are discussed separately on pages 129 to 133. ANALYSIS OF EXPERIMENTAL DATA Number of Pigs per Litter The number of pigs farrowed per litter is the result of two factors: (1) the number of eggs produced and fertilized and (2) the prenatal 1942} CROSSBREEDING IN SWINE 125 mortality of the embryos. There is no evidence that crossbreeding affects the number of eggs produced and fertilized. That the prenatal mortality was not lower in crossbred litters than in purebred litters can be determined from data on twelve experiments summarized in Table 1. The average litter sizes were derived from 2,619 purebred litters and 1,515 crossbred litters. Prenatal mortality was no lower in the crossbred litters than in the purebred litters if TABLE 1. SIZE OF LITTERS IN CROSSBREEDING EXPERIMENTS WITH SWINE BY VARIOUS INVESTIGATORS Number of litters of Pigs per litter of Reference* Pure breed with larger litters Pure breed with smaller litters Cross- breds Pure breed with larger litters Pure breed with smaller litters Cross- breds 3 30 338 558 9.7 8.0 8.0 11 11 11 11 79 54 34 34 35 79 79 35 42 7 11 5 9.6 10.7 9.9 9.9 9.5 9.6 9.6 9.5 9.2 11.6 8.4 8.2 12 12 12 530 530 171 132 171 132 628 86 82 10.2 10.2 9.7 8.6 9.7 8.6 9.3 10.4 9.6 4 4 14 14 14 14 14 14 10.4 10.2 10.2 10.4 10.4 10.6 10 10 38 10 36 16 58 10 10.6 10.1 9.8 9.0 10.0 8.0 Total Average 1 538 1 081 1 515 io.i 9^4 9^5 Numbers refer to literature citations on page 136. crossbreeding did not affect the number of eggs that were produced and fertilized. The crossbred litters contained at farrowing an aver- age of 9.5 pigs, whereas the smaller-litter purebreds had an average of 9.4 pigs per litter and the larger-litter purebreds had an average of 10.1 pigs per litter. In only three experiments did the size of the crossbred litters exceed the litter size of the larger-litter pure breeds, whereas in four experiments it was below the average of the smaller-litter pure breeds. In one experiment the crossbred average equalled the average of the larger-litter pure breeds and in another it equalled the average of the smaller-litter pure breeds. In the other three experiments the crossbred average was intermediate between the averages of the two purebred groups. From these data it is obvious that crossbreds are not superior to the better purebreds in size of litter produced. 126 BULLETIN No. 489 Birth Weight Four experiments have been reported from experiment stations in the United States and two from foreign countries in which adequate data were collected to show the effect of crossbreeding on birth weight TABLE 2. AVERAGE BIRTH WEIGHTS OF PUREBRED AND CROSSBRED PIGS Number of pigs of Birth weight of pigs of Reference Pure breed with heavier pigs Pure breed with lighter pigs Cross- bred matings Pure breed with heavier pigs Pure breed with lighter pigs Cross- bred matings 3 7 291 609 2 704 792 4 664 397 Ib. 2.71 3.27 Ib. 2.53 2.93 Ib. 2.77 3.24 10 10 14 14 403 101 167 157 353 144 147 36 580 80 ^301 115 2.40 3.54 2.67 2.80 2.38 3.02 2.45 2.58 2.45 3.12 2.67 2.49 Total Average 1 728 4 176 6 137 2.90 2.65 2.79 of pigs. These experiments, involving nearly 6,000 purebred pigs and more than this number of crossbreds, are summarized in Table 2. The average birth weight of all crossbred pigs in these experiments (2.79 pounds) was intermediate between the average birth weights of the pigs of the purebred groups (2.90 pounds, the average of the pure breeds having heavier litters, and 2.65 pounds, the average of the other pure breeds). In two experiments the crossbred pigs were slightly heavier at birth than the pigs of either of the pure breeds, in one they were lightest of the three groups, in one the crossbreds and heavier of the purebreds weighed the same, while in the remaining two experiments the average birth weights of the crossbred pigs were intermediate between the weights of the two pure breeds. This summary indicates that, on the average, good purebreds can be expected to weigh at birth as much as crossbreds, but that there may be considerable variation in results from experiment to experiment. Survival Ability Survival ability was reported for 24,162 purebred and 9,935 cross- bred pigs in 15 experiments. These data are summarized in Table 3. As an average for all the experiments, the same percentage of crossbred pigs survived as survived in the higher-surviving purebred groups. In only one case was the percentage survival of the crossbred 1942} CROSSBREEDING IN SWINE 127 pigs less than that for the lowest of the parent strains. In nine cases the survival of the crossbreds exceeded that of both parent stocks, TABLE 3. SURVIVAL ABILITY OF PUREBRED AND CROSSBRED PIGS Number of pigs of Percentage survival Reference Higher-sur- viving pure breed Lower-sur- viving pure breed Cross- bred matings Higher-sur- viving pure breed Lower-sur- viving pure breed Cross- breds 7 609 792 397 72 70 73 It 333 758 386 84 83 89 11 758 578 81 83 74 78 11 758 337 92 83 72 88 11 333 337 41 84 72 90 12 1 135 5 406 5 840 82 76 83 12 1 659 5 406 894 84 76 86 12 1 659 1 135 787 84 82 83 4 143 145 145 96 80 94 4 143 145 149 96 80 89 22 51 34 47 96 82 89 10 403 353 580 72 67 77 10 101 144 80 65 54 70 14 167 147 301 57 48 60 14 36 157 115 69 65 55 Total 8 288 15 874 9 935 Average 80 72 80 and in five cases the survival of crossbreds was intermediate between the survival of the two groups of purebreds. Weaning Weight Adequate data on the weaning weights of 23,655 purebred and over 9,500 crossbred pigs have been assembled from 15 different experi- ments and are summarized in Table 4. Again the average value for the crossbreds is intermediate between the values for the two purebred stocks. In seven experiments the crossbred pigs were 1 to 4 pounds heavier at weaning than the heavier of the two purebred strains. In four others the crossbred pigs were the lightest of the three groups; in one other the average weaning weights of the crossbred and of the heavier purebred group were the same, whereas in three experiments the average weights of the crossbreds fell between the average weights of the two pure breeds or equalled that of the lighter pure breed. These results suggest that under similar conditions crossbred and good purebred pigs in general can be expected to weigh approximately the same at weaning time. 128 BULLETIN No. 489 [July, TABLE 4. WEANING WEIGHTS OF PUREBRED AND CROSSBRED PIGS Number of pigs of Weaning weight of pigs of Reference Pure breed with heavier pigs Pure breed with lighter pigs Cross- bred matings Pure breed with heavier Pigs Pure breed with lighter pigs Cross- bred matings 7 609 792 397 Ib. 51 Ib. 48 Ib. 51 11 11 11 11 758 758 337 337 333 578 758 333 386 81 92 41 39 39 40 40 36 36 39 36 35 40 42 38 12 12 12 5 406 5 406 1 135 1 135 1 659 1 659 5 840 894 787 21 b 21 b 21b 21b I 9 b 19 b 20b 18 20 b 4 4 145 143 143 145 145 149 28 28 28 28 29 29 22 34 51 47 28 25 29 10 10 353 101 403 144 580 80 39 46 38 45 43 49 21 21 10 3* 20 10 24 () 29 31 28 29 28 28 Total Average 15 522 8 133 9 519 33.4 31.7 33.3 These figures represent number of litters. The number of pigs was not reported. b These weights were taken when pigs were six weeks old. c Data not reported. Average Daily Gain Nine experiments have been reported in which pigs of the two pure breeds and the resulting crossbred pigs were fed for the purpose of measuring their rates of gain. These experiments, involving 1,063 purebred pigs and 794 crossbreds, are summarized in Table 5. The average daily gain for the crossbreds was slightly greater TABLE 5. AVERAGE DAILY GAIN OF PUREBRED AND CROSSBRED PIGS Reference Number of pigs of Daily gain Faster- Slower- gaining pure gaining pure breed breed Cross- Faster- Slower- bred gaining pure gaining pure matings breed breed Cross- breds Ib. Ib. Ib. 22 20 20 20 1.26 1.08 1.34 10 10 136 20 235 20 476 20 .30 .35 .21 .31 1.34 1.45 17 17 17 17 47 87 74 71 40 104 89 45 32 85 70 48 .27 .22 .23 .95 .11 .12 .06 .91 1.09 1.18 1.26 1.89 14 14 18 16 8 13 16 27 .50 .79 .45 .62 1.58 1.79 Total Average 489 574 794 1.430 1.319 1.'436 1942] CROSSBREEDING IN SWINE 129 than for the better of the purebred groups. This difference, .006 pound per head daily, is too small to have either practical or statistical sig- nificance. In four of the nine experiments the crossbreds failed to gain at a more rapid rate than the better pure breed. Economy of Feed Utilization Data from all the experiments in which the amount of feed con- sumed per unit of gain was recorded are summarized in Table 6. As was found in measuring most of the other characteristics, the value for the crossbreds lies between the values for the pure breeds. In half the experiments (three) the crossbred pigs made slightly more TABLE 6. FEED FED PUREBRED AND CROSSBRED PIGS PER 100 POUNDS GAIN Number of pigs of Feed per 100 pounds gain Reference More eco- Less eco- Cross- More eco- Less eco- Cross- niiiiiic.il pure nomical pure bred nomical pure nomical pure breds breed breed matings breed breed Ib. Ib. Ib. 22 20 20 20 300 317 320 10 235 136 476 375 403 370 10 20 20 20 404 425 387 17 47 40 32 308 308 316 14 8 18 16 423 427 414 14 16 13 27 389 410 404 Total 346 247 591 Average ... ... ... 366 382 368 economical gains than either of the purebred groups. In two tests the crossbreds consumed more feed per unit of gain than either purebred group, and in one the value for the crossbreds was intermediate between the values for the purebred groups. No evidence is found in these results that crossbreds gain more economically than good purebreds. Danish Experiments Object of Experiments. The extensive application of crossbreed- ing to commercial swine production in Denmark during many years is frequently cited as evidence of its value. However, such cross- breeding was first undertaken, not to produce pigs that would make more rapid and economical gains, but to improve the carcass qualities of the Danish hog, the Landrace. This hog did not then produce a 130 BULLETIN No. 489 [July, carcass that met the exacting requirements of the British market, the principal market for Danish bacon. Beginning about 1895 the Danes imported Large Yorkshire hogs and began an elaborate government-subsidized swine-breeding program to improve the type of carcass they were producing for export. The aim of the program was to introduce into the Landrace breed the superior carcass qualities of the Yorkshire and yet to retain the hardi- ness and fecundity of the Landrace hogs. After this aim was ac- complished the Yorkshires were to be eliminated and crossbreeding discontinued. (See Reference 2 on page 136 herewith.) The Danish swine-breeding program attempted, among other things, to provide a measure of the value of breeding stock by measuring the performance of the offspring as bacon producers. Four pigs of a litter were sent to a testing station at weaning, where they were fed under standard conditions to bacon weights. Representatives of the group were then sent to a bacon factory, where the necessary carcass measure- ments were taken. Published reports of these tests provide a large amount of data on the feed consumption and gains of the pigs that have been thus handled. Data Selected. For the purpose of the present study, only those tests from the Danish program are of interest in which pigs of both parental pure breeds and of their crosses were tested at the same station the same year. These records provide data on 17,656 pure- bred Landrace and Yorkshire pigs and 786 Landrace X Yorkshire crossbred pigs. The 32 tests were made at five different stations during the period from 1911 to 1932. These data are here treated in the same way as the similar data already presented. The results are given in Tables 7 and 8. In each table the data are arranged in chronologic order and simple averages have been made of the 32 entries. Daily Gain. The crossbred pigs did not outgain the more rapid- gaining purebred group but did gain as much (1.38 pounds per head daily). In 18 of the 32 tests the crossbreds outgained both groups of purebreds ; they were the slowest gainers eight times. In the remaining six cases the gains of the crossbreds either equalled the gain of one of the pure breeds or was intermediate between the two. Economy of Gain. The records for economy of production (Table 8) tell about the same story as the records for daily gains. The average amount of feed eaten per 100 pounds of gain made by the crossbred pigs (344 pounds) lies between the amounts consumed by each of the two parental purebred groups (341 pounds and 350 1942} CROSSBREEDING IN SWINE 131 TABLE 7. AVERAGE DAILY GAIN OF PUREBRED AND CROSSBRED PIGS AT DANISH PIG- TESTING STATIONS Number of pigs of Daily gain Year and station Faster- gaining pure breed Slower- gaining pure breed Faster- Slower- Cross- gaining gaining Cross- bred pure pure breds mating? breed breed 1911-12E 448 208 112 1 040 24 76 344 340 88 40 148 44 64 204 192 432 200 44 164 468 156 476 108 36 248 32 152 380 512 304 80 24 760 7 500 32 48 48 24 16 16 12 20 4 4 16 4 4 16 16 4 12 8 12 4 92 20 28 32 44 48 44 4 4 108 12 12 768 Ib. .32 .23 .30 .28 .30 .30 .30 .28 .28 .32 .30 .34 .32 .36 .36 .39 .36 .43 .43 .50 .43 .52 .39 .52 .32 .50 .52 .47 .45 .39 .43 .50 Ib. .28 .19 .23 .21 .28 .23 .25 .23 .21 .30 .25 .34 .30 .36 .34 .39 .25 .41 .41 .47 .41 .50 .28 .50 .28 .47 .39 .39 .45 .39 .39 .43 Ib. .36 .34 .28 .34 .21 .21 .34 .30 .45 .45 .39 .03 .36 .30 .28 .63 .45 .47 .25 .54 .50 .52 .32 .43 .34 .52 .43 .39 .34 .45 .43 .54 1912-13B 1 232 1913-14B 260 1915-16B 268 1915-16E .... 136 1916-17B 4 1921-22B 8 1921-22E . . . . ... 148 1922-23B 304 1922-23E 240 1923-24B 356 1924-25B . ... 352 1925-26E 288 1926-27E 308 1927-28B 40 1927-28E 368 1927-28L 644 1927-28Ha . . 620 1928-29B . . . . 16 1928-29E 396 1928-29L. .. 36 1928-29Ha . . . . . 568 1929-30B 396 1929-30E 196 1929-30L . ... 536 1929-30Ha 504 1929-30Ho g 1930-31 B . . 16 1930-31 E . ... 268 1930-31L 624 1930-31 Ho 484 1931-32L 84 Total . .10 156 1 . 38 1 . 34 1 . 38 B = Bregentred Station, Station, Ho = Hong Station. E = Elsesminde Station, L = Lojstrub Station, Ha = Haroldskjaer pounds). In 14 of the 32 tests the crossbred pigs ate somewhat less feed per pound of gain than the more economical purebreds ; in 9 years they were the least economical of the three groups ; and in 9 years they ranked between the purebred groups or equalled one of them. The available records of Danish crossbreeding show that well- selected purebred pigs gain as rapidly and as economically as crossbreds. Theoretical Advantage in Danish Cross. During the study of the Danish records an observation was made which suggested that an advantage might well be expected from crossing Landrace and York- shire pigs. These two breeds apparently gain at about the same rate. 132 BULLETIN No. 489 [July, TABLE 8. FEED FED PER 100 POUNDS OF GAIN MADE BY PUREBRED AND CROSSBRED PIGS AT DANISH PIG-TESTING STATIONS Number of pigs of Feed per 100 pounds gain More Less More Less Year and station economi- economi- Cross- economi- economi- Cross- cal pure cal pure bred cal pure cal pure breds breed breed mat ings breed breed Ib. Ib. Ib. 1911-12E. . 448 208 32 378 381 391 1912-13B 1 232 112 48 379 380 353 1913-14B 260 1 040 48 372 380 371 1915-16B 24 268 24 363 375 362 191S-16E 136 76 16 353 357 374 1916-17B. . 4 344 16 374 378 370 1921-22B 8 340 12 338 352 327 1921-22E 88 148 20 367 371 370 1922-23B 40 304 4 335 346 334 1922-23E 148 240 4 369 374 323 1923-24B. . 356 44 16 352 355 332 1924-25B 64 352 4 341 344 415 192S-26E 204 288 4 360 363 336 1926-27E 192 308 16 346 357 360 1927-28B 40 432 16 324 335 373 1927-28E. . 200 368 4 338 351 330 1927-28L 44 644 12 328 333 309 1927-28Ha 164 620 8 333 334 335 1928-29B 16 468 12 323 331 339 1928-29E 156 396 4 336 346 338 1928-29L... 36 476 92 315 329 319 1928-29Ha 108 568 20 330 332 328 1929-30B 396 36 28 338 351 362 1929-30E 196 248 32 329 337 343 1929-30L 32 536 44 330 342 341 1929-30Ha . . 152 504 48 331 337 335 1929-30Ho 8 380 44 312 338 334 1930-31 B 16 512 4 315 339 331 1930-31E 268 304 4 333 342 338 1930-31L 80 624 108 310 334 308 1930-31 Ho. . 484 24 12 336 343 336 1931-32L 760 84 12 330 331 302 Total 6 360 11 296 768 Average 341 350 344 B = Bregentred Station, Station, Ho = Hong Station. E= Elseminde Station, L= Lojstrub Station, Ha = Haroldskjaer (Altho the actual average daily gains reported for the Landrace pigs were above those of the Yorkshire in about 66 percent of the tests, statistically the average difference in gain was not significant.) However, the average difference in feed consumed per pound of gain was highly significant, the Yorkshires using less feed than the Landrace. This difference affords an opportunity to obtain a cross that would maintain the rate of gain common to both and combine it with the Yorkshire's economy of feed utilization. In performance, however, the cross between these two breeds gained no more rapidly than the better purebreds and failed to gain as economically as did the Yorkshires. 1942} CROSSBREEDING IN SWINE 133 CORN HYBRIDIZING METHODS NOT APPLICABLE The great success that has been achieved with hybrid corn has without doubt influenced the thinking of livestock breeders with re- spect to crossbreeding swine. Can the same degree of success be expected from crossbreeding swine that has been experienced with hybrid corn? Anyone who is well acquainted with the procedures in the two fields would not expect this to be necessarily the case, as the methods used are basically different. The production of hybrid corn is based on bringing together highly inbred lines that have been purified genetically by several generations of self-fertilization. Crossbreeding of swine, on the other hand, utilizes existing breeds of hogs without any attempt at previous inbreeding. Those who assume that the two pro- cedures rest on similar genetic principles would have to assume also that present-day breeds of swine are already highly inbred an assump- tion which is well known to be untrue. Another assumption might be that the different breeds of swine are already at a higher level of genetic purity with respect to desirable characters than are the different varieties of corn. If one breed were relatively pure for certain desirable characters and other breeds for other desirable characters, then crossbreeding might be expected to bring together an increased number of desirable characters and thus actually create superior individuals. There is no evidence from the crossbreeding experiments reported in this study to suggest that dif- ferent breeds of swine possess such differences in desirable characters. If, then, the principle of crossbreeding swine is not the same as that by which hybrid corn is produced, what is the possibility of ap- plying the principles of hybrid corn production to swine breeding? This question has not been answered yet, tho it is being actively investigated by the U. S. Regional Swine Breeding Laboratory. A considerable number of theoretical as well as practical difficulties stand in the way of rapid progress in this direction. In the first place success of hybrid corn breeding rests upon the fact that corn can be self- fertilized generation after generation. Swine cannot be self-fertilized at all, and to reach with them a degree of genetic purity such as is obtained in only four generations of self-fertilization would require eleven generations of brother-sister mating, the closest form of con- tinued inbreeding possible with swine. Brother-sister mating of swine has not proved generally practicable, and even where it has been ac- complished it has been continued only with difficulty. Secondly, purified strains of corn, when once produced, can ap- 134 BULLETIN No. 489 [July, patently be maintained generation after generation in a condition of considerable genetic uniformity. Not so a strain of swine, for the two-parent system essential to reproduction in swine insures the presence within the strain of considerable variation. It is doubtless possible to develop and maintain strains of swine at relatively high levels of inbreeding, say as high as 40, 50, or 60 percent. A cross between certain of these inbred individuals would very likely produce excellent crossbred pigs. But this occurrence, would in itself give no assurance whatever that other individuals in future generations of these lines, even tho maintained at the same level of inbreeding, would also produce desirable crossbreds. Maintaining a given level of inbreeding, a relatively easy thing to do, will not guarantee uniform breeding results. Uniform results require the maintenance of the same genetic constitution from generation to generation, an achievement that is impracticable, if not impossible, owing to the biparental method of reproduction in swine. It is not sufficient that inbred strains of animals (or plants) when crossed produce offspring whose performance excels the performance of even the superior of the two inbred parental lines. To be of com- mercial importance, the crosses must be superior also to the populations of the pure breeds from which the two inbred lines were derived. A practical difficulty standing in the way of duplicating with swine the results obtained with hybrid corn is that swine can neither be multiplied at such a rapid rate nor handled in such large numbers as is possible with corn, and large numbers are essential in solving com- plex breeding problems. An overwhelming majority of inbred lines of corn give such poor results when tested that they are discarded. Some workers say that not more than one inbred line of corn in a thousand becomes commercially useful. The per-capita value of hogs would make such extensive elimination prohibitively expensive in practical swine-breeding operations. SUMMARY AND CONCLUSIONS The available literature on crossbreeding swine, which records the experimental use of more than 50,000 animals, is reviewed in this bulletin. Unfortunately not all the results of the experiments were suitable for use in this study because some of these experiments were not designed to yield all the required data, whereas in other experi- ments complete data may have been collected but were not reported. In order to give a complete picture of the value of crossbreeding 1942] CROSSBREEDING IN SWINE 135 swine, an experiment should measure separately the performance of each of the purebreds and of the resulting crossbreds. The advantage of crossbreeding, if such exists, may conceivably show up in a larger number of pigs per litter farrowed ; in heavier pigs at birth ; in greater vigor and survival of pigs ; in larger pigs at weaning time ; or in ability of pigs to make more rapid and economical gains. The available data show that in the case of four of the six factors just enumerated the average value for the crossbred pigs is intermediate between the values for the two pure breeds ; the average percentage survival of the crossbred pigs exactly equals that of the higher-surviving pure breeds, whereas the figure for average daily gain of all crossbred pigs is .006 pound above the figure for the more rapid-gaining pure breed. These averages do not support the belief that hybrid vigor can be expected in the majority of crosses between breeds of swine. What appears from these averages to have occurred is not so sug- gestive of hybrid vigor as of a grading-up process of the poorer pure- breds toward the better purebreds. The averages show only that the crossbreds approach but do not excel the better purebreds. If this is true, crossbreeding has nothing to offer the breeder with a highly improved, carefully selected herd. On the other hand, crossing less productive animals with animals of higher productivity might be expected to yield a crossbred that would excel the poorer parent. But mating poor purebreds with good animals of the same breed would be expected to improve the poor purebreds as much as crossing them with good animals of another breed. Before the wholesale crossbreeding of swine can be advocated as a worthwhile practice, crossbreeding must first be shown to be defi- nitely useful and then a way must be devised to predict with a high degree of accuracy what the successful crosses will be. The consider- able amount of research that has been done thus far has only cleared away some of the preliminary problems. The two crucial problems just mentioned remain to be solved; and they can be solved, if at all, only by research that is more skilfully planned and conducted than past crossbreeding experiments. 136 BULLETIN No. 489 BIBLIOGRAPHY 1. CANADIAN EXPERIMENTAL FARMS, Agassiz [B. C.]. 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