The Regional Swine Breeding Laboratory and the cooperating State agricultural experiment stations in Illinois, Indiana, Iowa, Minnesota, Missouri, Nebraska, Oklahoma, and Wisconsin have a total of 46 lines If swine. These include 19 Poland China, 11 Duroc-jersey, 4 Hampshire, 8 Chester White, and 1 Landrace line, and 3 lines that are being developed from crossbred foundations.

Eight new lines are being developed by the Bureau of Animal Industry at Beltsville. Another line is being developed in cooperation with the Maryland Agricultural Experiment Station, and two additional lines are being developed at Miles City, in cooperation with the Montana Agricultural Experiment Station.

Thirty-two lines of Rambouillet sheep are being developed at the Western Sheep Breeding Laboratory, Dubois, Idaho. In addition, the Bureau of Animal Industry is developing 10 lines of Columbia and 10 of Targhee sheep at the United States Sheep Experiment Station, which also is located at Dubois.

A few lines of beef cattle are being developed by the Bureau of Animal Industry and the Montana Agricultural Experiment Station at the United States Range Livestock Experiment Station at Miles City. Plans have been developed to establish up to 30 or 35 lines in cooperation with several State experiment stations in the range-cattle area, and lines of beef and dual purpose cattle are being established at Beltsville.

Breeding work with livestock progresses slowly, for obvious reasons. The reproductive level is low, compared with plants, and the time required for a generation is long. The long time per generation is illustrated by the following estimates made by Jay L. Lush : Horses, 10 to 13 years; beef cattle, 4.5 to 5 years; dairy cattle, 4 to 4.5 years; sheep, 4 to 4.5 years; swine, about 2.5 years. Also, with the exception of swine, a large proportion of the female progeny reared must be retained as replacements in order to maintain numbers. Lush gives the following estimates of the percentages of females that must be retained for this purpose: Horses, 35 to 45; beef cattle, 40 to 55; dairy cattle, 50 to 65; sheep, 45 to 55; swine, 10 to 15. While these factors, over which the breeder has no control, place limits on the rate of progress, they also make it imperative that the most effective methods of selection and breeding be used if the breeder is to have much real genetic progress to show for each generation of breeding effort. And despite the handicaps that limit the rate of progress in animal-breeding experimentation, much has been learned in recent years from the work with inbreeding of livestock.

That work has not progressed to a point where broad generalizations can be made, and many details of application of results must yet be worked out, but the results to date indicate that the breeder can make effective use of this tool in speeding up improvement. The work with swine naturally has gone ahead more rapidly than the work with sheep and beef cattle. To illustrate what is being accomplished, I outline some of the results in the Regional Swine Breeding Laboratory, which was established in 1937, with W. A. Craft as director, and the eight State experiment stations that are conducting projects in this regional effort, under the leadership of J. L. Krider ( Illinois) , J. R. Wiley (Indiana), Jay L. Lush (Iowa), L. M. Winters (Minnesota), L. A. Weaver (Missouri), M. L. Baker (Nebraska), O. S. Willham (Oklahoma), and A. B. Chapman (Wisconsin).

Increased inbreeding has generally been accompanied by some deterioration in the productivity of swine. This was expected, in view of the results obtained earlier with laboratory animals and corn. It has generally been more difficult to maintain vitality and litter size than growth rate in lines of swine that were being inbred.

Crossing of inbred lines has usually corrected the decline in performance that accompanied inbreeding, and in some cases appears to result in a considerable increase over the performance of non-inbred stock.

Inbred lines developed from widely unrelated stock have produced more favorable results when crossed than lines developed from related stock. Inbred lines from different breeds have given more favorable results in crosses than those from the same breed.

The mating of inbred boars of selected inbred lines to non-inbred sows appears to give a little increase in the performance of the pigs, in comparison with pigs from similar sows and sired by non-inbred boars.

Inbreeding of 30 to 40 percent appears to be enough to make it possible to determine the value of a line of swine, and to make lines differ genetically, particularly if the lines are from unrelated stock. This is equivalent roughly to about 2 generations of brother-sister mating, 4 generations of half-brother-sister coatings, or 12 generations of breeding in which single first cousins are mated.

Lines that were inbred 30 to 40 percent have been found to differ in physiological characters that were not evident in the appearance of the animals. For example, boars in different lines at the Minnesota station have been found to differ in the amount of male hormone excreted in the urine and in the rate of development of the testes.

Selection for one character may in some cases give rise to a change not desired by the breeder in another character. There is some evidence, for example, that maximum rate of fattening seems to be opposed to litter size and milking ability in sows. Some of the breeder's effort is canceled by the compromise necessary in selection between various characters. Studies now in progress have revealed that hereditary factors of the individual pig that cause rapid and economical gains when the pig is full-fed to 225 pounds, and that also lead finally to high ratings for conformation in the live pig, are largely the same features that cause rapid deposition of fat, resulting in fat carcasses at the time of slaughtering. These results tend to emphasize that selection based on conformation at market weight according to present standards constitute selection for fatter hogs because the differences in width, depth, and plumpness of body, which loom large in making choices, are largely differences in amount of fat.