Thus, it is evident that some compromise must be made in selections, and that care must be exercised to avoid selecting in an undesired direction. Sows possessing and transmitting the ability to fatten rapidly tend to be poor in suckling ability, and thereby reduce the gains and increase the feed requirements of their pigs during the period of fattening. It appears now, therefore, that in selection for rapid and economical gains, the indirect selection for fat carcasses associated with the most rapid gains may be offset if selections are based on individual gains of pigs during the early part of the growth period, perhaps at 85 to about 112 days of age, when more of the weight is muscle than later, and by giving much attention to suckling ability of sows, which is indicated by litter size and weight of pigs at 3 to 8 weeks of age.

Not all lines developed in any program with swine, or with other classes of livestock, will be valuable for use in livestock improvement. For example, it is becoming apparent that only a part of the 46 inbred lines of swine on hand in the Regional Swine Breeding Laboratory program will merit maintenance and use for improving purebred herds and for extensive use in pork production. Some wastage of lines is inevitable. Many inferior lines of corn have been discarded, and the same will apply to inbred lines of livestock. E. W. Lindstrom of Iowa made a survey of the results of inbreeding of corn (maize) in 1939, and estimated that only 2.4 percent of a total of about 30,000 inbred lines developed in the United States during several previous years had proved useful.

The expense of developing an inbred line of livestock is of course much greater than for a line of corn. For this reason, it is generally desirable to guide the development of lines as much as possible by selection, and to test them thoroughly before deciding to retain or discard them. But for the same reason, it is necessary to be ruthless in discarding lines, once it is clearly demonstrated that they can make no worthwhile contribution to improvement, rather than to follow a natural desire to retain expensive (but not valuable) stock with the hope that it may prove useful.

The outstanding inbred lines of swine may be used for crossing with noninbred stock or for crossing with other inbred lines to produce market hogs, or they may be used for crossing with other lines to develop still better lines from which stock will be available for use in commercial production or for improving purebred herds. The situation will be somewhat different for cattle and sheep than for swine. Since such a large proportion of the female offspring in inbred lines of cattle and sheep must be retained for replacements, the numbers that can be used for crossing with males from other lines for commercial production will be small. Experimental work has started to test the various ways of utilizing inbred lines, and further results should be had before recommendations are made.

One of the most urgent problems in connection with the use of inbred lines is the development of methods of preserving them and guarding their purity. The experiment stations cannot continue to maintain established lines indefinitely. Just as they developed many inbred lines of corn and placed them in the hands of organizations that were producing hybrid seed commercially, the inbred lines of livestock which they develop must be placed in the hands of private breeders who will expand numbers and make stock available to commercial livestock producers. Limited attempts are already being made to place some inbred swine in the hands of private breeders. Ways to guard the purity of the lines are yet to be found.

It is possible that the existing breed associations and their purebred-breeder members will take over this work as an additional function of the purebred association for each breed involved. This would be a logical development, and in the initial stages might be carried out with the help and guidance of the experiment stations that are developing the lines. If the associations do not expand their services to meet this need as it arises, other methods must be found. It is difficult to predict how rapidly the need will expand, but it should be remembered that the extensive hybrid-seed corn industry of today had its beginning only in 1926, when the first seed company was organized for the commercial production of hybrid corn, and that the first appreciable expansion began about 1932, when hybrid-seed production was taken up by several new companies.