ductiveness of sows, vitality of pigs, growth rate, economy of gain, and desirability of carcasses. Attention has been focused on a meat-type hog from the beginning.

Trials relating to methods of selection include rating of animals on appearance by scoring, rating by an index of merit based on traits under selection, litter testing, progeny testing, and measuring fatness of animals. Physiology problems investigated relate to breeding behavior of boars, number of eggs ovulated by gilts and by sows, and embryonic survival of pigs.

Results indicate that as inbreeding progressed with selection in the various populations tested, litter size, survival, and growth rate tended to decline, and a few lines have survived more than 50 percent inbreeding. Records suggest that lines started from crossbred foundations have held up better than those within a pure breed.

Lines which were inbred slowly declined more slowly than those which were inbred rapidly. Results with several lines are good enough to support the view that a herd can be maintained as a closed herd for many years to produce seed stock-boars particularly.

Lines which were retained have been tried in top-crosses, line-crosses, and breed-crosses. The traits which have been affected adversely by inbreeding have responded favorably to crossing-both line-crossing and breed-crossing. Line culling has been based almost entirely on the line's own performance. Economy of gain and desirability of carcasses do not appear to be as severely affected by inbreeding. Good results have been obtained from crossing the best performing inbred lines even when the inbreeding of the lines averaged no more than 25 to 35 percent. Currently, increasing emphasis in the regional project is being placed on breeding methods that improve carcass quality, where selection procedures rather than inbreeding alone are followed.

Nutrition studies of the Department have included examination of the effects on carcass quality of the various nutrients and diets under study. For example, studies have been concerned with different levels or qualities of protein or vitamins as well as fiber content or restrictive feeding. Physiological studies have investigated the effects of the modification of the hormone balance and of certain environmental factors upon hog carcass quality.

Present work.-Two long-time breeding experiments with swine are currently under way at the Department's Beltsville Agricultural Research Center. The two experiments are entitled "Reciprocal Recurrent Selection for Hybrid Vigor in Swine" and "Selection for High and Low Degrees of Fatness in Swine." In these, emphasis is placed on the methodology aspects in order to find procedures for improving carcass quality as well as general performance.

Other important experiments are concerned with identification and certification of meat-type hogs, swine semen preservation, and physiological studies.

The reciprocal recurrent method of selection of parents for use in crossing is based primarily on the performance of the offspring of the crosses and not on the performance of individuals in the parent strains themselves. The animals used to propagate their respective strains are those boars and sows which produce the best cross progenies when first mated with animals of the other strain. Theoretically, recurrent selection should provide continued improvement in the performance of crossbred pigs. It is a method of selection aimed at maximizing the amount of hybrid vigor which usually results from crossing unrelated strains or breeds.

The two principal objectives of the recurrent selection program are to determine the method's practical usefulness for improving the performance of crossbred market hogs and to develop strains for eventual use as seed stock in markethog production. The important elements of hybrid vigor being sought are sow productivity, pig viability, rate of growth to marketweight and feed efficiency. Also considered are carcass traits such as loin eye muscle area, yield of preferred cuts, carcass backfat thickness, and other more detailed carcass composition

measures.

The principal advantage of recurrent selection over the method of inbred line crossing, at first used by corn breeders in hybrid seed production, is that it eliminates the need of developing and maintaining inbred lines. Inbred lines can be difficult to maintain and must be extensively tested in order to find those lines which produce superior hybrids.

The practical value of recurrent selection is now being tested with corn, poul try, various laboratory animals, and swine. In addition to the Beltsville swin projects, similar tests are in progress at other stations. These include Federal

States projects at the Missouri and Oklahoma Agricultural Experiment Stations, and a project at the Montana Agricultural Experiment Station.

The Beltsville project was started in 1955 with two strains which were first crossed with one another for litters in 1956. The results obtained to date show rather substantial advantages for crosses compared with the individual performance of each of the two strains selected for study. However, several generations of crossing and selection will be necessary, before the practical value of the recurrent selection procedure can be evaluated.

In the project section for high and low degrees of fatness in swine, 4 lines of swine are being developed, 2 lines of Durocs, and 2 lines of Yorkshires. Selection of parent stock to keep in the four breeding herds is based on backfat thickness as determined by the live probe. The plan is to develop a fat group (or line) and a lean group (line) of hogs from each breed. The primary purpose of this experiment is to determine the usefulness of the backfat probing technique as a tool in the development of superior meat-type strains of swine. This experiment will serve to illustrate the amount of progress that may be possible nsing on-the-farm probing of live animals without the aid of swine test stations. The experiment has progressed to the second generation in the Duroc breed and is now entering the first generation in the Yorkshire breed. Detailed carcass analyses are being made.

The identification and certification program for meat-type hogs was initiated by the Department in cooperation with a number of Corn Belt and other agricultural experiment stations and the purebred swine record associations. The primary purpose was to assay herds of purebred hogs to locate and identify meattype animals and strains that consistenly produce superior offspring. These animals and strains are to be used for breeding stock on farms.

The on-the-farm probing of backfat thickness in live animals was begun with spring litters of 1954. Probing records have been obtained on over 5,000 hogs on 82 farms in 5 States. Purebred hogs from nine different breeds have been probed. On-the-farm probing is being carried on also outside the Department's identification and certification project, by various State and county extension workers and others.

Results of a recent test on the preservation of swine semen for use in artificial insemination emphasized the possibilities for future dissemination within this country of superior germ plasm needed to increase meat-type hog breeding stock. At Beltsville, tests have been conducted on the keeping qualities of semen when shipped long distances by air. Samples of semen were collected and processed daily and flown to Beltsville over a 15-day period in December 1956. In each case, 30 to 40 hours had elapsed from the time the semen was collected until the sows were inseminated. Of the 24 sows used in the test, 11, or 46 percent, farrowed litters of strong, healthy pigs. The conception rate should increase rapidly as the techniques of insemination are improved.

The boar semen was treated with sodium citrate and egg yolk as diluters. In the tests, the semen was shipped in small collapsible plastic bottles that were used in the inseminating process.

Methods of carcass selection for leanness are being evaluated. Changes which may occur in other traits when leanness alone is the basis of selection of breeding stock are also being studied. Trials are being undertaken to test the effectiveness of selection in a closed herd, with inbreeding held at low level (less than 10 percent) in a pure breed, as compared with that in a similar herd started from a crossbred foundation.

Methods of measuring backfat thickness in live animals (the metal probe and lean meter probe) have been developed and tested. These tools for probing backfat thickness on live animals are being used widely in the swine industry in helping to locate meat-type breeding stock and to cull the fat animals.

Future work.-Permanent improvement in swine must be achieved through breeding. Results of work done emphasize the importance of hybrid vigor to the commercial hog producer. More knowledge is needed and is being sought about genetic interactions with respect to traits in hogs. The question of whether remaking a breed periodically by crossing it with another and practicing subsequent selection is more effective than continued selection within an established breed, offers possibilities that should be investigated. Physiology problems need and will get further exploration. Artificial insemination holds promise. Facts are needed on how to bring a group of gilts into heat within a 24-hour period. One problem-that of making best use and application of what is known Low-receives and will receive constant consideration. Wider use of testing procedures is indicated. Added information is needed to determine the most effective testing procedures.

Future research will seek the most efficient methods for solving existing and potential problems. The experiments in recurrent selection and selection for high and low degrees of fatness should yield some much-needed information.

MEAT AND CARCASS COMPOSITION STUDIES

Laboratories of three research divisions of the Department have performed and are performing studies leading to a better understanding of the physical makeup of the hog. The goal of the work is to find improved methods of measuring or estimating quality. The work of the laboratories is built around measurement of (1) pork cuts, (2) carcasses, and (3) live hogs. Measurements of live hogs are studied to find improved ways of estimating the quality of the carcasses and the cuts. Measurements of pork carcasses are studied to find improved ways of estimating the quality of individual cuts without having to break the carcass. Individual cuts are studied to provide data for defining characteristics associated with good eating quality.

Human Nutrition Research Division.-In the study of the quality of individual cuts, research in this Division has been initiated to provide data for defining the minimum fat content associated with good eating quality of pork and the nutritive values of meat-type and fat-type pork. In one project, data will be obtained on the relationships of cooking quality, yield, flavor, juiciness, and tenderness characteristics of cooked pork from different types of hogs. The physical and chemical composition and histological structure of raw and cooked meat will be studied. Chemical analyses will be made to provide data on nutrients such as proteins, amino acids, vitamins, and minerals to be found in pork cuts representative of the entire carcass.

Such comprehensive data when related to genetic and other factors in production can be useful to producers and processors, as well as to consumers of pork. The resulting data on edible yield, for example, would enable calculation of the relative value to consumers of trimmed meat from fat-type hogs compared to trimmed meat from meat-type hogs. Results of these studies could also be used to aid the development or improvement of grade standards for cuts of pork. The possibility of packer standardization of fat content of the highly trimmed and boned pork product of the future could be considered.

Animal Husbandry Research Division.-Quick, reliable procedures to determine composition of the live animal, in terms of fat, lean, and bone, are being sought by the Meat Quality Laboratory of this Division. Such procedures would enable the producer to market his animals at the time they reach this optimum finish and would permit sorting, selling, and processing of uniform lots of animals. They would also be most valuable to the breeder in the selection of replacement breeding animals that would transmit the desired proportions of lean, fat, and bone to their offspring.

For example, it would be useful if the cross-sectional area of the loin muscle could be estimated in breeding stock. One approach to the solution of this problem involves the fact that electrical impulse rates and sound transmission rates through fat, lean. and bone are different. Thus, measurements of fat and lean thicknesses within the live hog may be determined through ultrasonic or electronic impulses. Other methods of estimating internal quality of live hogs are being tested. The measurement of blood volume and output of creatine (protein degradation product) in urine and enzymatic activity in biopsy samples and certain procedures involving specific gravity measurements are being investigated. Another procedure being tested involves an anesthetic which the animal's fat absorbs to some extent. Thus, the thinner animals remain asleep longer. This method will probably continue to be too time consuming for practical use in the market, but it has research possibilities.

In addition to studying the live hog for clues to carcass and meat quality, the Meat Quality Laboratory has experimented with and designed carcass measurements for estimating the quality of trimmed primal cuts and has worked on methods of quality evaluation of individual cuts.

A large volume of data on carcass measurement and composition is now being assembled. A comprehensive analysis of these data will aid in the development of an index of factors that characterizes a meat-type hog and in studying heritability.

A detailed carcass evaluation is being made of offspring of the animals in the two experiments, reciprocal recurrent selection for hybrid vigor in swine, and selection for high and low degrees of fatness in swine. Quality factors such as flavor, intramuscular fat and yield of preferred cuts are receiving attention. These analyses will supply data to improve knowledge of the heritability of

fatness in swine and will provide information about the degree of fatness most desirable in pork.

Marketing Research Division.—The Quality Evaluation Section of this Division is cooperating with the Meat Quality Laboratory in studying methods of rapid evaluation of body composition of live hogs. Current research is mainly concerned with the development of electronic instrumentation and equipment for nondestructive measurement of lean and fat content of live hogs, carcasses, and pork cuts, for use in marketing hogs on the basis of merit. Such measurements should aid in providing a sound objective basis for grade standards and, in addition, have potentials for development of automatic sorting and handling equipment.

Experimental work has been undertaken on measurement of the natural radioactivity of pork cuts. All animal tissues have a slight but definite natural radioactivity arising from the presence of infinitesimal amounts of the radioactive element potassium 40. This element appears to be associated mainly with lean tissues and not with bone or fat. Only limited data, using hams, have been obtained to date. However, in these instances, the expected correlation between lean yield and radioactivity was found. If promising results are obtained in further tests, the research emphasis will be shifted to the development of practical instrumentation.

Grade standards for pork carcasses have been developed that objectively define the ratio of lean to fat cuts as a part of grade. Similar data relating to measurement of other qualities, such as tenderness, might contribute much toward further improvement in carcass standards, besides providing an objective basis for determining standards for pork cuts. Methods of measuring and predicting tenderness and other attributes are being studied.

Contribution of this Branch to the project on identification and certification of meat-type hogs will be partly directed to develop an objective basis for grade standards of feeder pigs with particular reference to indexes of their meat-type potential.

Other work. A number of the State experiment stations are continuing work on developing methods of measuring carcass and meat quality. For example, photometric measurement of fat-lean ratio of the surface of a meat slice may be possible and may have some applications. The work of research laboratories in industry may, as a byproduct of the study of other problems, result in discoveries useful in evaluating meat and carcass composition. For example, a private firm is studying the possible use of ultrasonic equipment, originally designed to find flaws in the interior of heavy metal castings, for measuring back-fat thickness of live hogs.

SWINE AND PORK GRADES

Past work. A system of classifying and grading market hogs was originally developed by the Department in 1918 for use in market reporting. Revisions were made, consistent with changes in production and marketing conditions, and tentative standards were issued in 1930. Further revisions were effected in these tentative standards for swine in 1940. Similarly, tentaive standards for grades of pork carcasses and cuts were issued by the Department in 1931 and slightly revised in 1933. These tentative standards formed the basis for market news reports, but grade was seldom a factor in swine and pork marketing.

Increasing evidence of a shift in consumer preference toward leaner pork, as early as 1940, indicated a need for changes in the grade standards. Consequently, studies were initiated to develop grade standards that identify differences in the lean and fat proportions of swine and pork carcasses. Measurement and cutting data obtained at Beltsville were analyzed to determine relationships that would provide an objective basis for standards. In 1946, additional measurement and cutting data were obtained under commercial conditions in a study conducted by the University of Minnesota Agricultural Experiment Station.1 Analysis of these data established that backfat thickness in relation to weight or length of carcass provided a reliable guide to yields of lean cuts and fat cuts as well as to quality of pork.

1 Engelman, Gerald, Dowell, A. A., Ferrin, E. F., and Anderson. P. A., Marketing Slaughter Hogs by Carcass Weight and Grade, Minnesota Agricultural Experiment Station Technical Bull. 187, April 1950.

Engelman, Gerald, Dowell, A. A., and Olson, Robert E., Relative Accuracy of Pricing Butcher Hogs on Foot and by Carcass Weight and Grade, Minnesota Agricultural Experiment Station Technical Bull. 208, June 1953.

The study of Marketing Slaughter Livestock by Carcass Weight and Grade, conducted by the Minnesota Agricultural Experiment Station, was later adopted as a regional project by the North Central Livestock Marketing Research Committee. Considerable work has been done by the experiment stations of the region on both hogs and cattle."

In addition to the Department and the State experiment stations, others have been working on ways to improve the accuracy of both live and carcass grading methods. One meatpacker conducted four rather large-scale experiments with carcass grading during the period 1933 to 1948. From 1948 to the present, this firm has purchased hogs by carcass weight and grade on an optional basis. Other packers also have done pioneering work in this field as well as in the field of improving the accuracy of live grading.

United States Department of Agriculture analysis of data gathered for the original Minnesota study provided the basis for development of the present United States official standards for barrow and gilt carcasses. The new standards for grades of slaughter barrows and gilts and the resulting carcasses were proposed by the Department in 1949. Extensive field testing and demonstations of the application of the standards were conducted prior to the adoption of official standards in 1952. Continuing shifts in consumer preference toward leaner pork and changes in production and marketing patterns led to revision of the standards in 1955, whereby the fatness permitted in the two top grades was substantially reduced. Official standards similar to those for barrows and gilts were adopted in 1956 for grades of slaughter sows and sow carcasses.* Future work.-An area of activity that will receive attention during 1958 concerns development of standards for grades of feeder pigs. The increase in the number of organized feeder pig sales has created a need for a uniform means of identifying differences in pig quality. Proposed standards for grades of feeder pigs will be developed for this purpose.

At present, most major pork cuts are rarely graded or sorted on a quality basis. If there were price differences between cuts of different grades or qualities, there might be even greater incentive for the production of meat-type hogs. Proposed standards for grades of pork cuts will be developed and field tested. There are many difficulties surrounding the use of grades for individual cuts of pork. Even in Canada, where carcass grades have been used since 1934, grades are not used at the wholesale and retail levels.

Investigation will be made of available information on measuring the muscling in pork carcasses, with the goal of improving grade standards. Research has shown that backfat thickness in relation to weight or length, as now used in the standards, explains about 70 percent of the variation in yields of lean cuts from pork carcasses. Provision in the standards for consideration of other factors, such as thickness of muscling, may be possible, making the grade standard an even more precise tool for identifying differences in hogs and in pork

carcasses.

A lean-to-fat index may be developed that is more closely related to value than is yield of cuts. It also may be possible to develop more objective standards for live hogs, achieving a higher correlation between live grades and carcass values.

Two of the objectives in swine and pork grade standard activities of the Department are (1) the development of more objective means of measuring differences in quality characteristics or attributes of an animal, carcass, or cut; and (2) the determination of the relationships of various quality aspects to palatability or eating satisfaction. Information of this nature is a prerequisite to more meaningful standards for pork cuts.

Evidence that wholesale cuts can be given the same grades as the carcasses from which they came is not conclusive. Carcass grades were designed to sort carcasses according to their value to the packer on the basis of relative amounts

2 North Central Livestock Marketing Research Committee, Objective Carcass Grade Standards for Slaughter Hogs, Minnesota Agricultural Experiment Station Bull. 414, June 1952. 3 U. S. Department of Agriculture, AMS. Official United States Standards for Grades of Pork Carcasses (Barrow and Gilt), Service and Regulatory Announcement No. 171, July 1955.

U. S. Department of Agriculture, AMS, Official United States Standards for Grades of Slaughter Swine (Barrows and Gilts), Service and Regulatory Announcement No. 172, July 1955.

4U. S. Department of Agriculture, AMS, Establishment of Official United States Standards for Grades of Certain Pork Carcasses and Certain Swine. Reprint from Federal Register of August 18, 1956.