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Records for pigs included in an experiment on reciprocal recurrent selection conducted from 1956 through 1971 at the USDA Beltsville Agricultural Research Center were analyzed to obtain estimates of heritabilities and genetic correlations and to derive prediction equations for estimating weight of lean cuts (WTLC) and percentage of lean cuts of shrunk slaughter weight (LCPC). Lean cuts growth rate (LCGR) was then estimated as WTLC/age of pig at slaughter. The base population consisted of two unrelated crossbred strains. A total of 1,294 records of F1 and F2 crossbred pigs were analyzed with one barrow and one gilt from each litter. Estimates of heritabilities and genetic correlations were computed with sire components of variance and covariance from a nested analysis of variance with an assumed model of years, strain-lines within years, sire within strain-lines, dams within sires and residual. Degrees of freedom were 307 for sires in strain-lines, 270 for dams in sires and 646 for residual. Heritability (h2) estimates were .42 ± .13, .41 ± .13 and .27 ± .18 for WTLC, LCPC and LCGR, respectively, and .71 ± .16, .38 ± .13, .31 ± .13 and .25 ± .15 for carcass length, average backfat thickness, longissimus muscle area and ADG in BW, respectively. These estimates were apparently the first published genetic estimates involving LCGR based on carcass data. It was recommended that prediction equations to estimate WTLC, LCPC and LCGR for use in swine testing programs be derived from current meat-type pigs.

Two Duroc and two Yorkshire lines of pigs that had been selected at Beltsville Agricultural Research Center for 12 and 10 generations, respectively, for either thinner or thicker backfat were mated to produce all possible pure lines and reciprocal crosses in 1967, 1969 and 1970. Data for littermate gilts and barrows from 136 litters were analyzed to estimate genetic and maternal influence on individual pig weights at birth, 21 d, 56 d and 140 d of age; age at 79.4 kg; average backfat thickness at 79.4 kg and postweaning average daily gain (56 d to 79.4 kg). Pure-line gilts differed among breed-lines (P<.05 or P<.01) for all traits except weight at 56 d. Gilts of the two low-fat lines were heavier than gilts of the two high-fat lines through 56 d of age, but Yorkshire low-fat gilts were lightest at 140 d, were oldest at 79.4 kg and had the slowest daily gain, in addition to the least backfat. The Duroc low-fat line gilts were heaviest at 140 d, youngest at 79.4 kg and were second thinnest in backfat. Among pure-line barrows, the low-fat lines were heaviest at birth, at 21 d and at 140 d and were thinnest in backfat. Line-cross gilts were heavier than pure-line gilts at all four ages, were younger at 79.4 kg and higher in daily gain. Among barrows, line crosses were heavier in all weights except at 21 d, were younger at 79.4 kg and were higher in daily gain than pure lines. Differences between pure lines and line crosses in backfat were not significant for either sex. Heterosis varied from 6.5 to 16.7% among weights and growth traits. Pigs of both sexes differed among breed-lines in general combining ability for all traits except 21-d weight, and differed in maternal ability for weights through 56 d and for backfat. Specific combining ability (SCA) was significant only for intra-breed crosses for weight at 21 d, and for inter-breed, intra-line crosses for 21- and 56-d weights and for age at 79.4 kg among gilts, with no significant effects in SCA for any trait among barrows. General combining ability was not correlated with maternal effects for any trait except 21-d weight, for which they were positively correlated (r>.80).

Analyses of variance were computed for records on growth and body composition traits made in 1983 by 255 boars and gilts in selected and control lines of Durocs and Yorkshires and their reciprocal crossbreds. Previous selection over a period of several generations was mainly on an index of sow productivity including preweaning litter sizes and weight. Animals in the select lines were selected for high index values; animals in the control lines were selected to average near the mean index values of that year and line. Breeding animals in all four lines during that period were basically randomly selected with regard to growth rate or body composition traits. The same boars sired both purebred and crossbred litters in 1983. Traits analyzed were average daily gain (ADG) during a standard test period from 56 d of age to 90.7 kg and average backfat thickness (ABF) and longissimus muscle area (LMA) from ultrasonic scans at 90.7 kg. Crossbred pigs had greater (P<.01) ADG than purebred pigs, but did not differ (P>.05) in ABF or LMA. Heterosis was 8.2% for ADG. Crossbreds with Yorkshire dams had thinner (P<.01) ABF and larger (P<.01) LMA than crossbreds with Duroc dams. Boars had greater (P<.05) ADG, thinner (P<.01) ABF and smaller (P<.01) LMA than gilts. Correlations between 38 half-sib family averages of purebred and crossbred pigs of the same sex and the same sires were .07, .37 and .24 for ADG, ABF and LMA, respectively. Implications of the above and additional findings for swine breeding strategies are discussed.

Performance records were analyzed for 1,869 purebred Duroc and Yorkshire gilts tested in littermate groups of two to four pigs in 703 pens. Traits studied were average daily gain during a standard test period from 56 d of age to 90.7 kg (ADG), average daily feed consumption during the test period (ADF), average backfat thickness (ABF) measured from polaroid photos of ultrasonic scans at 90.7 kg and efficiency of feed conversion for the whole test period (feed conversion efficiency, FCE). The main objectives were: 1) to estimate pertinent genetic and phenotypic parameters and 2) to discuss applications of the findings to the swine industry in the United States. A balanced 16% crude protein diet in pelleted form was provided ad libitum during the entire test period. Nested analyses of variance were computed with both the individual and pen mean records. Sire components of variance and covariance, adjusted for level of inbreeding, were used to estimate heritabilities and genetic and phenotypic variances, covariances and correlations. Heritabilities estimated from individual records were .098 for ADG and .423 for ABF. Estimates from pen means were .105 for ADF and .061 for FCE. Genetic correlations of FCE with ADG, ADF and ABF were estimated as −.520, −.520 and .694, respectively, while phenotypic correlations for the same traits were −.240, .570 and .212, respectively, all from pen means. Genetic and phenotypic correlations of ADG with ABF from individual records were .176 and .254, respectively. Implications for swine testing programs were discussed, including a proposal that would include FCE indirectly in an index with ADG and ABF, based on genetic covariances of FCE with ADG and ABF, thereby removing the need to measure FCE directly.

Selection involving six cycles of reciprocal recurrent selection (RRS) was conducted over a period of 12 yr, followed by a 4-yr evaluation phase. Selection in two unrelated selected strains (BSand LS) was based on an index of sow productivity for preweaning litter traits in the parental strains and on postweaning gain, carcass backfat thickness (BF) and yield of five preferred cuts (percentage PC) in their reciprocal strain-cross progeny (BSLSand LSBS). In addition, concurrent selection in two control strains (BCand LC) derived from the same base populations was limited to the dam's preweaning litter productivity. Standardized selection differentials among parents when based on their cross-progeny averaged .28 for percentage PC and −.09 for BF. These values were far short, as expected, of 1.02 standard deviations, the theoretical maximum selection differential based on single-trait truncation selection and the average percentage of animals selected to produce the next generation among those tested. Responses to selection during the selection phase were estimated by the regression on generations of deviations of carcass traits in BSand LSstrains and in their cross-progeny from means for contemporary BCand LCstrains. Results showed significant negative trends in deviations for both percentage PC and BF among cross-progeny and negative but nonsignificant regressions among the parental strains. The main divergence in the results from expected was in the sharply negative (P<.01) response of percentage PC among cross-progeny. In the evaluation phase, heterosis effects in the selected strain crosses and in the control strain crosses were essentially zero for percentage PC and BF, indicating minimal non-additive genetic effects for these traits. Differences between parental selected and control strains were mostly nonsignificant but were largely significant between selected and control strain crosses, suggesting a positive response to RRS.

Main and interaction effects of breed, line, sex and season on 15 carcass and two related performance traits were evaluated in 649 Duroc and Yorkshire pigs. Selection of replacement animals in spring litters was based on an index of sow productivity and in fall litters on an index of average daily gain, scan backfat and loin eye area. Control lines were also maintained in each breed in each season. Five generations of selection were represented in spring pigs and three generations in fall pigs. Various statistical models were used to test interaction effects. Breed × fall select vs control lines was the only interaction that was significant for more than one or two traits. A reversal in order of the least-squares means for the select and control lines between the two breeds resulted in the significant interaction effects in eight of nine carcass traits. The Duroc select line was leaner than the Duroc control line but the reverse was generally true for Yorkshires. In main effects, Durocs were generally leaner than Yorkshires and gilts were leaner than barrows. Durocs also exceeded Yorkshires in both gross and lean cuts average daily gain, and barrows were higher than gilts in both traits. Partial regressions of tratis on years suggested possible negative correlated responses to the selection for sow productivity practiced among spring pigs. Residual phenotypic correlations among the traits generally agreed with previous reports. In addition, lean cuts average daily gain was more closely associated with gross average daily gain in slaughtered pigs than with carcass measures of leanness such as loin eye area or percentage lean cuts.

Quadratic regression equations were fitted to preweaning weight records for 3,587 Duroc and Yorkshire pigs in 533 litters farrowed in the spring and fall from 1974 through 1978 at the Beltsville Agricultural Research Center. The objective was to derive factors for adjusting pig and litter weights to uniform ages for use in evaluating sow productivity. Only records for litters with at least two boar and two gilt pigs alive at weaning at 42 d of age were included. Records for up to four healthy pigs of each sex in each litter were used. Pig weights were recorded shortly after birth and at approximately 21 and 42 d of age. Actual weights were transformed to natural logarithms prior to analysis. Quadratic regression equations were fitted for each of the 1,066 litter-sex subgroups. The computed least-squares mean and linear and quadratic regression coefficients from all subgroups were analyzed with a model that included effects of years, seasons, breeds, sex and all interactions. On the basis of significant season and year X season X breed interaction effects, the quadratic coefficient was required for a good fit of the equation to the data. The equation Ŷ = .2127 + .07545 (days of age)—.000558 (days of age)2, where Ŷ = estimated pig weight in natural logarithms, was then used to calculate multiplicative factors for adjusting preweaning pig and litter weights to ages of 14, 21,28, 35 and 42 d.

Direct and interaction effects of breed of dam, line, sex and diet on objective and subjective measures of fresh and cooked samples of the longissimusmuscle were studied. The effect of breed of dam in reciprocal crossbred pigs was significant only for color and firmness rating of the fresh sample. Cooked samples from control line crossbreds were significantly more tender according to the Warner-Bratzler (WB) shear test and palatability evaluation by the taste panel than samples from low-fat line crossbreds. Barrows had more intramuscular lipid than gilts. Cooked samples from barrows were more tender according to the WB shear test and taste panel. Diet differences significantly affected marbling rating, lipid percent and juiciness. Pigs fed a 12% protein diet exceeded pigs fed a 16% or 16% equivalent diet for each of these traits. Backfat thickness was largely independent of intramuscular lipid.Color and firmness ratings were significantly correlated with WB shear and with taste panel tenderness rating, but correlations were too low to be of much predictive value. Marbling rating was significantly associated with lipid percent and juiciness rating of the cooked sample, but not with flavor or tenderness ratings. Lipid percent was correlated with juiciness but not with WB shear or other sensory measures of the cooked sample. The WB shear was significantly correlated with cooked sample flavor and tenderness. No single objective or subjective measure of the fresh sample appears to be highly enough correlated with palatability measures to be a reliable predictor of pork quality.

Main and interaction effects of breed of dam, line and sex of pig and diet on traits related to body composition were studied in a balanced design. Duroc and Yorkshire parents were crossed reciprocally within low-fat lines and control lines from a previous long-term study in which selection was based on backfat thickness. Pigs were fed ad libitumone of three corn-soybean meal diets, 1, 2 or 3, containing, respectively, 12% (crude) protein, 16% protein or 12% protein, supplemented with lysine and methionine to approximate their levels in diet 2. Pigs were slaughtered at 100 kilograms.Crossbred pigs from Yorkshire dams had significantly leaner carcasses and deposited lean tissue faster than crosses from Duroc dams, giving evidence of an important maternal influence. Crossbred pigs of the low-fat line were significantly leaner at slaughter but did not exceed control line crosses in rate of deposition of lean tissue. Gilt carcasses were leaner than barrow carcasses but barrows deposited lean tissue faster.Pigs fed diets 2 and 3 produced significantly more lean tissue and at a faster rate than crossbred pigs fed diet 1. Further results also indicated that synthetic amino acids were highly effective in supplementing the basic 12% protein diet.For some body composition traits, differences between low-fat and control line crosses were larger in pigs fed diets 2 and 3 than in pigs fed diet 1, with implications for testing programs. Further results indicated that rate of deposition of lean tissue may be largely independent of body composition at slaughter.

Half-carcasses of 128 market swine were analyzed for moisture, ether extract (EE), protein and ash. The longissimusmuscles were analyzed for EE and some organ weights were measured. A balanced 25of treatments included shire (Y) breeds, high (H) and low (L) backfat lines within each breed, sex (female and castrated male) and two levels each of dietary protein (20 and 14%) and energy (3.6 and 3.2 kcal ME/g). Each of the breed-line groups differed significantly from each other in carcass chemical composition ranging from the highest protein and lowest EE in YL through DL, YH to DH which had the lowest protein and highest EE. Males were significantly lower in protein and higher in EE than females. A breed-line × sex interaction (P<.01) for all components except ash showed that the greatest difference between sexes was in the L lines. Carcasses from pigs fed the 20% protein diet were higher in moisture and lower in EE (P<.05) than those of pigs fed the 14% diet. No other diet effects and no interaction of diet with breed-line variables were significant. The EE of the longissimusmuscle was significantly lower in YL and higher in DH than in the other breed-line groups and higher (P<.01) in males than in females. Differences in organ weights (liver, heart, kidney, stomach) among the breed-line groups were significant and the liver and kidneys of pigs fed the 20% protein diet were heavier (P<.01) than those from pigs fed the 14% diet.

Direct and interaction effects of breed (Duroc and Yorkshire), line (high- and low-backfat), sex (barrows and gilts) and levels of diet protein (20 and 14%) and energy (3.6 and 3.2 kcal of M.E./g) on swine carcass traits, with ad libitumfeeding, were examined in a 25factorial arrangement of treatments. Each of 32 subclasses consisted of two pens, each with two pigs, all taken off test at 100 kg for slaughter.Breed × line interaction effects were significant for most traits. Except for carcass length and rate of growth in trimmed lean cuts (LC gain), these interactions were apparently due to different lengths of previous selection periods in the two breeds. The usual differences in carcass traits were noted between the high- and low-backfat lines.Breed × line and/or line × sex interactions were significant for all carcass traits except percent bone of ham. This re-emphasizes the need to consider sex effects in swine testing.

Purebred Duroc and Yorkshire boars and gilts, farrowed in spring litters from 1974 through 1982 and in fall litters from 1974 through 1978, were maintained as closed select and control lines descended from the same base population. Spring-farrowed pigs were selected mainly on an index of sow productivity traits, whereas selection among fall-farrowed pigs was mainly on an index of pig performance traits. Basic traits analyzed were age of pig at 91 kg, postweaning average daily gain in weight, average backfat thickness (ABF) and longissimus muscle area (LMA), with ABF and LMA measured from ultrasonic scans at 91 kg. Also analyzed were estimated weight of trimmed wholesale lean cuts at 91 kg live weight and lean cuts growth rate from birth to 91 kg. Standardized selection differentials indicated that no significant selection pressure was applied to the four basic traits in the population. A nested analysis of variance of intraclass correlations among paternal half-sib families was computed with 1,930 gilt records, providing estimates of heritabilities and genetic and phenotypic correlations among the six traits. Also, estimates were computed for the portion of total phenotypic variance due to maternal-related covariances among littermates and the portion due to random environmental variances among individuals. In addition, estimates of the population parameters were computed from regressions of boars and gilts on sires, dams and mid-parental values with 974 boar and 1,686 gilt deviation records. Composite parameter estimates were then computed from the separate values weighted by the inverse of their standard errors.

Records for a total of 732 daughter-dam pairs were analyzed to estimate the genetic correlations of pig performance traits with sow productivity traits, with implications to the development of specialized sire and dam lines for use in crossing. Major pig performance traits analyzed included average daily gain from 56 d of age to a final weight of 90.7 kg (ADG), average backfat thickness at 90.7 kg (BF) and a performance index (PI) consisting of ADG and BF. Major sow productivity traits included number of pigs born alive in a litter (NA), litter size (N21) and litter weight (W21) at 21 d of age and two sow productivity indexes, one with NA, N21and W21(SPI-3) and one with NA. and W21(SPI-2). All records were expressed as deviations from breed-line-year-season means of this population. Genetic correlations were computed from daughter-dam covariances. The mean genetic correlation of PI with SPI consisted of two correlations, that of daughters' PI with dams' SPI and that of dams' PI with daughters' SPI. The mean genetic correlation of PI with SPI-3 and SPI-2 was .07 ± .12, suggesting that concurrent improvement in both PI and SPI would not be restricted by selection within a single composite line. The genetic correlation of daughters' PI with dams' SPI (−.18 ± .13) was appraised as more critical than the reciprocal correlation of dams' PI with daughters' SPI (+.28 ± .13). This appraisal is based on the fact that only one generation separates a daughter's PI from her dam's SPI, as compared with two generations in the reciprocal covariance. However, the −.18 correlation was not significantly different from zero, indicating that formation of specialized sire and dam lines for use in crossing would be only marginally more effective at best for improving the overall efficiency in pork production than use of a single composite line, aside from the heterosis effects from crossing the lines. Indexes were proposed for combining PI and SPI for use either in specialized sire and dam lines or in a single composite line.

Prediction equations were derived to estimate weight of lean cuts (WTLC) in pig carcasses, based on data from 649 pigs, including 163 Duroc gilts, 161 Duroc barrows, 161 Yorkshire gilts and 164 Yorkshire barrows, all produced and processed at Beltsville. Chilled carcasses varied in weight from 68 to 78 kg. The WTLC was for the four standard closely trimmed cuts of picnic and blade Boston shoulders, loins and hams based on whole carcass weights. Various combinations of independent carcass variables (length, backfat thickness, longissimus muscle area and weight), were included in exploratory regression models to compare equations for accuracy of predicting WTLC. Based on R2and standard errors of estimates, only carcass length of the four variables could be excluded from prediction equations with essentially no loss in accuracy. Separate prediction equations were derived for the different breed, sex and breed-sex subgroups and for all subgroups pooled. Predictions were compared with actual WTLC in the different subgroups. As expected, specific subgroup equations provided accurate estimates of actual WTLC in pigs of that subgroup. However, WTLC estimated by two different pooled prediction equations differed (<.01) from actual WTLC in subgroups of Duroc barrows, Yorkshire gilts, all Durocs and all Yorkshires. In separate analyses, the effects of longissimus muscle area and carcass weight differed (<.01) between barrows and gilts, but no practical advantage was noted for including a quadratic effect in the prediction equation. It was recommended that the problem of biased estimates be addressed by further research to derive prediction equations to estimate lean cuts or lean tissue content for use in more specific populations, primarily with barrows and gilts.

Genetic and phenotypic parameters for sow productivity traits were derived from analyses of records for 999 purebred Duroc and Yorkshire litters at the Beltsville Agricultural Research Center. Included were 682 spring-farrowed litters and 317 fall-farrowed litters. Primary traits examined were number of pigs born alive in the litter (NA), litter size at 21 d of age (N21), litter weight at 21 d (W21) and an index (I21) combining the three traits. Also analyzed were total number of pigs farrowed alive or stillborn, litter weight of pigs born alive and litter size and litter weight at 42 d of age (weaning). Selection was based mainly on the sow productivity index among litters in the spring season and mainly on performance traits among fall-farrowed pigs. Select and control lines were maintained in both breeds in both seasons. Yorkshires started at higher levels of productivity than Durocs in spring 1975, showed more fluctuation and ended in spring 1982 at about their starting levels. Durocs started at lower productivity levels and recorded generally significant increases by 1982, with comparable selection differentials for the two breeds. Heritabilities, estimated from daughter-biparental (mid-dam, paternal granddam) regressions using records expressed as deviations from contemporary breed-line means, varied widely among the breed-line groups. Suggested composite heritability estimates were 5,10 and 20% for NA,N21and W21, respectively. Genetic correlations, computed from daughter-biparen-tal covariances, were high and positive (.7 or above) among all traits. Based on estimated genetic gains, it was proposed that W21, alone or in combination with N21and(or) NAin an index, be used as the primary selection criterion to bring about genetic improvement in sow productivity traits.

Selection involving six cycles of reciprocal recurrent selection (RRS) was conducted over a period of 12 years, followed by a 4-year evaluation period. Selection in two unrelated selected strains (BSand LS) was based on an index of sow productivity for preweaning traits in the pure strains and on postweaning gain, carcass backfat thickness and yield of preferred cuts in their reciprocal cross-progeny (BSLSand LSBS). In addition, concurrent selection in two control strains (BCand LC) derived from the same base populations was based mainly on the dam's preweaning litter productivity. This study compared responses in postweaning growth traits in the BSand LS strains and in their cross-progeny with postweaning growth in the BCand LCstrains. Standardized selection differentials among parents for growth traits were generally larger when based on performance of their cross-progeny rather than on their own performance and, as expected, were far short of potential maximum based on single-trait truncation selection. Responses to selection during the selection period were estimated by the regression on generations of deviations of performance of BSand LSstrains and their cross-progeny from means for contemporary BC and LCstrains. Results for postweaning gain showed a significant positive response to RRS among cross-progeny, but a significant negative response in the parental BSand LS strains. In the evaluation phase, comparisons of heterosis effects in the selected and control strains largely supported conclusions from the selection phase. A more definitive evaluation of the effectiveness of the selection practiced must await further planned studies.

The effects of various genetic and environmental factors on sow productivity traits were examined in 483 first-parity litter records of Duroc and Yorkshire sows in 7 year-seasons at the Beltsville Agricultural Research Center. Traits examined included litter size and weight at birth and at 21 days of age, sow weight gain during gestation, sow weight change during lactation, and length of gestation period. Age of sow, quadratic effects of sow weight at breeding and quadratic effects of sow weight gain during gestation did not significantly affect basic litter traits. Weight of sow at breeding affected only litter weight at birth. Sow weight gain during gestation had a positive association with litter size and weight at birth and with litter weight at 21 days. A sow productivity index expressed as a contemporary deviation had a heritability of 21 ± 9%, based on the regression of daughters on comparable midparental values. Milk yield significantly influenced lactation weight change by the sow. Little variation in gestation length was noted, but larger litter size had a slight negative association with that trait.

Records of 278 Duroc and 276 Yorkshire gilts confined to pens with concrete floors and fed a 16% protein diet ad libitumfrom 8 weeks of age until taken off test at 91 kg were analyzed for feet and legs (F-L) soundness as visually appraised. Front and rear F-L were scored separately from zero to 10 (best). Other traits were weight per day of age at start of test (W/A), average daily gain (ADG) on test, back-fat (BF) and loin eye area (LEA), the last two measured ultrasonically at 91 kilograms. Front and rear F-L scores (00B1; standard deviations) averaged 7.99 ± 1.32 and 7.88 ± 1.09, respectively. Components of variance and covariance were computed for sires within breed and year-season (104 df), dams within sires (89 df) and progeny within dams (353 df). Heritability estimates for front and rear F-L scores (00B1; standard errors) were 26.9 ± 20.5% and 15.2 ± 24.1%, respectively, with sire components and 27.9 ± 10.8% and 38.9 ± 11.2%, respectively, with sire + dam components. Genetic correlations between W/A, ADG, BF and LEA with front F-L scores were .035 ± .159, .334 ± .171, .350 ± .178 and −.327 ± .210, respectively; with rear F-L scores, they were .087 ± .139, .093 ± .158, .052 ± .166 and −1.86 ± .191, respectively. Results indicate slow to moderate rates of progress in F-L soundness by direct selection and no conflict with ADG but problems with BF and LEA.

Data on carcass measurements and chemical composition of young pigs approximately 9 weeks of age are presented. Pigs used in the study were from Duroc and Yorkshire breeds and included representatives of lines within each breed selected for high or low fatness. In addition, some pigs from crosses within the selected lines were studied as well as pigs from Duroc and Yorkshire herds considered representative of the present-day swine population. Dressing percentage, carcass length, backfat thickness, loin eye area and carcass chemical composition differed significantly among breed groups. Although pigs were slaughtered at about 62 days of age, slaughter weights differed among lines and individuals and significantly influenced many of the traits measured.

Direct and interaction effects of breed of dam, line of pig, sex of pig and diet on growth, feed consumption and feed conversion were investigated in a 2 × 2 × 2 × 3 factorial arrangement of treatments. Each of the 24 subclasses consisted of two pens, each with two pigs, on test from 8 weeks of age to 100 kg in weight. Pigs were allowed ad libitumone of three corn-soybean meal diets containing (1) 12% crude protein, (2) 16% crude protein or (3) 12% crude protein plus supplemental lysine and methionine to approximate their levels in diet (2). Duroc and Yorkshire parents were crossed reciprocally within low-fat lines or control lines from a previous long-term study in which selection was based on backfat thickness.Breed of dam (B) had no significant effect on average daily gain (ADG), but crosses with Duroc dams ate more feed per day (ADF) and had poorer feed conversion ratios (FC) than pigs with Yorkshire dams. Control-line crosses had higher ADG and ADF than low-fat crosses, but the crosses did not differ in FC. Barrows averaged higher in ADG and ADF than gilts, but sex (S) effects on FC were essentially zero. Diets 2 and 3 did not differ for ADG or FC, but both exceeded diet 1 for these traits. B × line of pig (L) interaction effects were nonsignificant. L × S effects were significant for ADG; B × S, L × diet (D) and S × D interactions significantly affected FC. Low-fat crosses had better FC on diet 2, but control-line crosses were more efficient in FC on diet 1. Barrows had better FC than gilts on diet 1, but no significant sex differences in FC were noted on diets 2 and 3.

Direct and interaction effects of breed, line, sex, diet protein and energy levels on swine growth and feed conversion were investigated in a 25factorial arrangement of treatments. Each of the 32 subclasses consisted of two pens, each with two pigs. The 128 pigs were placed on test at 9 weeks of age and taken off test for slaughter at 100 kilograms.Barrows and gilts of lines previously selected for up to 17 generations for either thicker or thinner backfat in Durocs and Yorkshires were fed diets with 20 or 14% protein and either 3.60 or 3.20 kcal of M.E./g. Average air temperature and water evaporation rates were recorded.Durocs outgained Yorkshires. High-fat lines ate more feed than low-fat lines. Low-fat Durocs outgained high-fat Durocs, but the reverse was true for Yorkshires, resulting in an interaction of breed with line for gain. Barrows gained 7% faster (P< .01) and consumed 6% more feed (P< .01) than gilts.

Duroc and Yorkshire lines which had been selected for 10 or more generations for either thicker or thinner backfat were mated in all possible combinations. Data from 164 litters were analyzed to estimate genetic and maternal influence on litter size, total litter weight and average pig weight at birth and at 21 and 56 days of age.Pure lines differed significantly for most traits, with the Yorkshire low-fat line ranking first in all traits except pig weight at birth and at 56 days. The Duroc low-fat line performed best for the latter two traits and ranked second for the other seven traits.Line crosses averaged significantly above pure lines for all nine traits, indicating significant mean positive heterosis effects. Percent heterosis increased with age at measurement, ranging from 11 to 40%.Lines differed significantly in maternal influence on total litter weight and average pig weight at all three ages, in line crosses. Low-fat lines of both breeds excelled in this respect for each trait. However, lines evidenced little difference in maternal influence on litter size.Lines differed significantly in average breeding value (general combining ability, GCA) only for average pig weight at birth and at 56 days of age. The Duroc high-fat line was highest in GCA but lowest in maternal ability for litter size at any age. Durocs evidenced more GCA than Yorkshires for litter weight through 56 days.Significant specific heterosis (specific combining ability, SCA) was noted for average pig weight at each age and for litter weight at 21 and 56 days. Inter-breed crosses of similarly-selected lines evidenced most SCA in ranking above intea-breed crosses and inter-breed crosses of oppositely-selected lines in all traits except litter size at birth.Inter-breed crosses of low-fat lines exceeded crosses of high-fat lines for all nine traits, significantly so for litter and pig weights. Individual line crosses, including reciprocals, varied widely, both within and between traits.The study indicated that long-term selection based on backfat thickness produced significant changes in the lines in their maternal and general and specific combining ability. Also, further evidence was obtained for a negative correlation of additive genetic and maternal effects. Thus, both factors need to be considered when evaluating individual dams and lines, strains or breeds for crossing.

Originally, Wright (1921, 1922) proposed the quantity, F, as an inbreeding coefficient indicating “the departure from the amount of homozygosis under random mating to complete homozygosis.” F has since been used as a measure of such departure relative to a specified foundation stock, not necessarily random-bred (Wright, 1951). The most important inbreeding coefficient (Wright, 1951, 1955) and the one of concern here is the F-value pertaining to neutral autosomal disomic loei. In the strictest sense, F measures the changes in homozygosis (or, homozygosity) due only to parental relationship. Malecot (1948) showed that these changes in homozygosity also can be expressed in terms of probability.In biological populations, “true” F usually is indeterminable. The inbreeding coefficient, Fp, here, computed from parental relationships, commonly is used as an estimate of F (Wright, 1922; Lush, 1948). More information is needed on its correspondence with the actual state of homozygosity of an individual or population, particularly as it is affected by type of gene action and selection practices.

Other papers (Winter, Cummings and Stewart, 1947; fine and Winters, 1952) have reported on portions of the extensive swine breeding experiments conducted at the Minnesota Agricultural Experiment Station since the formation of the Regional Swine Breeding Laboratory in 1937. The primary purpose of the present paper was to summarize certain aspects of an experiment on inbreeding which was completed by 1958. These concern mainly the effects of inbreeding on litter productivity, including litter size, total litter weight and average pig weight at birth and weaning. The results can be related to similar studies for other stations in the Regional Laboratory (Bereskin et al.,1968a) and to other reports in the literature.Materials and MethodsSources and Description of Data. Upon entry into the Regional Laboratory in 1937, several Minnesota substations initiated programs of producing inbred lines in the Poland China breed. Also, crosses of Tamworth and Landrace were used to develop the Minnesota No. 1 breed, and Yorkshires and Polands were crossed to produce the Minnesota No. 2 breed (Winters et al.,1943).

The development of Mendelian Genetics shortly after 1900 led to the view that inbreeding decline in plant and animal populations was due to an increasing frequency of deleterious recessive mutants in the homozygous state. It was further speculated that by developing a number of inbred lines, segregation in hereditary differences among the lines would result. However, animal breeding theory remained largely rudimentary and the experiments elementary until a series of classic papers by Sewall Wright on quantitative genetic theory (Wright, 1921, 1922) were published.Wright's papers did much to stimulate inbreeding studies with swine at various agricultural experiment stations. At the Iowa station, Lush (Lush et al., 1948) interpreted Wright's theories to indicate that a balance might be achieved whereby the simultaneous, conflicting effects of inbreeding and selection might cancel each other, permitting the slow attainment of genetic homozygosity without a deterioration of phenotype. To test this, an experiment was started in 1930 in which responses to maximum possible intraline selection under regimes of intense, mild and very slight inbreeding were compared.

The primary basis for the popularity of crossbreds in commercial pork production is the advantage of crossbreds over the straightbred mid-parental values. This crossbred advantage, or (positive) heterosis, generally has been noted for traits associated with “mothering ability” of crossbred females. These traits include numbers and weights of pigs in a litter at birth and at weaning. Also, survival rate and general vigor of crossbred pigs generally have reflected this heterosis effect. Other traits, such as post-weaning growth rate, have shown lesser degrees of crossbred advantage. Carcass traits of crossbreds usually have equalled mid-parental values, indicating little, if any, heterosis (Fredeen, 1958).An experiment to evaluate further the effects of crossing the Duroc and Yorkshire breeds was initiated at the Iowa Agricultural Experiment Station in 1964.

These findings clearly demonstrate that although selection was based on an index of phenotypic values computed from the entire genome, independent loci responded separately with changes in gene frequency. This response depended on selection pressure, initial gene frequency, model of gene action and effective population number. For example, at loci controlled by the additive model, gene frequency of the favored allele rose quickly, with larger increases in the larger-sized lines. A similar trend was noted at loci under control of dominance models. Where the overdominance models prevailed and selection was operative, gene frequency of 0.5 was maintained. At other loci where more complicated models were employed, gene frequency changes were more subtle, depending on the various factors enumerated.Significant differences were noted among replicates of each population in regressions of gene frequency on generation number, for each section of the genome. Clearly demonstrated was the low predictability of changes in gene frequency for a particular small segment of a population over a period of only a few generations.

Records for over 10,000 litters farrowed at experiment stations in eight midwestern states cooperating in the former Regional Swine Breeding Laboratory were analyzed. Litters included were farrowed from the mid 1940's to the early 1960's. Over 7,000 litters, on test to 154 days, averaged 71.8% of pigs born alive surviving to weaning at 56 days and 66.3% surviving to 154 days, while 93.5% of those alive at weaning survived to 154 days of age.Stations, years and seasons showed variable influence on survival rate to weaning in the different states, but year × season interaction had a highly significant effect in each state tested. Males averaged about 30 g heavier at birth than females of the same litters. However, females had a significantly higher survival rate, from 5 to 9% above males. Survival rate of pigs of either sex rose significantly with higher birth weight. In addition, above-normal birth weight apparently had no detrimental effect on pig survival. Highest survival rates occurred in litters of 8.6 pigs born alive.