Single Trait Selection Research Articles

Single trait selection for yearling fleece weight or liveweight in Romney sheep—direct responses

Abstract An experiment was carried out to study the responses to selection for yearling fleece weight and liveweight in Romney sheep. There were two selection lines, one selected for fleece weight (FW) and the other for liveweight (LW), and a control (CO) line was maintained. Data from lambs born 1967–90 were analysed to quantify the direct responses to selection. By 1990 there had been an average of 9 generations of selection with an average generation interval of 2.7 years. Realised heritabilities and selection responses were estimated from the deviation of the selected lines from a control line and by restricted maximum likelihood (REML) techniques. Realised heritabilities were 0.36 ± 0.04 for both fleece weight and liveweight respectively when estimated using the regression of response on selection differential. The corresponding REML estimates of heritabilities were 0.39 ± 0.03 for fleece weight using data from FW + CO and 0.42 ± 0.03 for liveweight using LW + CO. The direct responses to selection, d...

A note on consequences of single-trait selection for insulin-like growth factor 1 (IGF-1) in beef heifers

Eight sets of identical twin heifers and their first three calf crops were used to determine effects of single-trait selection for blood serum insulin-like growth factor 1 (IGF-1). Twin heifers were offspring of Simmental sires and Hereford × Simmental or Hereford × Angus dams. All 16 heifers were mated with the same Simmental bull in each year of the study to produce calves born in 1989, 1990 and 1991 (no. = 16, 13 and 10, respectively). Although differences were noted in performance of progeny from the four twins with the highestv.the four with the lowest IGF-1 concentrations, they zoere not large enough to recommend single-trait selection for blood serum IGF-1 in beef heifers.

Feed intake, body weight and milk production: genetic analysis of different measurements in lactating dairy heifers

Phenotypic and genetic correlations between measurements of feed intake, body weight and production within and between 4 periods in the first 15 weeks of lactation have been estimated. Measurements were available on 358 dairy heifers during 2 week periods starting at 2, 5, 9 and 13 weeks after calving. Heifers were progeny of 38 sires. The diet consisted of 6 kg concentrates and roughage ad libitum. Genetic parameters were estimated by REML fitting an animal model. Heritability of roughage intake decreased from 0.32 in week 2 to 0.18 in week 13. Fat protein corrected milk production (FPCM) had a heritability of 0.33 in week 2 which increased to 0.47 in week 13. Heritability of feed conversion changed from 0.17 to 0.29 from week 2 to 13. The largest differences in heritability were found between week 2 and week 5. The genetic correlation between roughage intake and feed conversion changed from 0.24 in week 2 to −0.57 in week 13. The genetic correlation between measurements in week 2 and 9 was 0.47 for energy intake, 0.80 for roughage intake and 0.99 for FPCM production. Estimated genetic correlations and heritabilities have been used to determine the relative efficiency of single trait selection using measurements in one single period. It is concluded that for an accurate evaluation of animals for feed intake and feed conversion measurement during at least two periods are needed.

Body composition and organ weights in fleeceweight-selected and control Romney rams

Abstract Rams from the Massey University fleeceweight-selected (FW) and control (C) lines of Romney sheep (13 per line) were slaughtered at 16 months of age to determine responses in body composition to 35 years of single trait selection for greasy fleeceweight. FW rams were heavier than C rams and, at a common liveweight, had heavier hearts, livers, thymus glands, and adrenal glands but a lower reticulo-rumen weight. Carcasses of FW rams were heavier and longer than those of C rams but there was no difference between the lines in dressing-out percentage. At a common carcass weight, FW and C rams had a similar content of water, protein, and fat in the carcass. Subcutaneous fat depths and soft tissue depth GR were generally lower in FW rams and there was a redistribution of tissue in the carcasses of these rams compared with C rams such that hind leg weight was increased at the expense of the shoulder, loin, and rack. Hind legs of the FW rams had higher weights of dissectible muscle and bone, lower weights of dissectible fat, a heavier and longer femur, and lower muscularity (depth of muscle relative to length of an adjacent bone) at a common carcass weight than those of C rams. It is concluded that responses in body composition to selection for greasy fleece-weight are small but generally favourable, except for the decrease in muscularity.

Genetic and phenotypic parameter estimates for selection to improve lamb carcass traits

Abstract The variance components needed to calculate genetic and phenotypic parameters for selection to improve carcass composition in white-faced sheep were estimated from a sire model using REML on data from 1602 Romney and Romneycross lambs. The sires were either Romney (n = 60), Border Leicester (n = 18), Poll Dorset (n = 14), or Coopworth (n = 10) with approximately 16 progeny per sire and all dams were Romney. Lambs had an average liveweight of 31.0 kg at an average age of 205 days. Carcasses were dissected into lean, subcutaneous fat, intermuscular fat, bone, and waste. The multivariate mixed linear model included birth date of the lamb as a linear covariate, year-rearing group, breed of sire, sex of lamb, birth rank, and slaughter group as fixed effects, and sire as a random effect. Traits were categorised as either lean, fat, or skeletal dimension indicators. Heritability estimates for lean indicator traits obtained by dissection averaged 0.36. The lean indicator trait with the highest heritability was longissimus muscle width (h 2 = 0.63). Heritability estimates for fat depth measurements averaged 0.23. Heritability of skeletal dimension traits averaged 0.43. Expected responses in a selection objective, comprising increased lean weight and decreased fat weight, were compared for selection indices consisting of hot-carcass weight and one of 26 carcass measurements, In general, both lean and fat indicator traits contributed to a reduction in fat response. The index comprising hot-carcass weight and weight of lean tissue, obtained by dissection, from the leg joint, produced the greatest economic response in the selection objective. The economic response from a nondissection trait was from selection on the index which comprised hotcarcass weight and longissimus muscle width, A. The increase in lean tissue response over that predicted from single trait selection on HCW was small (0.04 kg) relative to the decrease in FAT (0.20 kg). None of the dimension traits produced a negative predicted response in fat. In the range of predicted responses from the two trait indices, there was greater variation in the fat responses than in the lean responses.

Genetic Parameters of Conformation Traits, Milk Yield, and Herd Life in Holsteins

Genetic parameters were estimated simultaneously for 5 herd-life traits, 15 conformation (type) traits, and milk yield measured in first lactation for 128,601 Holstein cows. Heritabilities of all traits were higher in registered than in grade cows. Genetic correlations of linear type traits with first lactation yield ranged from ∼.48 for udder depth to .54 for dairy form. Genetic correlations among milk yield and herd-life traits were all positive except for milk-corrected herd life in grade cows. Udder traits had largest absolute genetic correlations with herd-life traits, followed by body traits and feet and leg traits. Some traits associated with body size and foot angle differed between registered and grade cows.Estimates of genetic trends from obtained parameters revealed greatest progress for milk yield from single-trait selection but also the largest changes for some type traits and milk-corrected herd life in an undesirable direction. Relative milk to type ratios of between 2:l and 3:1 yielded 90% of the gain in milk yield with no change or slight improvement in type traits and functional herd life. Selection for type traits associated with herd life appears to be warranted to improve days of functional herd life or to decrease involuntary culling of dairy cows.

Correlated Responses in Reproduction Accompanying Selection for Milk Yield in Jerseys

Reproductive traits of heifers and primiparous cows from a long-term selection project were analyzed to determine correlated response to single-trait selection for milk yield. Data were from 1056 daughters (765 selection, 291 control) of 37 bulls (17 selection, 20 control). Traits in heifers were ages at first observed estrus and at first breeding, services to conception, interval from first service to conception, and length of first gestation. Traits in primiparous cows were ages at first calving and at first breeding after calving; services to conception; length of second gestation; and intervals from calving to first observed estrus, to first breeding, and to conception, from first service to conception, and from first to second calving. Analyses for services to conception in heifers and primiparous cows were categorical using models containing genetic group and generation. Analyses of other traits were by linear mixed models using fixed effects of genetic group, generation within group, and year-season of birth. Sires were assumed random and nested within genetic group. The mean square for sires within group was used to test for group differences. No significant differences were found between genetic groups in traits measured in heifers; however, the interval from first service to conception approached significance (control superior). In primiparous cows, differences between genetic groups were significant for the intervals of calving to first breeding and Calving to conception and for length of second gestation (control superior). For other traits, reproductive performance of the control was better but not significantly different from that of the selected group. Reproductive performance should be monitored during selection for high milk yield.

Genetic interrelations among grain quality indicators and agronomic traits for oat

Test weight, seed weight, and groat percentage are three common measures of grain quality for oat (Avena sativa L.). There is considerable disagreement, however, about the utility of each of these traits as measures of grain quality, so it is important to understand the genetic interrelations among them and between them and other agronomic traits. In this study, 50 random F2-derived F3 lines from each of 13 biparental oat crosses were evaluated. Genotypic variances, heritabilities, and genotypic correlations for test weight (TW), seed weight (SW), groat percentage (GP), grain yield (GY), harvest index (HI), plant height (PH), and date of heading (DH) were used to predict and compare direct and indirect gains from single-trait selection. Direct selection was always the most effective method for improving single traits; average predicted improvement of population means was 4% for TW, 10% for SW, 1% for GP, and 14% for GY when 10% selection intensity was applied. Genotypic correlation coefficients, averaged for all crosses, were 0.04 for TW with SW, 0.37 for TW with GP, -0.09 for SW with GP, and 0.29, 0.36, and 0.30 for GY with TW, SW, and GP, respectively. Thus, direct selection for TW, SW, or GP should not greatly affect the other two grain quality indicators. For the seven traits considered, there seemed to be no large advantage or disadvantage, in terms of correlated responses, associated with selection for any of the grain quality indicators.

Correlated Response in Growth and Body Measurements Accompanying Selection for Milk Yield in Jerseys

Growth and body measurements from a long-term selection project were analyzed to determine correlated responses to single-trait selection for milk yield. Data were from 1056 daughters (765 selection, 291 control) of 37 bulls (17 selection, 20 control) and included BW and measures of heart girth, chest depth, wither height, and length from withers to pins and from withers to hooks taken at 6 mo, 15 mo, first calving, end of first lactation, and maturity. Other data were birth weight, change in measurements and weights from first calving to end of first lactation, monthly rate of gain from 1 to 13 mo of age, and age reaching breeding weight (250kg). Principal component scores were calculated from standardized measurements at each age. The first three principal components had meaning (size, length vs. girth, and height vs. girth). All analyses used linear mixed models with fixed effects of genetic group, generation within group, year-season of birth or calving, parity of dam, and birth status (multiple or single birth). Sires were assumed to be random and nested within genetic group. Mean squares for sires was used to test for group differences. Generation did not differ in any analysis and was removed from all models. Selection cows were heavier, larger in some measurements, and had greater overall size at 6 mo of age. Selection cows had greater monthly rate of gain and attained breeding weight at an earlier age. Genetic groups did not differ for any other measurement or weight. Control cows gained more weight and increased more in some measurements between first calving and end of first lactation. Selection for milk yield did not result in an undesirable correlated response in any growth or body measurement.

Effect of selection for litter size and body weight on hormone-induced ovulation rate in mice2

Genetic differences in natural vs hormone-induced ovulation rates were compared in immature female mice from five lines that had undergone long-term single-trait and antagonistic index selection for litter size and(or) 6-wk BW. Lines used were control (K); high litter size (L+); high BW (W+); low litter size and high BW (L-W+); and high litter size and low BW (L+W-). Natural ovulation rate at a mean age of 34.3 d and hormone-induced (5 IU of pregnant mare's serum gonadotropin followed 2 d later by 5 IU of human chorionic gonadotropin) superovulation rate at a fixed age of 31 d were obtained. Total number of eggs ovulated was affected by line (P less than .001), treatment (P less than .001), and line x treatment interaction (P less than .001). Line differences were subsequently tested within treatment because of the significant line x treatment interaction. Line differences were important (P less than .001) for natural ovulation, hormone-induced ovulation, and response to hormones. Mean natural ovulation rates for K, L+, W+, L-W+, and L+W- were 14.1, 19.8, 15.1, 13.6, and 16.4, respectively. Selection changed ovulation rate by 40, 16, 7, and -4% in the L+, L+W-, W+ and L-W+ lines, respectively (P less than .01). Hormone-induced ovulation rates in K, L+, W+, L-W+, and L+W- were 32.3, 24.6, 19.6, 20.9, and 22.1, respectively. Exogenous hormones increased ovulation by 18.2, 4.8, 4.6, 7.3, and 5.7 ova for K, L+, W+, L-W+, and L+W-, respectively (P less than .001). Lines with lower natural ovulation rates had higher responses to superovulation. Increased ovulation rate due to treatment ranged from 24.3% in L+ to 129% in K. These results indicate significant differences among lines in ovarian response to exogenous hormones.

Direct Response in Yield and Correlated Response in Components Accompanying Selection for Milk Yield in Jerseys

In 1967, the Jersey herd at the Dairy Experiment Station, Lewisburg, TN was divided into two groups on the basis of ancestry, type, and breeding value for milk as part of a project to determine effects of single trait selection for high milk yield on yield and correlated traits. Control group was mated randomly to 20 unproven young sires selected randomly from those available from breeding studs in 1967. Selection group was mated to sires selected solely on the basis of their high transmitting ability for milk. Selection sires were selected at intervals and used for 4 yr. By the end of the project (1984), lactation information was available on 672 daughters (520 selection and 152 control) of 37 bulls (17 selection and 20 control). Differences in breeding values for milk, fat, and fat test as calculated from the PTA reported in the July 1989 USDA genetic evaluations and differences in first lactation mature equivalent production of milk, fat, fat test, and 4% FCM were examined. Linear mixed models were used for all analyses and contained the fixed effects group, generation within group, and year. Sires were random, nested within group, and used to test for group differences. Groups differed for all traits. Selection was superior to control in breeding value for milk and fat (828 and 31kg) and for production of milk, fat, and 4% FCM (1066, 42, and 1061kg). Control was superior to selection in breeding value and production fat test (.15 and .12%). Group differences existed within generation class for all yield traits but not for fat percentage. Realized response closely matched or exceeded expected response as estimated from pedigree information.

Comparison of simulated and theoretical results in adult MOET nucleus schemes for dairy cattle

Six discrete generation of single trait selection in a closed adult Multiple Ovulation and Embryo Transfer (MOET) nucleus breeding scheme in dairy cattle were modelled using Monte Carlo simulation. The six schemes simulated had 16, 32 or 64 dams and four or eight sires selected. A hierarchical mating design was used with four daughters and one son per dam. Response to selection ranged from 0.21 to 0.3 phenotypic standard deviations per generation. Selecting eight sires instead of four resulted in no loss in response, except with 16 dams, and brought about substantial reductions both in inbreeding and in the variance of response. Genetic gain was predicted by two deterministic models. The first, assuming infinite population sizes and base generation variances, overestimated simulated responses by 40–47% with eight sires and by 63–67% with four sures selected. The responses predicted by the second model, which accounted for the effect of finite population sizes on selection intensities and the effects of inbreeding and selection on genetic variances and accuracies of selection, were within 5% of the simulated responses. Simulated inbreeding rates were 4.2–4.8% and 7.4–7.9% per generation with eight and four sires selected, respectively. These values were 1.8–2.7 times higher than predicted by simple random selection formulae.

Optimising potato breeding. II. A model for optimising single-stage and single-trait selection

A model for optimising single-stage and single-trait selection is presented. The model is based on the expected selection response model, extended with parameters describing the nature of the environment and with variables allowing a manipulation of selection schemes. As an example, the model is used to optimize tuber yield selection in potato breeding. Estimated parameter values have an uncertainty, which may bias the model outcome. Sensitivity analysis shows that the sensitivity to unrealistic parameter values for optimal allocation is less than the sensitivity for the selection response.

The importance of family sizes in adult multiple ovulation and embryo transfer (MOET) nucleus breeding schemes in dairy cattle

ABSTRACTThe importance of family sizes in adult multiple ovulation and embryo transfer (MOET) nucleus schemes with discrete generations of single trait selection was examined using Monte Carlo simulation. Two areas were investigated. Firstly, the number of sons and daughters per dam was varied in schemes using hierarchical mating designs. With four or eight sires and 32 dams selected, increasing the number of sons per dam from one up to four achieved 1 to 8% higher rates of response but at the expense of increased variation in response and 10 to 56% higher rates of inbreeding. With four or eight sires and 16, 32 or 64 dams selected, the number of daughters was set to two, four or eight (with one son per dam in each case). For schemes transferring equal numbers of embryos, responses were lower with two daughters per dam but were fairly similar with four or eight daughters per dam while inbreeding rates increased as fewer sires and dams were selected. Secondly, the effects of variation in family sizes due to biological factors and chance were investigated with eight sires and 32 dams selected and with hierarchical or factorial (two or four sires per dam) mating designs. When all selected cows yielded embryos, changes in family sizes due to differences in sex ratios, in survival rates of embryos to selection and to variation in the number of embryos per donor reduced response by 1 to 4%. However, when 20% or 33% of the superovulated females yielded no embryos, thus requiring the use of genetically inferior replacements, response was reduced by a further 9 to 13%

Correlated response in reproduction, growth and composition to selection in gilts for extremes in age at puberty and backfat

Females ( n=50) were selected for extremes in early vs. late sexual maturity (12 EP, 13 LP) based on age at first detectable estrus; or, high vs. low backfat (13 HF, 12 LF) based on backfat at the tenth rib adjusted to 105 kg basis. These selected females were then heat checked daily, bred during scheduled breeding periods and allowed to farrow. Reproductive performance was then monitored. More EP than LP females demonstrated a normal cycling pattern. Also, more HF than LF females had normal cycling patterns. Early puberty (EP) female farrowed more litters per sow ( P < 0.05) and more pigs per litter ( P < 0.10) than LP. Low-fat females farrowed fewer pigs alive ( P < 0.10) than HF. Early puberty (EP) and HF females had the highest Sow Productivity Indexes (SPI), while LP females were intermediate. Low-fat females had the lowest SPI ( P < 0.05). These results indicate that single-trait selection for extreme leanness or late puberty could be detrimental to reproductive performance. Progeny (371 gilts and 173 barrows) from these litters were then performance tested. The correlated responses in the offspring to selection for extremes for puberty in the dams indicates that selection for early puberty in the females will result in faster gaining offspring with an increased amount of backfat. The correlated responses in offspring of the backfat lines indicates that selection for low backfat thickness in cycling females will decrease fat and increase the amount of lean deposited by her offspring. Also, LF offspring reached puberty and market weight at an earlier age.

Selection strategies for developing smooth bromegrass cell wall ideotypes

A random sample of 80 families of the B8HD smooth bromegrass (Bromus inermis Leyss.) population were tested in three environments for forage yield and cell wall constituents. Expected progress from one cycle of family selection was computed for single-trait selection and multiple-trait restricted selection. Expected gains were compared to desired goals and actual results from one cycle of phenotypic selection. Desired goals were: Model I = reduced lignin and cellulose, with increased hemicellulose, resulting in no change in cell wall content; Model II = reduced lignin and cellulose with no change in hemicellulose; or Model III = reduced lignin, cellulose, and hemicellulose. Single-trait selection for high hemicellulose in first harvest or low cellulose in second harvest had the best expected responses, of any single trait, for Model I. Possible undesirable effects of selection for low cellulose would be a reduction in forage yield potential. Multiple-trait restricted selection was judged to be more effective, with responses all in the desired direction, by specifying increased hemicellulose in index development. Selection in second harvest was expected to have similar responses as first harvest, except for a greater increase in forage yield. Development of Models II or III is expected to be difficult due to a negative correlation estimate between first and second harvest cell wall concentration.

Single and Multiple Traits Selection in a Segregating Population of Wheat, Triticum aestivum L.

AbstractThe breeding materials used in this study were the F3, F4 and F5‐generations of the cross between Giza 158 × Sonora 64 (Triticum aestivum L.). The objective of this study was to compare the relative merits of Smith‐Hazel, desired gain selection indices, independent culling levels and single trait selection in improving grain yield, heading date and several agronomic traits. Highly significant differences among F3 families and a satisfactory genotypic coefficient of variability were obtained for all the traits studied. The genotypic correlations were high between yield and each of spike weight, kernels/spike and spikes; plant, intermediate with 1000 kernel weight and very low with heading date, plant height and spike1 length‐After two cycles of selection, the results of the gains realized indicated that the most effective method for improving yield was the Smith‐Hazel index (SH7) of seven traits followed by the desired gain index of seven traits (DG7), SH5, independent culling levels, DG5 and direct selection (or grain yield/plant. Direct selection for heading date, plant height and spike length was the best method for improving these traits, but undesirable correlated responses in the other traits were obtained.

LH receptor induction and ovulation rate in mice selected for litter size and body weight.

Female mice from lines which had undergone long-term single trait and antagonistic index selection for litter size and body weight were analysed for ovulation rate and LH receptor induction. Compared to randomly selected controls, selection for large litter size increased ovulation rate (60%; P less than 0.001) and decreased LH receptor induction per microgram ovarian DNA (87%; P less than 0.01). Selection for large body weight increased ovulation rate (18%; P less than 0.001), but did not lead to a significant correlated response in LH receptor induction. Index selection for large litter size and small body weight increased ovulation rate (14%; P less than 0.01) and decreased LH receptor induction (72%; P less than 0.01), while index selection for small litter size and large body weight did not significantly alter either ovulation rate or LH receptor induction. LH receptor quantities in testes of males from the 5 lines did not exhibit the among-line profile which was observed in ovaries of females. These results confirm the role of ovulation rate in mediation of the positive genetic correlation between litter size and body weight in mice. Increased ovulation rate in mice selected for large litter size may be due to mechanisms associated with LH receptors as well as factors related to growth. In contrast, increased ovulation rate in mice selected for large body weight may be due exclusively to factors related to growth.

Sire-Daughter and Dam-Daughter Paths: Research Geneticists’ View

The sire-daughter path of genetic improvement dominates the dam-daughter path. Within-herd management practices intended to enhance genetic progress should thus concentrate on the sire-daughter path. Studies of genetic progress via the sire-daughter path from historic and recent estimates show that rates for milk yield have increased in recent years; however, rates are still only half to two-thirds that theoretically possible from single-trait selection. Perceived importance of linear type traits, registry status, and reproductive management considerations influence genetic merit of service sires. Establishment of herd breeding goals and management of the breeding herd by fertility groups appear to be useful approaches to improve genetic merit of service sires. Impact of proposed changes in sire selection methods on demand for proven bulls and sample sires is discussed.

Expected early genetic gain from selection for milk yield in dairy cattle

A matrix program to predict short term genetic gain from single trait selection for milk yield was developed. Rate of genetic gain was calculated as the annual change in the mean breeding value of all producing females. Several parameters sets representing various selection policies were used to examine situations pertinent to dairy populations of the United States. Approach to the asymptotic rates of genetic gain within the model varied with the choice of parameters, but even with consistent selection policies, predicted total genetic gain in the first 10 years was only half of the expected from classical theory. Considerable year to year variation in the rate of gain occurred. Early gains were more dependent on female selection decisions than gains during the steady state. In a two-phase model, the approach to the linear rate of gain in the second phase was accelerated by starting with an ongoing improvement program, but considerable delays still existed. Selection for sex- limited traits such as milk yield, which require pedigree selection and a waiting time for progeny test results reached asymptotic rates more slowly than previously assumed.