Single Trait Selection Research Articles

Recent Technology of Genetic Selection Indices in Sheep Breeding

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Temporal trends in reproductive performance in Irish dairy herds and associated risk factors.

Irish dairy herd fertility has been declining since the 1980s. The extent, nature and causes of this decline in fertility and the current status of Irish dairy herd fertility were described. An increase in calving interval of approximately one day per year has been recorded. The principal components of this trend have been an increased incidence of postpartum endocrinopathies, reduced expression of oestrus and a fall in conception rate. Both submission rate and calving-to-service interval have increased slightly over time. Significant risk factors associated with these trends have been strain substitution within the Holstein-Friesian breed and single trait selection for milk production. Critically, these changes have been reflected in loss of body condition. Contributory factors included increased herd size and possibly increased use of DIYAI. The most recent Irish study showed that 48% of cows conceived to first service and 14% of cows were not pregnant at the end of the industry-average 15-week spring breeding season. However, the top quartile of herds achieved a first-service conception rate of 59%, illustrating the wide variation between herds. These phenotypic trends were attributed to both genetic and environmental factors and their interactions. Recent Irish dairy herd fertility performance falls short of the targets set for seasonal compact calving.

Genetic improvement of mastitis resistance: validation of somatic cell score and clinical mastitis as selection criteria.

Mean daughter deviations for clinical mastitis among second-crop daughters were regressed on predicted transmitting abilities for clinical mastitis and lactation mean somatic cell score in first-crop daughters to validate the predictive ability of these traits as selection criteria for reduced incidence of clinical mastitis. A total of 321 sires had 684,897 second-crop daughters, while predicted transmitting abilities were calculated for 2159 sires, based on 495,681 records of first-crop daughters. Predictive ability, as a measure of efficiency of selection, was 23 to 43% higher for clinical mastitis than for lactation mean somatic cell score. Compared to single-trait selection, predictive ability improved 8 to 13% from utilizing information on both traits. The relative weight that should be assigned to standardized predicted transmitting abilities from univariate genetic analyses were 60 to 67% for clinical mastitis and 33 to 40% for lactation mean somatic cell score. No significant nonlinear genetic relationship between the two traits was found.

Selection strategies for limiting the increase in ascites while increasing growth in broilers

The objective of the current study was to compare the changes in a fitness trait when selection is performed for 5, 10, and 20 generations on a production trait that influenced its expression. Responses to single-trait selection for growth based on phenotype or animal model predictions were compared by computer simulation. Two-trait index selection was performed when a trait, related to the fitness trait, was included in the index with the production trait. The phenotypic expression of the fitness trait among the sibs was also considered as a selection factor for single-trait and two-trait index selection. For a fixed increase in the expression of the fitness trait, mass selection produced a larger increase in the production trait than did use of standard animal model best linear unbiased prediction under single-trait selection. The reduction in the genotypic mean of the fitness trait was accompanied by an increase in its phenotypic expression. The use of sib information and an indicator trait reduced the level of expression reached by the fitness trait.

Genetic parameters of production, feed intake, body weight, body composition, and udder health in lactating dairy cows

Records on milk yield, body weight, feed intake, SCC, mastitis incidence, and indicators of body composition were analysed both as repeated measures within lactations and as aggregated lactation measures. The data comprised 603 lactations on 293 dairy cows on an experimental farm in Denmark. Most genetic parameters were in agreement with similar results from the literature. Minimum body weight is reached earlier in lactation than peak yield and much before peak feed intake. Genetic selection for higher milk production will lead to cows with lower body weight gains during lactation and therefore cows that are less well prepared to meet the energy requirements of the subsequent lactation. There were positive genetic correlations between mobilisation of body reserves and SCC indicating that increased production leads to metabolic stress and a change in the immune system. Single trait selection for production will lead to increases in SCC and thereby in mastitis frequency. Cows with large fat depots had a higher milk production. However, selection for increased milk production leads to lower weight gains during lactation leaving the cow less well prepared for the next lactation. This study showed large variation in the variables attempting to describe the shape of lactation curves for milk yield, body weight, and feed intake. Therefore, encouraged continuation in recording these variables, and physiological measures to understand the fundamental biological complex of the energy balance in lactating dairy cows are needed. The variables for repeated measures within lactation may be analysed using random regression models in order to study feed forward mechanisms from records in early lactation to records in late lactation.

Indirect selection criteria against clean wool colour in Corriedale sheep and their effects on wool production traits

The potential of greasy wool colour subjective assessment Visual Colour Score (VCS) and the yellow predictive test (YPC) as indirect selection criteria for reduction of clean wool colour (CWC) in Corriedale sheep was examined. The heritability of these wool colour traits and the wool production traits, greasy (GFW) and clean fleece weights (CFW), and mean fibre diameter (MFD) and the phenotypic and genetic correlations among these traits were estimated from a Corriedale flock using AIREML procedures. A high genetic correlation between YPC and CWC was observed, indicating that YPC could be a suitable indirect selection criterium for CWC. However, direct selection against CWC was predicted to produce faster genetic improvements in CWC than that expected under indirect selection via YPC. Single trait selection based on VCS or YPC were expected to reduce the response in CWC to 51% and 49% of that estimated for direct selection. The positive genetic correlations of CWC, YPC and VCS with CFW and MFD would cause a reduction in both MFD and CFW to result from selection that reduces wool colour. The results showed that the most effective way to genetically improve CWC was through indirect selection to reduce MFD, CFW or GFW, followed by direct selection, but the premiums for CWC in the Corriedale breed may not be sufficient to justify the expected losses in CFW.

Bivariate Animal Model Estimates of Genetic, Phenotypic, and Environmental Correlations for Production, Reproduction, and Somatic Cells in Jerseys

Correlations were obtained between 18 response variables of a Jersey herd (Florida Agricultural Experiment Station, Gainesville) for 374 first lactations. Estimates were from the use of multivariate, derivative-free, restricted maximum likelihood procedures with the simplex method of partial maximization. Estimates agreed closely with those obtained previously by other methods in this and other dairy populations. All correlations between yields were high and positive; those between yields and days from parturition to first service were negative and near zero. Correlations between yields and somatic cell scores were moderate and negative; those between yields and constituent percentages in general were negative, except for the yield and percentage of the same constituent. Genetic correlations between chloride content and somatic cells and between measures of somatic cells were 1.0. Results suggest that single-trait selection for milk yield should result in correlated increases in constituent yields with slight decreases in percentage composition of constituents and somatic cell counts.

Optimization of the Number of Cycles and Intensity of Selection, and Population Size in Mass Selection: Selection for Single Traits in Outcrossing Plant Populations.

Optimum number of cycles and rate of selection and optimum population size to give the highest cost efficiency in mass selection for allogamous crop plants were investigated. The optimum values of these variables were obtained based on the numerical calculations of a new selection efficiency index, S/C, where S stands for the probability that the desired genetic improvement is achieved in the target population, and C is the cost expended for that end. Selection procedures that maximize S/C will give the greatest opportunity of success under a certain total resource investment. The index S/C was calculated based on Monte Carlo simulations with a model in which the target population was initiated as a hybrid population of two cultivar lines that were genetically fixed for the trait concerned, and populations of a constant size were generated for subsequent selection cycles via random crossing between the plants selected with a constant rate in each cycle. The genes involved were assumed to be inherited independently and have to epistatic interaction. The calculations over practically possible ranges of the related variables led to the following conclusions. If, as would be the case in most selection projecrts for breeding a new commercial variety, the expenditure of time (years) rather than resource expense for managing the selection is important, a few to several cycles of selection with a rate of around 1%, testing a few thousand plants per cycle, should be nearly if not exactly the optimum. If the desired improvement is not achieved in these cycles, the population should be discarded and a new one should be tested. By contrast, if the time expenditure is not important, as in the case of the selection for new breeding stock lines, selection with several or more cycles, a rate in the range of 10 to 20% and a population size smaller than 100 should be costefficient. Dominance is not an important modifier of the optimum values unless the desirable genes are dominant or recessive unidirectionally at the majority of the loci involved.

Using recent versus complete pedigree data in genetic evaluation of a closed nucleus broiler line

Stochastic simulation was used to study the effect of using full data and pedigree structure vs more recent data and pedigree structure to obtain best linear unbiased predictors (BLUP) of breeding values for single trait selection. Simulations used heritabilities of 0.10 and 0.50, with a population structure of 20 sires each mated to two dams, each producing 10 progeny, with 11 hatches from an unselected base population under both discrete and overlapping generations. Selection of parents was based on BLUP of breeding values using an animal model. The use of the last two generations of data and pedigrees gave the same selection response as when using full data and pedigree structure, for both heritabilities. Under discrete generations with use of only the last generation data and pedigree, which is similar to phenotypic evaluation, response to selection decreased by 21 and 3.8% at Generation 10 compared to selection response when using the full data and pedigree for heritabilities of 0.10 and 0.50, respectively. Corresponding decreases in inbreeding were 72 and 37%. The amount of central processing unit time for genetic evaluation when using the last six, four, and two generations of data and pedigree was reduced to 70, 40, and 11% of that when using the full data set, for a heritability of 0.10 and discrete generations. Very similar values were observed for a heritability of 0.50 and also under overlapping generations.

Casein polymorphism and relation between milk production traits

Casein polymorphism and relation between milk production traits

Genetic trends for milk yield of Jerseys and correlated changes in productive and reproductive performance.

Estimates of genetic trends in 24 measures of milk and constituent yields, somatic cell counts, and reproduction were obtained from 935 records of 374 Jerseys in a single herd. Data were obtained from a designed project for single-trait selection from 1969 through 1987. One line was subjected to selection solely for milk yield and included 259 cows; an unselected control line included 115 cows. Estimates of trends were based on differences in linear phenotypic trends between lines for first lactations, all lactations, and for 305-d and total records. The genetic changes in milk yield for these four data sets were 1.22 to 1.48%/yr (36.8 to 41.0 kg per cow yr) and 0.54 to 1.64%/yr for five constituent yields. Except for the percentages of minerals plus lactose, all constituent percentages decreased by 0.05 to 0.60%/yr. The ratios of protein to fat and solids-not-fat to fat increased 0.30 to 0.54%/yr, respectively. The number of services required per conception increased (0.17%) in first parity records and in all data (0.69%). The intervals from parturition to first estrus and from parturition to first service decreased in first lactation (1.19 and 0.82%) annually but increased (1.25 and 0.01%) in all data. Age of heifers at first estrus decreased by 0.44% annually. Most of the five measures of somatic cells decreased in first lactations but increased for all data. Estimates of realized genetic correlations of 14 measures of constituent yield and composition (four correlations each) agreed well with values expected from the literature. The results quantified change in milk yield, constituent yields and percentages, reproductive performance, and somatic cell counts in a single herd and should prove useful in the development of selection programs for dairy cattle.

Genetic Variation and Predicted Gain from Selection for Winterhardiness and Turf Quality in a Perenial Ryegrass Topcross Population

Improved winterhardiness in turf‐type perennial ryegrass (Lolium perenne L.) could expand its area of adaptation for turf and seed production, but breeding efforts have been hindered in part by a lack of identifiable winter‐hardy germplasm. This research was conducted to estimate genetic variation for winterhardiness in a perennial ryegrass topcross population and to determine the feasibility of simultaneously improving turf quality and winter field survival. ‘NK‐200’, a perennial ryegrass cultivar with acceptable winterhardiness in northern Minnesota, was topcrossed to a large sample of improved turftype cultivars and 243 of the resulting half‐sib families were selected for evaluation. Genetic variances, correlations, and predicted gain from single and multiple trait selection were stimated for winterhardiness and turf quality characteristics of half‐sib progeny evaluated at two Minnesota locations in 1994 and 1995. Significant additive genetic variation and moderate narrow‐sense heritability estimates were obtained for winterhardiness (σ2A = 0.64 ± 0.11;h2 =0.59 ± 0.10). Fiftyfive half‐sib families had higher or equivalent winterhardiness than NK‐200 suggesting transgressive segregation. Significant additive genetic variation was found for turf quality, crown rust (Puccinia coronata Corda) resistance, and leaf texture, but turf quality and leaf texture were undesirably correlated with winterhardiness (rA = −0.15 and −0.25, respectively). Gain from selection estimates revealed that single trait selection could rapidly improve winterhardiness and turf quality. However, some form of multiple trait selection would be necessary to improve winterhardiness and turf quality traits simultaneously. These results indicate that the potential exists to develop improved

Genetics of disease resistance inBos tauruscattle

SummaryThis review summarises evidence for genetic variation ofBos tauruscattle to diseases encountered under temperate conditions, including internal and external parasitism, susceptibility to mycotoxic diseases (tall fescue toxicosis, facial eczema, ryegrass staggers), mastitis, ketosis, pasture bloat, leukosis, tuberculosis, foot and mouth, brucellosis and BSE. Averaging mean heritability estimates reviewed from 8 diseases (weighted equally) gave a value of 0.21, indicating that measurable genetic variation for disease traits inBos tauruscattle is somewhat less than that for production traits, such as milk yield or body weight. Many estimates, however, have high standard errors, and there could be an upward bias resulting from non-reporting of zero or non-significant estimates.Few single-trait selection experiments have been conducted to study the genetics of disease resistance traits in cattle. For the disease traits where selection is being applied extensively, index selection for improved disease resistance and increased production is more common than single-trait selection. Results from a long-term (25 year) divergent selection experiment with resistance/susceptibility to pasture bloat in cattle in New Zealand are reviewed. Four single-year experiments comparing progeny of ‘high’ versus ‘low’ sires for resistance to disease are also reviewed, one in Australia studying faecal nematode egg counts, one in the USA involving the mycotoxic disease, tall fescue toxicosis, a third in New Zealand involving the mycotoxic disease, facial eczema, and a fourth in the USA involvingBrucella abortus.

Efficiency of Secondary Traits for Improving Maize for Low‐Nitrogen Target Environments

Consideration of secondary traits could improve selection efficiency under stress conditions. This study assesses the value of secondary traits for improving lowland tropical maize (Zea mays L.) for low‐N target environments. Nineteen experiments grown with no N applied at CIMMYT, México, between 1986 and 1995 were analyzed for grain yield, anthesis‐silking intervai (ASI), number of ears per plant, leaf chlorophyll concentration, and an estimate of leaf senescence. Broad‐sense heritabilities of traits, genetic correlations between secondary traits and grain yield, and predicted response of grain yield to selection for single or multiple traits were calculated. Multiple traits were combined using an unrestricted selection index (Smith‐Hazel index). Broad‐sense heritabilities averaged 0.46 for grain yield, 0.52 for ASI, 0.44 for ears per plant, 0.35 for leaf chlorophyll concentration, and 0.60 for leaf senescence. Genetic correlations of secondary traits with grain yield averaged −0.47 for ASI, 0.78 for ears per plant, 0.24 for leaf chlorophyll concentration, and 0.42 for leaf senescence. Direct selection for grain yield was superior to selection for single secondary traits in most experiments. When information on all traits was combined in a Smith‐Hazel index, selection efficiency improved by 14% on average over selection for grain yield alone. Among secondary traits, ears per plant and leaf senescence discriminated high‐yielding genotypes the best. Leaf chlorophyll concentration, and in some instances ASI, provided information on environmental variation within experiments. We conclude that secondary traits can increase the efficiency of selection for grain yield in maize breeding programs targeting low‐N environments.

Selection for yield, protein, and oil in soybean crosses between adapted and introduced parents

Soybean [ Glycine max (L.) Merr.] breeders can sometimes increase genetic variability in their germplasm through wide crosses with plant introduction (PI) material, and in that way lead to slow progress in developing high-yielding cultivars. A study was conducted to evaluate the potential of crosses of adapted × introduced soybean lines for improving yield and protein and oil concentrations. Five soybean cultivars adapted to South Dakota were crossed in selected combinations with four PI lines to develop F 3-derived lines. The adapted and PI lines were selected for wide variation in protein and oil concentration. Thirty F 3:5 lines and parents from each of eight different crosses, including two adapted × adapted, were evaluated in a replicated group balanced block experiment. Significant genetic variability for yield occurred among progenies of two adapted × PI crosses, and for protein and oil concentrations among progenies of two adapted × PI crosses. No significant genetic variability for any trait was found among progenies of adapted × adapted crosses. Progenies of adapted × adapted crosses generally yielded more seed and had higher oil concentrations than those from adapted × PI crosses. PI parents with high trait expression produced populations with high expression for that trait, and combined better with some adapted parents than with others. With single-trait selection, simultaneous improvement of yield and oil concentration was more successful than for yield and protein concentration, or for protein and oil. Increases in the sum of protein and oil concentrations were small, and were achieved through increased protein concentration. In addition to measuring the trait of interest before crossing with PI lines, it seems that genetic variability of populations and combining ability of adapted parents should be determined. Relationships of yield, protein, and oil should be determined for each cross, even if only one trait is of interest in the selection program.

Flesh quality improvement in fish through breeding

In salmonids, which are fat fish, phenotypic and genetic parameters in flesh quality traits have been studied in several investigations while little has been done in other fish species. For traits like fat percentage, area of fat depot, and flesh colour in salmonids, relatively large genetic variation has been found. There is an unfavourable genetic correlation between body weight and the traits fat percentage and area of fat depot. The expected response to selection in quality traits, considering single trait selection, varies between 5 and 15% per generation. Quality traits can usually be recorded only on dead animals, and therefore family selection mush be practised in a breeding programme.

Molecular marker-assisted selection for malting quality traits in barley

Selection for malting quality in breeding programs by micromalting and micromashing is time-consuming, and resource-intensive. More efficient and feasible approaches for identifying genotypes with good malting quality would be highly desirable. With the advent of molecular markers, it is possible to map and tag the loci affecting malting quality. The objective of this study was to assess the effectiveness of molecular marker assisted selection for malting quality traits. Two major quantitative trait loci (QTL) regions in six-row barley for malt extract percentage, α-amylase activity, diastatic power, and malt β-glucan content on chromosomes 1 (QTL1) and 4 (QTL2) have been previously identified. The flanking markers, Brz and Amy2, and WG622 and BCD402B, for these two major QTL regions were used in marker-assisted selection. Four alternative selection strategies; phenotypic selection, genotypic selection, tandem genotypic and phenotypic selection, and combined phenotypic and genotypic selection, were compared for both single and multiple trait selection in a population consisting of 92 doubled haploid lines derived from ‘Steptoe’ × ‘Morex’ crosses. Marker assisted selection for QTL1 (tandem genotypic and phenotypic selection, and combined phenotypic and genotypic selection) was more effective than phenotypic selection, but for QTL2 was not as effective as phenotypic selection due to a lack of QTL2 effects in the selection population. The effectiveness of tandem genotypic and phenotypic selection makes marker assisted selection practical for traits which are extremely difficult or expensive to measure such as most malting quality traits. It can substantially eliminate undesirable genotypes by early genotyping and keeping only desirable genotypes for later phenotypic selection.

Multiple Trait Selection in a Recurrent Selection Population in Oat (Avena sativa L.)

Single trait selection is often utilized to maximize genetic gain in recurrent selection systems. However, traits negatively correlated to the primary trait can deteriorate with single trait selection. Multiple trait selection can be used to prevent or correct correlated trait deficiencies. Our objective was to compare the effectiveness of several multiple trait selection schemes with selection for grain yield alone. Five multiple‐trait selection schemes were evaluated for their effect on total genotypic worth and individual trait responses in an “opened” oat (Avena sativa L.) recurrent selection population. Grain yield, days‐to‐heading, plant height, and reaction to barley yellow dwarf virus (BYD) were studied in several environments. The phenotypic correlations with yield were 0.44 and 0.60 for heading date and plant height, respectively. There was no association between yield and BYD. A restricted index, a Smith index, a modified base index, a multiplicative index, and independent culling were evaluated and compared with selection for yield alone. The restricted and Smith selection indices, when compared with selection for yield alone, selected genotypes with improved heading date, plant height, and BYD with only negligible decreases in yield. Greater correction for heading date and plant height was achieved with the modified base index, the multiplicative index and independent culling, but yield decreased accordingly. Realized gains in total worth for the restricted index were 15% above, and the Smith index 14% above, the gain realized by selection for yield alone. Restricted and Smith index selection were effective at preventing secondary traits from deteriorating through selection for grain yield.

Single trait selection for yearling fleece weight or liveweight in Romney sheep—correlated responses in liveweight, fleece traits, and ewe reproduction

Abstract An experiment was carried out to study direct and indirect responses to selection in Romney sheep. There were two selection lines, one selected for greasy fleece weight (GFW) and the other for liveweight (LW), maintained alongside a control line (CO). Data from lambs born in 1967–90 were analysed to quantify the correlated responses to selection. By 1990, there had been an average of nine generations of selection with an average generation interval of 2.7 years. The correlated responses analysed included yearling and ewe greasy and clean fleece weights, washing yield, yearling and ewe fleece quality traits (staple length, mean fibre diameter, fibre diameter variation, loose wool bulk, Commission Internationale de l'Eclairange (CIE) Y value (brightness) and CIE Y—Z value (yellowness)), birth weight, weaning weight, yearling liveweight, ewe pre‐mating weight, and ewe reproductive rate (fertility, litter size, weaning percentage, and lamb survival). Correlated responses were estimated as the deviati...

Modified Convergent Improvement: A Breeding Method for Multiple Trait Selection

Breeders need efficient multiple trait selection schemes that allow selection for each trait within its respective optimum environment. This report describes such a breeding procedure entitled, “modified convergent improvement,” and compares theoretical predictions of genetic gain per cycle (Δq/cycle) for the following schemes: modified convergent improvement, tandem selection in two dissimilar populations followed by a population cross (strain crossing), recurrent selection for single traits in dissimilar populations followed by a multiple population cross (multiple strain cross), and independent culling. Estimates of Δq/cycle were calculated for both additive and dominant gene action when selecting for three to six traits. Modified convergent improvement (Δq/cycle = 0.13) and independent culling (Δq/cycle 0.11) exhibited highest average Δq/cycle for traits controlled by recessive genes (P≤ 0.05). Independent culling exhibited the highest average Δq/cycle when selecting for traits controlled by dominant genes (Δq/cycle = 0.13) or by additive gene action (Δq/cycle = 0.20). Modified convergent improvement proved superior to independent culling when selecting for traits controlled by additive gene action or by recessive genes when gene frequency was low (q = 0.04−0.12). Theoretical Δq/cycle values for multiple strain cross and strain crossing were consistently lower than those obtained for either independent culling or modified convergent improvement.We propose that modified convergent improvement is a superior method for multiple trait selection when traits must be evaluated in dissimilar environments or during the initial stages of germplasm development. This procedure confers additional benefits including reducing inbreeding potential and/or disequilibrium over that observed in single population breeding procedures.