Estimates of genetic parameters and selection strategies to improve the economic efficiency of postweaning growth in lambs

The objectives of this study were to estimate (co)variance components for growth and feed efficiency measures, and to compare selection strategies to improve economic efficiency of gain. Variance components for pre- and postweaning growth, body weight, and measures of feed efficiency were estimated from data collected on 1,047 Targhee lambs over 7 yr. Approximately 21 d after weaning, lambs were group-fed for 4 wk, with ad libitum access to a diet of 37% whole barley grain and 63% pelleted alfalfa hay. Lambs were then individually fed for 6 wk. Lambs were then returned to group feeding for another 4-wk period. The mean feed conversion ratio (gain/intake) for the individual feeding period was 0.11. Mean postweaning ADG for the total 14-wk feeding period was 0.26 kg. (Co)variance components were estimated from single- and two-trait animal models using REML. The selection strategies compared included direct selection, index selection, and restricted index selection. Estimates of (co)variances derived from single- and two-trait models were similar, except for mid-test body weight. Preweaning growth had a low heritability estimate (0.03 ± 0.04) compared with postweaning growth measures (0.25 to 0.39), but all measures of growth were highly correlated (r2 \u3e 0.98). Heritability estimates of measures of gain efficiency were variable (total feed intake = &#;0.39; feed conversion ratio = &#;0.26; residual feed intake = &#;0.26). Total feed intake was strongly correlated genetically with feed conversion ratio (0.79) and residual feed intake (0.77). The estimate of genetic correlation between feed conversion ratio and residual feed intake was low (0.23). Comparison of selection strategies showed the superiority of index selection (ADG, total feed, body weight) for economic improvement compared with other strategies. Economic response to direct selection for ADG was at least twice that for direct selection for feed conversion ratio or against total feed intake, and that for restricted indices (selecting against residual feed, while holding body weight and/or gain constant). Selection for ADG may be a practical approach for indirectly improving efficiency of gain in lambs

ESTIMATION OF GENETIC RELATIONSHIPS AMONG MILK RECORDS FOR FIRST THREE LACTATIONS USING REML FOR AN ANIMAL MODEL

In dairy cattle breeding, genetic relationships among lactation records are of special interest because most selection operated on first lactations. This selection also complicates estimation of genetic parameters. Techniques which give estimates unbiased by selection should be used. Estimation was done using EM-type REML for an animal model neglecting relationships across herds. Records were from 3,070 Holstein cows which had the fist lactation recorded. Estimates after 17 rounds of iteration for heritabilities and genetic correlations were: h21=.33, h22=.32, h23=.33, r12=.88, r13= .83, r23= .86. Within herd-year-season phenotypic standard deviations were 1,223 kg, 1,323 kg, and 1,265 kg

Type Appraisal: II. Variation in Type Traits Due to Sires, Herds, and Years

Variance components were estimated from type appraisal data to determine the importance of year, herd, sire, and herd x sire effects upon 49 body, udder, and management traits. Estimates were based on over 16,000 appraisals on daughters of Holstein artificial insemination sires. The variation explained by these effects never exceeded 34% of the total variance. Year effects were almost nonexistent (-2 to 3%). Herd effects were small for all traits except feeding speed, body weight, intensity and persistency of edema, and ketosis, and never exceeded 25%. Most appraisal traits had low heritabilities. The estimate for milking speed was .23 while estimates for other management traits were less than .08. The estimates for body weight and upstandingness were .40 and .39. Other body traits having estimates from .16 to .21 were sharpness, height of thurls, depth of body, levelness of rump, tightness of shoulders, and height of tail setting. The heritability estimates for udder traits were low. Estimates for only three of 21 udder traits exceeded .14. These were strength of rear attachment, rear teat spacing, and depth of udder

RANDOM MODELS WITH DIRECT AND COMPETITION GENETIC EFFECTS

Livestock producers often select for animals which are genetically superior for yield. Competition among animals in the same pen may affect yield of pen mates. If competitiveness has a genetic component, selection should be for direct genetic effects for yield and for genetic effects of competitiveness on yield of penmates (Muir and Schinkel, 2002). This simulation study examined estimates of variance components from models which ignored competition effects. A population structure of 642 related animals was created. Random effects were residual and pen effects and direct and competition genetic values with genetic correlation. Conclusions, based on 400 replications for 16 different sets of variance parameters, were that competition effects, if ignored, may inflate estimates of pen variance and of direct genetic variance and that ignoring pen effects may increase estimates of the genetic correlation and both genetic variances. Key words: Associative Effects, Genetic Correlation, REM

ASPECTS OF SELECTION FOR PERFORMANCE IN SEVERAL ENVIRONMENTS WITH HETEROGENEOUS VARIANCES

Dairy cattle evaluation schemes routinely assume homogeneous variance with respect to environment. Increasing evidence suggests the presence of systematic changes in variance components associated with mean level of performance. Best linear unbiased prediction procedures that account for heterogeneity are reviewed. The consequences of incorrectly assuming homogeneity for evaluation are demonstrated for a progeny test and an artificial breeding program that screens dams of sires from heterogeneous populations. Selection assuming homogeneity can be very efficient when heritability, and therefore accuracy of selection, is greatest in the more variable environment. Conversely, appreciable reduction in response results when heritability is greater in the less variable environment

AN ALTERNATIVE FOR MIXED MODEL ANALYSES OF LARGE, MESSY DATA SETS (MTDFREML)

Portable Fortran based programs (MTDFREML) were developed using a derivative-free algorithm to obtain REML estimates of (co)variance components. Computations are based on Henderson\u27s mixed model equations for multiple-trait models with missing observations on some traits and incorporation of relationships among relatives. Many fixed and random factors are allowed with number of levels dependent on computer memory. Data sets with more than 40,000 genetic effects have been analyzed. Options allow solving MME at convergence. Constraints are automatically imposed. Expectations, standard errors of contrasts of solutions for fixed effects and prediction error variances of solutions for random effects can be obtained. Dimensions can be changed to match data with computer capability. A Fortran compiler is necessary. No fee is charged but the University of Waterloo must certify a license has been obtained for sparse matrix subroutines (SPARSPAK) used in the program. As an example, birth weights of 4891 progeny of 389 sires nested within 12 breeds and of 2893 dams nested within 3 breeds of dam were analyzed to estimate components of variance due to sires and dams and to estimate differences among breeds of sires. For MTDFREML the analysis was trivial but for PROC MIXED the analysis was impossible unless dams were dropped from the model

Estimates of genetic parameters and selection strategies to improve the economic efficiency of postweaning growth in lambs

The objectives of this study were to estimate (co)variance components for growth and feed efficiency measures, and to compare selection strategies to improve economic efficiency of gain. Variance components for pre- and postweaning growth, body weight, and measures of feed efficiency were estimated from data collected on 1,047 Targhee lambs over 7 yr. Approximately 21 d after weaning, lambs were group-fed for 4 wk, with ad libitum access to a diet of 37% whole barley grain and 63% pelleted alfalfa hay. Lambs were then individually fed for 6 wk. Lambs were then returned to group feeding for another 4-wk period. The mean feed conversion ratio (gain/intake) for the individual feeding period was 0.11. Mean postweaning ADG for the total 14-wk feeding period was 0.26 kg. (Co)variance components were estimated from single- and two-trait animal models using REML. The selection strategies compared included direct selection, index selection, and restricted index selection. Estimates of (co)variances derived from single- and two-trait models were similar, except for mid-test body weight. Preweaning growth had a low heritability estimate (0.03 ± 0.04) compared with postweaning growth measures (0.25 to 0.39), but all measures of growth were highly correlated (r2 \u3e 0.98). Heritability estimates of measures of gain efficiency were variable (total feed intake = &#;0.39; feed conversion ratio = &#;0.26; residual feed intake = &#;0.26). Total feed intake was strongly correlated genetically with feed conversion ratio (0.79) and residual feed intake (0.77). The estimate of genetic correlation between feed conversion ratio and residual feed intake was low (0.23). Comparison of selection strategies showed the superiority of index selection (ADG, total feed, body weight) for economic improvement compared with other strategies. Economic response to direct selection for ADG was at least twice that for direct selection for feed conversion ratio or against total feed intake, and that for restricted indices (selecting against residual feed, while holding body weight and/or gain constant). Selection for ADG may be a practical approach for indirectly improving efficiency of gain in lambs

THRESHOLD SIRE MODELS FOR ESTIMATING GENETIC PARAMETERS FOR STAYABILITY IN BEEF COWS

Stayability is the ability of a beef cow to remain in production to a specified age. In this study, the interest was in determining the genetic relationship between stayability to an early age with stayability to a later age. A nested threshold sire model for stayability was used here to estimate the genetic relationship between stayability to different ages. Genetic correlations were estimated among six different stayability traits using records from 1,868 Hereford cows. The model included period and year of birth as fixed factors and sire as a random factor. The numerator relationship matrix accounted for all known relationships among sires. Penalized quasilikelihood estimates were obtained using a probit link function. Estimates of heritability on the original scale were small and ranged from 0.09 to 0.17. Estimates of genetic correlations were low to moderate and variable in sign. Results indicate that selection for stayability to an early age would have a limited impact on stayability to later ages

Genetic Parameters of Reproductive Traits in Sheep

Reproductive traits from 7642 ewes were recorded from 1975 to 1983. The ewes were of five breeds (Dorset (D), Finnsheep (F), Rambouillet (R), Suffolk (S) and Targhee (T)) and two composite lines [C1 (1/2F + 1/4R + 1/4D) and C2 (1/2F + 1/4S + 1/4T)]. Genetic parameters were estimated for six basic and seven composite traits. The basic traits were conception rate (CR), total number of lamb born (NLB), number of lambs born alive (NLBA), number of lambs alive at weaning (NLAW), litter mean weight per lamb born (LMWLB) and litter mean weight per lamb weaned (LMWLW). The composite traits were ratio of lambs surviving to weaning relative to NLB (LSW = NLAW/NLB), number of lambs born per ewe exposed (NLBEE = CR × NLB), number of lambs weaned per ewe exposed (NLWEE = CR × NLAW), total litter weight at birth (TLWB = NLB × LMWLB), total litter weight at weaning (TLWW = NLAW × LMWLW), total litter weight at birth per ewe exposed (TLWBEE = CR × NLB × LMWLB) and total litter weight at weaning per ewe exposed (TLWWEE = CR × NLAW × LMWLW). Year, age of ewe, breed of ewe, hormone treatment and season of breeding were used as fixed effects. Direct and maternal genetic effects, permanent environmental effects of ewe and mate of ewe were considered to be random effects. A derivative-free algorithm was used to obtain REML estimates of genetic and environmental parameters. Estimates of heritabilities for animal genetic and permanent environmental and maternal genetic effects were mainly small due to the typical high influence of environmental factors on reproductive traits and to non-normal distributions of traits. Mate of ewe effects were not important for any trait. Important genetic correlations were found between some traits. Some estimates of genetic correlations do not seem to have a biological explanation. Nevertheless, these estimates of genetic correlations among traits may provide a basis for deriving selection indexes for reproductive traits

Genetic Parameters of Reproductive Traits in Sheep

Reproductive traits from 7642 ewes were recorded from 1975 to 1983. The ewes were of five breeds (Dorset (D), Finnsheep (F), Rambouillet (R), Suffolk (S) and Targhee (T)) and two composite lines [C1 (1/2F + 1/4R + 1/4D) and C2 (1/2F + 1/4S + 1/4T)]. Genetic parameters were estimated for six basic and seven composite traits. The basic traits were conception rate (CR), total number of lamb born (NLB), number of lambs born alive (NLBA), number of lambs alive at weaning (NLAW), litter mean weight per lamb born (LMWLB) and litter mean weight per lamb weaned (LMWLW). The composite traits were ratio of lambs surviving to weaning relative to NLB (LSW = NLAW/NLB), number of lambs born per ewe exposed (NLBEE = CR × NLB), number of lambs weaned per ewe exposed (NLWEE = CR × NLAW), total litter weight at birth (TLWB = NLB × LMWLB), total litter weight at weaning (TLWW = NLAW × LMWLW), total litter weight at birth per ewe exposed (TLWBEE = CR × NLB × LMWLB) and total litter weight at weaning per ewe exposed (TLWWEE = CR × NLAW × LMWLW). Year, age of ewe, breed of ewe, hormone treatment and season of breeding were used as fixed effects. Direct and maternal genetic effects, permanent environmental effects of ewe and mate of ewe were considered to be random effects. A derivative-free algorithm was used to obtain REML estimates of genetic and environmental parameters. Estimates of heritabilities for animal genetic and permanent environmental and maternal genetic effects were mainly small due to the typical high influence of environmental factors on reproductive traits and to non-normal distributions of traits. Mate of ewe effects were not important for any trait. Important genetic correlations were found between some traits. Some estimates of genetic correlations do not seem to have a biological explanation. Nevertheless, these estimates of genetic correlations among traits may provide a basis for deriving selection indexes for reproductive traits