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,3
* Department of Animal Science, University of Nebraska, Lincoln 68583-0908;
and
USDA, ARS, U.S. Meat Animal Research Center, Lincoln, NE 68583-0908; and
and
USDA, ARS, U.S. Sheep Experimental Station, Dubois, ID 83423
2 Correspondence:
A218 Animal Science (phone:402-472-6409; fax:402-472-6362; E-mail:
khanford2{at}unl.edu).
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Abstract |
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 Top Abstract IntroductionMaterials and MethodsResults and DiscussionImplicationsLiterature Cited |
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Key Words: Breeding Value • Genetic Correlation • Heritability • Litter Size • Prolificacy • Weaning Weight
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Introduction |
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TopAbstractIntroductionMaterials and MethodsResults and DiscussionImplicationsLiterature Cited |
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The main objective of this study was to document genetic trends in production traits of the Targhee breed at the U.S. Sheep Experiment Station (USSES), Dubois, ID, over a 49-yr period (1950 to 1998), where selection was based on weaning performance under range conditions. The production traits examined included prolificacy, weight, and wool traits. A secondary objective was to compare genetic trends estimated from single- and seven-trait analyses.
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Materials and Methods |
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TopAbstractIntroductionMaterials and MethodsResults and DiscussionImplicationsLiterature Cited |
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During the 49-yr period (1950 to 1998), the Targhee breed at USSES was subjected to different selection criteria, all generally related to increasing weaning weight. Selection favored wool and growth traits in the early years (approximately 1950 to 1969), then individual lamb weaning weight and litter size (from 1969 to 1976), and finally weaning weight of the lamb or total litter weight weaned of the ewe (from 1976 to 1998). A random-bred control line was also maintained for many of these years. A total of six outside rams were introduced, all for the 1989 through 1991 breeding seasons. Lines have been rerandomized several times as new selection criteria were imposed on the flock. Rams in control lines that were superior for the selection traits were often used in the appropriate selection lines. The effects of specific selection objectives could not be accounted for because of the rerandomization of breeding animals over the years of this study. The genetic trend in this flock, however, may represent general, but changing, selection emphases of the American sheep industry over this time period.
The numbers of records per trait, as well as unadjusted means and standard deviations, are presented in Table 1
. Ercanbrack and Knight (1998) and Hanford et al. (2002) previously described management of the flock.
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. Lower survival rates of single-born lambs compared to twin-born lambs were likely due to the greater proportion of single born lambs being reared by younger ewes (Snowder et al., 2001). A summary of numbers of ewes by age, litter size at both birth and weaning, and survival by age of ewe is presented in Table 3.
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Wool Traits.
Greasy fleece weight (kg) and fleece grade (U.S. spinning count) were obtained annually at shearing in late May. Fleece grades were subjectively determined by certified graders according to U.S. wool grade standards (Pohle, 1963). Staple length (cm) was measured prior to shearing at midside without stretching the fiber. Staple length was primarily measured on lambs and rams only. Staple lengths for ewe lambs were available from 1977 through 1991. Only wool data from ewes and ewe lambs with lambing records were included in these analyses.
Statistical Analysis
(Co)variance components for each trait were estimated from single-trait analyses using models described in Table 4. (Co)variance components between traits were estimated from two-trait analyses with the models described in Table 4 combined with appropriate covariances between random effects in the model for the pairs of traits. Breeding values of individual animals were estimated from single-trait analyses and were also estimated from a seven-trait analysis, using the within-trait co(variances) from single-trait analyses and between-trait correlations from two-trait analyses. Means of estimated breeding values by year of birth were calculated from the seven-trait analysis and compared with the corresponding means of estimated breeding values from single-trait analyses.
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Single-Trait Analysis
The single-trait linear model was
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where y is the vector of observations, ß is the vector of fixed effects, X is a design matrix relating fixed effects to y, a is a vector of additive genetic effects of animals, m is a vector of maternal genetic effects, p is a vector of permanent environmental effects corresponding to the ewes, with incidence matrices Za, Zm, and Zp relating the effects to y, and e is a vector of random residual effects. Nonadditive genetic effects were assumed not to exist.
Expected values and (co)variance structures for random effects were assumed to be:
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where A is the numerator relationship matrix, Ip and In are identity matrices with order equal to the number of ewes (p) and number of records (n), and 
2a,2m,2p, and 2e are the direct additive genetic, maternal additive genetic, permanent environmental, and residual components of variance, respectively, and am is the covariance between direct and maternal additive genetic effects. The full model was used for the weight traits, whereas a model with the maternal genetic effects removed was used for the prolificacy and wool traits. A permanent environmental effect was not included in the model for staple length because staple length was recorded only at 1 yr of age.
Fixed effects included in the model for the prolificacy traits were age of ewe in years at lambing (1 to 12) and year of lambing (1950 to 1998). Records of all ewes that were bred and present at lambing were included. Therefore, number of lambs at birth or at weaning could be zero. Analyses of litter size at birth included only parturitions that resulted from single-sire pen matings. Litter size at weaning included only lambs that resulted from single-sire pen matings that were present with their biological mother at weaning. Models for litter size at weaning included the fixed effect of foster code (1, if the ewe did not raise a foster lamb; 2, if ewe did raise a foster lamb). Foster lamb records were not included in the record of either the birth dam or the foster dam for litter size at weaning.
The model for birth weight also included the fixed effects of gender of lamb and type of birth (1 to 4), whereas the model for weaning weight included the fixed effects of gender of lamb and type of birth and rearing. One of eight types of birth and rearing combinations was assigned to each lamb to account for a lamb born as a single, twin, triplet, or quadruplet, and reared as a single, twin, or triplet.
Year of production and number of lambs weaned were included as fixed effects in the model for all three wool traits. Because animals could have more than one measurement for fleece weight and fleece grade, the additional fixed effect of age (yr) at shearing was added to the model for these two traits. Julian day of year shorn was included as a linear covariate for all three wool traits.
Two-Trait Analyses
Traits were analyzed by pairs to estimate covariance components. In addition to (co)variance structures for single-trait models, covariances between the two traits depended on the models for the traits (Table 4
). For two-trait analyses for litter size at weaning with each of the wool traits, the fixed effect of number of lambs weaned included in the model for wool traits was dropped from the model due to apparent confounding with the litter size weaned trait.
Correlations between permanent environmental effects were estimated between prolificacy traits and wool traits recorded in the same year of production. To estimate environmental correlations between an animal‘s own birth weight, weaning weight, and yearling staple length and her prolificacy and wool traits, a permanent environmental effect was included in the model for birth weight, weaning weight, and yearling staple length. This assignment of a permanent environmental effect to those traits that were measured only once for each animal was done to force the covariance between environmental effects into the covariance between permanent environmental effects rather than to the covariance between residual effects when one of the traits was measured more than once. Although the environmental covariance across traits can be forced into permanent environmental effects, interpretation requires some caution when one trait, such as birth weight, cannot have repeated measures (Okut et al., 1999). Because of the complete confounding between the permanent environmental and residual effects, variance due to those effects can go to either component of variance, which also makes interpretation of correlations among permanent environmental effects difficult (Bromley et al., 2000). The environmental variance for the single measured trait was calculated by summing variance components for permanent environmental and residual effects. The environmental correlation between traits was calculated with the formula presented by Okut et al. (1999). For pairs of traits measured in the same year for each ewe (litter size at birth, litter size at weaning, fleece weight, and fleece grade), covariances between both permanent and temporary environmental effects were estimated from bivariate analyses.
Seven-Trait Analysis
Estimates of (co)variances from single-trait analyses and estimates of correlations from two-trait analyses were used to set up mixed-model equations to estimate breeding values for the seven traits simultaneously. A 9 x 9 genetic (co)variance matrix and an 11 x 11 environmental (co)variance matrix were constructed. There were two types of permanent environmental covariances between traits included in the environmental (co)variance matrix. The first type was the permanent environmental covariance estimated from traits with repeated records. The second type was where the permanent environmental effect was completely confounded with the temporary environmental (or residual) effect because the trait was measured only once. Because the variance due to the permanent environmental and residual effects can go to either component of variance, a fraction of the total environmental variance (0.0001) was arbitrarily assigned to the residual variance for traits measured only once and the remainder was assigned to the permanent environmental variance.
Each (co)variance matrix had to be adjusted to be positive definite by applying a singular value decomposition to each of the two matrices as follows:
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where D is a diagonal matrix of eigenvalues and P is a matrix of eigenvectors for V (either the genetic or environmental (co)variance matrix). Any eigenvalue in D that was negative was replaced with a small positive value (0.0001) to create a modified diagonal matrix, D*. A new (co)variance matrix was then calculated as
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so that V* is positive definite.
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Results and Discussion |
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TopAbstractIntroductionMaterials and MethodsResults and DiscussionImplicationsLiterature Cited |
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Weight Traits.
Estimates of direct heritability for both birth weight (0.25) and weaning weight (0.22) were moderate. The estimate of maternal heritability for birth weight was about twice as large as that for weaning weight (0.20 vs. 0.11). Estimates of genetic correlation between direct and maternal effects were small for both birth weight (0.09) and weaning weight (-0.04). Variance due to permanent environmental effects associated with the dam as a proportion of total variance was similar for birth weight (0.08) and for weaning weight (0.06). Estimates of both direct and maternal heritability for birth weight were similar to the estimates (0.27 and 0.25, respectively) reported for Columbia sheep by Hanford et al. (2002), whereas our estimates of both direct and maternal heritability for weaning weight were higher than the estimates they reported (0.16 and 0.08, respectively). The estimate of direct heritability for birth weight was in general agreement with the weighted mean of estimates of 0.19 for dual-purpose breeds reported by Fogarty (1995) and the estimate of 0.22 for Targhee sheep reported by Bromley et al. (2000), but was higher than other previously reported estimates ranging from 0.07 to 0.17 (Jurado et al., 1994;Näsholm and Danell, 1996;Gut et al., 2001). The estimate of direct heritability for weaning weight was also in general agreement with the weighted mean of estimates of 0.20 for dual-purpose breeds reported by Fogarty (1995) and the estimate of 0.19 reported by Al-Shorepy and Notter (1996), but was higher than other previously reported estimates of 0.15 for Swedish Finewool sheep by Näsholm and Danell (1996) and 0.16 for Targhee sheep by Bromley et al. (2000). The larger estimate of maternal heritability for birth weight compared with the estimate for weaning weight supports the conclusion of Robison (1981) that maternal genetic effects generally are important for measurements of weight at younger ages and diminish with increasing age. This diminishing maternal genetic effect on lamb weight over time has been reported by others (Al-Shorepy and Notter, 1996; Näsholm and Danell, 1996;Bromley et al., 2000).
Wool Traits.
Estimates of direct heritability for wool traits were moderate to large (0.54, 0.41, and 0.65, for fleece grade, fleece weight, and staple length, respectively). Estimates of variance due to permanent environmental effects of the ewe as a proportion of total variance were 0.09 for fleece weight and 0.06 for fleece grade. These results are similar to those reported for fleece weight and fleece grade for the Columbia breed by Hanford et al. (2002) of 0.53 and 0.41 for heritability estimates and of 0.14 and 0.11 for fractions of variance due to permanent environmental effects, but larger than the heritability estimate of 0.55 reported for staple length. Heritability estimates for fleece weight and fleece grade agreed with mean estimates of 0.52 and 0.40, respectively, reported by Bromley et al. (2000). The heritability estimate for staple length was larger than the mean estimate of 0.54 reported by Bromley et al. (2000). The estimate for fleece weight was larger than the weighted mean of 0.36 summarized by Fogarty (1995), but smaller than the 0.66 reported by Saboulard et al. (1995) for clean fleece weight in western whiteface ewes. The estimate for fleece grade was smaller than the weighted mean of 0.52 reported for fiber diameter by Fogarty (1995).
Estimates from Bivariate Analyses
Estimates of genetic correlations from bivariate analyses between prolificacy, weight, and wool traits are presented in Table 6.
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The estimate of correlation between permanent environmental effects of ewes was large and positive for litter size at birth with litter size at weaning (0.73). This estimate was somewhat larger than the estimate of 0.52 for the Columbia breed (Hanford et al., 2002).
Within Weight Traits.
The estimate of direct genetic correlation between birth weight and weaning weight was moderate and positive (0.52), in agreement with the estimate of 0.56 for the Columbia breed (Hanford et al., 2002). The estimate was higher than the weighted mean of 0.39 between birth weight and weaning weight summarized by Fogarty (1995). The moderate estimate of direct genetic correlation between birth weight and weaning weight suggests that animals with above-average weaning weight would tend to be above average in genetic merit for birth weight. The estimate of maternal genetic correlation between birth weight and weaning weight was also moderately positive (0.44) and somewhat smaller than the estimate of 0.58 for the Columbia breed (Hanford et al., 2002). The moderately positive maternal genetic correlation indicates that Targhee ewes that are above average in genetic merit for producing lambs with heavier birth weights will tend to produce lambs with heavier weaning weights. Estimates of genetic correlations between direct and maternal effects were both small to moderate (-0.09 and 0.26) and in general agreement with estimates presented by Bromley et al. (2000) and Hanford et al. (2002). The estimate of correlation between permanent environmental effects of the dam for birth and weaning weight was moderately large and positive (0.44) and in agreement with the estimate of 0.46 for the Columbia breed (Hanford et al., 2002).
Within Wool Traits.
Estimates of direct genetic correlations between pairs of wool traits were positive between fleece weight and staple length (0.54) and negative between fleece grade and both fleece weight (-0.47) and staple length (-0.69), in agreement with previous estimates (Saboulard et al., 1995;Bromley et al., 2000;Hanford et al., 2002).
The negative (unfavorable) estimate of the genetic correlation between fleece grade and fleece weight was in general agreement with positive (unfavorable) estimates between fleece fiber diameter and fleece weight previously published (Iman et al., 1992;Fogarty, 1995). The negative genetic correlation between fleece weight and grade suggests that selection for fleece weight would decrease genetic merit for fleece grade (increased fiber diameter). The positive estimate of genetic correlation (0.54) between fleece weight and staple length agreed in direction with the estimate of 0.20 between yearling fleece weight and staple length reported for Merino sheep by Atkins (1997). The positive genetic correlation indicates that staple length would increase as a genetic response to selection for increased fleece weight. Although Atkins (1997) reported a negative (favorable) genetic correlation between yearling fiber diameter and staple length (-0.10), the negative (unfavorable) genetic correlation between fleece grade and staple length estimated in this study indicates that staple length would decrease as a genetic response to an increase in fleece grade (fiber diameter becomes finer).
Prolificacy and Weight Traits.
Estimates of genetic correlations among prolificacy and weight traits ranged from 0.00 between both litter size traits and birth weight to 0.20 between litter size at birth and weaning weight, which were in general agreement with estimates presented for the Columbia breed by Hanford et al. (2002). The estimate of the genetic correlation between birth weight and litter size at birth (0.00) was smaller than the average for four breeds (0.12) reported by Bromley et al. (2000) and the average of estimates (0.30) reviewed by Fogarty (1995). The estimate of the genetic correlation between birth weight and litter size at weaning (0.00) was smaller in magnitude than both the average for four breeds (-0.12) reported by Bromley et al. (2000) and the average of estimates (0.34) summarized by Fogarty (1995).
The estimate of genetic correlation between weaning weight and litter size at birth (0.20) was in agreement with the average of estimates (0.20) reported by Fogarty (1995) and with the average for four breeds (0.16) reported by Bromley et al. (2000) for average daily gain to weaning and litter size at birth. Positive correlations suggest selection for weaning weight may increase genetic merit for litter size at birth. The estimate of genetic correlation between weaning weight and litter size at weaning (0.15) was the same as the average for three breeds (Polypay, Rambouillet, and Targhee) reported by Bromley et al. (2000) for average daily gain to weaning and litter size at weaning (0.14), but was substantially different from the estimate they reported for the Columbia breed (-0.82). The estimate was also smaller than the average of estimates (0.34) reported by Fogarty (1995).
Estimates of correlations between direct genetic effects for prolificacy traits and maternal genetic effects for weight traits were moderate to large and in general agreement with the estimates for the Columbia breed (Hanford et al., 2002) and with the average for four breeds reported by Bromley et al. (2000), with the exception of the correlation between litter size at birth and birth weight of -0.11 reported by Bromley et al. (2000).
Prolificacy and Wool Traits.
Estimates of genetic correlations between prolificacy traits and wool traits ranged from -0.19 between both litter size traits and fleece weight to 0.11 between litter size at weaning and fleece grade. These estimates were in general agreement with estimates presented for the Columbia breed by Hanford et al. (2002) and with the average of four breeds reported by Bromley et al. (2000). One exception was between litter size at weaning and staple length (0.05) where both Bromley et al. (2000) and Hanford et al. (2002) reported moderate negative correlations (-0.33 and -0.20, respectively). In agreement with conclusions hypothesized by Hanford et al. (2002), small negative correlations between fleece weight and the litter size traits indicate ewes that are genetically predisposed to produce larger litters also tend to produce lighter fleeces.
Weight and Wool Traits.
Estimates of genetic correlations ranged from -0.06 between birth weight and fleece grade to 0.24 between either birth or weaning weight and fleece weight and were similar to those reported for the average of four breeds by Bromley et al. (2000) and to those reported for the Columbia breed by Hanford et al. (2002). Positive correlations for fleece weight with birth and weaning weight (0.24 for both) suggest that genetic factors influencing animal growth also influence wool growth.
Estimates of Individual Breeding Values and Genetic Change
Means of estimates of breeding value by year of birth calculated from both single-trait analyses and from the seven-trait analysis are plotted in Figures 1 and 2 for prolificacy traits, in Figures 3 and 4 for weight traits, and in Figures 5 to 7 for wool traits. Means of estimates of breeding value by year are deviations from the means of estimates of breeding value for animals born in 1950 (1977 for staple length).
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). From 1980 to 1998, average estimates of breeding values from the seven-trait analysis increased at a greater rate than average estimates from the single-trait analysis, so that by 1998, the average estimate of breeding value from the seven-trait analysis was 0.2 lambs greater than that from the single-trait analysis. This difference for litter size at birth may be due to the positive direct correlation for litter size at birth and litter size at weaning (0.77) and the increase of litter size at weaning. The mean estimates for litter size at birth increased about 0.6 lambs from 1950 to 1998. The plots of the mean estimates of breeding value by year of birth for litter size at birth followed a pattern similar to those from single- and seven-trait analyses for the Columbia breed (Hanford et al., 2002).
Means of estimates of breeding value by year of birth for litter size at weaning from the single- and the seven-trait analyses also were similar from 1950 to 1980 (Figure 2). From 1980 to 1998, average estimates of breeding values from the seven-trait analysis increased at a greater rate than average estimates from the single-trait analysis, so that by 1998, the average estimate of breeding value from the seven-trait analysis was 0.2 lambs greater than from the single-trait analysis. As with litter size at birth, this difference for litter size at weaning may be due to the positive direct correlation between the two traits and the increase of litter size at birth. The mean of estimates of breeding value for litter size at weaning increased by 0.4 lambs during the study period, which was less than the increase for litter size at birth. The plots of the mean estimates of breeding value by year of birth for litter size at weaning followed a pattern different from those from single-trait and seven-trait analyses for the Columbia breed (Hanford et al., 2002). For the Columbia breed, the average estimates of breeding value calculated from the single-trait analysis were greater than the average estimates of breeding value calculated from the seven-trait analysis, which was thought to be due partly to the introduction of outside Columbia rams, which negatively impacted weaning weight, which in turn was correlated with litter size at weaning (0.24).
Weight Traits.
Means of estimates of breeding value for birth weight by year of birth from the single-trait analysis were slightly less from 1958 to 1976 than means of estimates of breeding value from the seven-trait analysis (Figure 3). During that period, the average estimate of breeding value for birth weight increased by 0.3 kg. The mean of estimates of breeding value for birth weight increased more than 0.2 kg between 1977 and 1978. After 1978, and until 1992, means of estimates of breeding value from the single-trait analysis were about 0.2 kg less than means of estimates of breeding value from the seven-trait analysis. Selection was not directly applied for birth weight. The 0.2 kg difference between the two means of estimates of breeding value for birth weight may be due to the positive genetic correlations between birth weight and litter size at birth and also weaning weight. The difference between means of estimates of breeding value from single- and seven-trait analyses decreased after 1992. Means of estimates of breeding value for birth weight increased about 0.5 kg during the study period. The plots of the mean estimates of breeding value by year of birth for birth weight followed a pattern similar to those from single- and seven-trait analyses for the Columbia breed (Hanford et al., 2002).
Means of estimates of breeding value for weaning weight by year of birth from single-trait analysis were slightly less from 1956 to 1977 than means of estimates of breeding value from the seven-trait analysis (Figure 4). During this period, the average estimate of breeding value for weaning weight increased about 4.0 kg. After 1978, and until 1992, means of estimates of breeding value from the single-trait analysis were about 1.0 kg less than means of estimates of breeding value from the seven-trait analysis. This difference between the two average estimates of breeding value for weaning weight may be due to the positive direct correlation for birth weight and weaning weight (0.52) and the increase of birth weight. During the 49-yr period, the mean of estimates of breeding value by year of birth for weaning weight increased about 7.5 kg. The plots of the mean estimates of breeding value by year of birth for weaning weight followed a similar pattern to those from single- and seven-trait analyses for the Columbia breed (Hanford et al., 2002).
Wool Traits.
Means of estimates of breeding value for fleece weight by year of birth from single- and seven-trait analyses showed a fairly consistent pattern with means of estimates from the seven-trait analysis higher than those from the single-trait analysis (Figure 5). The larger means of estimates of breeding value from the seven-trait analysis may be due to the large negative genetic correlation of -0.47 between fleece weight and fleece grade and small positive correlations between fleece weight and both birth (0.24) and weaning weight (0.24). Means of estimates of breeding value for fleece weight did not vary much from the base year from 1954 to 1977. From 1977 to 1980, the means of estimates of breeding value increased by 0.5 kg compared with the base year. Means of estimates of breeding value then were between 0.4 and 0.5 kg heavier than the base year until about 1991, when means of estimates of breeding value decreased to 1.0 kg below the base year estimates by 1997, although they rebounded to 0.3 kg below the base year estimates in 1998. The plots of the mean estimates of breeding value by year of birth for fleece weight followed a pattern similar to those from single-trait and seven-trait analyses for the Columbia breed (Hanford et al., 2002), except that they reported an increase of 0.3 kg above the base year by the end of the study period.
Means of estimates of breeding value by year of birth for fleece grade were similar between single- and seven-trait analyses (Figure 6) and varied less than 1 spinning count from the base year throughout the study period. The plots of the mean estimates of breeding value by year of birth for fleece grade followed a pattern similar to those from single- and seven-trait analyses for the Columbia breed (Hanford et al., 2002).
Means of estimates of breeding value by year of birth for staple length were similar between single- and seven-trait analyses (Figure 7) and differed by less than 0.5 cm from the base year throughout the study period. The plots of the mean estimates of breeding value by year of birth for staple length followed a different pattern from those values for the Columbia breed (Hanford et al., 2002). For the Columbia breed, the yearly means of breeding values from the seven-trait analysis were greater than those from the single-trait analysis, which was thought to be due to the high correlation between staple length and fleece weight (0.55) and the increase in fleece weight during the study period.
Estimated breeding values averaged over year of birth did not appear to differ substantially between estimates of breeding values obtained from single- and seven-trait analyses, except for traits which were highly correlated with another trait that had responded to selection. Estimates of breeding value for litter size at birth and litter size at weaning from the seven-trait analysis tended to be higher relative to estimates from single-trait analyses, which may be due to the high genetic correlation between them (0.77). Estimates of breeding value for birth weight and weaning weight from the seven-trait analysis also increased relative to estimates from single-trait analyses due to the high genetic correlation between them (0.52). Estimates of genetic correlations less than 0.5 did not have a noticeable impact on means of estimates of breeding value of other traits.
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Implications |
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TopAbstractIntroductionMaterials and MethodsResults and DiscussionImplicationsLiterature Cited |
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Footnotes |
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3 Current address: Roman L. Hruska U.S. Meat Animal Research Center, Clay Center, NE 68933-0166.
Received for publication August 27, 2002. Accepted for publication November 22, 2002.
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Literature Cited |
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TopAbstractIntroductionMaterials and MethodsResults and DiscussionImplicationsLiterature Cited |
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lósarz, and T. Szwaczkowski. 2001. Genetic trends of some performance traits in Whiteheaded Mutton sheep. Czech. J. Anim. Sci. 46:363–369.
Sons, New York.
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