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Environmental and genetic effects on conformation, precocity and musculature traits at weaning in Suffolk lambs
Small Ruminant Research 102 (2012) 131–134
Contents lists available at ScienceDirect
Small Ruminant Research journal homepage: www.elsevier.com/locate/smallrumres
Environmental and genetic effects on conformation, precocity and musculature traits at weaning in Suffolk lambs Adriana Luiza Somavilla, Laila Talarico Dias ∗ , Rodrigo de Almeida Teixeira Departamento de Zootecnia, Campus Agrárias, Universidade Federal do Paraná, Rua dos Funcionários, 1540, Curitiba, PR 83035-050, Brazil
a r t i c l e
i n f o
Article history: Received 18 October 2010 Received in revised form 4 July 2011 Accepted 12 July 2011 Available online 11 August 2011 Keywords: Genetic correlation Growth Heritability Sheep Visual score
a b s t r a c t The aim of this study was to investigate and estimate the influence of type of lambing, age at weaning and age of the dam at lambing on conformation (C), precocity (P), musculature (M), and to estimate the heritability coefficients and genetic correlations among these traits, in Suffolk lambs. For the analysis of environmental effects and genetic parameters, the contemporary group was considered as a fixed effect, whereas covariates included the linear effects of type of lambing and age at weaning and the linear and quadratic effects of the age of dam at lambing. Lambs born as singletons received the highest scores in all three studied traits when compared to those from multiple lambings. Similar effects were observed for late-weaning lambs. 4–5 years old dams promoted the highest scores for their lambs. Heritability coefficients were 0.15, 0.18, and 0.16 for C, P, and M, respectively, indicating possibility of genetic gain through individual selection. Estimates of genetic correlation between these traits were positive and of high magnitude, varying from 0.83 to 0.95, showing that the studied traits are controlled, in part, by the same sets of genes and, therefore, there is the possibility of correlated response. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Selection based only on body weight not always provides the best individuals for meat production because it might lead to the choice of large-bodied, long-legged, late individuals with weak muscle development and little subcutaneous fat, which are undesirable traits for the meat production systems. For that reason, visual scores of conformation, precocity, and musculature are frequently used in beef cattle in addition to weighing with the goal of obtaining animals more suitable for beef production (Queiroz et al., 2009). In sheep industry, the most commonly used visual scores are about body condition, which is correlated to percentage of fat (Sanson et al., 1993) and conformation
∗ Corresponding author. Tel.: +55 41 3350 5704; fax: +55 41 3350 5625. E-mail addresses: adri [email protected] (A.L. Somavilla), [email protected] (L.T. Dias), [email protected] (R.A. Teixeira). 0921-4488/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.smallrumres.2011.07.004
(Nsoso et al., 2000). Recently precocity and musculature traits scores have been used and are carried out at weaning, in addition to conformation, in weighing routines. A problem with using visual scores in sheep is that the animals need to be sheared before observation and carried out so as to facilitate the visualization of body development and, consequently, the results of the analyses. Regardless of the animal species, visual scores are often influenced by environmental factors, such as the age of the dam, the age of animal, birth season and sex, therefore, these factors must be considered in breeding programs (Fernandes et al., 2001). Moreover, identifying environmental effects that influence economically important traits is crucial when developing better statistical modeling of genetic effects. To determine which traits to use as selection criteria for increased meat production, knowledge of genetic parameters is necessary. According to Weber et al. (2009), heritability estimates for visual scores in beef cattle are
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A.L. Somavilla et al. / Small Ruminant Research 102 (2012) 131–134
usually moderate and indicate the possibility of response to individual selection. The aim of this study was to investigate the influence of contemporary group, type of lambing, age at weaning and age of the dam on conformation, precocity and musculature scores at weaning, and to estimate the heritability and genetic correlation coefficients for these traits in Suffolk breed lambs. 2. Materials and methods Records from 4315 Suffolk lambs born between 1992 and 2007 from a commercial herd were analyzed. During the dataset edition were excluded animals without information about performance, genealogy, or that were born under dams aged >9 years old (AD), as well as individuals with body weight less than 20 or greater than 50 kg (BW) and weaned prior to 60 days or later than 120 days of age (AW). The contemporary group (CG) was defined by including birth year and sex inasmuch as many individuals did not have information about management group at weaning. CG with less than 15 animals was excluded. After edition the dataset consisted of 2560 animals that were offspring of 84 rams and 914 dams, distributed in 26 CG. Animals were sheared a week prior to weighting to permit the best evaluation of their scores. The capacity for meat production of each animal was evaluated by conformation (C) observing the animal as if it were slaughtered at the time. Precocity (P) score was obtained based on the level of body development, as indicated by the ribs arching and legs structure. Musculature score (M) focused on muscle development in strategic regions of the body, such as the leg and shoulder. Scores varied from 1 to 5. The medium animal within the contemporary group received score 3 for all the traits. The observations were realized by a single qualified evaluator. The study of environmental effects was carried out using the least squares method implemented by GLM procedure of SAS (Statistical Analysis System, 2002), according to the following model: Yijkl = + CGi + BTj + AWk + AD1 + AD21 + eijkl where Yijkl = the observed value of the dependent variables (C, P or M); = overall mean of the dependent variable; CGi = effect of the i-th contemporary group; BTj = effect of the j-th type of birth; AWk = linear effect of the k-th age at weaning; ADl = linear effect of the l-th age of the dam at the time of lambing; AD2l = quadratic effect of the l-th age of the dam at the time of lambing; eijkl = random error associated with each observation. To explain the changes in dependents variables (C, P and M) based on changes of AW and AD was used a simple regression model. For genetic parameters estimates, the effect of the contemporary group was considered as fixed and as covariates were included the linear effects of type of birth and age at weaning, as well as the linear and quadratic effects of the age of the dam. The model can be described in matrix form as: Y = Xˇ + Za + e
Table 1 Summary of the analysis of variance for scores of conformation (C), precocity (P) and musculature (M) at weaning in Suffolk lambs. Sources of variation
Contemporary group Type of lambing Age at weaning Age of dam Linear Quadratic Residuals
DF
MS C
P
M
25 1 1
6.94** 186.99** 128.70**
8.89** 176.67** 144.77**
7.71** 147.03** 108.12**
1 1 770
31.16** 28.26** 0.78
27.24** 24.13** 0.91
21.59** 19.97** 0.78
DF: degrees of freedom; MS: mean square. ** P < 0.01.
where Y = vector of observations (C, P, or M); ˇ = vector of fixed effects; a = vector of direct additive genetic effects; X and Z = incidence matrices associated with the fixed and direct additive genetic effects to each animal; e = vector of random residual. Heritability and genetic correlation estimates were obtained using the derivative-free restricted maximum likelihood, using in the software MTDFREML (Boldman et al., 1995), for bivariate analyses.
3. Results The means and standard deviations obtained for covariates AW and AD were 92.96 ± 14.55 days and 3.71 ± 1.74 years, respectively. The average number of the lambs were born by ewe (2.8) and AD were justified by 83% of dams prior to 5 years old. Furthermore 67.6% individuals were born by multiple births. The analysis of variance for C, P and M shows that all the variation sources considered in the model had a significant effect (P < 0.01) on traits of C, P and M (Table 1). In the case of type of lambing, the frequency of scores equal to or higher than 3 was more common in animals from single birth when compared to those from multiple birth (Fig. 1). Fig. 2 shows the effect of age at weaning and the age of the dam at the time of lambing on C, P, and M scores at weaning evaluation. It is possible to observe that sheep with delayed weaning had the best scores for all three traits, probably due to the improved body development in older animals. Moreover, visual scores increased with the age of the dam up to around 4 years and later decreased as they became older, with nine-year old females weaning offspring with worse scores than younger dams (less than 4 years old).
Fig. 1. Frequency distribution of conformation (C), precocity (P), and musculature (M) scores at weaning for Suffolk lambs.
A.L. Somavilla et al. / Small Ruminant Research 102 (2012) 131–134
133
Fig. 2. Effects of age at weaning and the age of dam on scores of conformation (C), precocity (P), and muscultature (M) in Suffolk lambs. Table 2 Heritability coefficients (on diagonal), genetic correlation (upper diagonal) and environmental correlation (lower diagonal), with respective standard errors for conformation, precocity, and musculature at weaning for Suffolk lambs. Traits
Conformation
Precocity
Musculature
Conformation Precocity Musculature
0.15 ± 0.04 0.83 ± 0.01 0.78 ± 0.12
0.95 ± 0.03 0.18 ± 0.04 0.79 ± 0.11
0.83 ± 0.06 0.90 ± 0.04 0.16 ± 0.04
Heritability coefficients for C, P, and M at weaning and their respective environmental and genetic correlations are shown in Table 2. The magnitude of the heritabilities was moderate for all three studied traits, indicating possibility of response to individual selection (Table 2). Environmental and genetic variances estimates were 0.68 and 0.12; 0.77 and 0.17; 0.66 and 0.13 for C, P and M at weaning, respectively, confirming the large environmental influence on the visual scores. Genetic correlations were positive, favorable, and high (Table 2). 4. Discussion Kippert et al. (2006) studying a Charolais herd, related that the environmental effects should be included in genetic parameters analyses for economic important traits as visual scores. The present results confirm this because all environmental effects studied were substantial and significant on conformation (C), precocity (P) and musculature (M). The significance of type of lambing effect shows that lambs born from single birth received higher scores in comparison to those from multiple birth. A similar result was observed by Fernandes et al. (2001) who, when studying the weight of Morada Nova lambs, demonstrated that animals born from single birth had higher growth than those from multiple birth. This may be related to the increased availability of milk. For age at weaning, Lewis et al. (2002) concluded by regression analysis that older lambs are heavier than younger. This difference is the factor promoting most non-genetic variation in visual scores (Cardoso et al., 2001). Thereby, although the average age at weaning is in agreement to literature (Rashidi et al., 2008), it must be standardized and the amplitude reduced before selection for visual scores. In case of the age of the dam, Mysterud et al. (2002) demonstrated that ewes aged between 3 and 4 years
produced heaviest lambs and after that reproductive senescence can be observed. This can be attributed to the decrease in milk production by the female, which takes place after the fourth or fifth lambing (Carolino et al., 2003). Jorge et al. (2004) related similar results for Nelore cattle at weaning and showed an increase in the scores for offspring until a plateau (dams aged between 6 and 7 years old) and then a subsequent decrease. Koury Filho (2005) estimated higher heritability values of 0.13, 0.25, and 0.23 for C, P, and M, respectively, at weaning for Nelore cattle but also concluded that in this phase of growth there is an additive genetic variation in the population and, therefore, it’s possible to obtain genetic gain in those traits through individual selection. However, according to Forni et al. (2007), given the moderate heritability coefficients found in beef cattle studies the direct selection for visual scores traits would provide low annual gains. Moreover how the visual scores are in part subjective measurements and suffer considerable environmental influence, the evaluation of genetically superior animals for conformation, precocity and musculature is thus not easy. To minimize the influence of environmental effects, Kippert et al. (2006) suggested measures that improve environmental control, such as the standardization of feeding and sanitary management and, in particular, the training of evaluators. Estimates of genetic correlations obtained in the present study were positive, favorable, and of high magnitude, corroborating the results observed by Weber et al. (2009), who concluded that traits evaluated through scores are, in part, controlled by the same set of genes and therefore selection for one of the traits will result in correlated favorable response for the remaining traits. These results were expected because the all traits are related to the body development. Despite lacking a precision definition, the most commonly score used to lamb production is the conformation, which is poorly related to meat yield (Nsoso et al., 2000). Therefore, with high and favorable correlations among the studied visual scores, favorable responses in precocity and musculature at weaning are expected to follow from selection for conformation.
5. Conclusions To avoid biased estimates of the genetic parameters for visual scores, the environments effects of contemporary
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group, type of lambing, age at weaning and age of the dam should be considered in estimation models. Individual selection should give genetic gains to the population, however with low annual gains. Genetic correlations indicate that selection for conformation could improve precocity and musculature traits at weaning. References Boldman, K.G., Kriese, L.A., Van Vleck, L.D., 1995. A Manual for Use of MTDFREML. USDA-ARS. Clay Center, NE, 120 p. Cardoso, F.F., Cardellino, R.A., Campos, L.T., 2001. Fatores ambientais sobre escores de avaliac¸ão visual à desmama em bezerros Angus criados no Rio Grande do Sul. Braz. J. Anim. Sci. 30 (2), 318–325. Carolino, N., Gama, L., Dinis, R., Sá, T., 2003. Características produtivas da ovelha Serra da Estrela. Arch. Zootec. 52, 3–14. Fernandes, A.A.O., Buchanan, D., Selaive-Villaroel, A.B., 2001. Avaliac¸ão dos fatores ambientais no desenvolvimento corporal de cordeiros deslanados da rac¸a Morada Nova. Braz. J. Anim. Sci. 30 (5), 1460–1465. Forni, S., Federici, J.F., Albuquerque, L.G., 2007. Tendências genéticas para escores visuais de conformac¸ão, precocidade e musculatura à desmama de bovinos Nelore. Braz. J. Anim. Sci. 36 (3), 572–577. Jorge Jr., J., Dias, L.T., Albuquerque, L.G., 2004. Fatores de correc¸ão de escores visuais de conformac¸ão precocidade e musculatura, à desmama, para idade da vaca ao parto, data Juliana de nascimento e idade à desmama em bovinos da rac¸a, Nelore. Braz. J. Anim. Sci. 33 (6 (Suppl. 2)), 2044–2053.
Kippert, C.J., Rorato, P.R.N., Campos, L.T., Boligon, A.A., Weber, T., Gheller, D.G., Lopes, J.S., 2006. Efeito de fatores ambientais sobre escores de avaliac¸ão visual à desmama e estimativa de parâmetros genéticos, para bezerros da rac¸a Charolês. Cienc. Rural 36 (2), 579–585. Koury Filho, W., 2005. Escores visuais e suas relac¸ões com características de crescimento em bovinos de corte. PhD Thesis. Universidade Estadual Paulista, Faculdade de Ciências Agrárias e Veterinárias, Jaboticabal. Lewis, R.M., Emmans, G.C., Dingwall, W.S., Simm, G., 2002. A description of the growth of sheep and its genetics analysis. Anim. Sci. 74, 51–62. Mysterud, A., Steinheim, G., Yocooz, N.G., Holand, O., Stenseth, N.C., 2002. Early onset of reproductive senescence in domestic sheep, Ovis aries. Oikos 97, 177–183. Nsoso, S.J., Young, M.J., Beatson, P.R., 2000. A review of carcass conformation in sheep: scores, genetic control and development. Small Rum. Res. 35, 89–96. Queiroz, S.A., Costa, G.Z., Oliveira, J.A., Fries, L.A., 2009. Efeitos ambientais e genéticos sobre escores visuais e ganho de peso à desmama de animais formadores da rac¸a Brangus. Braz. J. Anim. Sci. 38 (2), 277–283. Rashidi, A., Mokhtari, M.S., Safi Jahanshahi, A., Mohammad Abadi, M.R., 2008. Genetic parameter estimates of pré-weaning growth traits in Kermani sheep. Small Rum. Res. 74, 165–171. Sanson, D.W., West, T.R., Tatman, W.R., Riley, M.L., Judkins, M.B., Moss, G.E., 1993. Relationship of body composition of mature ewes with condition score and body weight. J. Anim. Sci. 71 (11), 1112–1116. SAS, 2002. Statistical Analysis System User’s Guide. SAS Institute Inc., Raleigh, NC. Weber, T., Rorato, P.R.N., Lopes, J.S., Comin, J.G., Dornelles, M.A., Araújo, R.O., 2009. Parâmetros genéticos e tendências genéticas e fenotípicas para características produtivas e de conformac¸ão na fase prédesmama em uma populac¸ão da rac¸a Aberdeen Angus. Braz. J. Anim. Sci. 38 (5), 832–842.
Contents lists available at ScienceDirect
Small Ruminant Research journal homepage: www.elsevier.com/locate/smallrumres
Environmental and genetic effects on conformation, precocity and musculature traits at weaning in Suffolk lambs Adriana Luiza Somavilla, Laila Talarico Dias ∗ , Rodrigo de Almeida Teixeira Departamento de Zootecnia, Campus Agrárias, Universidade Federal do Paraná, Rua dos Funcionários, 1540, Curitiba, PR 83035-050, Brazil
a r t i c l e
i n f o
Article history: Received 18 October 2010 Received in revised form 4 July 2011 Accepted 12 July 2011 Available online 11 August 2011 Keywords: Genetic correlation Growth Heritability Sheep Visual score
a b s t r a c t The aim of this study was to investigate and estimate the influence of type of lambing, age at weaning and age of the dam at lambing on conformation (C), precocity (P), musculature (M), and to estimate the heritability coefficients and genetic correlations among these traits, in Suffolk lambs. For the analysis of environmental effects and genetic parameters, the contemporary group was considered as a fixed effect, whereas covariates included the linear effects of type of lambing and age at weaning and the linear and quadratic effects of the age of dam at lambing. Lambs born as singletons received the highest scores in all three studied traits when compared to those from multiple lambings. Similar effects were observed for late-weaning lambs. 4–5 years old dams promoted the highest scores for their lambs. Heritability coefficients were 0.15, 0.18, and 0.16 for C, P, and M, respectively, indicating possibility of genetic gain through individual selection. Estimates of genetic correlation between these traits were positive and of high magnitude, varying from 0.83 to 0.95, showing that the studied traits are controlled, in part, by the same sets of genes and, therefore, there is the possibility of correlated response. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Selection based only on body weight not always provides the best individuals for meat production because it might lead to the choice of large-bodied, long-legged, late individuals with weak muscle development and little subcutaneous fat, which are undesirable traits for the meat production systems. For that reason, visual scores of conformation, precocity, and musculature are frequently used in beef cattle in addition to weighing with the goal of obtaining animals more suitable for beef production (Queiroz et al., 2009). In sheep industry, the most commonly used visual scores are about body condition, which is correlated to percentage of fat (Sanson et al., 1993) and conformation
∗ Corresponding author. Tel.: +55 41 3350 5704; fax: +55 41 3350 5625. E-mail addresses: adri [email protected] (A.L. Somavilla), [email protected] (L.T. Dias), [email protected] (R.A. Teixeira). 0921-4488/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.smallrumres.2011.07.004
(Nsoso et al., 2000). Recently precocity and musculature traits scores have been used and are carried out at weaning, in addition to conformation, in weighing routines. A problem with using visual scores in sheep is that the animals need to be sheared before observation and carried out so as to facilitate the visualization of body development and, consequently, the results of the analyses. Regardless of the animal species, visual scores are often influenced by environmental factors, such as the age of the dam, the age of animal, birth season and sex, therefore, these factors must be considered in breeding programs (Fernandes et al., 2001). Moreover, identifying environmental effects that influence economically important traits is crucial when developing better statistical modeling of genetic effects. To determine which traits to use as selection criteria for increased meat production, knowledge of genetic parameters is necessary. According to Weber et al. (2009), heritability estimates for visual scores in beef cattle are
132
A.L. Somavilla et al. / Small Ruminant Research 102 (2012) 131–134
usually moderate and indicate the possibility of response to individual selection. The aim of this study was to investigate the influence of contemporary group, type of lambing, age at weaning and age of the dam on conformation, precocity and musculature scores at weaning, and to estimate the heritability and genetic correlation coefficients for these traits in Suffolk breed lambs. 2. Materials and methods Records from 4315 Suffolk lambs born between 1992 and 2007 from a commercial herd were analyzed. During the dataset edition were excluded animals without information about performance, genealogy, or that were born under dams aged >9 years old (AD), as well as individuals with body weight less than 20 or greater than 50 kg (BW) and weaned prior to 60 days or later than 120 days of age (AW). The contemporary group (CG) was defined by including birth year and sex inasmuch as many individuals did not have information about management group at weaning. CG with less than 15 animals was excluded. After edition the dataset consisted of 2560 animals that were offspring of 84 rams and 914 dams, distributed in 26 CG. Animals were sheared a week prior to weighting to permit the best evaluation of their scores. The capacity for meat production of each animal was evaluated by conformation (C) observing the animal as if it were slaughtered at the time. Precocity (P) score was obtained based on the level of body development, as indicated by the ribs arching and legs structure. Musculature score (M) focused on muscle development in strategic regions of the body, such as the leg and shoulder. Scores varied from 1 to 5. The medium animal within the contemporary group received score 3 for all the traits. The observations were realized by a single qualified evaluator. The study of environmental effects was carried out using the least squares method implemented by GLM procedure of SAS (Statistical Analysis System, 2002), according to the following model: Yijkl = + CGi + BTj + AWk + AD1 + AD21 + eijkl where Yijkl = the observed value of the dependent variables (C, P or M); = overall mean of the dependent variable; CGi = effect of the i-th contemporary group; BTj = effect of the j-th type of birth; AWk = linear effect of the k-th age at weaning; ADl = linear effect of the l-th age of the dam at the time of lambing; AD2l = quadratic effect of the l-th age of the dam at the time of lambing; eijkl = random error associated with each observation. To explain the changes in dependents variables (C, P and M) based on changes of AW and AD was used a simple regression model. For genetic parameters estimates, the effect of the contemporary group was considered as fixed and as covariates were included the linear effects of type of birth and age at weaning, as well as the linear and quadratic effects of the age of the dam. The model can be described in matrix form as: Y = Xˇ + Za + e
Table 1 Summary of the analysis of variance for scores of conformation (C), precocity (P) and musculature (M) at weaning in Suffolk lambs. Sources of variation
Contemporary group Type of lambing Age at weaning Age of dam Linear Quadratic Residuals
DF
MS C
P
M
25 1 1
6.94** 186.99** 128.70**
8.89** 176.67** 144.77**
7.71** 147.03** 108.12**
1 1 770
31.16** 28.26** 0.78
27.24** 24.13** 0.91
21.59** 19.97** 0.78
DF: degrees of freedom; MS: mean square. ** P < 0.01.
where Y = vector of observations (C, P, or M); ˇ = vector of fixed effects; a = vector of direct additive genetic effects; X and Z = incidence matrices associated with the fixed and direct additive genetic effects to each animal; e = vector of random residual. Heritability and genetic correlation estimates were obtained using the derivative-free restricted maximum likelihood, using in the software MTDFREML (Boldman et al., 1995), for bivariate analyses.
3. Results The means and standard deviations obtained for covariates AW and AD were 92.96 ± 14.55 days and 3.71 ± 1.74 years, respectively. The average number of the lambs were born by ewe (2.8) and AD were justified by 83% of dams prior to 5 years old. Furthermore 67.6% individuals were born by multiple births. The analysis of variance for C, P and M shows that all the variation sources considered in the model had a significant effect (P < 0.01) on traits of C, P and M (Table 1). In the case of type of lambing, the frequency of scores equal to or higher than 3 was more common in animals from single birth when compared to those from multiple birth (Fig. 1). Fig. 2 shows the effect of age at weaning and the age of the dam at the time of lambing on C, P, and M scores at weaning evaluation. It is possible to observe that sheep with delayed weaning had the best scores for all three traits, probably due to the improved body development in older animals. Moreover, visual scores increased with the age of the dam up to around 4 years and later decreased as they became older, with nine-year old females weaning offspring with worse scores than younger dams (less than 4 years old).
Fig. 1. Frequency distribution of conformation (C), precocity (P), and musculature (M) scores at weaning for Suffolk lambs.
A.L. Somavilla et al. / Small Ruminant Research 102 (2012) 131–134
133
Fig. 2. Effects of age at weaning and the age of dam on scores of conformation (C), precocity (P), and muscultature (M) in Suffolk lambs. Table 2 Heritability coefficients (on diagonal), genetic correlation (upper diagonal) and environmental correlation (lower diagonal), with respective standard errors for conformation, precocity, and musculature at weaning for Suffolk lambs. Traits
Conformation
Precocity
Musculature
Conformation Precocity Musculature
0.15 ± 0.04 0.83 ± 0.01 0.78 ± 0.12
0.95 ± 0.03 0.18 ± 0.04 0.79 ± 0.11
0.83 ± 0.06 0.90 ± 0.04 0.16 ± 0.04
Heritability coefficients for C, P, and M at weaning and their respective environmental and genetic correlations are shown in Table 2. The magnitude of the heritabilities was moderate for all three studied traits, indicating possibility of response to individual selection (Table 2). Environmental and genetic variances estimates were 0.68 and 0.12; 0.77 and 0.17; 0.66 and 0.13 for C, P and M at weaning, respectively, confirming the large environmental influence on the visual scores. Genetic correlations were positive, favorable, and high (Table 2). 4. Discussion Kippert et al. (2006) studying a Charolais herd, related that the environmental effects should be included in genetic parameters analyses for economic important traits as visual scores. The present results confirm this because all environmental effects studied were substantial and significant on conformation (C), precocity (P) and musculature (M). The significance of type of lambing effect shows that lambs born from single birth received higher scores in comparison to those from multiple birth. A similar result was observed by Fernandes et al. (2001) who, when studying the weight of Morada Nova lambs, demonstrated that animals born from single birth had higher growth than those from multiple birth. This may be related to the increased availability of milk. For age at weaning, Lewis et al. (2002) concluded by regression analysis that older lambs are heavier than younger. This difference is the factor promoting most non-genetic variation in visual scores (Cardoso et al., 2001). Thereby, although the average age at weaning is in agreement to literature (Rashidi et al., 2008), it must be standardized and the amplitude reduced before selection for visual scores. In case of the age of the dam, Mysterud et al. (2002) demonstrated that ewes aged between 3 and 4 years
produced heaviest lambs and after that reproductive senescence can be observed. This can be attributed to the decrease in milk production by the female, which takes place after the fourth or fifth lambing (Carolino et al., 2003). Jorge et al. (2004) related similar results for Nelore cattle at weaning and showed an increase in the scores for offspring until a plateau (dams aged between 6 and 7 years old) and then a subsequent decrease. Koury Filho (2005) estimated higher heritability values of 0.13, 0.25, and 0.23 for C, P, and M, respectively, at weaning for Nelore cattle but also concluded that in this phase of growth there is an additive genetic variation in the population and, therefore, it’s possible to obtain genetic gain in those traits through individual selection. However, according to Forni et al. (2007), given the moderate heritability coefficients found in beef cattle studies the direct selection for visual scores traits would provide low annual gains. Moreover how the visual scores are in part subjective measurements and suffer considerable environmental influence, the evaluation of genetically superior animals for conformation, precocity and musculature is thus not easy. To minimize the influence of environmental effects, Kippert et al. (2006) suggested measures that improve environmental control, such as the standardization of feeding and sanitary management and, in particular, the training of evaluators. Estimates of genetic correlations obtained in the present study were positive, favorable, and of high magnitude, corroborating the results observed by Weber et al. (2009), who concluded that traits evaluated through scores are, in part, controlled by the same set of genes and therefore selection for one of the traits will result in correlated favorable response for the remaining traits. These results were expected because the all traits are related to the body development. Despite lacking a precision definition, the most commonly score used to lamb production is the conformation, which is poorly related to meat yield (Nsoso et al., 2000). Therefore, with high and favorable correlations among the studied visual scores, favorable responses in precocity and musculature at weaning are expected to follow from selection for conformation.
5. Conclusions To avoid biased estimates of the genetic parameters for visual scores, the environments effects of contemporary
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