Visually assessed traits, such as conformation (C _PW ), precocity (P _PW ) and musculature (M _PW ), have been included in beef cattle breeding programs in order to select the animals with higher meat yield, since selection based only on body weight would not be enough ( Faria et al. , 2009 ). Furthermore, it is expected an improvement on the selection efficiency by avoiding long-legged individuals and animals with inadequacy in their muscular development ( Kippert et al. , 2006 ). In addition, reproductive efficiency of the ewes, which is associated to the number of weaned lambs per dam and to the overall herd profitability, is considered one of the most important factors in lamb production systems ( Kern et al. , 2010 ). The age at first lambing (AFL) is a common and easy-to-measure selection criterion which is related to the age at puberty ( Montoya, 2010 ). It is worth to mention that the younger the ewe at first lambing, the longer is its reproductive life, resulting in more offspring produced and faster return on investment ( Short et al. , 1994 ). Other traits such as lambing intervals, litter size and litter weight could also be included as selection criteria ( Mokhtari et al. , 2010 ; Vanimisetti & Notter, 2012 ; Zishiri et al. , 2013 ). However, due to lack of data in Brazilian sheep production, it is necessary the use of correlated traits to obtain better results in reproduction.

Male reproductive traits have also been widely used as selection criteria, mainly because they have larger progenies, and therefore, the prediction of their genetic values is more accurate. It is well known that scrotal circumference (SC) is often favorably correlated to both weigh performance and AFL ( Toe et al. , 2000 ). Thus, in order to achieve better results in a breeding program, coefficients of genetic correlation between growth and reproductive traits and the prediction of correlated responses to selection should drive the inclusion of growth and reproductive traits on selection indexes ( Boligon et al. , 2008 ).

The aim of this study was to estimate the genetic parameters for conformation, precocity, musculature, age at first lambing, and scrotal circumference in Suffolk lambs at 180 days of age.

The variance and covariance components for C _PW , P _PW and M _PW , obtained from bivariate analyses including AFL and SC are presented in Table 3. The heritability coefficients obtained by uni and bivariate analyzes were similar for C _PW , P _PW , M _PW , and SC, 0.08 ± 0.03, 0.12 ± 0.03, 0.10 ± 0.03 and 0.22 ± 0.07, respectively. To AFL the estimates of the heritability were moderate (0.20 ± 0.06) with uni and low to high with bivariate analyzes, therefore, the most probably value for the heritability is that closer to the value obtained in the univariate model. It was observed low coefficients of heritability for C _pw and M _pw , which were combined with high environmental variations. This could indicate a strong influence of non-genetic factors on the visually assessed traits, thus limiting the response to direct selection. For P _PW , AFL and SC it was observed moderate coefficients of heritability, indicating that faster genetic progress could be obtained.

The genetic correlation between C _PW and AFL was negative, moderate and favorable, indicating that selection for C _PW can reduce the average age at first lambing, and therefore, improving the sexual precocity of the flock. Among P _PW and AFL, and, M _PW and AFL the correlations were of low magnitude, positive and unfavorable, then, higher scores of P _PW and M _PW will also increase the AFL in the ewes.

The genetic correlation coefficients between SC and the visual traits were high, positive and favorable, suggesting that selection for SC will result in a correlated response for C _PW , P _PW and M _PW through indirect selection, or vice-versa. Furthermore, the correlation between SC and AFL was moderate, positive and unfavorable, showing that selection for SC may have negative correlated response on sexual precocity of females, as observed by Martínez-Velázquez et al. (2003) with information of 12 Bos taurus breeds at the Meat Animal Research Center (USDA).

It is clear that the late exposure of females to reproduction resulted in higher average AFL, which can be justified by the reproductive management adopted by the Brazilian farming systems. Mating strategies, such as the identification of precocious females ( e.g. AFL at 1 year old) would decrease the AFL mean ( Snowder, 2007 ). Furthermore, breeding younger ewes would increase their reproductive lifespan ( Kern et al. , 2010 ), favoring discard rates and replacement costs ( Mekkawy et al. , 2009 ). Although prolificacy in Suffolk sheep is maximal between 4 and 5 years of age ( Notter, 2000 ), it is important to expose the females to reproduction earlier, as this will allow the identification of the early females.

Similar to the variance components results obtained, Qureshi et al. (2010) estimated low coefficient of heritability (0.13) for AFL in Kajli sheep, a Pakistanian dual-purpose breed, indicating small genetic gain by direct selection. The low genetic variances observed for reproductive traits could reflect the high influence of environmental effects ( Rosati et al. , 2002 ).

Regarding the visually accessed traits, Janssens & Vandepitte (2004) observed that the Suffolk sheep musculature had moderate heritability (0.29) which could be used as a selection criterion for carcass yield, while heritability for growth traits ranged from 0.34 and 0.57. However, their results differed from the obtained in the present study probably because the authors used a larger scale scores (1 to 9), allowing greater variability among the evaluated animals, which may have resulted in higher variance.

Genetic association between AFL and body development has been reported in beef cattle. For instance, Boligon & Albuquerque (2010) estimated the genetic correlation between age at first calving (AFC) and visually assed traits (conformation, precocity and musculature) at 550 days of age in Nelore. The coefficients (SE) were -0.21 (0.02), -0.26 (0.01) and -0.18 (0.03), for the three cited traits, respectively. The authors concluded that there was an association between body development and reproductive traits, thus selection for visual scores could affect the response to selection for age at first calving. Based on the moderate and favorable genetic correlations between SC and visual traits, the authors concluded that direct selection for SC could be more efficient, although the selection for the visual traits could also influence fertility and sexual precocity. It is worth to mention that visual traits definition varies from study to study.

For SC, the coefficient of heritability estimated in this study ( Table 3) was low to moderate and reflects the lack of genetic variability at 180 days of age. Also, the results obtained were lower compared to those estimated from other breeds, such as Rambouillet (0.41) ( Matos et al. , 1992 ), Highlander (0.45) ( Toe et al. , 2000 ) and Makooei (0.32) ( Abbasi & Ghafouri-Kesbi, 2011 ). Furthermore, Fossceco & Notter (1995) and Al-Shorepy & Notter (1996) showed that coefficients of heritability vary according to the age of the animals, and suggested that measurements at 90 days of age would be the most adequate selection criterion due to the higher genetic variability.

Regarding the genetic correlation between SC and AFL, a moderate, positive and unfavorable estimate was obtained. This result differ from that reported by Toe et al. (2000) , who obtained high, negative and favorable (-0.57 and -0.78 at 9 and 12 months, respectively) in Highlander sheep and concluded that selection for SC is efficient to obtain genetic gain on sexual precocity of females. These differences could also be by the fact of assuming different parameters in the model.

The main focus of this study was to explore the genetic correlation between visually assessed and reproduction traits in a Brazilian Suffolk sheep herd, since animals have been often selected due to their conformation, precocity and musculatures scores. The direct selection for conformation, precocity e musculature at 180 days of age and age at first lambing will result in a slower genetic progress, probably due to the high influence of environmental effects. For scrotal circumference, it is possible to obtain genetic gain in sexual precocity through direct selection in Suffolk rams while promoting genetic gain for conformation, precocity and musculature. Also, the selection for age at first lambing will provide favorable correlated response to selection on conformation in Suffolk sheep.