Phenotypic and genetic relationships between testicular size and live weight in young Merino rams.

Abstract

Proc. Aust. Soc. Anim. Prod. Vol. 16 PHENOTYPIC AND GENETIC RELATIONSHIPS BETWEEN TESTICULAR SIZE AND LIVE WEIGHT IN YOUNG MERINO RAMS I.W. PURVIS*, T.N. EDEY*, R.J. KILGOUR** and L.R. PIPER*** SUMMARY Data on testicular diameter (TD) and live weight (LW) of rams from a random-breeding Merino flock have been utilized to estimate phenotypic and genetic correlations between these traits at various ages. Weaning LW was highly correlated genetically with TD at 5 months of age but lowly correlated with TD at 8 and 12 months. LW at 12 months was moderately to highly correlated, genetically, with TD at 5, 8 and 12 months of age. Ram selection indices which incorporate hogget liveweight as a criterion, could therefore be expected to exert positive selection pressure on pre- and post-pubertal TD. Phenotypic correlations were all positive and mid-range. (Key words: Phenotypic and genetic correlations , live weight , testicular diameter, Merino rams). INTRODUCTION The increasing use of objective selection criteria by Australian sheep studs and the formation of several large open-nucleus group breeding schemes committed to objective measurement and performance recording, has seen increasing use of sire selection indices which incorporate measures of male reproductive traits. Peart (1982), for example, has reported that the largest of the group breeding cooperatives, the Australian Merino Society (AMS), has included testicular size as a selection criterion for rams in the nucleus. Live weight is also included in the AMS criteria. Studies of testicular growth in rams have identified a sigmoid pattern, with rapid growth occurring during the pubertal stage of development (Watson et al. 1956). Live weight growth, in contrast, is relatively linear up to 12 months of age. The growth of the testes has been shown to be controlled largely by the gonadotrophic hormones (Courot and Ortavant 1981) and these hormones have only an indirect influence on whole body growth. Given the desire to select for both testicular size and live weight, it is important to establish the phenotypic and genetic relationships between these traits at various stages of development, so as to identify the optimum time at which to select. The optimum time takes consideration of the relationship between the traits, the heritabilities and the effect on generation interval. The study described in genetic relationships between live weight at weaning and 12 flock grazed at Trangie, New this paper aimed to identify the phenotypic and testicular size at 5, 8 and 12 months of age and months of age, in a large random-breeding Merino South Wales. * Dept of Animal Science, University of New England, Armidale, NSW 2351 ** Dept of Agriculture, Trangie Agricultural Research Centre, Trangie, NSW 2823 *** CSIRO Division of Animal Production, Armidale, NSW 2350. 311 Proc. Aust. Soc. Anim. Prod. Vol. 16 MATERIALS AND METHODS Animals and environment The random-breeding Trangie D flock is composed of 14 sub-flocks of 100 ewes and was established on the basis of being representative of the Merino breeding industry of New South Wales. Detailed descriptions of the background of this flock and the environment and sheep management at Trangie'Agricultural Research Centre have been given by McGuirk et al. (1978) and Dun (1964) respectively. Experimental Ram lambs born in July/August of 1979 (n = 201), 1980 (n = 215), 1981 (n = 190) and 1982 (n = 164) formed the basis of this study. Testicular diameter (TD) and live weight (LW) were measured, generally monthly, between 4 and 12 months of age by the methods previously described by Purvis et al. (1984). Data from the 5, 8 and 12 month TD, and weaning and 12 month LW measurements are considered here. They are representative in the case of TD of prepubertal, pubertal and post-pubertal stages of sexual development and, for LW, representative of ages at which weighing is normally practised in commercial enterprises. Statistical Data were analysed using Least Squares ANOVA methods, and effects included in the model are detailed in Table 2 of the Results. The within-sire and between-sire components were calculated for each trait by equating the 'error' and between-sire, within-strain/line/year mean squares to their expectations. Paternal half-sib genetic and phenotypic correlations were calculated from the appropriate variances and covariances, and standard errors of the former calculated according to the methods of Tallis (1959) and Swiger et al. (1964). RESULTS Unadjusted mean TD at 5, 8 and 12 months and LW at weaning and 12 months of age, along with measures of dispersion, are presented in Table 1. Table 1 Unadjusted means, SD's and CV%*s of testicular diameter (cm) and live weight (kg) at various ages in Trangie D flock rams born 19791982 (n = 770) 312 Proc. Aust. Soc. Anim. Prod. Vol. I6 Testicular diameter increased at a rate of 0.62 cm/month between 5 and 8 months of age but between 8 and 12 months the growth rate had declined to 0.25 cm/month. The between-animal variability in TD at 5 and 8 months of age was much greater than that for the same trait at 12 months of age and greater than for LW at weaning and 12 months of age. Analysis of variance mean squares for TD and LW at the different ages are presented in Table 2, and phenotypic and genetic correlations between TD and LW are presented in Table 3. Table 2 Analysis of variance mean squares for testicular diameter (cm) at 5, 8 and 12 months and live weight (kg) at 5 and 12 months of age of D flock rams (n = 770) Table 3 Phenotypic and genetic ( +SE) correlations between live weight (kg) at 5 and 12 months and testicular diameter (cm) at 5, 8 and 12 months of age in Trangie D flock rams (n = 770; sire df =106; k= 4.39)* average number of progeny per sire. 313 Proc. Aust. Soc. Anim. Prod. Vol. 16 The same factors caused both TD and LW to vary except that variation between lines within strains was a significant source for LW but not for TD. DISCUSSION estimates repor'ted here reveal t.hat at The genetic correlation weaning and at 12 months of ag 3 TD and LW have a high degree of common 78) genetic control. Live weight at 12 months of age was also closely related to TD at 8 months and moderately correlated with TD at 5 months of age. Ram selection indices which incorporate hogget live weight as a criterion, could therefore be expected to exert positive selection pressure on pre- and postpubertal TD. Weaning LW, although closely correlated with TD at 5 months of age, was only poorly correlated with this trait at later ages. When compared to 12 month LW, selection on weaning LW would be expected to result in little correlated response in post-pubertal TD. Testicular size has been proposed as a ram selection criterion, not only because of its direct relationship with ram fertility but also for its role as When an indirect selection criterion for female prolificacy (Land 1973). selection for TD is at an early age, adjustment for LW is often made (e.g. Land et al. 1980), as a means of removing temporary environmental effects. Preliminary results (Purvis et al. unpublished) suggest that, although such adjustment increases the heritability of TD, it also results in a reduction in the r between TD and the female trait (e.g. ovulation rate) such that little or no 45 change in the f emale trait would result from selection on adjusted TD. Such a finding highlights the necessity for all relationships between selection criteria to be considered before adjustments, aimed at removing temporary environmental effects, are made. ACKNOWLEDGEMENTS The authors wish to thank the staff of the Trangie Agricultural Research Station. IWP held an Australian Wool Corporation Postgraduate Scholarship while these studies were carried out. REFERENCES COUROT, M. and ORTAVANT, R. (1981). J. Reprod. Fert. Suppl. 2: 47. DUN, R.B. (1964). Aust. J. Exp. Agric. Anim. Husb. & 376. LAND, R.B. (1973). Nature 241: 208. LAND, R.B., CARR, W.andE, G.J. (1980). In 'Selection Experiments in Laboratory and Domestic Animals: p. 343, editor A. 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