Variation in testis diameter and serving capacity within and between 14 Merino lines.

Abstract

Animal Production in Australia Vol. 15 VARIATION IN TESTIS DIAMETER AND SERVING CAPACITY WITHIN AND BETWEEN 14 MERINO LINES I.W. PURVIS* , R.J. KILGOUR+, T.N. EDEY* and L.R. PIPER # SUMMARY Testis diameter at five, eight and 12 months and serving capacity at 18 months were studied in three year of birth groups of rams from a flock representing 14 Merino lines. The most important sources of 'variation for the testis size traits were year, strain, litter size, birth date and sire, whilst for the serving capacity measures, the effects of year and year x strain interaction were the only significant sources of variation. INTRODUCTION Male reproductive performance is an important component of any sheep breeding enterprise through its direct effect on reproductive efficiency. Increased interest has focussed on male reproductive traits since Land (1973) argued that the quantitative expression of reproductive performance in males and females may be correlated genetically. In the Australian Merino little is known about the extent and nature of variation in male reproductive traits. This paper presents estimates of the variation in testis diameter and serving capacity within and between the 14 separate lines of a Merino flock grazed at Trangie, NSW. A similar analysis of within and between line sources of variation for ovulation rate of two year old ewes from this same flock was presented by Piper et al. (1980). MATERIALS AND METHODS Animals and environment The 14 different random breeding lines of which the flock is composed, were chosen on the basis of strain and line to be representative of the Merino breeding industry of NSW. Each line comprises 100 ewes which are mated to three sires purchased each year from a major stud of their line. A detailed description of the background of this flock has been given by McGuirk et al. (1978), and Dun (1964) has described the environment and sheep management at the Trangie Agricultural Research Centre. Experimental Lambs were born in July/August of 1979, 1980 and 1981 and weaned at four to five months of age. Each drop of ram lambs ran as a single monosexual mob from weaning until 18 months of age. With some variation between years, testis measurements were taken monthly from five months to 12 months of age. Net testis diameter was calculated as the mean diameter of both testes after correction for skin thickness. Net testis diameters at five, eight and 12 months of age representing pre-pubertal, pubertal and post-pubertal stages of testis growth, are presented in this paper. In April of 1981, 1982 and 1983, when each successive year of birth group was 18 months of age, the rams were tested for serving capacity as previously * 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. 545 Animal Production in Australia Vol. 15 described by Kilgour and Whale (1979). Each ram received 2 x 20-minute tests and 2 x one-hour tests. Numbers of mounts and services (ejaculations) were recorded and an index based on numbers of mounts and serves was calculated for each animal for each test. Index values were allocated on the following basis: 0 - no mounts or services; 1 - one to five mounts, but no serves; 2 - greater than five mounts but no serves; 3 - one serve; 4 - two or three serves; 5 - four to six serves; 6 greater than six serves. The sum of the 2 x one-hour test service and index values are reported in this paper. Statistical Only animals with complete testis diameter and serving capacity records were included in the analysis. Missing records in most cases reflected individual animal health problems or death. The data were examined by least squares ANOVA methods. The model included effects due to year (year of birth and year of measurement evaluated jointly), strain, year x strain interaction, line within strain, year x line interaction, sire within line x year subclass, litter size at birth, age of dam and date of birth. Line and sire were regarded as random effects whilst the remaining effects were regarded as fixed. Comparisons between individual means within main effects (Table 2) were made using Duncan's Multiple range test. These comparisons were only carried out when the ANOVA mean square for the main effect was significant. RESULTS The analysis of variance for the full model is presented in Table 1 and the least squares means of the three testis diameter traits and the two measures of serving capacity are given in Table 2. TABLE 1 Analysis of variance mean squares for testis diameter at five, eight and 12 months of age, number of services and index of serving capacity 546 Animal Production in Australia Vol. 15 Year of birth/measurement effects were the major source of variation for all traits except testis diameter (TD) at five months of age. The rams born in 1979, whilst having smaller testes at five months of age, had significantly larger testes at 12 months than the other two year of birth groups (Table 2). The decline in testis size from 8 to 12 months of age in the 1981 born rams probably reflects the effects of severe drought conditions prevailing during late autumn and winter of 1982. TABLE 2 Least squares means (&S-E.) of testis diameter and serving capacity according to years, strains and litter size at birth For each trait different superscripts between rows within effects denote significant differences (P<O.OS) Whilst differences at five and eight months months of age. Likewise age so that by 12 months ly from that of singles. between strains in TD were highly significant (P<O.OOl) of age, there was no difference between strains at 12 the influence of litter size at birth on TD declined with the TD of multiple born lambs did not differ significant- Aside from year differences, the only other significant source of variation in the measure of serving capacity (SC) was an interaction between strain and year for number of serves (P<O.OS). The variation between lines within strains was only of significance (P<O.OS) for TD at 12 months of age. The other random effect, sires, accounted for highly significant components of the total variation in eight and 12 months of age testis size. Adjustments made to account for age differences by the regression on birth date, were highly significant for the three testis size traits. DISCUSSION These data highlight, once again, the importance of environmental influences in genetic studies of grazing animals in areas such as central-western NSW where both the incidence and reliability of rainfall is low. The year differences found in this study are such that these age based measures of testis size and serving capacity may not be reflecting the same physiological mechanisms in each of the three years. This has important implications for the efficiency of selection 547 Animal Production in Australia Vol. I.5 programmes which incorporate male reproductive traits in their selection criteria. The age range over which TD was measured in this study encompasses the pubertal period for Merino rams (Watson et al. 1956) and it may be that the observed differences in TD between strains are largely a function of differences in rate of attainment of sexual maturity. Additional analyses of testis size in this flock will take account of liveweight and penis development in order to more fully examine the influence of sexual maturation on testis size. Further work is being carried out to establish the nature of the relationship between testis size in sexually immature rams and mature age sperm production. The relationship between testis size and sperm production in mature rams is already established (Knight 1973). The highly significant sire component of variance for testis size 'at eight and 12 months of age, suggests that these traits may be highly heritable. By contrast, the analyses of the SC measures, where neither the strain, line nor sire effects were significant, suggest that serving capacity has only a relatively small genetic component. However, since 36.5% of the 606 rams tested did not serve during the 2 x one-hour tests and 20.2% had zero index scores (no mounts or serves), the efficiency of the pen-test technique as an indicator of potential libido and mating dexterity for heterosexually inexperienced rams must be questioned. ACKNOWLEDGEMENTS The authors sincerely thank the staff of the Trangie Agricultural Research Centre for their assistance and cooperation. 1-W. Purvis held an Australian Wool Corporation postgraduate scholarship whilst this study was being carried out. REFERENCES DUN, R.B. (1964) Aust. J. Exp. Agric. Anim. Husb. 4: 376. KILGOUR, R.J. and WHALE, R.G. (1979) Aust. J. Exp.=Agric. Anim. Husb. 2: 5. -_KNIGHT, T.W. (1973) Ph.D. thesis. University of Western Australia. LAND, R.B. (1973) Nature, Lond., 241: 208. 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