Abstract:
Proc. Aust. Soc. Anim. Prod. Vol. 19 MANIPULATION OF SEX TO MODIFY GROWTH AND CARCASS COMPOSITION OF PRIME LAMBS S. A. SPIKERA, L. J. CUMMINSA and F.D. BRIEN. B *Dept of Food and Agriculture, Pastoral and Veterinary Institute, PB105, Hamilton, Vic. 3300. BSouth Australian Dept of Agriculture, GPO Box 167 1, Adelaide, S.A. 5001. SUMMARY Two experiments were carried out to examine the effect of sex modification on prime lamb production. In both experiments induced cryptorchidism increased growth rate and reduced carcass fatness, the largest effect was observed when cryptorchids were compared with ewe lambs at heavy slaughter weights. At light and medium slaughter weights there were only small differences between cryptorchids and wethers. An attempt to improve ewe lamb performance by androgenisation early in gestation was not successful. Keywords: lambs, cryptorchids, androgenisation, carcass characteristics. INTRODUCTION The use of entire males (rams or cryptorchids) as prime lambs will reduce carcass fatness and increase liveweight gain to slaughter (Corbett et al. 1973; Lee et al. 1990). Ewe lambs are often overfat, particularly at heavy carcass weights. De Haan et al. (1987) and Jenkins et al. (1988) have shown that by altering the pre-natal steroid environment within the dam at about the time of sexual differentiation, there are changes in the growth pattern and carcass composition of ewe lambs due to androgenisation. This paper reports on 2 experiments conducted at the Pastoral Research Institute (PRI) Hamilton one comparing cryptorchid, wether and ewe lambs and the other comparing cryptorchid, wether, ewe and androgenised ewe lambs when slaughtered at several liveweights. Both experiments included twinreared and single-reared lambs. MATERIALS AND METHODS Experiment 1 In 1985, 51 Corriedale, (CC), 66 Romney Marsh x Corriedale, (RC) and 64 Border Leicester x Corriedale (BC) 4-year-old ewes were joined to Poll Dorset rams in February. Lambs were tagged and weighed at birth. At marking half the male lambs from each ewe group were randomly allocated to be cryptorchidised using rubber rings to remove the scrotum. One third of the total lambs from each sex group was randomly allocated to be slaughtered at 35 kg (L), 45 kg (M) or 55 kg (H) full liveweight. The L lambs and a proportion of the M lambs were slaughtered before weaning whereas all the H lambs were weaned and shorn. Upon reaching the pre-determined slaughter weight lambs were fasted for 24 h, weighed and then slaughtered 24 h later. At slaughter, hot carcass weights (including kidney fat and kidneys), hot GR (total tissue depth 110 mm from the mid line of the carcass over the 12th rib), hot C fat depth (45 mm from the mid-line over the 12th rib) and carcass length (from the lower side of the gambrel to the point of the shoulder) were measured. After chilling and transfer to the Institute' s meat laboratory, carcasses were halved, trimmed, had the kidney fat, kidney and channel fat removed and cut into leg, chump, loin, rib loin, forequarter and flank. The eyemuscle (M. longissimus dorsi) area was measured over the 12th rib. The individual joints were then further trimmed of subcutaneous fat and other trim to the estimated equivalent of a GR 8-10 mm carcass; all were weighed pre- and posttrimming. The total trim, including kidney and channel fat, is expressed as total trim fat percentage (` ITF%). Experiment 2 Eighty-eight RC ewes were joined after oestrus synchronisation in March 1986 to Suffolk rams fitted with marker crayons to record actual mating dates. After joining, the ewes were randomly allocated to a control treatment or to 1 of 3 testosterone enanthate treatments of 37.5 mg, 112.5 or 225 mg (75 mg/mL given as a subcutaneous injection) on day 25 of gestation. At birth all lambs were inspected for the normality of their external genitalia. The testosterone treatment of the dams modified the apparent sex of female lambs but had no effects on the sex of males (consistent with the data of Clark et aZ. 1976). At the lowest dose of testosterone only 9% of females were androgenised, at the middle dose 44% and at the highest dose 67% were affected. This resulted in comparisons based on 39 normal females and 14 androgenised ones. Androgenisation caused a range of 163 Proc. Aust. Sot. Anim. Prod. Vol. 19 changes to the external genitalia of ewe lambs, including the presence of a false scrotum and an increase in the ano-genital distance. This resulted in extensive areas of urine staining which would reduce skin values and make the sheep very susceptible to fly strike. Slaughter management and carcass analysis was carried out as for experiment 1. The lambs in this experiment were slaughtered at the L and M weights only. All data were analysed using Genstat version 5 (Lawes Agricultural Trust, Rothamstead Experimental Station 1987) analysis of variance procedures. In both experiments the ewes and lambs grazed improved perennial pastures. RESULTS Data for both experiments are presented in Table 1 as the slaughter group x sex interaction and in Fig. 1 as main effects. Table 2 shows the statistical significance (P < 0.05) of the treatments. Sex of lamb had a significant (P < 0.0.5) effect on most lamb measurements in both experiments. In experiment 1 breed of dam, litter size and the slaughter group x sex interaction were often significant, while in experiment 2 litter size and testosterone dose were occasionally significant. Fig. 1. The 95% confidence limits for means of the main effect of sex on slaughter age (days) and GR depth (mm) for cryptorchid (Cr), wether (W) and ewe (E) lambs (experiment 1, open symbols) and Cr, W, E and androgenised ewe (A) lambs in experiment 2 (solid symbols). Cryptorchids grew faster (represented as slaughter age) than wethers which, in turn, grew faster than ewes. In experiment 1 the sex difference in growth rate between sexes increased as the lambs grew. Androgenization did not improve growth rate of ewe lambs (experiment 2). Age at slaughter was increased by about 11% for twin reared lambs compared with singles and reduced by about 8% in lambs born to BC ewes compared with those born to RC or CC ewes. Measures of carcass fatness presented in the data in Table 1 gave a consistent pattern. As expected, significant differences (P c 0.05) in fatness due to the major variables (slaughter group and lamb sex) were measured. In experiment 1, slaughter group accounted for an average of 10% of the total sum of squares in the different fat measures (3-24%) and sex for an average of 17% (4-28%), while in experiment 2 these variables accounted for an average of 3 1% (23-41%) and 12% (6-19%), respectively. In both experiments the other main effects and interactions each accounted for less than 4% of the variation in fatness. Of the carcass joints reported here, the loin and forequarter required more trimming than the leg. Lamb sex had significant effects on trimmed leg and forequarter joint weights, ewes had lighter joints than cryptorchids. In experiment 1, the slaughter group x sex interaction reflected an increase in the difference between the sexes in fatness at heavier weights. Thus in the light and medium slaughter groups cryptorchids were not significantly leaner than wethers, nor were androgenised ewes significantly leaner than normal ewes. Sex had no effect on eye muscle area, carcass weight/length ratio or lean/fat and lean/bone ratios from a complete loin dissection. At similar liveweights twin reared lambs were slightly leaner than singles, whilst lambs from CC ewes were slightly leaner than those from crossbred ewes in experiment 1. 164 Proc. Aust. Sot. Anim. Prod. Vol. 19 Table 1. Means of measurements taken on cryptorchid (Cr), wether (W), ewe (E) and androgenised ewe (A) lambs and their carcasses Table 2. Statistical significance (P < 0.05) from analysis of variance on the effects of slaughter group (G), sex (S), litter size (Es), ewe breed (B) and testosterone dose rate (T) on slaughter age (Sl. age), carcass weight (Car. wt) dressing% (Dress. %), total trim fat (TTF) and other carcass factors 165 Proc. Aust. Sot. Anim. Prod. Vol. 19 DISCUSSION At traditional slaughter weights (L and M) the advantage of cryptorchids was seen as a reduction in slaughter age by a mean of 2 weeks (3-32 days) , but not as a significant (P < 0.05) reduction in carcass fatness. Androgenisation of ewe lambs in our experiment did not improve growth rate or reduce carcass fatness. It did cause potential stock management problems associated with constant urine staining of the breech and rear belly areas on the lambs which also reduced skin values. These factors will prevent its adoption by producers. In addition, consumers may not readily accept lamb meat from animals modified in this way. In this environment (Hamilton, Victoria) the production of heavy lamb is difficult due mainly to poor lamb growth rate over summer, autumn and winter. Growth of unweaned lambs to the light slaughter weight was 208 g/day (over spring), from then to the medium slaughter weight growth of weaned lambs declined to 116 g/day and over autumn and winter growth declined further to 32 g/day for the heavy slaughter group. Based on our data and those of Lee et al. (1990) the benefits of cryptorchidism would only be worthwhile when producers develop feeding systems for heavy lamb production but they should be aware of the potential for some of these lambs to be fertile; ewe lambs in a subsequent experiment at the PRI were pregnant to cryptorchids. ACKNOWLEDGMENTS The authors acknowledge the financial support of the MRC (AMLRDC) for this work and the assistance of Messrs M. Hannah and J. W. D. Cayley with the analysis of the data, Mr A. J. Clark with the carcass dissection and MS N. D&fen and Miss A. Nagorcka for preparation of the manuscript. REFERENCES CLARKE, I. J., SCARAMUZZI, R. J. and SHORT, R. V. (1976). J. Embryol. Exp. Morph. 36: 87-99. CORBETT, J. L., FURNIVAL, E. P., SOUTHCOTT, W. H., PARK, R. J. and SHORTHOSE, W. R. (1973). Anim. Prod. 16: 157-63. DE HAAN, K. C., BERGER, L. L., KESLER, D. J., McKEITH, F. K., THOMAS, D. L. and NASH, T. G. (1987). J. Anim. Sci. 65: 1465-70. JENKINS, T. G., FORD, J. J. and KLINDT, J. (1988). J. Anim. Sci. 66: 1179-85. LEE, G. J., HARRIS, D. C., FERGUSON, B. D. and JELBART, R. A. (1990). Aust. J. Exp. Agric. 30: 743-7.