Sequential relationships of weight and estimates of carcase parameters in fast growing lambs

Abstract

Proc. Aust. Soc. Anim. Prod. 1994 Vol. 20 SEQUENTIAL RELATIONSHIPS OF WEIGHT AND ESTIMATES OF CARCASE PARAMETERS IN FAST GROWING LAMBS D.G. HALL, P.J. HOLST, A.F. LUFF, J.E. MORGAN, J.D. COSTELLO and H.I. NICOL NSW Agriculture, PO Box 242, Cowra, N.S.W. 2794 SUMMARY Liveweight, C fat depth (45 mm from the mid-line over the 12th rib), GR (total tissue depth 110 mm from the mid-line of the carcase over the 12th rib) and other measurements were obtained on 4 occasions from 2 weeks after weaning until slaughter at an average liveweight of 49 kg on 76 ewe and cryptorchid second cross lambs. At slaughter cryptorchids were 10 kg heavier and leaner by 0.4 mm in C fat depth, 2 mm in GR and 0.4 of a fat score. To slaughter ewes at the same GR or fat score as cryptorchids the ewes would need to be sold 16 to 24 days earlier, when they could be 14 kg lighter. Thus the 2 sexes need to be sold into markets with different specifications for weight and sex. The data demonstrate the wide variation that exists even within a small group of lambs born over 6 weeks and grazed together at all times. Keywords: lambs, liveweight, GR, fat. INTRODUCTION The export and domestic demand for large, lean lamb is gradually increasing (Thatcher 1992). To meet this market, producers need to know when to market their lambs to avoid penalties for fat. However, the information available to producers is not comprehensive because fat levels have usually been only measured at slaughter, if at all. With the availability of real-time ultrasound equipment to measure fat and tissue depths, sequential measurements can now be obtained. This paper reports on liveweight and estimates of carcase parameters of 76 lambs on 4 occasions from weaning until slaughter at an average of 49 kg liveweight. MATERIALS AND METHODS Mature Border Leicester x Merino ewes at the Agricultural Research Station, Cowra were joined to 20 Poll Dorset rams. The rams had a range of LAMBPLAN Estimated Breeding Values (EBV' for s) weight and leanness. Sixty-six single and 14 twin lambs were born between 9 March and 19 April 1992. Lambs were tagged at birth and weight, sex and birth date recorded. Four lambs died prior to weaning on 24 June 1992. Male lambs were cryptorchidised using rubber rings to remove the scrotum at the end of lambing. The lambs were grazed together on the best pastures available at all times. Measurements were obtained by the same operator (A.F. Luff) on 7 July, 4 August, 1 and 29 September (days 189, 217, 245 and 273 respectively). Measurements included liveweight, C fat depth (45 mm from the mid-line over the 12th rib) measured by ultrasound (Delphi Backfat Meter Model 1017a), fat score by manual palpation, and GR (total tissue depth 110 mm from the mid-line of the carcase over the 12th rib) and eye muscle depth both measured using real-time ultrasound (Aloka Echo Camera Model SSD-500). Measurements were obtained at a similar time on each occasion, within 1 hour off pasture. After slaughter, carcases were weighed and GR was measured manually with a GR knife. Statistical analysis Variables were analysed on each occasion by analyses of variance with the factors sex, litter size, lamb birth date, the sire group and appropriate interactions included. Regression relationships for each variable were established between liveweight or time from day 189 to slaughter with sex and sex x liveweight or sex x time as factors. Similarly liveweight was related to time and sex with a sex x time interaction. Correlations were obtained for carcase GR with preslaughter GR and C fat depth. AI1 analyses were done using GENSTAT 5. RESULTS At the same age cryptorchids were heavier and leaner measured by estimates of C fat depth, GR and fat score but had similar eye muscle depths compared to the ewes (Table 1, Figure 1). Litter size and lamb birth date significantly affected liveweight, C fat depth, GR and fat score on some occasions (P < 0.05) but had no significant affect on eye muscle depth. Sire group and interactions never significantly affected any variable. Increases per kg were 0.087 mm for C fat depth, 0.30 mm for GR, 0.07 of a fat score and 0.3 mm for eye muscle depth (Table 2). 160 Proc. Aust. Sot. Anim. Prod. 1994 Vol. 20 Table 1. Mean (2 se) liveweight (kg), C fat depth (mm, 45 mm from the mid-line over the 12th rib), GR (mm, total tissue depth 110 mm from the mid-line of the carcase over the 12th rib), fat score and eye muscle depth for ewe and cryptorchid lambs on 4 occasions Figure 1. Relationship of final (day 273) liveweight with C fat depth (mm, 45 mm from the mid-line over the 12th rib) (a) and GR (mm, total tissue depth 110 mm from the mid-line of the carcase over the 12th rib) (b) for ewe (diamond) and cryptorchid lambs (circles) Carcase weight was 26.7 (s.e. + 0.5) kg for cryptorchids and 22.2 (s.e. + 0.5) kg for ewes (P c 0.001). Carcase GR did not differ between the sexes with 14.4 (5 0.6) mm for cryptorchids and 14.6 (5 0.5) mm for ewes. Liveweight increased over time at different rates for each sex (P < 0.001). The equations were: Cryptorchid liveweight = 34.5 (2 0.8) + 0.237 (2 0.014)day . . . . . . . . . Equation 1. Ewe liveweight = 29.5 (5 0.5) + 0.168 (5 O.OOl)day . . . . . . . . . Equation 2. The regression relationships for each carcase parameter estimate (Table 2) show the factors time and sex, and liveweight and sex, were always significant (P c 0.01). The interactions of time x sex or liveweight x sex were non-significant. The equations including time (Table 2) indicate that to slaughter ewes at the same GR or fat score as cryptorchids, the ewes needed to be sold 16 to 24 days earlier (GR, 1.08/0.0689; fat score, O-39/0.0163). Alternatively, from the equations including livetieight in Table 2, GR and fat score were similar for cryptorchids and ewes when ewes were 14 kg lighter (GR, 4.18/0.3011; , and fat score, 0.93/O-0682) than the cryptorchids. The correlation between preslaughter GR and carcase GR was 0.32 (P c 0.01) and the means were similar (Table 1 and text). The correlation of preslaughter C fat depth with carcase GR was 0.64 (P < 0.01). 161 Proc. Amt. Sot. Anim. Prod. 1994 Vol. 20 Table 2. Regression equations (2 se) for the relationships between C fat depth (mm, 45 mm from the midline over the 12th rib), GR (mm, total tissue depth 110 mm from the mid-line of the carcase over the 12th rib), fat score and eye muscle depth and either time (days, where 0 = day 189,2 weeks after weaning) or liveweight (kg) for ewe and cryptorchid lambs DISCUSSION For ewes to be sold at the same fat levels, measured by GR or fat score, as cryptorchids the ewes needed to be sold about 3 weeks before the cryptorchids. This result was also supported by the C fat depth measurements. However, at this time the ewes would have been about 14 kg lighter than the cryptorchids depending on the nominated fat level and age of slaughter. The most obvious difference 0 between the sexes was at the actual slaughter occasion when crvptorchids were 10 kg heavier but were leaner by 0.4 mm in C fat depth, 2 mm in GR and 0.4 of a-fat score. The GR and carcase weight differences are similar to those of Lee et al. (1990). The figures demonstrate the wide variation that exists even within a small group of lambs, sired by a range of rams from 1 stud, born over 6 weeks and grazed together at all times. This variation is usually allowed for by lamb producers when they sell lambs in 'drafts' and our results demonstrate that selling sexes separately will substantially increase the chances of obtaining consistent specifications with each sale lot. To sell the ewes 14 kg lighter than the cryptorchids would normally mean the ewes would be penalized for being too light and thus selling at similar fat levels is not feasible. Consequently the 2 sexes need to be sold into markets with different specifications for weight and sex. The correlation of live GR with carcase GR was lower than that reported by McEwan et al. (1989) and Hopkins et al. (1993) and there was no obvious explanation. However, the correlation of C fat depth with carcase GR was higher and above the level of 0.4 of Hopkins et al. (1993). ACKNOWLEDGMENTS Financial assistance was provided by the Meat Research Corporation. S. Chang, NSW Agriculture, Orange, is thanked for statistical advice. REFERENCES HOPKINS, D.L., PIRLOT, K.L., ROBERTS, A.H.K. and BEATTIE, A.S. (1993). Amt. J. Exp. Agric. 33: 707-12. LEE, G.J., HARRIS, D.C., FERGLJSON, B.D. and JELBART, R.A. (1990). Amt. J. Exp. Agric. 30: 743- 7. McEWAN, J-C., CLARKE, J.N., KNOWLER, M.A. and WHEELER, M. (1989). Proc. AU. Sot. Ahim. Prod. 49: 113-9. THATCHER, L.P. (1992). Proc. Amt. Sot. Anim. Prod. 19: 173430. 162