The effects of age, year and lambing performance on greasy wool production in Merino ewes in north west Queensland.

Abstract

Proc. Aust. Soc. Anim. Prod. (19 74) 10; 367 THE EFFECTS OF AGE, YEAR AND LAMBING PERFORMANCE ON GREASY WOOL PRODUCTION IN MERINO EWES IN NORTH WEST QUEENSLAND MARY ROSE* Summary Estimates were made of the effects of age, year and lambing performance on greasy fleece weight in Merino ewes aged 15 to lO$ years for the period 1966-72. Greasy fleece weight increased to a maximum of 3.75 kg at 4& years and declined gradually thereafter not falling below the 2; year production level until 8& years. Pregnancy lowered greasy fleece weight more than lactation. The effects of each were 0.22 and 0.12 kg respectively which represented six and three per cent of the mean for ewes which failed to lamb. Differences between years were very marked. The maximum betweenyear difference of 1.36 kg (1971 and 1972) was equivalent to 39 per cent of the mean for all years. The effects of age and lambing performance have been used to pred ict the wool production in flocks of differing age structures. I. INTRODUCTION In a breeding flock where optimum age structure, both for genetic improvement and current productivity, is to be determined, an estimation of productivity for each age of the ewe's life is required. Estimations of changes in wool production with age for Australian Merinos have been published (Brown et al. - 1966; Mullaney -w 1969). ' et al. Rose (1972) reported age-specific death rates and reproduction rates for an experimental flock of Merino ewes at Julia Creek, north west Queensland. This paper presents the changes with age and the effect of year on greasy wool production for this flock and discusses these age effects and the penalties of pregnancy and lactation on wool production in flocks with differing age structures. * Department of Primary Industries, Brisbane, Queensland, `4000. 367 II. MATERIALS AND METHODS Sheep and Management The experimental flock was run on Toorak Sheep Field Research Station, Julia Creek. The .environment and flock history have been described previously (Beattie 1961; Rose 1972). Ewes were first mated at li years, then annually until death. In the period of this study, ewes were joined in October-November, lambed in MarchApril, and lambs were weaned at shearing in June-July. Observations Data were obtained from records of greasy fleece weight and lambing performance of ewes aged 18 to lO$ years for the per.iod 1966-72. Greasy fleece weights were adjusted proportionally where necessary to 365 days' growth. For convenience ages are spec.ified as yearly increments from l& years. Ewes born from 1966 to 1971 were two to three months younger and ewes born prior to 1966 two to three months older than the specified age at shear.ing. There were< e.ither eight or nine age groups represented each year and each age group was represented in either five or s.ix years. The numbers of observations in each age group over the seven year period ranged from 441 at 13 years to 180 at 105 years; a total of 3 655 observations. ( C1 Method of Analvs,is The effects of age, year and lambing performance were estimated by the least squares method. The effects of drop (year of birth)) year of measurement and age are confounded and in this paper year of measurement and drop effects were eliminated jointly as year effects. The type of birth and the age of the ewe's dam are known the ewe's lifetime productivity but these effects were not included .in the as ewes born as twins and ewes born to 14 year old ewes were very few flock. No interactions were .included, as preliminary analyses indicated effects were small compared with the main effects. III. RESULTS AND DISCUSSION to affect model in this that their ( a) Age effects Table 1 presents the age effects for greasy fleece weight. Greasy fleece weight .increased to a maximum of 3.75 kg at 4$ years and declined thereafter not falling below the 2i year production level until 8& years. Compared with other published data (Brown et al. 1966; Mullaney et al. 1969) `Toorak' ewes atta.ined their maximum greasy wool production at a later age and production declined less rap.idly w.ith age. The wool production of 1; year old ewes was very low. 368 TABLE 1 Age effects for greasy fleece weight (b) Effects of pregnancy and lactation The effect of pregnancy was estimated from the difference between values for ewes which did not lamb and those which lambed and lost their lamb. The effect of lactation was calculated by the difference between values for ewes which lambed and lost and those which reared a lamb. These values are shown in Table 2. The possible sources of bias listed by Brown et al. (1966) would apply to these estimat.ions although there was no year in wh.ich joining did not take place. TABLE 2 Effects of pregnancy and lactation on greasy fleece weight . The effect of pregnancy on greasy fleece weight was greater than that of lactat.ion (0.22 compared w,ith 0.12 kg) . The combined effect of pregnancy and lactation was a decrease of 0.34 kg which was nine per cent of the mean value for ewes which failed to lamb. Pregnancy accounted for a six- per cent loss and lactat.ion a loss of three per cent. Of ewes rearing lambs the percentage which reared two lambs was 1.7, Z.l.and 0.9 for 1966, 1969 and 1970 respectively. In other years no ewe reared two lambs so rear.ing tw.ins .is not an important influence on this estimate of the effect of rais.ing one lamb. 369 The estimate of nine per cent for the comb.ined effects of pregnancy and lactation corresponds w,ith that of nine per cent for Mer,inos in western V.ictor.ia (Mullaney-et al. 1969) but is much less than the 17 per cent reduction estimated by Brown -et al. (1966) for Peppin Merinos at `Cunnamulla. Their estimate for pregnancy effect and that for lactation effect (11 and 6 per cent respectively) were each approximately twice as great as the corresponding effect for the `Toorak' flock. The mean lactation effect of 15.8 per cent reported by Corbett (1964) for adult Merino ewes at Armidale was considerably larger than either of these estimates. (c) Year effects TABLE 3 Year effects for greasy fleece wei&t Year effects are shown in Table 3. The maximum between-year difference was 1.36 kg (1971 and 1972) which was a change equivalent to 39 per cent of the mean for all years. These values reflect the very variable seasonal conditions of the area although it is not possible to estimate to what extent as these effects are the confounded effects of drop and year of measurement. IV. PREDICTION OF WOOL PRODUCTION FOR FLOCKS OF VARYING AGE STRUCTURES Using the effects est.imated, the wool production can be predicted for flocks with different age compositions, and with varying proportions of ewes .in each lambing performance class. The mean proportions of ewes in each lambing performance class for the ?Toorak' flock in the period 1965-72 were calculated and using the method described by Brown et al. (1966) ) the age effects were adjusted for the differing proportions at each age. Deviations of flocks of ewes aged 2$ to x years were calculated from a flock consisting entirely of 2i year old ewes. This was the age of ewes at shear.ing after they f.irst entered the breeding flock. The death rates applied were for the `Toorak' flock .in the period 1965-70 and were based on numbers at shearing each year. Year effects were eliminated. These values appear in Table 4. Deviations from a flock consist.ing ent.irely of 14 year old ewes were calculated also as they show more clearly the impact of culling on current productivity .in a breeding flock l.ike the TToorakT one. These values appear in Table 4. 370 TABLE 4 Values based on the `Toorak' flock data show that no flock with any of the age structures exam.ined was below the mean greasy wool production of a flock of 24 year old ewes. This pattern differs from those recorded by Brown et al. (1966) both for the `Gilruth Plains' flock and for flocks recorded by R.iches (1958)whose data they also examined. Th.is emphasises the marked effect of the environment on the factors controlling greasy fleece weight production in breeding flocks and the need to know the pattern for diiferent areas. Dev.iat.ions from a 14 year old flock show that culling of aged ewes would not have a great impact on the current level of greasy wool production in a flock of similar performance. This .is an important fact in an area where, in order to maintain flock numbers, .it .is not practicable to cull or cast for age (Rose 1972). V. ACKNOWLEDGEMENTS It is a pleasure to thank Miss Roslyn Grant who has maintained the records and assisted in the collation of the data. I would also like to thank the many staff members of Toorak Sheep Field Research Station who have collected these records. VI. REFERENCES . BEATTIE, A.W. (1961). Qd Jo agric. Sci. 18 : 437. BROWN, G.H., TURNER, Helen Newton, YO%G, S.S.Y., and DOLLING, C .H.S. (1966). Aust. J. agric. Res. 17 : 557. CORBETT, J.L. (1964). Proc. Aust. So&Anim. Prod. 5 : 138. MULLANEY, P.D., BROWN, G.H., YOUNG, S.S.Y., an&YLAND, P.G. (1969). Aust. J. agric. Res. 20 : 1161. RICHES, J.H. (1958). C.S.I.R.O.=st.. D,iv. Anim. Hlth Divl Rep. No. 7 (Ser. SW-3). ROSE, Mary (1972). Proc. Aust. Soc. Anim. Prod. 2 : 48. - 371