Relationship between joining age, bodyweight, fertility and productivity of fine wool Merino ewes in southern N.S.W.

Abstract

RELATIONSHIP BETWEEN JOINING AGE, BODY WEIGHT, FERTILITY AND PRODUCTIVITY OF FINE WOOL MERINO EWES IN SOUTHERN N.S.W. T. G. KENNEDY* and J. P. KENNEDY* Ewes joined to rams for the first time at a mean age of 18 months had a lower body weight at 30 months than ewes joined at 30 months, and a small difference was still present at 42 months in two of the three flocks studied. There was no significant difference in performance following joining at 30 months of age between ewes first joined at 18 months of age and ewes first joined at 30 months of age. A positive relationship was found between body weight at joining of maiden 18 month old ewes and fertility, but not for 30 month old ewes being joined for the second time. When compared with greasy wool production of dry ewes, it was estimated that pregnancy followed by lactation reduced greasy fleece weight by 11.2%) whereas pregnancy alone reduced greasy fleece weight by 4.3%. Summary I. INTRODUCTION A survey made on the Southern Tablelands of N.S.W. showed that on 70%. of farms, fine wool Merino ewes were not joined with rams for the first time until they were about 30 months of age (Anon. 1962). The majority of farmers believed that ewes in the area were not sufficiently well grown at 18 months of age to be bred, and that pregnancy from this age would permanently stunt their growth. In this paper, the productivity of fine wool Merino ewes on the Southern Tablelands, joined for the first time at 18 months of age (early-joined) and 30 months of age (late-joined) is compared. II. MATERIALS AND METHODS In March 1965, approximately 1,700 two-tooth ewes in three commercial fine wool Merino flocks were allocated at random to two groups. On each property, ewes in an early-joined group were joined with rams for the first time at a mean age of 18 months, and those in the late-joined groups at a mean age of 30 months. On property B, only 100 of the 400 early-joined ewes were used for the comparison with late-joining. The other 300 ewes were used in a separate trial to determine the effect on body weight ' of supplementary feeding around the period of lambing, but were included in the analysis to determine the relationship between body weight and reproduction. Except for the first joining period, all ewes on a property grazed as one flock. They were weighed off pasture before and after joining and before weaning of the lambs. *School of Wool and Pastoral Sciences, The University of New South Wales, Kensington, N.S.W. 215 (a) Animals (b) Sheep Management TABLE 1 IMean body weights of the ewes before joining in three years The approximate dates of shearing on properties A, B and C respectively were February, September and October, and of the start of lambing were October 1, September 15 and September 1. Greasy fleece weights were obtained for all ewes at shearing. In the analysis of the effects of pregnancy and lactation on wool production in the early-joined group of property A, mean greasy fleece weights were adjusted for differences in mean body weight at the start of joining. Before weaning, the udder of each ewe was examined and the ewe was classified as being either `wet', `wet-dry' or `dry' (Dun 1963). Ewes classified as `wet' were considered to have had a pregnancy followed by a lactation, those classified as `wet-dry' were considered to have had a pregnancy only, and those classified as `dry' had not lactated recently and were considered to have been barren at the particular lambing. TABLE 2 Mean greasy fleece weights of ewes for the period 19651967 216 TABLE 3 Lambing percentages in 1966 of early-joined ewes and of late-joined ewes III. RESULTS (a) Eflect of early-joining on growth, wool production, fertility and survival rate At 30 months of age (1966), the body weights of the early-joined ewes were 2.2, 3.1 and 3.0 kg less than those of the late-joined ewes on properties A, B and C respectively (Table 1). One year later, the early-joined ewes were still significantly lighter on properties A and C. Significant differences in greasy fleece production between early- , and latejoined groups were found on all properties at the 1966 shearings (Table 2). Of the ewes present at weaning in 1966, the proportion that were either `wet' or `wetdry' in the two groups did not differ significantly (chi-square test) on any property (Table 3). The mean survival rate of all ewes was 78.2% to the start of joining in 1967, and did not differ significantly between joining treatments on any property. (b) Efiects of pregnancy and lactation on wool production Following adjustment by covariance analysis (Snedecor 1956) for differences in body weight at joining, the mean greasy fleece weight of the `wet' and `wet-dry' ewes in 1965 was 11.2 and 4.3% lower respectively than that of the `dry' ewes (Table 4). TABLE 4 Mean greasy fleece weights of early-joined ewes (property A, Feb., 1966), adjusted for differences in body weight at the start of joining 196.5 217 TABLE 5 Mean body weight at start of joining 1965, in relation to mating and. reproductive performance (c) Relationship of body weight at joining to the occurrence of mating and to fertility In 1965, on property A, 22.3 % of the early-joined ewes were not marked by the rams and, of those marked, 26.0% were classified at weaning as being `dry'. On property B the corresponding figures were 7.6% and 23.6% and on property C, 1.0% and 26.8%. I On properties A and B, ewes that remained unmarked by the rams were significantly lighter at the start of joining than those marked but which were subsequently `dry'. `Dry' ewes in turn were significantly lighter than `wet' and `wetdry' ewes on property A but not B (Table 5). Only one ewes on property C was unmarked. The mean body weight at joining of the `dry' ewes, whether or not they actually mated, did not differ significantly from that of the `wet-dry' and `wet' ewes, except at the early joining on properties A and B in 1965 when the `dry' ewes were 1.6 and 1 .O kg lighter respectively (P<O.OOl). IV. DISCUSSION Early-joining, as opposed to late joining of ewes appeared to have only slight effects on wool production, fertility, mature body weight, and none on ewe mortality. Although at the start of joining in 1967, significant differences in body weight existed between early-joined and late-joined ewes on two of the three properties, these differences were small, probably of little economic importance and might riot be permanent. In 1966, no difference was found between groups in the proportion of ewes that lambed although the mean body weight of early-joined ewes at the start of joining was from 2.2 to 3.1 kg less than that of those late-joined. The positive relationship between body weight at the start of joining in 1965 and fertility is in agreement with the results of Coop (1962), McInnes, Grainger and Smith (1967) and Killeen (1967). Observations made on properties A and B in 1965 indicated that, among the barren ewes, some ewes mated but apparently did not conceive whereas other ewes, on average the lightest, did not mate. The body weight of the latter ewes varied greatly, ranging from 18 to 35 kg on property A, and from 22 to 33 kg on property B. Watson and Gamble (1961) found a large variation in the body weight of Merino ewes at first oestrus, the mean for spring218 born ewes being approximately 33.5 kg at a mean age of 186 days. The minimum body weight of the ewes marked by the rams in the present trial was 22 kg, but since no records of the. body weights of the ewes were available before joining at 18 months, it is possible that the ewes had a higher body weight at some time before the first joining period. The finding that lactation had a greater effect on wool production than did pregnancy is in agreement with the results of Ray and Sidwell (1964). Corbett (1964) found a 15.9 % difference in wool production over a 14-week lactation period by ewes which had lambed but had their lambs removed at birth compared with similar ewes which were lactating. Assuming equal rate of wool production outside the treatment period, Corbett's (1964) lactating ewes produced approximately 4.5% less wool over the whole year than his non-lactating ewes. A reduction of 7.2% in 12 months greasy fleece weight was found in the trial reported here when lactating ewes were compared with ewes which had lambed but were not lactating. V. ACKNOWLEDGMENTS The authors wish to acknowledge the cooperation of Messrs. C. Crisp, 'Hardwicke', Yass; M. Darmody, 'Nanima', Yass; and R. Hill, Inveralochy Station, Goulburn. The trial was initiated by Mr. G. J. Judson, now at the University of New England, and one of us (T.G.K.) was financed by an Australian Wool Board Post-Graduate Research Scholarship. VI. REFERENCES . (1962). Regional Research and Extension Study, Southern Tablelands, N.S.W. Joint Planning Committee. Rep. No. 9, pp. 76. COOP, I. E. (1962). N.Z. .7. agric. Res. 5: 249. C ORBETT, J. L. (1964). Proc. Aust. Soc. Anim. Prod. 5: 138. D UN, R. B. (1963). Aust. 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