Genetic evaluation of crossbred lamb production. 5. Age of puberty and lambing performance of yearling crossbred ewes

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1 CSIRO PUBLISHING Australian Journal of Agricultural Research, 2007, 58, Genetic evaluation of crossbred lamb production. 5. Age of puberty and lambing performance of yearling crossbred ewes N. M. Fogarty A,F, V. M. Ingham B, A. R. Gilmour A, R. A. Afolayan A, L. J. Cummins C, J. E. Hocking Edwards D, and G. M. Gaunt E A The Australian Sheep Industry Cooperative Research Centre, NSW Department of Primary Industries, Orange Agricultural Institute, Orange, NSW 2800, Australia. B Agrisearch Services Pty Ltd, Orange, NSW 2800, Australia. C Department of Primary Industries, Primary Industries Research, Hamilton, Vic. 3300, Australia. D South Australian Research & Development Institute, Struan Research Centre, Naracoorte, SA 5271, Australia. E Department of Primary Industries, Primary Industries Research, Rutherglen, Vic. 3685, Australia. F Corresponding author. neal.fogarty@dpi.nsw.gov.au Abstract. The age and liveweight at puberty were evaluated in 2155 crossbred ewe lambs at 2 sites over 3 years. A separate dataset examined the lambing performance of 1177 crossbred ewe lambs that were joined naturally at 7 months of age at 2 sites over 3 years and also their lambing performances in the subsequent 2 years. The ewe lambs were the progeny of Merino dams and sires from several breeds including Border Leicester, East Friesian, Finnsheep, Coopworth, White Suffolk, Corriedale, and Booroola Leicester. Overall, 85% of the ewe lambs reached puberty (detected by teasers) in their first autumn at an average age of 248 days and liveweight of 42 kg. There was significant variation in age and weight at puberty (P < 0.01) among the site and year cohorts of ewes. Sire breed was significant for weight (P < 0.01) but not age at puberty. Of the crossbred ewe lambs joined at 7 months of age, 54% lambed with an average litter size of 1.31 and 49% of lambs were weaned from the ewes joined. There was a significant site effect for all reproduction traits (P < 0.01), with the higher performance at one site associated with higher liveweight at joining. Maternal sire breed was significant for all reproduction traits (P < 0.01) and there was considerable variation between sires within sire breeds. Ewes that reared lambs in their first year weaned 12% more lambs per year in their second and third years compared with ewes that failed to lamb or rear any lambs at 1 year of age. The results demonstrate that lamb weaning rates of 90% or more can be achieved from crossbred ewe lambs with optimal genetics and management. The opportunities for improving productivity by joining crossbred ewe lambs and the selection and management considerations required for successful joinings and lambings are reviewed and discussed. Additional keywords: progeny test, Border Leicester, Finnsheep, selection. Introduction Joining ewe lambs during their first year of age has been suggested for many years (McGuirk et al. 1968; Cannon and Bath 1969) as a way of increasing the lifetime reproduction of ewes (Dyrmundsson 1973). If successful, the practice could effectively reduce the cost of replacement ewes, be a means of rapidly increasing the size of the breeding flock following drought or when replacement ewes are in short supply, and reduce the generation interval in breeding flocks. The practice has limited adoption in Australia (Groves and Cummins 2002), although it is more widespread in New Zealand (Kenyon et al. 2004). Reports in the literature of the proportions of ewes successfully lambing at 1 year of age have varied greatly and have commonly been in the range of 60 80% (Dyrmundsson 1973, 1981), although lambing rates have been considerably lower in some of the early Australian studies (McGuirk et al. 1968; Tyrrell et al. 1974; Tyrrell 1976). Many factors contribute to the variation in reproductive performance of ewe lambs, including between- and withinbreed variation, age of ewes, liveweight and mating season (Dyrmundsson 1973, 1981). Often these factors may be confounded experimentally so that it is difficult to apportion variation to each. The hormonal regulation of puberty and reproduction in ewe lambs has been reviewed by Quirke (1981). Heavier liveweight (McGuirk et al. 1968) and older age at joining (Cannon and Bath 1969) result in higher lambing performance from ewe lambs. There have generally been no adverse effects on subsequent reproduction for ewes lambing at 1 year of age, although wool production and weight may be slightly reduced in the first year (Omar et al. 1977; Baker et al. 1978, 1981; McMillan and McDonald 1983; Moore et al. 1983). There have been considerable differences in the performance of ewe lambs from different breeds (Moore et al. 1983), with Finnsheep-cross ewes generally being more productive than those from other CSIRO /AR /07/090928

2 Puberty and lambing performance of yearling ewes Australian Journal of Agricultural Research 929 breeds and crosses (Laster et al. 1972; Hulet and Price 1975). There also appears to be genetic variation for fertility in ewe lambs, with Fossceco and Notter (1995) reporting a heritability of A comprehensive survey of 629 New Zealand farmers who were joining ewes in their first year (Kenyon et al. 2004) identified several factors that producers thought would ensure high lambing percentages. These factors included incorporation of Finnsheep or East Friesian genetics, high ram percentage ( %), high liveweight at mating, and managing single- and multiple-bearing ewes separately during lambing and lactation. This study examines the variation in age and liveweight at puberty and the lambing performance of crossbred ewe lambs joined at 7 months of age. The variation among several maternal sire breeds and individual sires and the effect of ewe liveweight on lambing performance at 1 year of age and subsequently at 2 and 3 years of age are also examined. The opportunities for improving productivity by joining crossbred ewe lambs and the selection and management considerations required for successful joinings and lambings are also reviewed and discussed to provide recommendations for advisors and producers. Materials and methods This study involves 2 subsets of data from crossbred ewe lamb progeny in the Maternal Central Progeny Test (MCPT; Fogarty et al. 2005). The first dataset examines the age and liveweight at puberty of first-cross ewe lambs at 2 sites (Cowra and Struan) over 3 years. The second dataset examines the lambing performance of first-cross ewe lambs joined at 7 months of age at 2 sites (Cowra and Hamilton) over 3 years and the lambing performance of these ewes from their subsequent 2 joinings at 19 and 31 months of age. Ewes and sites The crossbred ewe lambs in the MCPT were progeny of 91 sires from several maternal breeds and mainly Merino-base ewes, with the lambs born at 3 sites over 3 years (Fogarty et al. 2005). The sites and years of birth of the lambs were: Cowra Agricultural Research and Advisory Station on the central slopes of NSW (lat S) from 1997 to 1999, the Pastoral and Veterinary Institute, Hamilton, in western Victoria (lat S) from 1997 to 1999, and Struan Research Centre in south-eastern South Australia (lat S) from 1998 to The base ewes at Cowra were mature medium-wool Merino, at Hamilton approximately half the ewes were fine-wool Merino and half were Corriedale, and at Struan they were broad-wool Merino. The 91 sires were from the Border Leicester (n = 18), East Friesian (12), Finnsheep (12), Coopworth (9), White Suffolk (7), Corriedale (6), and Booroola Leicester (6) breeds. Other breeds among the remaining 21 sires included Hyfer (4), English Leicester (2), Gromark (2), Merino (2), Poll Dorset (2), Romney (2), Cheviot, South African Meat Merino (SAMM), South Hampshire Down, Texel, White Dorper, Wiltshire Horn, and a composite (Coronga) breed. To provide genetic links for evaluation, 3 of the sires were used at all sites and in all years, except that only 2 of the link sires were used at Struan in Further details on the management, genetic merit of the sires, source of base ewes, and insemination protocols used have been reported by Fogarty et al. (2005). At Cowra, each cohort of crossbred ewe lambs was split into autumn and spring joining groups by stratified randomisation based on weaning weight within sire group. The autumn groups were first joined at 7 months of age in February for lambing in July. The spring-joined groups were monitored for oestrous during their first autumn, but were not joined until the following spring (14 months of age). The 1997 cohort of ewe lambs was transferred to Orange (lat S) after weaning, where oestrus was monitored and the autumn group joined. This autumn joining group was returned to Cowra in mid pregnancy (late May) and the spring joining group was returned to Cowra before joining in September. At Hamilton, all the first-cross ewe lambs were joined at 7 months of age in autumn (March) for lambing in August of each year but oestrous monitoring records were not available. The 1998 cohort of crossbred ewes at Hamilton experienced vibriosis pre-lambing in 1999, which resulted in some abortion loss, and this cohort has been excluded from this analysis. The ewes at Struan were monitored for oestrus during their first autumn, but were not joined until the following late spring summer (19 months of age) after transfer to Rutherglen, Vic. Each cohort of crossbred ewes was mated to terminal sire rams for 3 years to assess their second-cross lamb production. The project was conducted under approval of the relevant Departmental Animal Ethics Committee at each site. Age and liveweight at puberty Data on age and liveweight at puberty were available for 2155 first-cross ewe lambs at Cowra and Struan, which were monitored for oestrus during their first autumn. Harnessed teasers were run with each cohort of ewes and weekly raddle marks recorded from late January to late May during their first year. The autumnjoined ewes at Cowra were joined to entire harnessed rams in late February and raddle marks recorded weekly until the end of joining. The numbers of ewes in the various cohorts, including the number of ewes that were not raddled and their liveweight in autumn are shown in Table 1. The date of recording of the first raddle mark for each ewe was defined as her date of puberty and her age from birth was also determined. The weight at puberty was interpolated from the liveweights available at Table 1. Numbers of ewes assessed for puberty and number not raddled, average date born and weight and date of weight during assessment for cohorts of crossbred ewes at Cowra and Struan Ewes Not raddled Born Weight Date (%) (kg) Cowra (15) 1/8/ /3/ (12) 11/7/ /3/ (29) 16/7/ /2/00 Struan (17) 16/6/ /3/ (11) 9/6/ /4/ (7) 13/6/ /4/01 Total (15)

3 930 Australian Journal of Agricultural Research N. M. Fogarty et al. approximately 2-monthly intervals in the autumn. The ewes that were not raddled in each cohort were excluded from the analyses of age and weight at puberty. Lambing performance Data on the lambing performance were available for 1177 firstcross ewe lambs that were joined naturally to entire rams at 7 months of age for 6 8 weeks at Cowra and Hamilton. These ewes were also joined at 19 and 31 months of age to assess subsequent lambing performance. Their second-cross lambs were tagged within 15 h of birth and the birthweight, dam identification, sex, birth-type (number of lambs in the litter), and date of birth recorded. These second-cross lambs were weighed and weaned at 12 weeks of age. Statistical analysis Data for age and weight at puberty were analysed with a linear mixed model and a residual maximum likelihood procedure using ASReml (Gilmour et al. 2002). The fixed effects in the model were site (Cowra, Struan), cohort of ewes (1, 2, 3), maternal sire breed of ewe (Border Leicester, East Friesian, Finnsheep, Coopworth, White Suffolk, Corriedale, Booroola Leicester, and Others), and type of birth and rearing of the ewe (single born and reared, multiple born and single reared, multiple born and reared). All 2- and 3-way interactions of the fixed effects were included in the initial models and non-significant (P > 0.05) interactions were removed from the final model. Sire was included as a random effect, which means the testing of the ewe breed fixed effect is relative to the remaining between-sire variance. Analysis of lambing performance for the ewes joined at 7 months of age included the following traits: fertility (ewes lambing or not), litter size (LS, number of lambs born from lambing ewes), lambs born (NLB, number of lambs born for all ewes joined), and lambs weaned (NLW, number of lambs weaned for all ewes joined). A logit-transformation was used for fertility and the reproduction traits were analysed using similar models and procedures to those described above. Sire means were predicted from the basic model and combined fixed sire breed with random sire effects. Results Puberty The crossbred ewe lambs at Cowra were generally born in mid to late July of each year (Table 1) and reached puberty in late February and early March of the following year at an average age of 226 days (Table 2). The 1997 cohort was born 2 3 weeks later than the other cohorts at Cowra and reached puberty at a similar month, which meant they were 3 weeks younger at puberty. The ewes at Struan were born about 1 month earlier in the year (mid June) and generally reached puberty a month later than those born at Cowra, resulting in a significant difference in age at puberty between the sites of 66 days (P < 0.01). The average time and age of puberty varied considerably among the cohorts at Struan and ranged from 4 March (2000 cohort, 263 days) to 30 April (1999 cohort, 326 days). Sire breed of the ewes was not significant for age at puberty (Table 2). Ewes at the Cowra and Struan sites were a similar liveweight at puberty, although there Table 2. Predicted means (± s.e.) of age and weight at puberty for crossbred ewes by various sire breeds Means followed by the same letter within columns and factor are not significantly different at P = **P < 0.01; n.s., not significant Age (days) Weight (kg) Sire breed n.s. ** Border Leicester ± ± 0.5cba East Friesian ± ± 0.9a Finnsheep ± ± 0.7cb Coopworth ± ± 0.8cba White Suffolk ± ± 0.9ba Corriedale ± ± 1.3d Booroola Leicester ± ± 1.2cba Others ± ± 0.5c Site ** n.s. Cowra ± 1.4b 40.8 ± 0.4 Struan ± 1.6a 41.6 ± 0.4 Cohort ** ** Cowra ± 2.0f 43.0 ± 0.5a Cowra ± 2.0d 38.9 ± 0.5d Cowra ± 2.3e 40.4 ± 0.5cb Struan ± 2.2b 41.5 ± 0.5b Struan ± 2.3a 43.0 ± 0.5a Struan ± 2.2c 40.2 ± 0.5c Overall ± ± 0.3 was significant variation among the cohorts (P < 0.01, Table 2). There were significant differences between the sire breeds of the ewes in average liveweight at puberty (P < 0.01), ranging from 37.1 kg for the Corriedale to 43.1 kg for the East Friesian-cross ewes. There were 15% of exposed ewes not raddled and failing to reach puberty in their first autumn. Lambing performance Of the crossbred ewes joined at 7 months of age at Cowra and Hamilton, 54% subsequently lambed with an average litter size of 1.31 and 49% of lambs of ewes joined were weaned (Table 3). The average liveweight at joining for the ewes at Hamilton was low (31.9 kg), which resulted in low lambing performance. At Cowra the ewes were heavier at joining (38.5 kg), which resulted in better lambing performance, although the heaviest cohort (1999, 41.4 kg) had the poorest lambing performance relative to the other cohorts at Cowra (Table 3). The 1997 cohort at Cowra had the best performance, with 66% of ewes lambing with 92% lambs born and 64% lambs weaned of ewes joined. The sire breed of the crossbred ewes was highly significant (P < 0.01) for all reproduction traits (Table 4). Fertility ranged from 0.71 ± 0.06 for the East Friesian to 0.24 ± 0.07 for the Corriedale-sired ewes, with the resulting NLW being 0.67 ± 0.07 and 0.19 ± 0.08, respectively. The Corriedale-cross ewes were the lightest of all the breed crosses at joining (31.4 kg) and East Friesian were the second heaviest (37.2 kg), although the White Suffolk (39.0 kg), which was the heaviest breed cross, only had moderate fertility (0.44 ± 0.08), low litter size (1.13 ± 0.12), and poor lamb survival, resulting in 0.27 ± 0.10 NLW. The highest litter sizes were for the Booroola Leicester (1.60 ± 0.08)

4 Puberty and lambing performance of yearling ewes Australian Journal of Agricultural Research 931 Table 3. Number of ewes joined, average date of birth and date joined (number of weeks joined), weight at joining (s.d.), and means (s.d.) for fertility, litter size (LS), and number of lambs born (NLB) and lambs weaned (NLW) per ewe joined for cohorts of crossbred ewes joined at 7 months of age at Cowra and Hamilton Ewes Birth Joined Weight (kg) Fertility LS NLB NLW Cowra /08/97 26/02/98 (6) 39.6 (5.0) 0.66 (0.48) 1.39 (0.56) 0.92 (0.80) 0.64 (0.69) /07/98 26/02/99 (6) 35.0 (4.3) 0.61 (0.49) 1.43 (0.54) 0.88 (0.82) 0.64 (0.71) /07/99 24/02/00 (8) 41.4 (5.0) 0.50 (0.50) 1.36 (0.48) 0.68 (0.76) 0.54 (0.67) Hamilton /08/97 31/3/98 (8) 32.5 (4.7) 0.48 (0.50) 1.19 (0.39) 0.57 (0.65) 0.41 (0.56) /08/99 17/3/00 (7) 31.1 (4.9) 0.46 (0.50) 1.19 (0.39) 0.55 (0.65) 0.30 (0.50) Overall (6.1) 0.54 (0.50) 1.31 (0.51) 0.71 (0.75) 0.49 (0.63) Table 4. Number of ewes, joining weight (s.d.), and predicted means (± s.e.) for fertility, litter size (LS), and number of lambs born (NLB) and lambs weaned (NLW) per ewe joined for cohorts of crossbred ewes by various sire breeds joined at 7 months of age at Cowra and Hamilton Means followed by the same letter within columns and factor are not significantly different at P = **P < 0.01 Ewes Weight (kg) Fertility LS NLB NLW Sire breed ** ** ** ** Border Leicester (6.4) 0.62 ± 0.05ba 1.25 ± 0.07cb 0.77 ± 0.07cb 0.59 ± 0.06a East Friesian (6.9) 0.71 ± 0.06a 1.29 ± 0.07cb 0.91 ± 0.08ba 0.67 ± 0.07a Finnsheep (5.4) 0.64 ± 0.05a 1.43 ± 0.07ba 0.91 ± 0.08ba 0.62 ± 0.06a Coopworth (5.9) 0.46 ± 0.06cb 1.27 ± 0.08cb 0.60 ± 0.09dc 0.51 ± 0.07ba White Suffolk (5.0) 0.44 ± 0.08dcb 1.13 ± 0.12c 0.50 ± 0.12edc 0.27 ± 0.10cb Corriedale (5.4) 0.24 ± 0.07d 1.09 ± 0.12c 0.27 ± 0.10e 0.19 ± 0.08c Booroola Leicester (5.9) 0.62 ± 0.06ba 1.60 ± 0.08a 1.02 ± 0.09a 0.62 ± 0.08a Others (5.5) 0.44 ± 0.05c 1.16 ± 0.08c 0.51 ± 0.08ed 0.34 ± 0.07cb Site ** ** ** ** Cowra (4.7) 0.57 ± 0.03a 1.36 ± 0.04a 0.79 ± 0.04a 0.59 ± 0.03a Hamilton (4.8) 0.45 ± 0.03b 1.16 ± 0.04b 0.53 ± 0.04b 0.31 ± 0.03b and Finnsheep (1.43 ± 0.07) cross ewes. The ewes at Cowra had significantly higher performance (P < 0.01) than those at Hamilton for all reproduction traits. Subsequent lambing performance The subsequent reproductive performance of ewes at their second and third joinings following different lambing status at the 7-month joining is shown in Table 5. Ewes that had lambed and weaned at least one lamb from the 7-month joining weaned significantly more lambs subsequently than ewes that were not pregnant or lambed and lost all lambs before weaning (1.27 ± 0.03 v ± 0.03, P < 0.05). This was due to the combined effects of better subsequent fertility and higher lamb survival. Sire progeny means The range in BLUP sire progeny means within breeds for the 57 sires based on their daughter s performance when they were joined at 7 months of age is shown in Table 6. For the Border Leicester, East Friesian, Finnsheep, and Booroola Leicester breeds, all the sires ranged from 0.5 to over 0.7 for fertility, whereas few of the other breeds had any sires over 0.5. There was a considerable range among sires for litter size within most breeds, with the highest Booroola Leicester sire being 1.82 ± 0.11 and the Finnsheep being 1.57 ± The highest Border Leicester and Coopworth sires were above 1.4 for litter size, which was greater than the lowest Booroola Leicester and Finnsheep sires. The sire with the highest value for NLW (0.89 ± 0.10) was from the East Friesian breed. Table 5. Effect of lambing status of crossbred ewes from their first 7-month joining on their subsequent average fertility, litter size (LS), and number of lambs born (NLB) and lambs weaned (NLW) per ewe joined from their second and third joining Means followed by the same letter within columns are not significantly different at P = 0.05 Lambing status Fertility LS NLB NLW Not lambed 0.89 ± 0.01b 1.70 ± ± ± 0.03b Lambed & lost 0.90 ± 0.02ab 1.77 ± ± ± 0.05b Lambed & weaned 0.93 ± 0.01a 1.70 ± ± ± 0.03a Overall mean 0.91 ± ± ± ± 0.03

5 932 Australian Journal of Agricultural Research N. M. Fogarty et al. Table 6. Range of sire predicted means (± s.e.) for crossbred ewe progeny lambing performance from joining at 7 months of age Sire breed Sires Fertility Litter size Lambs weaned Minimum Maximum Minimum Maximum Minimum Maximum Border Leicester ± ± ± ± ± ± 0.10 East Friesian ± ± ± ± ± ± 0.10 Finnsheep ± ± ± ± ± ± 0.09 Coopworth ± ± ± ± ± ± 0.10 White Suffolk ± ± ± ± ± ± 0.11 Corriedale ± ± ± ± ± ± 0.10 Booroola Leicester ± ± ± ± ± ± 0.12 Others ± ± ± ± ± ± 0.11 Discussion Puberty in ewe lambs is influenced by both genetic and environmental factors. Ewe lambs from most breeds will reach puberty in their first autumn or winter if they are sufficiently well grown to be heavier than a threshold liveweight, which may vary with breed (Dyrmundsson 1973; Quirke 1981), and ewes that express oestrus are heavier than those that do not (Hight et al. 1973). Some 85% of the crossbred ewe lambs in our study reached puberty in their first autumn at an average weight of over 42 kg. We found significant differences among sire breeds for average weight but not age at puberty, whereas there have been large breed effects for both traits reported in the literature (Dyrmundsson 1973, 1981; Young and Dickerson 1991). Despite the ewe lambs at Struan being born earlier in the year they reached puberty later than those born at Cowra and hence were over 2 months older at puberty. Factors that may have contributed to this difference between the sites include seasonal growth conditions, latitude, and genetic background of the Merino dams of these ewe lambs. In their review, Baker and Morris (1982) concluded that there were moderately high heritabilities for age and weight at puberty, which has been supported more recently (0.35 ± 0.06 and 0.26 ± 0.08, respectively; Alkass et al. 1994), and the number of cycles in the first year has a moderate heritability (0.35 ± 0.06; Meyer et al. 1994). There also appears to be heterosis for reduced age at puberty among crossbreds (Dyrmundsson 1973; Baker and Morris 1982). Over 50% of the ewes that were joined at 7 months of age subsequently lambed with an average litter size of 1.31 and resulting in 49% lambs weaned of the ewes joined, which is within the range of reports reviewed by Dyrmundsson (1973, 1981). This level of performance is higher than earlier Australian reports for Merino (Tyrrell 1976) and crossbred (McGuirk et al. 1968; Tyrrell et al. 1974) ewes joined at 7 months of age, although Cannon and Bath (1969) reported 71% lambing from older crossbred ewes (10 11 months of age at joining). The higher liveweight of ewes at Cowra than at Hamilton contributed to their better lambing performance, which is consistent with other reports (McGuirk et al. 1968; Gaskins et al. 2005). There were significant effects for sire breed with the East Friesian, Finnsheep, Border Leicester, and Booroola Leicester-cross ewes all having >60% fertility. It is not clear how much the differences in liveweight at joining contributed to these breed effects. There were still large differences among the breeds when liveweight at joining was included as a covariate in the analyses as the linear effect did not take account of the threshold nature of liveweight on puberty. It should be noted that the White Suffolk-cross ewes were heavier than all other crosses and only had 44% fertility. Breed differences in performance have been reported (Moore et al. 1983), with Finnsheep (Southam et al. 1971; Laster et al. 1972; Hulet and Price 1975; Gaskins et al. 2005), Booroola (Young and Dickerson 1991; Meyer et al. 1994), and Border Leicester (Hight and Jury 1976; McMillan and McDonald 1983) cross ewes outperforming other breeds and crosses in these various studies. At Cowra, 6 of the 36 sire groups of ewes that were evaluated over the 3 years weaned over 90% of lambs (of ewes joined). There was also a considerable range in performance among the sire predicted means within each breed, which indicates scope for selection and genetic improvement of 1-year-old lambing performance. Fossceco and Notter (1995) reported a heritability of ewe lamb fertility of Lambing at 1 year of age had no detrimental effects on subsequent lambing performance of the ewes at 2 and 3 years of age. We found that the ewes that reared at least one lamb in their first year subsequently weaned an average of 12% more lambs per year than ewes that did not lamb or lambed and lost all their lambs. This advantage in subsequent performance was largely due to better lamb survival and to slightly better fertility. Several other studies have reported either no detrimental effects on subsequent performance following lambing in the first year (Cannon and Bath 1969; Dyrmundsson 1973; Keane 1974; Omar et al. 1977; Moore et al. 1983) or improved performance (Baker et al. 1981; McMillan and McDonald 1983). Ewes that expressed oestrus in their first year compared with those that did not have also been reported to have higher ovulation rates and 15 23% more lambs born when joined in their second year (Hight and Jury 1976; Moore et al. 1978). The performance of ewes in their first year is generally lower than that from comparable adult ewes, with many factors contributing, in addition to those discussed above. Compared with adults, ewe lambs have a later onset of the breeding season (Dyrmundsson 1981) and express poor sexual behaviour with short oestrus periods (18 v. 29 h, Edey et al. 1978). Insemination failure (Quirke 1981) and poor ovum quality (McMillan and McDonald 1985) have been reported in young ewes, which results in high embryo losses (Quirke 1981; McMillan and McDonald 1985; Young and Dickerson 1991). Lower lamb survival from 1-year-old compared with adult ewes has also been reported. This is associated with lower birthweight, although maternal behaviour does not appear to be impaired (Dyrmundsson 1973) and lamb survival is improved at

6 Puberty and lambing performance of yearling ewes Australian Journal of Agricultural Research 933 subsequent lambings compared with ewes that have not lambed as 1 year olds (Dyrmundsson 1981; McMillan and McDonald 1983). Growth rate of lambs from 1-year-old ewes is also lower than that from adult ewes (Dyrmundsson 1981). This effect may only be for early growth to 4 weeks of age, which is associated with lower milk production as no effect of ewe age was found for lamb growth or milk production later in the lactation (Morgan et al. 2005). Our results clearly demonstrate that crossbred ewe lambs can be joined in their first year, although many factors contribute to the level of performance achieved and success of the practice. The genetic merit, including maternal background of the ewes, is important, with Finnsheep and Dorset crosses having extended cycling (Quirke et al. 1985), although our results have also shown that several other breeds can perform well. In self-replacing flocks, consideration could be given to including selection for early date of lambing (Smith et al. 1995) and ewe lamb fertility (Fossceco and Notter 1995) in the breeding objective. The most important management consideration is to ensure that the ewes are above their threshold liveweight for puberty at the time of joining in their breeding season, with our results indicating that ewes should be at least 42 kg, although there was variation among the breeds. Increasing liveweight above the threshold has been reported to increase the proportion of multiple births but not affect the conception rate (Quirke 1981; Gaskins et al. 2005). Early-drop lambs will perform better than later drop lambs as they will be older and heavier at joining (Dyrmundsson 1981). Ewe lambs have a later start to their breeding season than adult ewes (Dyrmundsson 1981) and joining ewe lambs later in the season will improve their performance. Mature and experienced rams should be used (Bichard et al. 1974) because ewe lambs have a short oestrus and express poor oestrous behaviour (Edey et al. 1978). Ewe lambs should not be joined with mature ewes as reduced ewe lamb conception rate has been reported (Hulet and Price 1975). Teasing the ewe lambs before joining may result in higher fertility (Kenyon et al. 2005) and an earlier average lambing date (Kenyon et al. 2006). Particular attention is required to nutrition and management during pregnancy and lambing to ensure optimum lamb birthweights and high lamb survival. While a recent report failed to show any effect of nutritional treatment on either lamb birthweight or lamb survival from ewe lambs (Morris et al. 2005), those ewes on the low nutrition treatment weaned lighter lambs. This is also supported by Morgan et al. (2005) who found that lamb growth was significantly affected by ewe weight in mid pregnancy. Consideration should also be given to early weaning (no later than 12 weeks) so that the young ewes have sufficient time to recover and gain weight for the next joining. Many of these recommendations were identified as important factors by New Zealand farmers who had experience with joining ewe lambs (Kenyon et al. 2004). Implications Crossbred ewe lambs can be successfully joined in the autumn to lamb in their first year. Lamb weaning rates of 90% or more can be achieved with optimal genetics and management, although 60% weaning may be more commonly expected. The successful joining of ewe lambs reduces the effective cost of replacement ewes, increases the lifetime productivity of ewes in the flock, and reduces generation interval. It is also a means of rapidly increasing flock numbers after drought or when supply of replacement ewes is limited. The use of pregnancy scanning following joining allows the culling and disposal of non-pregnant ewes, which can still be sold as slaughter lambs. The additional culling of ewes that fail to rear a lamb in their first year will further improve the subsequent lifetime lambing performance of the flock. Acknowledgments The MCPT was run by the NSW Department of Primary Industries, Department of Primary Industries Victoria, and the South Australian Research and Development Institute with the generous financial support of Meat and Livestock Australia. Commonwealth funding through the Australian Sheep Industry Cooperative Research Centre is also gratefully acknowledged. We also gratefully acknowledge the many other scientists, technical, and other support staff at the Centre for Sheep Meat Development, Cowra, the Orange Agricultural Institute, the Pastoral and Veterinary Institute, Hamilton, and Struan Research Centre, who have contributed to and supported the work over several years. 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