N. M. Fogarty

Genetic evaluation of crossbred lamb production. 1. Breed and fixed effects for birth and weaning weight of first-cross lambs, gestation length, and reproduction of base ewes

This is the first paper in a series that reports on a national maternal sire central progeny test program (MCPT) to evaluate the genetic variation for economically important production traits in first- and second-cross progeny of maternal and dual purpose (meat and wool) sires and the scope for genetic improvement. The MCPT program also provides direct linkages between breeds that will contribute information for across-breed genetic evaluation. Crossbred progeny by 91 sires from more than 7 maternal breeds (including Border Leicester, East Friesian, Finnsheep, Coopworth, White Suffolk, Corriedale, and Booroola Leicester) at 3 sites over 3 years with 3 link sires in common at each site and year were evaluated in this study. National estimated breeding values for the sires that were entered by industry breeders indicated that there was some selection for weight, but not other traits and the sires were generally representative of the maternal genetics available to the industry. There were 8377 base Merino and Corriedale ewes artificially inseminated with thawed-frozen semen from the 91 sires and 8193 crossbred lambs born and 6117 lambs weaned. Mixed model analyses of birth and weaning weight, lamb survival, and gestation length as well as fertility, litter size, and lamb weaning rate of the base ewes are reported. Sire breed was significant for birth weight (with a range of 4.0-4.4 kg at an average litter size of 1.8) and weaning weight (19.6-22.5 kg), as well as lamb survival (70-79%) and gestation length (147.1-150.3 days). Birth/rearing type was significant for all traits. Males were significantly heavier than females at birth and weaning, although there was no sex difference for lamb survival or gestation length when birthweight was included in the model. Pregnancy rate from the artificially inseminated base ewes varied from 40 to 84% at the 3 locations in the various years. Sire breed was significant for the number of lambs weaned per ewe lambing, but not for fetal number or litter size born. There were also significant differences in performance due to the source of the base ewes at each site.

Genetic parameters for visually assessed traits and their relationships to wool production and liveweight in Australian Merino sheep

Heritability was estimated for a range of visually assessed traits recorded on Merino sheep, together with the phenotypic and genetic correlations among the visually assessed traits and correlations of the visually assessed traits with measured wool production traits and liveweight. Data were derived from four research resource flocks, with a range of 12 958 to 57 128 records from animals with 478 to 1491 sires for the various traits. The estimates of heritability were high for the wool quality traits of handle, wool character and wool colour (0.33–0.34) and the conformation traits of face cover, neck wrinkle and body wrinkle (0.42–0.45), moderate for front leg structure (0.18) and low for back leg structure (0.13). Fleece rot score had low heritability (0.14), while classer grade was moderately heritable (0.20). Estimates of genetic correlations among the visually assessed wool quality traits were low to moderate in size and positive (0.17–0.47). Genetic correlation estimates among the assessed conformation traits were generally very low, except for the genetic correlations between scores for neck and body wrinkle (0.92 ± 0.01) and front and back leg structure (0.31 ± 0.09). Fleece rot score had low positive genetic correlations with neck and body wrinkle scores (0.18 ± 0.05 and 0.15 ± 0.05, respectively) and classer grade (0.26 ± 0.06). Classer grade was slightly positively correlated with the wool quality traits (0.17–0.45) and leg structure traits (0.21–0.25). The genetic correlations among the visually assessed traits were generally neutral to favourable. The visually assessed wool quality traits had low to moderate favourable genetic correlations with mean and coefficient of variation of fibre diameter (0.19 –0.47), but negative correlations with clean wool yield (–0.26 to –0.37). Face cover was unfavourably correlated with staple length (–0.27 ± 0.04) and liveweight (–0.23 ± 0.02). Neck and body wrinkle scores were genetically associated with higher greasy (0.33–0.39) and clean fleece weights (0.19–0.22), greater coefficient of variation of fibre diameter (0.24–0.26) and fibre curvature (0.27–0.28), but with reduced yield (–0.26 to –0.28) and staple length (–0.34 to –0.41). Fleece rot score was genetically correlated with clean fleece weight (0.26 ± 0.05) and coefficient of variation of fibre diameter (0.27 ± 0.04). Classer grade was favourably correlated with greasy and clean fleece weights (–0.41 to –0.43), staple length (–0.29 ± 0.04), liveweight (–0.36 ± 0.03) and coefficient of variation of fibre diameter (0.27 ± 0.03). Most genetic correlations between the visually assessed traits and the measured production traits and liveweight were close to zero and less than 0.2 in magnitude. This study provides accurate values for the parameter matrix required to incorporate visually assessed traits into breeding objectives and the genetic evaluation programs used in the Australian sheep industry, allowing the development of breeding objectives and indexes that optimally combine visually assessed performance and measured production in Merino sheep.

Growth and carcass composition of second-cross lambs. 2. Relationship between estimated breeding values of sires and their progeny performance under fast and slow growth regimes

Growth and carcass performance of 2316 ewe and cryptorchid second-cross lambs over 2 years was related to the LAMBPLAN estimated breeding values (EBVs) of their sires. The lambs (56-156 per sire) were sired by 20 Poll Dorset rams from one stud, which were selected to represent a range of EBVs for growth rate and leanness. Half of each lamb sex group was grown at a fast rate from weaning to slaughter at 40 kg liveweight for ewes or 48 kg for cryptorchids. The other half was grown at a slower rate to the same slaughter weights, 10-13 weeks later. Lamb measurements included liveweight approximately every 21 days to slaughter, and fat and muscle depth on live lambs using ultrasound and carcass fat depth. The regressions of lamb weight on sire EBV for weight increased as the lambs aged and were 0.243 ± 0.124 for fasted weight pre-slaughter and 0.158 ± 0.064 for carcass weight. Live lamb C fat depth (45 mm from the mid-line over the 12th rib), carcass C fat depth, and carcass GR (total tissue depth 110 mm from the mid-line of the carcass over the 12th rib) were all related to sire EBV for fat. The regression values for ultrasound C fat depth and carcass GR of 0.156 ± 0.049 and 0.366 ± 0.159 on the sire EBV for fat (mm/mm EBV) were higher than the expected values. The results demonstrate that over a range of sires, 2 years, 2 sexes, and different nutrition regimes there are significant advantages in progeny weight and leanness if sires are selected for high EBVs for weight and leanness.

Milk yield and milk composition from grazing primiparous non-dairy crossbred ewes

Milk production and milk composition were studied in 520 primiparous Merino crossbred ewes that were the progeny of 30 sires from several maternal crossing breeds including Border Leicester, Coopworth, Finnsheep and East Friesian. The ewes were born in 3 years (1997, 1998, and 1999), with 3 sires used in every year to provide genetic links. The crossbred ewe lambs were randomly assigned to autumn or spring joining groups and mated to Poll Dorset rams to lamb at 12 or 19 months of age. Ewes were milked on 3 days during their first lactation at approximately 3, 4, and 12 weeks after lambing. On each milking day, each ewe was initially injected with oxytocin, milked out by machine, and then hand-stripped. This procedure was repeated approximately 4 h later, with the milk weight and time recorded to extrapolate to daily milk yield, and composition of the milk was determined. Daily milk yield was analysed using REML mixed models procedures and 3 measures of milk production were predicted: peak milk yield at 21 days of lactation, total milk yield from 21 to 90 days of lactation, and length of lactation (days for daily yield to decline to 600 g/day). Milk composition traits (%fat, %protein, %lactose) and proportion of machine milk yield were analysed with similar models. Sire breed, number of lambs born and suckled, season, and ewe weight gain from mid-pregnancy until lamb weaning were all significant (P < 0.01) for daily yield, which declined from 2.1 kg/day at 21 days to 0.7 kg/day at 90 days of lactation. The Finnsheep-cross ewes had lower peak milk yield (1.84 ± 0.08 v. 2.09–2.19 ± 0.1 kg/day) and lower total yield of lactation (21–90 days) than all the other breed-cross ewes (78 ± 3 v. 92–107 ± 5 kg). The East Friesian-cross ewes had significantly longer lactations (128.8 ± 10.9 days) than the Border Leicester (98.1 ± 4.4 days) and Coopworth (93.7 ± 4.2 days), with the Finnsheep-cross ewes the shortest (80.1 ± 3.1 days). The East Friesian-cross ewes had the greatest and the Finnsheep-cross ewes the least total yield of lactation (107 ± 5 and 78 ± 3 kg, respectively). There were significant differences in peak milk yield for ewes bearing and suckling single lambs, twin-born and single-raised lambs, and twin-born and raised lambs (1.86 ± 0.04, 2.03 ± 0.09, and 2.37 ± 0.07 kg/day, respectively). There was a significant decline in daily milk yield with increasing ewe weight gain from mid-pregnancy to lamb weaning (–18.1 ± 4.2 g/kg). All the components of milk composition changed from the peak of milk production at 3 weeks to the end of lactation. The East Friesian-cross ewes had significantly lower milk %fat (by approx. 1 percentage point) at both 21 days and 90 days of lactation than ewes by all other sire breeds. The 30 sire progeny mean deviations for daily milk yield ranged from –292 to +276 g/day, with considerable ranges between sires within the sire breeds. The implications for lamb production and breeding programs are discussed.

A review of the genetic and epigenetic factors affecting lamb survival

Poor lamb survival pre-weaning is a major source of reproductive inefficiency in Australian sheep flocks. While nutrition and management options have been extensively researched and promoted to improve lamb survival, the present review focuses on the prospects for obtaining genetic gain and helps identify selection strategies for boosting such gains to improve overall reproductive efficiency in the Australian sheep industry. Estimated heritability for lamb survival using linear model analysis is low, although use of threshold models suggests that heritability could be higher, which, if true, could help explain the substantial genetic gains obtained in long-term selection experiments. Epigenetic mechanisms may hinder selection and quantitative trait-loci identification through confounding and/or masking genetic variances and co-variances. With sufficient information, these effects could be considered in genetic evaluations by identifying those components that are amenable to selection. Regarding indirect selection, finding effective criteria for improving lamb survival has proved elusive. Most measures of maternal behaviour, temperament and lambing difficulty researched are poorly correlated genetically with lamb survival. Of lamb behaviours and thermo-genic indicators studied, latency to bleat following handling by humans is moderately genetically correlated with lamb survival, as is neonatal rectal temperature. Industry application remains to be adequately explored for the more promising of these measures. Finally, in lieu of direct selection for lamb survival, there is merit in selecting for multiple-rearing ability or its equivalent, possibly with additional selection criteria for lamb survival and reproductive efficiency.

Genetic evaluation of crossbred lamb production. 2. Breed and fixed effects for post-weaning growth, carcass, and wool of first-cross lambs

The study evaluated post-weaning growth, carcass characteristics, and wool production of crossbred progeny of 91 sires from more than 7 maternal breeds (including Border Leicester, East Friesian, Finnsheep, Coopworth, White Suffolk, Corriedale, and Booroola Leicester). The sires were joined to Merino and Corriedale ewes at 3 sites over 3 years with 3 link sires in common at each site and year. Post-weaning weight at an average age of 200 days of 2841 ewes and 3027 wethers was analysed using mixed model procedures. The wethers were slaughtered at an average age of 214 days and carcass weight, fat, muscle, meat colour, and ultimate pH (24 h post-slaughter) were analysed. For ewes, hogget fleece weight, yield, fibre diameter, and faecal worm egg count (FEC) were analysed. Sire breed was significant (P < 0.01), with a range of 32.8–39.0 kg for post-weaning weight, 19.1–22.8 kg for hot carcass weight, and for carcass fat levels (11.1–17.2 mm at the GR site (FatGR) and 3.4–6.5 mm at the C site, adjusted to 22 kg carcass weight), with the East Friesian cross carcasses being very lean (FatGR 11.1 mm). Sire breed was significant for eye muscle area (P < 0.01) but not for eye muscle depth, meat colour L* (brightness), or ultimate pH. Sire breed was significant (P < 0.01) for greasy and clean fleece weight (CFW), yield, and fibre diameter (FD), with ranges of 0.9 kg (CFW) and 4.6 μm (FD). Sire breed was not significant for FEC. Type of birth and rearing classification was significant for most traits. National estimated breeding values for the sires that were entered by industry breeders indicated that they covered a range of genetic merit for most traits within their respective breeds and were generally representative of the maternal genetics available in the industry. Where there were large differences between the sire mean and breed mean breeding values the effects on crossbred progeny performance would be small and not expected to affect our conclusions about the breed differences reported. The results provide lamb producers with comparative information on sire breeds for growth, carcass and wool traits. The considerable variation among individual sires within the respective breeds will be reported in later papers.

Genetic evaluation of crossbred lamb production. 3. Growth and carcass performance of second-cross lambs

This study evaluated the growth and carcass performance of second-cross lambs that were the progeny of common terminal sires and a range of first-cross ewes. The first-cross ewes were the progeny of 91 industry sires from several maternal breeds including the Border Leicester, East Friesian, Finnsheep, Coopworth, White Suffolk, Corriedale, and Booroola Leicester breeds. The first-cross ewes were generated at 3 sites over 3 years, and 3 common maternal sires were used at each site and each year to provide genetic links for combined analysis. The 2726 first-cross ewes were mated naturally to common terminal sires for 3 years at each site to quantify the variation in maternal genetics on the performance of 11 341 second-cross cross lambs. The birthweight, weaning weight (at an average age of 100 days) and post-weaning weight (at an average age of 200 days) of the second-cross lambs were analysed using mixed-model procedures. The lambs were slaughtered (n = 8878) at an average target carcass weight of 22 kg and dressing yield, carcass fat, muscle, meat colour, and meat pH were analysed. Crossbred ewe breed (the maternal grand sire breed) was significant for birthweight (range of 0.9 kg), weaning weight (range 4.4 kg) and post-weaning weight (range 4.6 kg). Type of birth and rearing, age of dam, and sex effects were significant for all liveweights. Ewe breed was significant for hot carcass weight, carcass fat levels, and muscle dimensions, but not for meat colour and ultimate meat pH. There was a significant difference between wether and ewe lamb carcasses for most traits, with other fixed effects generally being small when carcass weight was included as a covariate. Lamb carcasses from East Friesian-cross ewes were very lean compared with carcasses from all other ewe breeds (~2 mm lower fat depth at the GR site at 22 kg carcass weight). This comprehensive study of maternal sire breeds provides an opportunity to exploit between-breed variation for some maternal growth and carcass traits. There was, however, considerable overlap among breeds and opportunities exist for additional improvement by exploiting between-maternal sire genetic variance. The role of improved growth and carcass performance in profitability of prime lamb enterprises is discussed.

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

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.

The relationship of lamb growth from birth to weaning and the milk production of their primiparous crossbred dams

The growth rate of 667 single cross lambs from birth to 4 weeks (Period 1) and 4 weeks to weaning at 12 weeks (Period 2) was measured, along with the milk production of their 520 primiparous crossbred ewe dams over 3 years. The lambs were the progeny of Poll Dorset rams and first cross ewes lambing for the first time at 12 and 19 months of age. The first cross ewes were the progeny of Merino ewes mated to 30 individual sires from several maternal sire breeds, including Border Leicester, East Friesian, Finnsheep and Coopworth. Milk production of the ewes was measured at ~3, 4 and 12 weeks of lactation using a 4-h milk test. The effect of ewe milk production on lamb growth rate in Period 1 included a significant interaction with lamb birth and rearing type (P < 0.001). These regressions were 52 ± 18 g/day per kg/day for single born and reared, 24 ± 17 g/day per kg/day for multiple born and single reared and 18 ± 17 g/day per kg/day multiple born and reared lambs. The regression of ewe milk production in Period 2 on lamb growth rate in Period 2 was also significant (P < 0.001), although milk production in Period 1 accounted for more variation in lamb growth. There were significant interactions of ewe weight gain (from mid pregnancy to lamb weaning) with season in Period 1 (P < 0.01) and year in Period 2 (P < 0.001). There was also a significant interaction (P < 0.001) of year with ewe mid-pregnancy weight for lamb growth in Period 2. Lamb birthweight had a significant effect on lamb growth rate in Period 2 (regression coefficient 6.68 ± 2.57 g/day.kg, P < 0.01).

Genetic evaluation of crossbred lamb production. 4. Genetic parameters for first-cross animal performance

The study estimated heritability for lamb growth and carcass performance, hogget ewe wool production, and worm egg count among crossbred progeny of maternal breed sires, as well as the genetic and phenotypic correlations among the traits. The data were from crossbred progeny of 91 sires from maternal breeds including Border Leicester, East Friesian, Finnsheep, Coopworth, White Suffolk, Corriedale, and Booroola Leicester. The sires were mated to Merino ewes at 3 sites over 3 years (and also Corriedale ewes at one site), with 3 common sires used at each site and year to provide genetic links. These sheep comprised part of the national maternal sire central progeny test program (MCPT) to evaluate the genetic variation for economically important production traits in progeny of maternal and dual-purpose (meat and wool) sires and the scope for genetic improvement. The matings resulted in 7846 first-cross lambs born, with 2964 wether lambs slaughtered at an average age of 214 days, and wool data from 2795 hogget ewes. Data were analysed using univariate mixed models containing fixed effects for site, year, sex and type of birth and rearing, dam source and sire breed, and random terms for sire and dam effects. Heritabilities and genetic correlations were estimated based on variances from progeny of 70 sires by fitting the same mixed models using a REML procedure in univariate and multivariate analyses. Estimates of heritability were low for lamb growth traits (0.07–0.29), meat colour and meat pH (0.10–0.23), and faecal worm egg count (0.10), moderate for carcass fat and muscle traits (0.32–0.47), and moderate to high for wool traits (0.36–0.55). Estimates of direct genetic correlations among liveweights at various ages were high and positive (0.41–0.77) and those between liveweights and most carcass and meat quality traits were small and varied in sign. Liveweights were moderately to highly positively correlated with most wool traits, except fibre diameter (–0.28–0.08). The study indicates that there is genetic variation for wool, growth, carcass, and meat quality traits, as well as for faecal worm egg count, with scope for selection within Australian maternal sire breeds of sheep.