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Context: Diarrhoea linked to helminth resistance is a major problem in sheep health. Aims: To test whether the distribution of mast cells and eosinophils along the gastrointestinal tract is related to helminth resistance and to susceptibility to diarrhoea. Methods: At weaning, lambs were selected according to extreme (high or low) estimated breeding values for diarrhoea score (50 lambs per genotype, balanced for sex), and for faecal worm egg counts (FWEC). They grazed in single-sex groups in separate but similar enclosures under normal farm conditions (dry summer, wet winter) from autumn until spring, during which time diarrhoea score and FWEC were recorded monthly. Lambs that presented extreme phenotypic values for diarrhoea score and FWEC in August and September were slaughtered (n = 38) at the age of 14 months. For the abomasum, duodenum, ileum, jejunum, caecum and colon, we recorded the densities of eosinophils and mast cells, the number of helminths, their species and developmental stages. The data were pooled across enclosures/sexes and regression analysis was used to assess the relationships among pre-slaughter phenotype, helminth counts, and densities of mast cells and eosinophils. Results: Eosinophil density was five- to seven-fold greater in the ileum and jejunum than in the abomasum, duodenum, or colon. Mast cell density was greatest in the abomasum and duodenum, and gradually declined along the jejunum, ileum, caecum and colon. In the most immunologically active site, the jejunum, increasing eosinophil density was related to a decrease in the number of helminths (P < 0.05). Similarly, in the next most active site, the abomasum, increasing eosinophil density was related to decreases in the numbers of fourth-stage larvae and the total number of helminths (P < 0.05). In the abomasum, increasing mast cell density was related to declines in the numbers of Teladorsagia circumcincta , total helminth counts (P < 0.05) and FWEC, but was not related to diarrhoea score. Conclusions: Cell-mediated immune mechanisms in the gastrointestinal mucosa enhance resistance to internal parasites but also increase susceptibility to the diarrhoea in sheep. Implications: Eosinophil and mast cell responses to helminth infection help explain 'hypersensitivity diarrhoea'. In sheep, the severity of helminth infection is related to eosinophil density in the mucosa of the abomasum and jejunum, the most immunologically active sites in the gastro-intestinal tract. The severity of helminth infection is also related to mast cell density in the mucosa of the duodenum. Eosinophil and mast cell responses to helminth infection help explain 'hypersensitivity diarrhoea' in helminth-resistant sheep. [ABSTRACT FROM AUTHOR]

Context: Breech strike is a serious disease particularly in wool sheep. Skin wrinkle and dags are known predisposing factors for breech strike, although a large part of the variation between sheep is unexplained. Aims: We used sensor buttons to determine whether there were differences in humidity and temperature in the breech area of Merino sheep exposed to blowflies without traditional preventive measures being used in the flock. Methods: Sixty-two Merino rams and 62 Merino ewes that had high or low breeding values for breech strike were each fitted with a sensor button to record hourly temperature and humidity in the breech over 49 days. The sensors were fitted before the onset of the blowfly season. The sensor was tied to a wool staple close to the skin in the breech and was removed when a sheep was struck. All remaining buttons were removed prior to shearing. Breech wrinkle, neck wrinkle, breech cover and faecal soiling (dags) were recorded on all the sheep. A total of 98 213 hourly temperature and humidity records were analysed with cubic smoothing splines to assess how humidity and temperature differed between struck and unstruck sheep, and whether the breech indicator traits affect temperature and humidity in the breech. Results: Time of day and day-to-day variation prior to being struck explained most of the variation of temperature and humidity in the breech of the sheep. Humidity and temperature increased with an increase in breech cover. The humidity in the breech area was, on average, 2% higher in struck sheep prior to being struck. There was a sharp increase in humidity in struck sheep in the 8 days leading up to detection of breech strike relative to unstruck animals, most probably due to exudate from the wound. Conclusions: Sheep prone to be struck have higher humidity in the breech than sheep less prone to be struck. Implications: Temperature in the breech does not qualify as a potential indicator trait for breech strike in wool sheep, whereas humidity needs further research to determine its effectiveness as a potential indicator trait. Sheep were fitted with sensors to record hourly temperature and humidity in the breech over 49 days before the onset of the blowfly season. Sheep susceptible to breech strike had higher humidity in the breech than resistant sheep. Further studies needs to be undertaken to determine whether humidity may qualify as a potential indirect indicator trait to differentiate between sheep resistant and susceptible for breech strike. [ABSTRACT FROM AUTHOR]

The peri-parturient period is crucial for controlling worms as the acquired immunity of ewes is disrupted, resulting in an increase in faecal worm egg counts. Two hypotheses were tested in this experiment — that ewes bred for worm resistance would have lower faecal worm egg counts than unselected control ewes, during late pregnancy and lactation, under similar but separate grazing areas; and also that numbers of infective nematode larvae would be lower on pastures grazed by resistant ewes than pastures grazed by unselected control ewes. Faecal samples were collected from resistant and unselected ewes in late pregnancy and early lactation, during the winter rainfall season, and analysed for numbers of Trichostrongylus colubriformis and Teladorsagia circumcincta. Pasture samples were taken 1 week before and 7 weeks after lambing started and analysed for infective larvae. In all sheep, worm egg counts rose 2 weeks prior to lambing and continued into lactation. Worm egg counts were significantly lower in the resistant ewes from 1 week before lambing to 2 weeks after lambing. There were no differences in egg counts between single- and twin-bearing ewes in the resistant line. However, twin-bearing control ewes had significantly higher egg counts than single-bearing control ewes. Following lactation, plots grazed by resistant ewes had substantially less contamination with T. colubriformis larvae, but there were no differences in numbers of T. circumcincta larvae. Our results demonstrate that sheep bred for worm resistance has lower worm burdens during the peri-parturient phase and that lambs born to resistant ewes face a lower larval challenge during their introduction to grazing. In our environment, selection for low worm egg counts has produced sheep highly resistant to T. colubriformis, but has had less impact on resistance towards T. circumcincta. [ABSTRACT FROM AUTHOR]

Genetic parameters for carcass and meat quality traits of about 18-month-old Merino rams (n = 5870), the progeny of 543 sires from three research resource flocks, were estimated. The estimates of heritability for hot carcass weight (HCW) and the various fat and muscle dimension measurements were moderate and ranged from 0.20 to 0.37. The brightness of meat (colour L*, 0.18 ± 0.03 standard error) and meat pH (0.22 ± 0.03) also had moderate estimates of heritability, although meat relative redness (colour a*, 0.10 ± 0.03) and relative yellowness (colour b*, 0.10 ± 0.03) were lower. Heritability estimates for live weights were moderate and ranged from 0.29 to 0.41 with significant permanent maternal environmental effects (0.13 to 0.10). The heritability estimates for the hogget wool traits were moderate to high and ranged from 0.27 to 0.60. The ultrasound measurements of fat depth (FATUS) and eye muscle depth (EMDUS) on live animals were highly genetically correlated with the corresponding carcass measurements (0.69 ± 0.09 FATC and 0.77 ± 0.07 EMD). Carcass tissue depth (FATGR) had moderate to low genetic correlations with carcass muscle measurements [0.18 ± 0.10 EMD and 0.05 ± 0.10 eye muscle area (EMA)], while those with FATC were negative. The genetic correlation between EMD and eye muscle width (EMW) was 0.41 ± 0.08, while EMA was highly correlated with EMD (0.89 ± 0.0) and EMW (0.78 ± 0.04). The genetic correlations for muscle colour with muscle measurements were moderately negative, while those with fat measurements were close to zero. Meat pH was positively correlated with muscle measurements (0.14 to 0.17) and negatively correlated with fat measurements (−0.06 to −0.18). EMDUS also showed a similar pattern of correlations to EMD with meat quality indicator traits, although FATUS had positive correlations with these traits which were generally smaller than their standard error. The genetic correlations among the meat colour traits were high and positive while those with meat pH were high and negative, which were all in the favourable direction. Generally, phenotypic correlations were similar or slightly lower than the corresponding genetic correlations. There were generally small to moderate negative genetic correlations between clean fleece weight (CFW) and carcass fat traits while those with muscle traits were close to zero. As the Merino is already a relatively lean breed, this implies that particular attention should be given to this relationship in Merino breeding programmes to prevent the reduction of fat reserves as a correlated response to selection for increased fleece weight. The ultrasound scan traits generally showed a similar pattern to the corresponding carcass fat and muscle traits. There was a small unfavourable genetic correlation between CFW and meat pH (0.19 ± 0.07). [ABSTRACT FROM AUTHOR]

Data from seven research resource flocks across Australia were combined to provide accurate estimates of genetic correlations among production traits in Merino sheep. The flocks represented contemporary Australian Merino fine, medium and broad wool strains over the past 30 years. Over 110 000 records were available for analysis for each of the major wool traits, and 50 000 records for reproduction and growth traits with over 2700 sires and 25 000 dams. Individual models developed from the single trait analyses were extended to the various combinations of two-trait models to obtain genetic correlations among six wool traits [clean fleece weight (CFW), greasy fleece weight, fibre diameter (FD), yield, coefficient of variation of fibre diameter and standard deviation of fibre diameter], four growth traits [birth weight, weaning weight, yearling weight (YWT), and hogget weight] and four reproduction traits [fertility, litter size, lambs born per ewe joined, lambs weaned per ewe joined (LW/EJ)]. This study has provided for the first time a comprehensive matrix of genetic correlations among these 14 wool, growth and reproduction traits. The large size of the data set has also provided estimates with very low standard errors. A moderate positive genetic correlation was observed between CFW and FD (0.29 ± 0.02). YWT was positively correlated with CFW (0.23 ± 0.04), FD (0.17 ± 0.04) and LWEJ (0.58 ± 0.06), while LW/EJ was negatively correlated with CFW (−0.26 ± 0.05) and positively correlated with FD (0.06 ± 0.04) and LS (0.68 ± 0.04). These genetic correlations, together with the estimates of heritability and other parameters provide the basis for more accurate prediction of outcomes in complex sheep-breeding programmes designed to improve several traits. [ABSTRACT FROM AUTHOR]

Resistance of sheep worms against the available anthelmintic drenches is increasing. Breeding sheep for resistance against gastrointestinal parasites is the only long-term solution. Research has shown that breeding for worm resistance is a highly feasible option and that genetic gains of more than 2% per year can be achieved without loss of production. Replicated field trials in which resistant animals were managed separately from unselected control sheep showed that there was a 10-fold difference in worm egg counts between resistant and control ewes. Resistant ewes were 18% heavier than the controls at the time of peak worm-challenge in a Mediterranean environment. At weaning, there was a five-fold difference in worm egg counts between resistant and control lambs, which resulted in an improvement of weaning weight of up to 22%. It is therefore better to include selection for worm resistance in breeding programs than to omit it for fear of diminishing future genetic gains in other traits. [ABSTRACT FROM AUTHOR]

Wool and meat contribute to profit in sheep enterprises and both need to be considered in breeding programs. The relative responses expected from selection for a range of traits are presented and the realised responses that have been achieved in Merinos and variation in maternal breeds are illustrated. Knowledge of genetic parameters is required for the development of complex breeding objectives and selection indexes, comprehensive genetic evaluation of animals and the design of effective breeding programs. A review of world literature has highlighted the lack of accurate estimates of genetic parameters, especially for genetic correlations between trait groups. Analyses of a combined dataset from seven Australian Merino resource flocks comprising over 2000 sires and up to 100,000 records for each of various traits have provided accurate estimates of parameters to fill these gaps in current knowledge. The results show that there are no major genetic antagonisms between wool and meat traits and that improvement of both can be achieved by using appropriate selection indexes. Sheep Genetics Australia now provides a common system for genetic evaluation of Australian sheep, including across-flock estimated breeding values for a comprehensive range of traits and several standard indexes for various wool and meat breeding objectives. [ABSTRACT FROM AUTHOR]