Study of variations, prediction of variance components and genetic parameters of survival distribution function in Kermani ewes

The Kermani sheep breed is one of the most important native breeds in Iran. It is well adapted to the harsh and undesirable environmental conditions of the southeastern part of the country. Therefore, consideration to the breeding of Kermani sheep and the improvement of the environmental and genetic conditions of this breed can contributed greatly to providing a part of the livelihood needs of the people. The longevity in the ewe is a high economic trait, because increasing the longevity will lead to a reduction in the culling rate as well as replacement costs in the herd and increasing of number of lambs. Considering that the investigation and identification of the important factors influence culling of ewes from the herd, which leads to a decrease in the yield and longevity of the animal, can be used to provide and develop a suitable strategy to promote the survival and longevity of the ewes. In addition, due to the absence of genetic parameters of production life length in Kermani sheep, the aim of this study was to study changes, prediction of components of variance and genetic parameters of survival distribution function in Kermani ewes.

In this research, longevity records of 772 heads of Kermani ewes (collected from 1989 to 2008) at the Kermani sheep station (Shahr Babak, Kerman province) were studied. The overall production life length was equal to the time between births to ewe culling. Frequency distribution for causes of culling and survival distribution function for ewes were estimated using R software. The Weibull model and Matvec software were also used for estimating variance component and genetic parameters of longevity traits as survival.

The results showed that the mean of ewe’s longevity was 4.72 years with average lambing of 3.4 in life time. The most important cause of culling was disease (53%), especially in the first three parity of the ewe. After disease, oldness, reproductive problems, low production, physical phenomenon, experimental slaughter, selling additional ewes and bad type (0.5%) are the accounted causes for remaining ewe culling, respectively. Considering that culling due to disease accounts for more than half of the deaths, prevention and treatment of diseases are important in this breed. Therefore, the type and timing of the prevalence should be investigated and preventive and therapeutic proceeding should be taken for diseases. The total amount of non-optional culling (illness, physical incidence, and aging) was about 83% of total casualties. It can be concluded that the deletion due to the low production was very small. At the age of two and three, the mortality rate was low, reaching its peak at the age of four, has declined since this age, with partial fluctuations between the ages of 5 and 6, and at the age of 7 and 8 years decline have happened. Therefore, most ewes were culled from the herd before reaching the age of 8. Most culling has occurred in pre-maturity ages, so it should be taken into consideration because it indicates that the ewes were eliminated before they reach the peak of production.The conditional culling probability increases with age, while cumulative survival rate decreased with age of ewe. The hazard ratio of death increased with age up to 6 years of old steadily and then increased strongly, and reached two at 7 to 8 years of old. The high hazard ratio indicated an early death or short longevity. Normally, inverse of this ratio expresses useful information. The reversal of this ratio at a certain time interval, provided that the hazard level remains constant at this interval, will indicate the time it takes to occur (death). For example, at a time interval of 3 to 4 years, the ewe's longevity will be equal to a reversal of 0.5298, equivalent to two years. But, for 7 to 8 years old, the reciprocal of two will be 0.5 years that indicating if the dangers remain constant at this interval, the ewe's life will be only 6 months, and ewes remaining in this group will be lost after 6 months. Trend of survival and hazard ratio at different ages of ewes can be provided as a model to breeders of this breed. Heritability on the logarithmic scale, the original scale and effective heritability of the risk ratio obtained from the sire model with the Weibull function was predicted at low to high (0.002-0.145). Although heritability estimates for longevity are low, but genetic improvement may be increased by selecting animals with higher inbreeding values.

The results of this study showed that the most effective factor in shortening the longevity and decreasing survival in Kermani ewes is culling due to disease, especially in pre-maturity. Therefore, in order to improve the longevity, necessary studies and necessary preventive should be taken regarding the type and timing of the disease. Genetically, if possible, it would seek to create genetic resistance to diseases and environmental conditions. On the other hand, the results of models with the Weibull function indicate that in order to improve the survival rate and longevity, in addition to improving the non-genetic factors, the selection should be considered within the breed. Also, high heritability traits with high genetic correlation with longevity traits are identified and indirect selection be done based on them.