factors affecting on longevity in northern khorasan kordish sheep

introduction the longevity is normally defined as the length of its productive life in the flock, which is the amount of time an animal spends producing (1). longevity reflects the ability of ewe to avoid being culled for low production, low fertility, illness and influences the number and cost of replacements required to maintain the flock size. the benefit of increasing longevity are increased average age of the flock, having more ewes available for sale at the end of their four parity, having more ewe lambs to sell, and higher productivity from a slightly older flock age profile (2). sheep population of khorasan province (10). in sheep production, longevity has an important influence on the economic returns. reliable estimates of non genetic effects of longevity are needed to aid establishing an efficient strategy for improving ewe productivity. thus, the objective of this study was the factors affecting on longevity in northern khorasan kordish sheep. we want to determine effect of environmental and non-genetic factors on longevity. material and methods in order to investigate on factors affecting longevity trait in northen khorasan kordish sheep, records of 7469 sheep (187 sire and 2258 dam) ) between 1990 to 2012 that were collected by breeding station of hossein abad in shirvan (this city is located in the north of mashhad) were used. flock has been kept under village system. breeding ewes were identified in the data set as those that lambed at 2 year of age and culled at 6 to 7 year of age duo to oldness. ram kept until a male offspring was available for replacement. in this study longevity was defined as the age of a ewe (in day) when it leaves the flock. ewes were generally removed from the flock due to poor production, low fertility, death and illness. ewes were identified as being removed from the flock if a lambing record was present at n year of age but not at n+1 years of age. all females were culled before reaching 7 years of age. conditional stayability measures were recorded as binary indicators of a lambing record at a given age (failure=1, success=0). overall stayability traits (stay n/2) were defined as the probability that a ewe that lambed at 2 year of age also lambed at n year of age for n equal to 3, 4, 5, 6, and 7 year of age. data for overall stayability only included individuals that had opportunity to have a lambing record at n year of age. marginal stayability traits (stay n/n-1) were defined as the probability that a ewe that lambed at n-1 year of age also lambed at n year of age for n equal to 3, 4, 5, 6, and 7 years of age. marginal stayability was missing for ewes without lambing records at n-1 years of age. also, longevity of ewes was calculated according to lambing date and culling date in flock. so, culling reasons are divided in to two groups: involuntary and voluntary culling. involuntary culling includes death, barren ewes and deficiency, and voluntary culling includes extension sale, excess ewes and ewe with inappropriate phenotype. the fixed effect included year of production, sex and birth type. the fixed effects to be considered was made after were when significant. tables of productive life were also determined table procedure of sas 9.1. result and discussion according to the results, 99% of productive ewes were culled due to the culling reasons but not due to their ages. the highest culling rate in ewes was due to disease (33.1%) and the lowest culling rate in rams was poisoning (0.11%). the disease had the highest culling rate in twins (33.5%). the highest percentage of involuntary culling was observed in barren ewes (1.65%). in voluntary culling, the highest percentage of culling was due to sales (55.75%). the effect of year and season of birth was significant on productive life. mean comparison test showed that ewes lambing in winter and spring had the highest and lowest of productive life, respectively (p<0.05). also the number of removed ewes from the flock decreased when increased age of ewes, but culling rate in ewes that have more than four ages. survival of ewes with having single type were more than ewes with twin type. ewes with one lamb had more survival) 1%( compared to ewes with 2 and 3 lambs (0.7-0.9%). results show that ewes with single type in all of lambing had the most percentage of longevity, while ewes with 2and 3 lambs showed the less survival. however the general policy trend to increased twin type, but according to the results, emphasis on twin type removed ewes from the flock in low age and could be decrease longevity in order to improving longevity, the management and environmental conditions should be improved. conclusion illness is the most important reason that cause to ewe left the flock to improve longevity. thus, in order to decrease the illness, it should be improve the environmental factors (health, feeding, management, poisoning) and genetic resistance to disease.