Survival Analysis of Makuie Sheep Breed’s Lambs Using Proportional Hazard Functions

Introduction: The ability of an animal to survive in the breeding period is called the survival trait. One of the most important features of survival analysis is unfinished (censored) observations. In other words, the event under study does not necessarily occur at the time of recording the observations. Thus, survival observations and data are generally incomplete, and events occur for some people but for others there is no observation for that event. In general, nonparametric regression methods (Kaplan-Meier estimator, Cox hazard ratio model) and parametric methods are used for this type of analysis more than other methods. Survival and longevity analysis is performed by linear, threshold and hazard ratio functions methods. Linear models are easier to use, but due to the nonlinearity of the survival trait, they are skewed, and the estimates of heritability with this method are low and close to zero. Threshold models are more accurate and obtain higher genetic estimates. In the analysis of hazard ratio method, the accuracy is higher than previous models due using the more information (use of censored data), but it is time consuming and its computational load is higher. Therefore, the aim of the present study was to estimate the genetic parameters and the effect of fixed factors on the survival of Makuie sheep lambs up to one and two years of age using the proportional hazard function under Weibull distribution.
Material and methods: In this study, 2332 records of Makuie lambs that collected between 1986 to 2010 in Makuie sheep breeding station, were used. In the first step, fixed factors affecting longevity (gender, type of birth, year of birth, month of birth, birth weight, weaning weight, weaning age and age of dam) Was investigated by LIFEREG procedure of SAS statistical software version 9.4 under Weibull distribution. Then significant effects were used in the survival analysis with the proportional hazard model. Survival up to one and two years of age were analyzed using the Weibull hazard model by SURVIVAL KIT software, version 6.12. SURVIVAL KIT software is able to provide components of genetic variance and can estimates breeding values for animals using mixed models. Survival from post-weaning to one year (for analysis up to one year of age) and two years (for analysis up to two years of age) were assessed by day.
Results and discussions: In the analysis of survival from weaning to the one year of age, 38.12% of observations and in the study of survival up to the two years of age, 13.68% of the observations were censored. According to the study, the year of birth had a significant effect on the survival of lambs. The year of birth affects the amount of rainfall and ultimately the availability of food resources by affecting the pasture, and thus changes the quality and amount of nutrition. survival up to one year of age Lambs born in 2004 and for survival up to two years of age lambs born in 1994 had the lowest risk of elimination. Lambs born in 2010 had the highest risk of elimination. Scrutiny the effect of birth month on the survival of lambs, it was found that for survival up to one year of age, lambs born in April had a higher risk of elimination. However, for survival up to two years of age, lambs born in May had the highest risk of elimination and the amount of risk for May compared to March was 1.96. Lambs born in May reach autumn at a young age and during a period of rapid growth. Because the studied flock is kept semi-densely, access to feed for these lambs is reduced and they are removed from the flock due to their low weight and insufficient growth rate. Birth weight is one of the most important components affecting the survival rate and directly affects the elimination rate. Threshold weights have the highest elimination risk ratio and the weight of 3.5-4.7 kg has the lowest elimination risk ratio for survival up to one and two years of age. There is a curved relationship between elimination risk and birth weight, so with increasing birth weight to medium weight (3.5-4.5 kg), the risk ratio decreases and then increases. Mortality at low weights is due to the inability to control body temperature and high mortality at high birth weights is due to infertility. Weaning weight is an important factor influencing survival after weaning. Similar to birth weight, weaning weight has a curved relationship with survival. So that with increasing the weaning weight from 10 kg to 19 kg, the risk of elimination for survival to one and two years of age decreases from 1.8 to 0.97 and 0.93, respectively, and then with increasing the weaning weight, risk ratio reaches to 2.53 and 2.38, respectively. Lack of growth-induced production encourages the elimination of low-weight weaned lambs. So the elimination risk ratio is high at low weights. But as the weaning weight increases, the desire to keep it for later periods will be greater, and thus the removal pressure will be lower. The average weight of weaning in the herd is equal to 19.01 kg, the results also show that the lowest elimination risk ratio and consequently the highest survival for weaning weight is 18-20 kg. Therefore, in order to increase the survival and reduce the elimination risk ratio, management and corrective programs can be designed to select the optimal weaning weight. For survival up to one and two years of age, the risk of elimination for male lambs was higher than females. In the period between weaning until the age of one year, the elimination risk ratio for male lambs was 2.17 relation to females and for survival to the age of two years, the elimination risk ratio of males was 2.61 Compared to females. This difference in elimination rate can be attributed to sex-related factors, sex characteristics that may not have yet been discovered, systemic diseases, different management practices for males and females, and high elimination rate for males due to surplus fattening. There are three types of births in Makuie sheep (singles, twins and triplets). Lambs born with twins and triplets have lower birth weight and weaning weight than single lambs. Due to the above topics, low birth weight and low weaning weight, reduces the survival and, increase the risk of elimination. Another reason for the reduced survival of multiple births is the limitation in dam milk production, which can be due to poor maternal genetics or inadequate nutritional intake. Heritability of survival to one and two years of age for Makuie sheep lambs according to the censorship ratio of 38.12% and 13.68% (respectively) was 0.062 and 0.079, respectively.
Conclusions: Since heritability was estimated at a low level, control of environmental factors and management will play an important role in improving the survival of Makuie lambs.