Genetic analysis of milk protein percentage using random regression model in Shirvan Kurdi sheep

The present study was conducted on Kurdi ewes of Hossein Abad Sheep Breeding Station of Shirvan. In order to estimate genetic parameters, 1065 test day records of milk protein percentage from 245 ewes were collected. Data collection was started from April 2012 up to the end of lactation period. For genetic analysis of data, test day models with random regression coefficient were utilized. In random regression model, parity, litter size, month of recording as fixed effects and age at recording and days in milk as covariates were included. Random regression test day model was carried out using WOMBAT software. Genetic and environmental (co)variance components of milk protein percentage were estimated using restricted maximum likelihood. Orthogonal Legendre polynomials with different orders were applied for taking into account of genetic and permanent environmental effects. Legendre polynomial with the order of 2 was the best fitting model.In this research, the average milk protein percentage in Kurdi ewes was 6.69±0.70. The lowest and highest additive genetic variances were at the beginning) 0.058) and the end (0.114) of lactation, respectively. Phenotypic and permanent environmental variances were high at the beginning and at the end of lactation. Estimates of heritability increased over the lactation. Minimum and maximum heritabilities were estimated at the beginning (0.096) and end (0.178) of lactation, respectively. The higher the distance between days of lactation, the less were the genetic and permanent environmental correlations. Generally, results indicated that Shirvan Kordi ewes had high milk protein production potential. Moreover, higher heritability for this trait at the end of lactation probably indicated higher contribution of additive genetic variation to the total variation and selection of ewes in this part of lactation would be preferred.