عنوان مقاله [English]
Introduction Milk solid no-fat is economically very important in cheese industry. Compared to the other kinds of milk, ewe’s milk contains higher amount of milk solids no-fat. Milk solids no-fat (MSNF) contains lactose, caseins, whey proteins, and minerals.
The use of test day records in random regression method has several benefits including flexibility to account for the environmental and genetic components of the shape of lactation, reducing generation interval and cost of recording by making fewer measurements, increasing the accuracy of genetic evaluation and direct correction for fixed effects. Therefore, the objective of the present study was to estimate genetic parameters for test-day milk solid no-fat percentage in Kurdi sheep of Shirvan using fixed and random regression models.
Materials and methods In the present investigation, genetic analysis of milk solid no-fat percentage was carried out using fixed and random regression models by Wombat software. Data included 1094 test day records of milk solid no-fat percentage collected from 250 ewes in Hossien Abad Kurdi sheep breeding station. Milking was carried out by hand milking combined with lamb suckling at 14 days interval starting from May to August 2012. Then, 50 ml of milk samples were immediately analysed by Ecomilk total to determine the milk solid no-fat percentage. Fixed effects of litter size, parity, month of recording and days in milk as covariate and random effects of direct genetic and permanent environmental effects were included in the models. General linear model was used to identify effective fixed effects on the trait by SAS 9.1 software. Variance and covariance components were estimated using restricted maximum likelihood procedure. In random regression model, orthogonal Legendre polynomials of order 2 for permanent environmental and additive genetic effects was fitted.
Results and Discussion Average milk solid no-fat percentage of Kurdi ewes was 11.83. Average heritability, repeatability, additive genetic variance, permanent environmental variance, phenotypic variance and residual variance of milk solid no-fat percentage were estimated as 0.06, 0.26, 0.029, 0.094, 0.0471, and 0.0348, respectively in fixed regression model. In a study on northern Thailand dairy cows, heritability, additive genetic variance and residual variance estimates were 0.133, 0.36, and 0.238, respectively. In random regression model, heritability of milk solid no-fat percentage was higher in second than first part of lactation period. The highest and the lowest heritability were estimated at 14 (0.068) and 112 (0.193) days, respectively. Repeatability of milk solid no-fat percentage was higher in second than first part of lactation period. The highest and the lowest repeatability were estimated at 14 (0.279) and 126 (0.732) days, respectively. In a study on milk solid no-fat percentage in Holstein cow in Korea, the range of heritabilities were from 0.305 to 0.489 and higher estimates of heritability for milk solid non fat percentage occurred in second half of lactation. Additive genetic variance in early lactation was lower than that of the end of lactation. The highest and the lowest additive genetic variance were estimated at 5 (0.013) and 126 (0.143) days, respectively. Permanent environmental variance was estimated higher in second than first part of lactation period and the highest and the lowest permanent environmental variances were estimated at 21 (0.05) and 126 (0.39) days, respectively. Phenotypic variance of milk solid no-fat percentage during lactation was not constant, the highest and the lowest permanent environmental variances were estimated at 5 (0.27) and 126 (0.79) days, respectively. Residual variance for the model considered to be constant for all days in milk (0.191). Genetic correlations between milk solid no-fat percentages at different days in milk were positive and ranged from 0.391 to 0.999. Permanent environmental correlation between Milk solid no-fat percentage at different days in milk were ranged from -0.343 to 0.999. Milk solid non fat percentage additive genetic and permanent environmental correlations between adjacent test days were more than between distant test days.
Conclusion Generally, results indicated that Shirvan Kurdi ewes had high milk solid no-fat 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.
Acknowledgements The authors would like to thank the devoted team of Hossien Abad Kurdi sheep breeding station for their continuing collaboration in milk recording.