تولید شیر

This research was carried out to investigate the heterogeneity of milk yield variance components in different production levels of holstein cattles. The first lactation milk yield records of 95945 Holstein cattles, which had calved in 651 herds through years 1991 to 2000, were used in this research. Data was collected by the Animal Breeding Center of Iran and adjusted for two-time milking per day. Records were classified into three classes: low (less than 5275 kg), average (5275-6874 kg), and high (more than 6874 kg) based on the level of production. Variance components and genetic parameters were estimated using single and three-trait animal models for each class and three classes with Derivative Free Restricted Maximum Likelihood methods (DF-REML). Bartlett test showed significant differences for variance components between classes. Milk yield heritability was estimated as 0.205, 0.276, and 0.196 for low, average, and high class, respectively, in both single and three-trait analysis. The gentic correlation coefficient between classes ranged from 0.80 to 0.85, Phenotypic correlation coefficients between classes were low and ranged from 0.17 to 0.19. The highest rank correlation coefficient between estimated breeding value by single and three-trait model was obtained in average class. The average changes in the rank of common animals in the top 1% of cows were greather than the top 5% of bulls. Adjustment for heterogeneity of variance components suggested selecting more top cows from herds with high level of production.

Using a deterministic model for dairy herd according to milk pricing system in Iran, the economic weights of milk production, fat percentage, protein percentage and herd life with minimized cost were identified, while restricted total input and total costs were variable in animal unit. By data simulation and the data obtained from 3 different dairy herds in Iran, the sensitivity of economic values with the assumption of 20% change in forage and concentrate price, forage to concentrate ratio, feed-cost, non-feed cost, base price of milk, free-market price of milk, average milk production, fat percentage, protein percentage and herd life were estimated. In the base condition, the economic weights of milk production, fat and protein percentage and herd life in three herds were -0.55, -0.24, -0.54 -34, -20, -11 -34, -37, -33 -0.37, 0.67, -0.39, respectively. The effect of change in different factors of production system on economic weights of three farms was variable. The economic weights of milk production, fat percentage, protein percentage and herd life had the most sensitivity to change in the average milk productoin, base price of milk and herd life, respectively.