تأثير ناهمگني اجزاي واريانس بر ارزيابي ژنتيكي مقدار پروتئين شير گاوهاي نر و ماده برتر هلشتاين

The effect of heterogeneity of variance components on genetic evaluation of protein yield in Holstein top sires and dams

هدف از مطالعه حاضر، بررسي ناهمگني اجزاي واريانس مقدار پروتئين شير در سه شكم زايش اول گاوهاي هلشتاين ايران با استفاده از ركوردهاي جمع‌آوري شده توسط مركز اصلاح نژاد و بهبود توليدات دامي طي سال‌هاي 1362 تا 1392بود. مجموعه داده‌ها به ترتيب شامل 141670، 115395 و 82529 ركورد در دوره‌هاي شيردهي اول، دوم و سوم بودند. داده‌ها با استفاده از روش دومارتن به سه دسته مديترانه‌اي، خشك بياباني و مرطوب دسته‌بندي شدند. ناهمگني واريانس‌ها با آزمون بارتلت بررسي شد. نتايج آزمون بارتلت قبل از تبديل داده‌ها در سه دوره شيردهي معني‌دار (0/0001

The objective of this research was to study the heterogeneity of variance components for protein yield from the first three lactations of Iranian Holstein cows which collected from 1983 to 2014 by the Animal Breeding Center and Promotion of Animal Products of Iran. Data set were 141670, 115395 and 82529 records in the first, second and third lactations. Data were classified into three categories of Mediteranean, Dry desert and Humid using Domarten method. The heterogeneity of vari ances was studied using the Bartlett’s test. The results of Bartlett’s test before data transformation in the first three lactations were significant (P<0.0001) which indicated the heterogeneity of variance in all three climatical regions. In order to redu ce or removing the heterogeneity of variances, transformation methods of Logarithmic, Square root and Box -Cox were used which the Square root transformation method caused homogenous variance for protein yield in the third lactation. Genetic parameters, her itability and variance components were estimated by VCE software. The model was included additive genetic effects of animal, herd -y e a r -season and age at calving. Heritability estimates varied from 0.15 to 0.19 and 0.12 -0.20 for untransformed and transforme d data, respectively. Square root transformation method caused 6.88 and 42.66% of top sires and dams excluded from the usual list, respectively. The results of this study indicated that variance components of milk protein yield in Holstein cows are heterog eneous. Therefore, it is suggested effects of heterogeneity of variance should be considered during genetic evaluation and selection of superior sires and dams.