بررسي عوامل مؤثر بر سلول‌هاي بدني شير و تخمين فراسنجه‌هاي آن در گاوهاي شيري هلشتاين استان خراسان رضوي به روش رگرسيون تصادفي

Factors Affecting Milk Somatic Cell Counts and Its Parameter Estimation in Holstein of Khorasan Razavi Province by Random Regression

هدف این تحقیق بررسی عوامل مؤثر بر صفت تعداد سلول‌های بدنی شیر و تخمین وراثت‌پذیری و همبستگی‌های آن با سایر صفات تولیدی بود. برای این منظور از اطلاعات 185688 ركورد روز آزمون تولید شیر و شجره گاوهای 54 گله استان خراسان رضوی استفاده شد. برای نرمال كردن ركوردهای صفت تعداد سلول‌های بدنی از آنها لگاریتم طبیعی گرفته شده و گاوهایی در برآورد مولفه‌های واریانس استفاده شدند كه حداقل 1 و حداكثر 10 شكم زایش داشتند. اثرات محیطی در مدل آماری شامل اثر گله × سال ركوردگیری (سطح 247)، نوبت ماهیانه شیردهی (15 سطح)، دوره شیردهی (10سطح)، ماه ركوردگیری (12سطح)، سال (9 سطح) و فصل زایش (4 سطح)، مقدار تولید شیر و توان دوم آن بودند. وراثت‌پذیری تعداد سلول‌های بدنی دوره اول شیردهی با استفاده از مدل روزآزمون و رگرسیون تصادفی و همبستگی‌های بین صفات با مدل دو صفته نرم افزار Wombat برآورد شدند. اثرات ثابت مدل شامل گله × سال ركوردگیری، درصد خلوص خونی هلشتاین و ماه شیردهی به‌عنوان كوواریت و اثر ژنتیك افزایشی حیوان و محیط دایمی مشترك با چندجمله‌ای لژاندر درجه 5 به عنوان اثرات تصادفی بود. تمام عوامل ثابت مدل اول در سطح 1 درصد معنی‌دار بودند. با پیشرفت مراحل شیردهی تعداد سلول‌های بدنی افزایش یافت. كمترین تعداد سلول‌های بدنی در فصل ركوردگیری پاییز و یا در گاوهایی كه در پاییز زایش كرده بودند، مشاهده شد. همچنین به نظر می‌رسد كه افزایش سن عاملی برای افزایش تعداد سلول‌های بدنی باشد. مقادیر وراثت‌پذیری این صفت از 043/0 تا 136/0 متغیر بود. كمترین میزان همبستگی سلول‌های بدنی با تولید شیر (011/0-) و بیشترین آن با درصد پروتئین شیر(442/0) بود.

Introduction Mastitis is recognized as one of the most costly diseases of the dairy industry. Somatic cell count (SCC) is a widely used indicator trait for mastitis, which in many countries is used as an indirect selection criterion for improving mastitis resistance. This is based on the positive genetic correlation between clinical mastitis (CM) and SCC. Recent estimates of the genetic correlation between mastitis and SCC range between 0.53 and 0.77 and the heritability is estimated between 0.05 and 0.31. Individual test day SCC are influenced by systematic environmental effects, such as stage of lactation, lactation number, season of calving, and season of recording. The aim of this study was to investigate factors affecting milk somatic cell score (SCC) and estimation of its heritability and correlation with the other productive traits. Materials and Methods A total of 185688 test day records of production and pedigree from 54 herds in Khorasan Razavi State were used. To normalize SCC, log transformation of natural records were considered from cows, which had at least one calving up to 10th. Environmental effects such as herdyear of record (247 levels), stage of lactation (15 levels), lactation number (10 levels), month of recording (12 levels), year of calving (9 levels) and season of calving (4 levels), and the 1st and 2nd regression coefficient of milk production were included in the model. Heritability of SCS was estimated by animal model test-day random regression and correlations between traits were analyzed by bivariate animal model of Wombat. Fixed effects in model included herdyear of record, Holstein gene percentage, and stage of lactation as covariate, as well as additive genetic effect and common permanent environmental effects as random effects with 5-degree of Legendre. Results and Discussion Analysis of variance revealed that all factors in models were significant (P <0.01). Stage of lactation had a significant effect on SCS, therefore, SCS was increased with the progress of lactation from the 2nd month of milking to the end of lactation curve. The lowest amount of SCS observed in autumn records or in cows calving in autumn, however the highest amount observed in the late of winter and early spring seasons. It is possible that the undesirable condition of the rearing stall could affect the SCS. According to the results, SCS increased when cows getting older from the first lactation to the 10th calving. We found heritability of SCS ranged from 0.043 to 0.136 in the first lactation by random regression animal model. This low heritability especially in the first mid-lactation implies the much more effect of temporary environmental factors on SCS value. The highest genetic, phenotypic and environmental correlation was between SCS and milk protein percent (0.442), milk protein percentage (0.153) and milk production (-0.174) respectively. The negative environmental correlation between yield and SCS indicates that good management and hygienic practices would be helpful in maintaining a satisfactory level of milk production as well as controlling the SCS of milk. SCS is as mastitis criterion, which increasing its amount leads to decrease milk production. The negative correlation can be explained by this fact that SCS is diluted by increasing of milk production during the lactation. A plausible explanation exists for the positive association of somatic cells and protein percentage. Lactose is apparently reduced in milk as sec increases for maintenance of osmotic balance within milk secreting cells. Positive relationship of SCS and protein percentage, therefore, may be an artifact that results in overestimation of protein percentage with increased SCS. Conclusion Genetic correlations provide information about expected changes in one trait as a correlated response to selection on another trait. This study revealed that cow with a high genetic milk production tended to produce less somatic cell score. Differences of this study with the others can be depended on data structure, population and definition of SCS. The negative correlation can be explained by this fact that SCC is diluted by increasing of milk production during the lactation. The same result was observed for correlations of fat percent of milk with SCS, which can be due to negative correlation between milk production and its fat percent. On the other hand, negative environmental correlation between yield and SCS in present study indicates that good management and hygienic practices would be helpful in maintaining a satisfactory level of milk production as well as controlling the SCS of milk.