بررسی منحنی تغییرات درصد چربی و درصد پروتئین و نسبت چربی به پروتئین در رکوردهای شیر روز آزمون گاوهای شیری ایران با استفاده از تکنیک آماری رگرسیون کوآنتایل

زمینه‌ی مطالعه: میزان تولید گاو در اوج تولید شیر، به‌جیره‌ی غذایی بعد از زایمان، توانایی ژنتیکی، شرایط بدنی حیوان در هنگام زایمان، نداشتن بیماری‌های عفونی و متابولیکی، بستگی دارد. نسبت چربی به پروتئین در رکوردهای شیر روز آزمون، سنجه‌ای کاربردی در رابطه با میزان سوخت و ساز چربی‌های بدن و میزان تعادل منفی انرژی است و می‌توان از آن به‌عنوان سنجه‌ی انتخاب، در جهت بهبود پایداری متابولیکی گاوهای شیری استفاده نمود. هدف: این تحقیق، با هدف بررسی منحنی تغییرات درصد چربی، درصد پروتئین و نسبت چربی به پروتئین در رکوردهای شیر روز آزمون گاوهای شیری ایران و با استفاده از رگرسیون چندجمله‌ای کوآنتایل اجرا گردید. روش ‌کار: تعداد 784532 رکورد روز آزمون متعلّق به 93259 راس گاو شیری شکم اوّل در 660 گله که طیّ سال‌های 1382 الی 1392 زایش داشتند، توسّط مرکز اصلاح نژاد دام و بهبود تولیدات دامی جمع‌آوری و مورد استفاده قرار گفت. صفت مورد بررسی، درصد چربی و درصد پروتئین و نسبت (درصد یا مقدار) چربی به پروتئین در رکوردهای شیر روز آزمون بود. مدل رگرسیون چندجمله‌ای کوآنتایل، توسّط نرم‌افزار sas بر میانگین رکوردها برازش داده شد که در آن، روز شیردهی، به‌عنوان متغیّر مستقل و هر یک از صفات مذکور به‌عنوان متغیّر پاسخ معرفی شدند. نتایج: گرچه بیشترین مقدار نسبت چربی به پروتئین در ماه‌های اوّل و دوّم شیردهی، و کمترین مقدار آن در ماه‌های ششم و هفتم دوره‌ی شیردهی مشاهده گردید، امّا به‌دلیل متفاوت بودن ضریب تابعیّت خطی و درجه دوّم روز شیردهی، بیشترین و کمترین مقدار صفت مزبور، در چندک‌های مختلف، تغییرات داشت؛ به‌گونه‌ای که بر حسب قدر مطلق مقادیر، کوچکترین و بزرگترین ضریب تابعیّت خطی روز شیردهی، به‌ترتیب در چندک‌های 5 و 95 مشاهده شد. مشابه با وضعیّت مزبور نیز برای ضریب تابعیّت درجه دوّم روز شیردهی به‌دست آمد. نتیجه‌گیری نهایی: یافته‌های تحقیق حاضر، نشان داد شیب بالارونده و پایین‌رونده منحنی تغییرات درصد چربی، درصد پروتئین و نسبت چربی به پروتئین در طول دوره‌ی شیردهی گاوهای شیری ایران، در چندک‌های مختلف از صفات مزبور، متفاوت است؛ و لذا توصیّه می‌شود که این تفاوت، در مدیریّت تغذیه و سلامت همچنین برنامه‌‌ی‌ اصلاح نژادی گاوهای شیری مورد توجّه قرار گیرد.

a study on the curve of far percentage, protein percentage and fat to protein ratio in test day milk records of iranian dairy cows using quantile regression statistical technique

a study on the curve of far percentage, protein percentage and fat to protein ratio in test day milk records of iranian dairy cows using quantile regression statistical techniqueabstractintroduction: while the physiological conditions of most animals are reflected in their body fluids, in dairy cows physiological conditions are reflected in milk combinations. when negative energy balance occur, production that caused body fat store destruction, goes in two major metabolic ways in liver and breast tissues. in breast tissues, free fatty acids increase fat to protein ratio and in liver, free fatty acid change metabolism leading to increase of ketone bodies. lactation persistency in dairy cows is one of inheritable economic trait and important characteristic of the lactation curve. the change of this trait is under control of different factors such as animal genetics structure. the amount of cow production at the peak of milk production depends on the calving physical condition, genetics, the absence of infectious and metabolic diseases, and postpartum diet. fat to protein ratio in early lactation is associated with performance during lactation. fat to protein ratio is also correlated with some diseases such as retained placenta, displacement of abomasum, metritis, endometritis, mastitis and culling. fat to protein ratio (fpr) in test day milk records is a good indicator for body fat metabolism and the level of negative energy balance, and that it could be used as a selection criterion to improve metabolic stability. objective: quantile regression has not been widely used for modeling of biologic characteristics of livestock and this research aimed to evaluate the shape of fat percentage, protein percentage and fpr in test day milk records of iranian dairy cows using a polynomial quantile regression model. material and methods: the data used in this study were provided by the animal breeding centre, iran. initial data set were edited by excel and foxpro software. the traits under consideration were fat percentage, protein percentage and fat to protein ratio (calculated based on the magnitude of the first two traits). all cows had value for all traits. final data comprised a total of 784,532 test day records collected from 93,259 first-parity dairy cows (progeny of 2741 sires and 79843 dams) distributed in 660 herds over the country and calved between 2003 and 2013. in the data, test day fat percentage and protein percentage had a minimum of 1 and 1.5 and a maximum of 7 and 7, respectively. by spss software, some statistical characteristics of the traits were calculated, and by sas software a quadratic polynomial quantile regression model (in the range of quantiles 5 to 95) was fitted to the data. based upon the fitted model, linear and quadratic regression coefficients were estimated for the effect of day of lactation on each trait in different quantiles. in the model, traits and day of lactation were response and independent variables, respectively. quadratic effect of day of lactation is considered due to nonlinear variation of the traits over the course of the lactation curve. results and discussion: mean fat percentage, protein percentage and fat to protein ratio over the lactation course were found to be 3.35%, 3.08% and 1.1, respectively. as expected, for each trait a nonlinear change was observed over the lactation period. the highest content fat to protein ratio occurs in the early period of lactation and the lowest was observed in the sixth and seventh months of lactation. in terms of absolute values, minimum (953.564) and maximum (1797.661) linear regression coefficient of days in milk were obtained in quantiles 5 and 95, respectively. the same trend was also observed for quadratic regression coefficient of days in milk. linear and quadratic regression coefficients of fat and protein percentages ranged over different quantiles so that minimum and maximum absolute values were observed in quantiles 5 and 95, respectively. conclusion: quantile regression model is an appropriated statistical technique for evaluate the effects of an independent factor which differently affects the shape of a response variable. in this research, the effect of days in milk on three traits (fat and protein percentage, as well as fat to protein ratio) was modeled by quantile regression. the findings of the present research indicate that the curve shape of fat percentage, protein percentage and fat to protein ratio are different for the quantiles suggesting that these differences are needed to be taken into account in nutrition management and cattle breeding scheme.keywords: dairy cow, modeling, quantile regression, ratio of milk compositiona study on the curve of far percentage, protein percentage and fat to protein ratio in test day milk records of iranian dairy cows using quantile regression statistical techniquea study on the curve of far percentage, protein percentage and fat to protein ratio in test day milk records of iranian dairy cows using quantile regression statistical techniquea study on the curve of far percentage, protein percentage and fat to protein ratio in test day milk records of iranian dairy cows using quantile regression statistical techniquea study on the curve of far percentage, protein percentage and fat to protein ratio in test day milk records of iranian dairy cows using quantile regression statistical techniquea study on the curve of far percentage, protein percentage and fat to protein ratio in test day milk records of iranian dairy cows using quantile regression statistical techniquea study on the curve of far percentage, protein percentage and fat to protein ratio in test day milk records of iranian dairy cows using quantile regression statistical technique