hadeer aboshady

Citation:

Abo-Shady, H. M., Genetic response of heat stress on performance of Spanish local dairy goat breed., , Valencia, Spain, Polytechnic University of Valencia, 2013.

Thesis Type:

Abstract:

In the present thesis the effect of heat stress on milk, fat, and protein yields and fat and protein contents has been studied in the Spanish Florida breed of dairy goats. It comprises three chapters: The first one is a bibliographic review of the state of the art. The second one is a phenotypic analysis carried out to determine the climatic variables most correlated to the production traits and to estimate the tolerance thresholds and slopes of the responses of the dairy traits studied to heat stress. The third chapter is a genetic analysis of formerly cited responses, undertaken with the objective to estimate the environmental and genetic (co)variance components of heat stress tolerance. A total of 185,675 test-day records belonging to 13,481 lactating goats distributed in 20 flocks, collected between 2006 and 2012, combined with maximum and average temperatures and the values of an index of temperature and relative humidity (THI), registered the day of milk recording and one and two days before in meteorological stations located less than 22 Km from the farms, were used for the phenotypic and genetic analysis. For the first study, a Ridge regression analysis and a GLM select analysis were carried out in order to select the climatic variables and dates that were recorded, having the highest correlations with the dairy traits under study. Then, tolerance thresholds and slopes of the regressions of these traits with the selected climatic variables were estimated with spline and polynomial models by means of Bayesian methods. Results shows that the maximum and average temperatures (mean of maximum and minimum temperatures) explain the change in dairy traits caused by climatic effects better than the THI. Temperatures registered the day of milk recording or one day before have more effect on the traits studied than those registered two days before. Generally, yields and contents of milk components decrease when temperature increases. However, milk yield seem to be less affected by high temperatures, being more affected by cold temperatures. Climatic variables have a higher effect on milk composition in high productive animals in respect to the rest. For the genetic analysis, a reaction norm animal model (RNM) was used to estimate the genetic and permanent environmental (co)variance components of the relation of dairy traits with THI, maximum and average temperature, modeling this relation by means of Legendre polynomials. Results show that the heritability of dairy traits tends to decrease for increasing values of the climatic variables. Genetic correlations between the intercept and the slope of each model, and between the first and the subsequent points in the scales of the climatic variables, provide evidences that selection for better milk performance will reduce heat stress tolerance. The genetic variation of the response to heat stress found in this analyses could be used to select animals according to their response to heat stress (robust, tolerant and non-tolerant).

Notes:

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