The genetic improvement of dairy cows in warm climates

In the present study the necessity and possibility of including heat tolerance among selection objectives for high yielding cows are discussed. After examination of the possible indices of heat tolerance, rectal temperature (RT) was chosen because it expresses the thermal balance properly and it is an index for which certain genetic parameters have been reliably estimated. These parameters include heritability (from 0.19 to 0.64), variability within individuals under hot environments (c.v.= 0.65-0.70%), rank coefficients (rs=0.67; P<0.05 and 0.94; P<0.01), and phenotypic and genetic correlations between milk yield, milk composition, feed intake and fertility. A selection scheme for high yielding dairy breeds (HY) based on measurement of RT on progeny testing bulls and their daughters is proposed. A simulated comparison of genetic progress on milk production achievable between the HY and local breeds (LB) selected by a classic model has been carried out. The simulation demonstrated the effectiveness of selection based on the RT of high yielding breeds, in which heat tolerance is improved in a few generations, while local breeds need several generations to reach comparable milk production levels. A molecular approach to exploit the knowledge of quantitative trait loci (QTL) for RT in HY or milk yield in LB is discussed.