Do not add InitialValue. Remove that it will work correctly.
Below i have added code you can try-
<telerik:radcombobox id="CountryCombo" runat="server" cssclass="MyComboBox" datasourceid="SqlDataSource1" datatextfield="Country_Name" datavaluefield="CountryID" emptymessage="Select Country"></telerik:radcombobox>
<asp:sqldatasource id="SqlDataSource1" runat="server" connectionstring="Data Source=FTPL01\SQL2012;Initial Catalog=ZricksDB;Persist Security Info=True;User ID=sa;Password=webuser" providername="System.Data.SqlClient" selectcommand="SELECT [CountryID], [Country_Name] FROM [Country_Master]"></asp:sqldatasource>
<asp:requiredfieldvalidator id="RequiredFieldValidator6" runat="server" controltovalidate="CountryCombo" errormessage="*"></asp:requiredfieldvalidator>