protected void DropDownList1_PreRender(object sender, EventArgs e)
        {
            if (!IsPostBack)
            {
                DropDownList1.Items.Insert(0, new ListItem("", "0"));
                DropDownList1.SelectedIndex = 0;
            }
        }

 <asp:RequiredFieldValidator ID="RequiredFieldValidator3" runat="server" ControlToValidate="DropDownList1" 
ErrorMessage="Ошибка"></asp:RequiredFieldValidator>