calculated value shows empty when exporting grid view using itextscharp

i am using itextscharp to export grid view to word,pdf and excel
  the problem is  when i export the grid view it show all data except the calculated column which is GridView1.Rows[index].Cells[5].Text
protected void Page_Load(object sender, EventArgs e)        {for (int index = 0; index < this.GridView1.Rows.Count; index++) { double sum = 0; sum += Convert.ToDouble(this.GridView1.Rows[index].Cells[3].Text) * Convert.ToInt16(this.GridView1.Rows[index].Cells[4].Text); this.GridView1.Rows[index].Cells[5].Text = Convert.ToString(sum); }} <Columns> <asp:BoundField DataField="Order_ID" HeaderText="OrderID" SortExpression="Order_ID" /> <asp:BoundField DataField="Cust_Name" HeaderText="Cust Na ...