ASP.net DataGrid -- convert string to Int

Hi,

i have a working DataGrid1 whose source is XML file.

lately, i realized that i could not able to use DateFormatString as the values returned from XML are all strings. (pls correct me if wrong)....below code doesn't apply any format
<asp:BoundField HeaderText="Start QTY" DataField="start_qty" DataFormatString="{0:N3}" ApplyFormatInEditMode="True" HtmlEncode="False"/>

can you pls suggest me how to convert from string to int or decimal

------- DataGrid -------
 rowDataSet.ReadXml(filepath);
            pnlGrid.DataSource = rowDataSet.Tables[0].DefaultView;
            pnlGrid.DataBind();
--------------------------------

 <asp:GridView ID="pnlGrid" runat="server" CellPadding="4" AutoGenerateColumns="False"                    EnableModelValidation="True" ForeColor="#333333" GridLines="None">
<FooterStyle BackColor="#5D7B9D" Font-Bold="True" ForeColor="White" />
                        <HeaderStyle BackColor="#5D7B9D" Font-Bold="True" ForeColor="White" />
                        <PagerStyle BackColor="#284775" ForeColor="White" HorizontalAlign="Center" />
                        <RowStyle BackColor="#F7F6F3" ForeColor="#333333" />
                        <SelectedRowStyle BackColor="#E2DED6" Font-Bold="True" ForeColor="#333333" />
                        <Columns>
                             <asp:BoundField HeaderText="Symbol" DataField="symbol" />
                          <asp:BoundField HeaderText="Start QTY" DataField="start_qty" DataFormatString="{0:N3}" ApplyFormatInEditMode="True" HtmlEncode="False"/>
                            <asp:BoundField HeaderText="End QTY" DataField="end_qty" />
 </Columns>
</asp:GridView>