Data Controls :: How To Get Value Of GridView BoundField Set To Visible Is False

I am using gridview with sqldatasource , now i have a column

<asp:gridview ID="MyGrid" runat="server" AutoGenerateColumns="False"  datakeynames="Seq"
DataSourceID="ExistingConsPhaseSource" >
<Columns>
<asp:boundfield datafield="Seq" headertext="Seq" readonly="True" Visible="False" sortexpression="Seq" />
<asp:templatefield headertext="Description" sortexpression="Description">

[Code] ....

I also have a button outside gridview , on its click event i want to delete all rows in a grid view. So far i have done like this :

protected void DelAllExisting_OnClick(object sender, EventArgs e) {
foreach (GridViewRow row in ExistingConsPhases.Rows) {
string query =
"UPDATE [dbo].[myTable] SET IsDeleted = 1, DeletedBy = @DeletedBy, DeletedDate = GETDATE(), DeletedByPrefix = @DelPrefix WHERE Seq = @Seq";
SqlCommand comm = new SqlCommand(query,_connection);
comm.Parameters.AddWithValue("@Seq", ???);
comm.ExecuteNonQuery();
_connection.Close();
}
}

What to type instead of ??? I want my bound field of Seq to be place here ? how ?