Cascading DropDownList not working when DateValueField is set in ASP.Net

HI there

I have 2 dropdownlist in my page

1--ddlTarde

2--ddlTradeClass

and 2 table in database

1-Trade table

2-Tradeclass

I bind ddlTrade from Tradetable and I want when users click on Item from ddlTrade it bind data from ddlTradeclass that TNAME=ddlTrade.selectedItem   in ddltradeClass  for this I used below code

for binding ddltrade

BindDropDownList(DDLtrade, "tradename", "Name", "id");


 protected void BindDropDownList(DropDownList DDL, string spname, string dataTextfield, string dataValuefield)
    {
        SqlCommand _cmd = new SqlCommand(spname, _cn);
        _cmd.CommandType = CommandType.StoredProcedure;


        _cn.Open();
        SqlDataReader _dr = _cmd.ExecuteReader();

        if (_dr.HasRows)
        {
            DDL.DataSource = _dr;
            DDL.DataTextField = dataTextfield;
           DDL.DataValueField=dataValuefield;
            DDL.DataBind();
        }
        _cn.Close();


    }

and this is ddlTrade_SelectedIndexChanged  event

  protected void DDLtrade_SIC(object sender, EventArgs e)
    {
        BindTrade();




    }


    private void BindTrade()
    {
        DDLclass.Items.Clear();
        DDLclass.Items.Add(new ListItem("click here", ""));
        DDLclass.AppendDataBoundItems = true;

        SqlCommand _cmd = new SqlCommand("tardeclass1", _cn);
        _cmd.CommandType = CommandType.StoredProcedure;
        _cmd.Parameters.AddWithValue("@Trade", DDLtrade.SelectedItem.Value);

        _cn.Open();
        DDLclass.DataSource = _cmd.ExecuteReader();
        DDLclass.DataTextField = "Name";
        DDLclass.DataValueField = "ID";
        DDLclass.DataBind();


        _cn.Close();

    }

and SP

ALTER procedure  [dbo].[tardeclass1]

@Trade nvarchar(30)
as
begin
select ID,Name
from tradeclass
where TradeN=@Trade or @Trade='0'

end

NOW my problem is that when I select Item from ddlTrade it didn't show any thing in ddlTradeClass but when I delete this code

DDL.DataValueField=dataValuefield;

from BindDropDownList Metod and change like below

  protected void BindDropDownList(DropDownList DDL, string spname, string dataTextfield)
    {
        SqlCommand _cmd = new SqlCommand(spname, _cn);
        _cmd.CommandType = CommandType.StoredProcedure;


        _cn.Open();
        SqlDataReader _dr = _cmd.ExecuteReader();

        if (_dr.HasRows)
        {
            DDL.DataSource = _dr;
            DDL.DataTextField = dataTextfield;
          // DDL.DataValueField=dataValuefield;
            DDL.DataBind();
        }
        _cn.Close();


    }

It worked correctly I mean when I click Item from ddlTrade in ddlTradeClass show data from TradeClass Table  

so here what should I do ?

I need DataValueField in ddlbinding

Why this problem happen?

BestRegards