ทำไมค่าใน dropdownlist ตอนบันทึกไม่เปลี่ยนไปตามที่เลือก

dbtestDataContext db = new dbtestDataContext();
protected void Page_Load(object sender, EventArgs e)
        {
      //เลือกค่าจากฐานข้อมูลมาแสดงใน dropdownlist
      var fa = from f in db.Faculties select f;
            if (fa.Count() > 0)
            {
                DropDownList1.DataSource = fa.ToList();
                DropDownList1.DataBind();
                DropDownList1.DataTextField = "Faculty1";
                DropDownList1.DataValueField = "ID_Faculty";
                DropDownList1.DataBind();
            }
        }

        protected void cmdAdd_Click(object sender, EventArgs e)
        {
            if ((TextBox1.Text.Trim() != "") && (TextBox2.Text.Trim() != "") && (TextBox3.Text.Trim() != "") && (TextBox4.Text.Trim() != ""))
            {
                Researcher r = new Researcher();
                r.PrefixName = TextBox1.Text.Trim();
                r.ResearcherN = TextBox2.Text.Trim();
                r.IDcard = TextBox3.Text.Trim();
                r.Position = TextBox4.Text.Trim();
                r.ID_Faculty = Convert.ToInt32(DropDownList1.SelectedValue); //dropdownlist
                db.Researchers.InsertOnSubmit(r);
                db.SubmitChanges();
                GridView1.DataBind();
            }
        }