Can you give this a try?
using (SqlConnection conn = new SqlConnection(conStr))
{
conn.Open();
SqlCommand cmd = new SqlCommand(sqlcmd, conn);
cmd.Parameters.AddWithValue("@Manufacture", manu);
SqlDataReader dr = cmd.ExecuteReader();
IList<string> modelList = new List<string>()
while (dr.Read())
{
modelList.add(dr[0].ToString());
}
ModelComboBox.DataSource = modelList;
}