ok, lets start with the obvious
here
DateTime LOAD_DATE = Convert.ToDateTime(row.Cells[6].Text.ToString());
DateTime LAST_UPDATED_DATETIME = Convert.ToDateTime(row.Cells[7].Text.ToString());
string message = string.Empty;
int x = obj.DeleteSupplierDetails(SOURCE_ENTITY, PLANT_CODE, SUPPLIER_NUMBER, SUPPLIER_GROUP_NUMBER, LOAD_DATE, LAST_UPDATED_DATETIME, out message);
LOAD_DATE
LAST_UPDATED_DATETIME
Are DateTime
Whereas here
public int DeleteSupplierDetails(string SourceEntity, string PlantCode, string SupplierNumber, int SupplierGroupNumber, String LoadDate, String LastUpdatedDateTime, out string message)
LoadDate and LastUpdatedDateTime are String