Hi plt102,
if possible, a couple of days before your pigs go to slaughterhouse feed them in the trailer and let them get used to it,if they have not been in one before (other than collecting them when they were small), it gets them used to going up the ramp and don't feel threatened. You can also feed only a little food the night before slaughter and feed in morning to get them in the trailer, the hungrier they are the more they are willing to go into the trailer first time. If my pig looses an ear tag before slaughter time I paint mark mine on the back and never have had any problems. HTH