On my job my manager told me I can't work this Thursday because of work slow down...?

I don't think it is discrimination but I do believe seniority should count as long as you both show the same amount of efficiency. Now if you should complain I don't think so at this point either- I think the time for that would have been as soon as he told you not to work. you could have (should have) stood up for yourself at that point. You did finish the apples which was your choice od work so you are "finished" he probably felt he was being fair in his actions and the other employee would have prob felt just as you do had he swapped you guys out at this time to let you have the work. The solution might have been for you to offer to help and get the job done in an hour and a half verses the the three hours you think it will take without you.