So we know that Bikurim was collected by tying a reed to the first fruit of a particular field when it would be found budding. We also know that Chazal say to give one 60th of one's produce as Bikurim. The question is how to do this properly. It would seem necesary to know how much fruit the field will yield in advance how to then know how many fruits to mark off. Am I missing something?
Experienced farmers can estimate in advance the Yield for the Year and there is usually only slight variation so he does know how much fruit the field will yield in advance .