Bechorot 54: What is a Herd? Tithing and Shepherds

Today our daf continues its consideration of tithing.  We learn that a person who owns a herd of at least ten animals must arrange for tithing.  Tithing is not required if there are many animals of different kinds, none reaching the ten number mark of a herd.  The Mishna teaches:

Animals are joined for purposes of tithing if the distance between them is no greater that the distance that a grazing animal can walk and still be tended by one shepherd.  What is the distance over which they can wander while grazing?  Sixteen mil.  If there was between tow groups of animals a distance of thirty-two mil, they do not combine for the purpose of tithing.  If however there was a herd in the middle of the distance of thirty-two mil, he brings all three flocks into one shed and tithes them at some point in the middle.  Rabbi Meir says: the Jordan River is regarded as forming a division with regard to the tithing of animals. 

Rabbi bar Sheila interprets in the Gemara calling on a passage in Sefer Yirmiyahu (33:12-13), where it states that a herd depends on the ability of the shepherd to see all of the animals and that the Sages had a tradition that it is possible for him to keep track of them up to sixteen mil.  

Steinsaltz discusses the measure of a mil.  The Mishnaic mil is based on the Roman mile which is approximately 1000 steps or 1490 metres.  A mil equals 2000 amma or cubits as defined in Masechet Eiruvin.  Today this is understood as being between 960 and 1200 metres.  The rabbis argue about whether a shepherd can see all animals while standing in the middle of the circumference of 16 mil so that he only has to see 8 mi in any direction or whether he could stand everywhere and see 16 mil.