1. Source that terumah cannot be taken ממין על שאינו מינו

A Mishnah teaches terumah may not be separated ממין על שאינו מינו – from food of one species for food of another species. This is derived from "כל חלב יצהר וכל חלב תירוש ודגן" – all the best of oil and all the best of wine and grain, where the passuk hints: תן חלב לזה וחלב לזה – give the best of this [oil] and the best of that [wine and grain], i.e., terumah for each separate species. Taking terumah separately from wine and from grain (where "חלב" is only written once), and for each type of דגן (e.g., wheat and barley), is derived from a kal vachomer: if תירוש ויצהר – wine and oil, which are not כלאים with one another (i.e., they may be planted together), cannot serve as maaser for one another, then wine and grain, or different types of grain, which are כלאים if planted together, certainly cannot serve as maaser for one another. Any other foods (such as beans and lentils) also cannot be taken as maaser for one another, although they are not כלאים with each other, because their entire maaser obligation is דרבנן, וכל דתקינו רבנן כעין דאורייתא תקינו – and whatever the Rabbis enacted, they enacted parallel to Biblical laws, which preclude separating maaser on another type.

1. Animals are combined for מעשר בהמה across an area of sixteen mil

The next Mishnah states: מעשר בהמה מצטרף כמלא רגל בהמה רועה – animals subject to מעשר בהמה are combined (to a group of ten) in an area as large as an animal’s grazing area, which is sixteen mil. If there were thirty-two mil between the groups of animals, they do not combine, and each group of fewer than ten animals is not obligated in מעשר בהמה. היו לו באמצע – If he had animals in the center between two groups which are thirty-two mil apart, they are all combined into a single group of ten to require maaser. The source that only animals within a grazing area are combined is the passuk "עוד תעבורנה הצאן על ידי מונה" – the flocks shall again pass before the hand of the one who counts them. This indicates that animals which can be counted by a single shepherd are considered one flock, וקים להו לרבנן – and it was known to the Rabbis דשיתסר מיל קא שלטא ביה עינא דרועה – that the shepherd’s eye reaches across an area of sixteen mil.

1. How many animals in the center combine groups on both sides

Rav says that if there are five animals on each end of thirty-two mil, they are only combined if there are five animals in the center, דהני חמש חזיא להכא וחזיא להכא – because these five are fit to combine with the five here to form a group of ten, and are fit to combine with five there to total ten. These fifteen are brought into a pen together, the first ten are counted, and the remaining five are still subject to מעשר בהמה. The same applies to nine animals in the outer groups and one in the center. However, if there were only five in each outer group and one in the center, they would not combine. Shmuel disagrees and says that even if there are five in each outer group and one in the center, חזינן לרועה כמאן דקאי הכא – we view the shepherd as if he is standing here with the center animal, since he often goes there to care for it, וקרינן ביה מונה – and we call him “the one who counts,” who can combine any animals within his sixteen mil. On the next Daf, Rav Pappa says that according to Shmuel, the presence of the shepherd (or even his belongings) in the center can combine the outer groups even with no animal in the center.