The idea of ma'aser (tithe) only applies if there are ten animals. If there are fewer than ten animals, there is no ma'aser on them at all. Even if there are ten newborn animals, but they are spread across different groups or flocks, he would not give ma'aser from them.
Here we'll talk about how far apart the animals can be from each other and still be considered part of the same flock, which means that the laws of ma'aser apply to them. The Mishnah says that if ten newborn animals, to which ma'aser behemah (animal tithe) would apply, are within one area where a grazing animal would be able to roam and still be watched by the same shepherd, that is one area, and it is considered one flock, and they're mitztaref. They combine with each other. If there are ten in total, then the ma'aser obligation kicks in.
How far can a grazing animal roam and still be watched by the same shepherd? Sixteen mil (halachic miles), which is 32,000 amot (cubits). In an open area, a shepherd can watch animals up to eight mil in either direction. An animal could drift apart while grazing. As long as they're within that same area, under the care of the same shepherd, they're considered part of one flock.
If a person has five newborn animals in one village and five others in another village, and they're no more than 16 mil apart, he would gather them into one pen and then take ma'aser from them. Hayah bein elu la'elu shloshim ushnayim mil, but if there was a distance of 32 mil between these and those, those newborn animals, einan mitztarfin, they do not combine to be part of a single flock. They're no longer considered one entity for ma'aser.
This din (law) is a little bit strange. We just said that shisha asar mil, 16 mil, is the distance you could have from one animal to another for them to be considered one flock. Here we're saying that more than 32 is considered too far apart. But what we're really doing here is setting up the next part of the Mishnah, which is going to tell us that if you have an animal in between—let's say you have 16,000 between one animal and the next, and then another 16,000 in the other direction—all of those would be considered connected, which is a distance of 32 between all of them. The ones on the extreme left and the extreme right would be considered part of the same flock because they have the one in the middle that connects them. That's what we'll say now.
Hayah lo ba'emtza, the owner had some animals in the middle, halfway between the 32,000 and the rest of the flock. He brings the outer animals together and takes the animals from them, because the ones in the middle bring everything together. The middle animals are within 16 mil of the ones on either side of them, and therefore connect everything. So the animals on both sides, through the connection of the animals in the middle, would combine and be considered one flock. If all of them are a combination of at least 10 newborns, the owner has to take ma'aser from them.
The Mishnah will bring down a Tanna (early rabbinic sage) who gives a situation where animals do not combine even if they're much closer than 16 mil. The Jordan River serves as a separation for the ma'aser of animals. If a person had five newborn animals on one side of the Jordan River and five on the other side, they do not combine to form a ma'aser. They do not form one group, even if the distance between them is less than 16 mil, because the Jordan River separates the animals on either side, even though they are less than 16 mil apart. The Jordan River is considered a break between the two groups, and therefore they are considered to be two separate flocks. If there are 10 in each group, that's enough to do ma'aser behemah on their own. But in combination with each other, there would be no ma'aser behemah.