IF A TRENCH BETWEEN TWO COURTYARDS WAS TEN HANDBREADTHS DEEP AND FOUR HANDBREADTHS WIDE, TWO ERUVS MAY BE PREPARED BUT NOT ONE, EVEN IF IT WAS FULL OF STUBBLE OR STRAW. IF, HOWEVER, IT WAS FULL OF EARTH OR GRAVEL, ONLY ONE ERUV MAY BE PREPARED, BUT NOT TWO.
The rabbis discuss this difference as having to do with intention – a trench filled with straw may be only a temporary holding place, so unless the owner declares the intention to keep it there (abandoned) it is not a valid filling. Earth, on the other hand, is considered abandoned in a trench and therefore the trench loses its ability to separate.
Then the Mishnah goes in the opposite direction: from depth to height:
IF A HEAP OF STRAW BETWEEN TWO COURTYARDS YARDS WAS TEN HANDBREADTHS HIGH, TWO ERUVS MAY BE PREPARED BUT NOT ONE. THE TENANTS OF THE ONE COURTYARD MAY FEED THEIR CATTLE AT THEIR SIDE AND THOSE OF THE OTHER COURTYARD MAY FEED THEIRS ON THE OTHER SIDE.
IF THE HEIGHT OF THE STRAW HEAP WAS REDUCED TO LESS THAN TEN HANDBREADTHS, ONE ERUVS MAY BE PREPARED BUT NOT TWO.
Straw here constitutes a valid separation, even if cattle are feeding from it. The rabbis make clear that one cannot pull the straw into a basket and feed to animals – the straw is mukzeh, something which cannot be handled, not because it is actually forbidden, but doing so may lead to a forbidden labor. But the animals can eat from it by themselves. If the eat so much on the Sabbath that the straw pile becomes lower than the required height, it no longer constitutes a division between the two courtyards.