Dear consuelo: The story was about a small town in your general vicinity. The time was early in WWII.
The fencerows meant the margins of the fields that could not be mowed by tractor cutter bar. Presumably
any trees would be removed;, leaving only saplings and brush. We had choke cherries. Even though you are unlikely to be interested in beef cattle, you might be interested in experience we had ;with choke cherries
around edge of our pasture.
My wife who raised Herefords noticed that a cow would not come out of the barn. And her urined was peculiar in that it looked extra yellow, and was foamy. She called the vet, who walked around pasture, and found places where cow had been eating chokecherry saplings, some with wilted leaves. Such leaves can contain a toxin that can injure liver, causing jaundice, which causes photosensitivity, so that the cow would actually get painful reaction of her nose and places where hair was short if she went out into sunlight.
But I still have no idea why similar saplings might be called cowcatchers.