You're not worried about your neighbor's dog pooping on your lawn.
Fences are there to keep things in, or to keep things out. Sometimes both. That's our understanding as human beings. Most animals, however, are slightly less clear on the concept. That's why barbed wire fences surrounding cattle pastures are supplemented with a hot wire. There's a distinctive sound the first time a calf discovers this. It's something along the lines of **zzZOT!** "&*()&!@#$ MOO!"
Sometimes animals are able to completely ignore fences. During free-range time, ol' Zap the rooster used to lead small expeditions under the barbed wire into the cattle pasture next door to find the assorted bugs that sought refuge there. And you may remember me contemplating whether I could hurdle the barbed wire fence while chasing a fox.
Sometimes fences break. Or gates are left unlatched. Or gremlins decide to cause mischief. Whatever the cause, a cow will see an opportunity and will take it. Followed by all of her family, friends, acquaintances, and "you-look-familiar-have-we-met?"s. And suddenly, just as the chickens followed their stomachs to bug-rich lands, these cows follow their stomachs to ungrazed pastures.
Under Nebraska law, upkeep of a fence is the mutual responsibility of the neighboring landowners, so even though we didn't install the fence between us and the cow pasture, Caryl spent about a week effecting preventative repairs to the barbed wire fence -- replacing posts, tightening the wire, changing the orientation of part of the fence from horizontal to vertical... That might be a blog entry for another day, but I don't think there were any photos of the process so maybe it won't be a blog entry -- we'll see.
At this point, I should mention a few things in the interest of complete honesty. The picture of the cow in front of a house? That's an extremely disused house adjacent to a corn field, and the cows had been moved there to graze on the yummies left behind after harvesting. If you pay close attention, you can see the hot wire that the farmer temporarily put up around the field.
And: when we took possession of Dunrovin Station, there was a cow patty between the house and the outbuilding -- but other than the evidence that it's happened in the past, the cows in the adjacent pasture have never escaped since we moved in (except for Houdini the calf). We've encountered other stray cows here and there, but none have pooped on our lawn in the last 2½ years. But it's always in the backs of our minds that it might happen, though.