I used to weedwack under all the fences and the ditch. My mower has a wide deck and can get better than 1/2 the grass/weeds under the rails and then I’d weedwack around the posts and whatever is left. Last summer at age 71 I started to just weedwack where anyone would see but along the perimeter I’d use Round Up. I’m getting lazy in my old age. 
I do have my landscape people weedwack the ditch out front as it’s fairly narrow at the base on one side of the driveway but not the other but I let them do the entire ditch.
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We mow and weedwhack. We’ve got 25 acres. We also have chickens, and eat the eggs.
We spray perimeter fence line with Walmart’s house brand of grass and weed killer. Works as well or better than Round Up, and far less expensive. Lasts 60-90 days.
The short answer is “yes.” However, if you prefer not to trust a random stranger on the internet, check the label of any product you want to use and you should find the grazing restrictions.
For example, the Grazon P+D label says: “There are no grazing restrictions for non-lactating dairy animals or other livestock including horses, sheep, goats, and other animals in the treatment area.”
We’ve done both. It’s really a constant battle…The spraying seems to last longer, the weed whacking is better aesthetically.
I have over 135 acres and have weed whacked mostly. I try to do all the fence lines once a year but haven’t even started yet this year. I am thinking of trying the spray this year, especially on places that don’t get grazed or aren’t up by the house.
Modern herbicides, properly used, are safe and effective. Don’t be mislead by the “screaming mimis” who decry anything that isn’t “natural.” Afterall, how is using a weed whacker or a bush hog “natural?” 
But you’ve got to read the instructions (preferably twice) and then abide by them. Do that and the odds of a bad result are extremely low.
G.
yes I spray my arena with “Barrier.” It is by the company that makes Pasture Pro. It works for several months.
i usually just weedwhack but this year I am spraying! Trying something new. May use the Barrier there too. I use a 2-4D product in the pastures for weeds. We have a lot of briars trying to grow up in some areas so I use ForeFront on those which is stronger than just 2-4-d. I also use in on the fenceline where blackberries want to grow. I have had good luck with the Forefront, my briar problem is way better than it was.
