I can't beleave that a herbicide can maintain it's potential to kill after being processed through a horse or chickens gut, plus the time it takes the horse or chicken to eat it after initial application.
Must use some way more better herbicide than I use. I have a hard time getting a good kill if the morning dew is still on the grass, or if it rains to soon after application. 
As I was commenting earlier. Hypotheticly--- I spray a fence line with 2,4d amine. Don't have the bottle in front of me so I am shooting from the hip. But anyhow I spray a fence line, with what amounts to about a 1% solution of active ingredient. The horse eats the grass, within minutes of me spraying it- still wet with spray, but that ain't all the grass he is going to eat that day, it may only be a small percentage of his total intake, for arguments sake, let's say it's 10% of his total intake of grass/hay/feed for the day. So we are at 1/10th of a 1% active solution. And that is to say that the active ingredient has not been broken down by exposure to sunlight, temperature, or moisture after application. Then we add the time factor, most all these herbicides break down in time, so how long does it take for a mouth full of grass eaten on Monday morning, to pass out the south end of a north bound horse, 24, 36, 48 hours. Then you are saying that the active ingredients will not be effected at all by the digestive action of the bacteria present in a horses gut? 
Like I said, maybe their are some "ground sterilizers" that would or could maintain potency, but the two herbicides I use have never demonstrated any ability to maintain any potency, just hours after application, when exposed to normal climate conditions, let alone time, climate, and digestive processes of a grazing animal. As proof of this I have grass growing all over my field right out of the horse apples, atleast a few of which are composed of the digested remains of grass that was sprayed with THE BIG EVIL ONE, and 2,4d amine.