It started creeping up from a canyon and choking out my pasture. Dumping cow manure on it to smother it out is not working.

I can't be sure from those photos, but it looks a bit like a vinca.

Yes, you're right, it's vinca minor.
http://commons.wikimedia.org/wiki/File:Vinca_minor_clump.jpg

Thanks! Now I need to find a way to get rid of "vinca minor."

They're also calling it "periwinkle."

Yep, it loves rich soil and growing under mulch so will do well with the cow manure you're feeding it - it doesn't like too much root disturbance so keep hand pulling, vigorous raking or harrowing. Will only spread if the runner-like stems contact the soil and are able to root, so yank 'em up before they can. Alternatively close mowing should do it too.

You could always harvest it.

Source: http://www.herbs2000.com/herbs/herbs_lesser_periwinkle.htm

Uses
Lesser periwinkle (Vinca minor) has a number of therapeutic uses, for instance it is used in the form of an astringent as well as an herb to stop hemorrhage. The astringent attribute of lesser periwinkle makes the herb an effective mouthwash for treating canker sores, sore throats and gingivitis. Following the discovery that the leaves of lesser periwinkle enclose an alkaloid known as vincamine, this herb has been employed in the treatment of arteriosclerosis as well as for curing dementia owing to inadequate flow of blood to the brain. Also, lesser periwinkle is very useful and effective in treating nosebleeds, internal bleeding and heavy menstrual bleeding

In traditional medicine, the dried herb of lesser periwinkle (its aerial parts and, occasionally, the entire Vinca plant) is employed to augment blood circulation, especially to the brain, improve metabolism in the brain as well as to cure cardiovascular ailments.

Put a border around it and call it a flower bed. That always works for killing flowers around my house.......