An attemept at Terza Rima:

[Leanne]

My Best Friend:

A holstered tale between two nervous knees: -- nice "tale" wordplay
O churlish cur why bite the hand of care? -- comma after cur
the lamb was not for eating if you please.

I call you friend and yet those fangs you bare.
You and me, we have a shepherd's job to do, -- too long, you could try "we two, we have a shepherd's job to do"
you're not supposed to be a wolf, just glare.

A year or so when it becomes a ewe -- grammatically this line and the next are not great, what about "A year to wait 'til it becomes a ewe"
and then we'll strike, but silent with a blade,
or axe, and have more meat to place in stew.

But no you greedy bastard, had you weighed
the situation up you would have heard
a whistle saying how you should have laid.

Generally a pretty good first terza rima (I love the interlocking rhymes, it's one of my very favourite schemes). The voice is great, speaking to the dog as a person, which I've heard many a farmer do. The poem would benefit from the traditional closing couplet to summarise -- as it is now it ends rather abruptly.

That's all I've got. I like it, and will like it even more when it's tweaked just that little bit.