A friend challenged me to find the origin or source of this French Proverb:

   "Laisse pisser la vache, c'est un animal qui pisse longtemp" 

  I would like to know what is the original source of the proverb, the
date of the work it first appeared in, and the steps you took to find
the information.

Dear toughcamper,

Thank you very much, you are really generous. I have hereby answered
the question, so there is nothing you'd have to do.

Again, thank you!
Regards,
Scriptor

Not an answer, just a remark: It seems to be a proverb with no source
that can be defined exactly. Obviously, it was first mentioned in a
printed source in 1867, in the "Dictionnaire de la langue verte"
(dictionary of slang). It was still shorter then, just "Laisse pisser
la bête" (Let the animal piss), an expression from the language of
cavalry soldiers and coach drivers meaning that it is necessary to
have patience and let nature take its way.

http://correcteurs.blog.lemonde.fr/correcteurs/2006/09/laisse_pisser_l.html#

Scriptor

As mentioned by Scriptor, a very similar expression is an old French proverb. 

"On dit qu'il faut laisser pisser le mouton parce que c'est une bête
qui pisse longtemps."

In English: "It is said that it's necessary to let the sheep piss
because it's a beast that pisses for a long time."

According to the French webpage linked below, the phrase had its
origin in the 19th Century or earlier. People knew that if draft
animals such as cattle, horses, or oxen were not allowed to take
breaks for the purpose of urination, this could cause medical
disorders. So, during long trips using animal-drawn conveyances, there
were stops along the way to allow the animals to relieve themselves.

The substitution of a sheep in the phrase, instead of a draft animal,
was done as a joke.

If you read French, there's a good deal of information here:
http://www.expressio.fr/expressions/laisser-pisser-le-merinos.php

Many citations of the French proverb may be found with a Google search:

://www.google.com/search?hl=en&q=%22b%C3%AAte+qui+pisse+longtemps%22

Scriptor-

  Your answer is sufficient to warrant the $10 fee I offered.  I'm not
sure how to credit you with this, but if you let me know how, I would
gladly pay you for your response.

  -ToughCamper