The OP is udder nonsense.
Not really a joke but my mom, when she was first dating my dad, asked him how the cows managed to stand on hillsides without falling over (she being from the flatlands of the midwest.) He helpfully told her that the cows legs were shorter on the left side than they were on the right and thus were doomed to always face the same way standing on the hill. My mom brought this for many years.
I got my gullibility gene from her.
Not so much a cow joke but it has the word “cow” in it:
What does Stedman say to Oprah when she wants sex?
How now brown cow.
Sorry if I offended anybody.
I really like this old one:
Two cows were standing around, munching gasss, and just taking it easy. Suddenly, one of them went, “MOOOOO!”
The other one looked up slowly, and said, “Y’know, I was just going to say that.”
Eutychus -
Now that you’ve proven your hypothesis, maybe you could put this thread out of its misery? 
Aw, one more? Please?
There were these two cows, chatting over the fence between their fields.
The first cow said, “I tell you, this mad-cow-disease is really pretty scary. They say it is spreading fast; I heard it hit some cows down on the Johnson Farm.”
The other cow replies, “Hell, I ain’t worried, it don’t affect us ducks.”
Euty, you are absolutely correct in your hypothesis. Your mom is not the least bit funny.
From, uh, Colors?
Two bulls, a father and son, are standing on a hill, looking down at a herd of cows in the field. Son says to the father, “Hey pop, let’s run down there and screw one of them cows.”
Father says to son, “Nah, boy. Let’s walk down, and screw all of 'em.”
[sub]I think Euty’s right… But Ice Wolf’s joke made me chuckle…[/sub]
There are 2 cows and 2 bulls atop a grassy hill grazing away.
A big burst of wind comes along and the 2 cows go rolling down the hill, whilst the bulls just stand and weave in the wind.
The cows confusedly trudge back up the hill and return to grazing. Again, a big burst of wind comes along, and the cows go rolling down the hill, and the bulls just weave gently in the wind.
After climbing back up the hill, one of the cows asks the bulls: “How is it that when a big burst of wind comes along, we cows go rolling down the hill, and you bulls just stand up here and weave in the wind?”
“It’s simple,” one of the bulls replied…
"We bulls wobble, but we don’t fall down…"
::d&r::
Maybe it was really a haggis? 
*“I never saw a Purple Cow,
I never hope to see one;
But I can tell you anyhow,
I’d rather see than be one.” *
-Mr. Gelett Burgess
and, of course, his follow-up poem, purportedly written years later:
Ah, yes! I wrote the ‘Purple Cow’—
I’m sorry, now, I wrote it!
But I can tell you anyhow,
I’ll kill you if you quote it!
Daniel
- Although the choice’s outrageous now.
They freeze whatever’s saleable.
I never thaw a Purple Cow.
I don’t think they’re available. *
–Peg Bracken.
Cecil’s “Cows are lousy tippers” column is very, very funny.
Why do cows wear bells?
Because their horns don’t work.
What is the most important use for cowhide?
It keeps the cow together.
Did you hear about the cow who couldn’t give milk?
She was an udder failure.
What does one call a sleeping cow?
A bull dozer.
A New York family bought a ranch out West where they intended to raise cattle. Friends visited and asked if the ranch had a name. “Well,” said the would-be cattleman, “I wanted to name it the Bar-J. My wife favored Suzy-Q, one son like the Flying-W, and the other wanted the Lazy-Y. So we’re calling it the Bar-J-Suzy-Q-Flying-W-Lazy-Y.” “But where are all your cattle?” the friends asked. “None survived the branding.”
You’re right - no good cow jokes…
NASA is planning to send a bunch of cows up on the next flight of the Space Shuttle, to test the effects of weightlessness on them.
The project will be known as “the herd shot 'round the world.”
What do you call a cow with only 3 legs?
…lean beef…
What do you call a cow that has had an abortion?
… de-calf-inated…
Oh, and Dazzling White Diamonds, the ‘weebles’ joke is one of my favourites
Bear in mind that do be Kosher an animal must not have been mistreated.
Have you seen the bales of hay that are rolled up instead of packed into a cube?
The Rabbinate in Memphis has said that beef from cows fed with this hay is not Kosher.
Why, you ask?
Because the cows do not get a square meal.
A city slicker is driving through the countryside and sees a farmer standing next to a cow with only three legs. He’s very curious about this, having thought such an animal would be destroyed, so he asks the farmer why the cow only has three legs.
“Let me tell you about this cow,” said the farmer. “This cow is the best cow in the world. About three years ago, we had a really bad house fire in the middle of the night. My whole family and I would have died in our beds if this cow hadn’t run into the house, woke us all up and led us through the smoke to safety.”
“Wow,” said the dude, obviously impressed.
“That’s not all,” continued the farmer. “Last year my tractor overturned in the back forty and pinned me beneath. This cow ran over, used a log as a lever to free me, and then let me ride her to the house, on account of my broken leg.”
“That is truly an amazing animal,” said the slicker. “But you still haven’t told me why she only has three legs.”
“Well,” said the farmer, “cow this good, you hate to eat her all at once.”
Do I remember the knock knock joke correctly that evilbeth referenced? I think it goes something like this…
Knock knock.
Who’s there?
Impatient cow.
Impatient cow wh
MOOOOOO!
Oh, snap!
And in that vein:
Yo mama’s so fat, Doc Martens had to kill three cows just to make her a pair of shoes.
A little city kid was playing in the yard on his uncle’s farm. He found several milk bottles laying in the grass, so he ran to his mother, yelling “Mom! Mom! I found a cow’s nest!”
I almost snorted milk out my nose when I first heard that. Not funny, indeed!