How hot is it in Texas? It’s so hot that “the cows are giving evaporated milk.”
 
This “evaporated milk” joke (and many others like it) has been on the internet since at least 1998. However, the “powdered milk” version of the joke dates back to at least the 1930s.
 
“What do you get from an invisible cow?“/“Evaporated milk” and “What do little ghosts drink?”/“Evaporated milk” are related “evaporated milk” jokes.
   
   
Stanford Texas Club: Humor
You Know You’re In Texas When…
The cows are giving evaporated milk.
 
It’s So Hot In Texas That…
The cows are giving evaporated milk.
     
11 September 1930, Burlington (NC) Daily Times, pg. 8, col. 2:
“It was so dry out there for three months, our cows gave powdered milk.”
(In Kansas —ed.)
 
1 January 1935, Greeley (CO) Daily Tribune, pg. 9, col. 6:
Pastures were so dry about Chipowa Falls, Wis., Guy R. Jowett said, that cows gave powdered milk. 
   
20 August 1936, Placerville (CA) Mountain Democrat, pg. 9, col. 1:
The Jackson County (Kansas) Signal observes that if it gets much drier in that vicinity, the cows will be giving powdered milk. 
 
21 October 1947, Portland (ME) Press-Herald, pg. 4, col. 2:
“Our wells are dry. Our cows are giving powdered milk.” 
 
11 October 1953, Joplin (MO) Globe, pg. 4A, col. 4:
SO DRY THAT SOME COWS
“GIVE POWDERED MILK”
Columbia, Mo. - The good humor of Missourians seems unsurpassed and the disasterous drought has been responsible for several wisecracks that are making the rounds.
 
Down in Texas county, for example, where the people seem to have a lot in common with the residents of that great state by the same name, the boys are saying, “It is so dry in Texas county that the cows are giving powdered milk.”
   
23 October 1955, Galveston (TX) Daily News, pg. 31, col. 4:
“It’s been so dry for so long in one part of West Texas that the cows give powdered milk.”
(From You Can Always Tell a Texan (But You Can’t Tell Him Much) by Boyce House—ed.)
   
13 April 1960, Dallas (TX) Morning News, section 1, pg. 1:
MOST PEOPLE just don’t understand West Texas.
 
Here comes Ray Eisenhauer, for instance, and says he hears it gets so dry in the Lariat country that the cows give powdered milk. 
 
Google Groups: alt.hahaha
From:  m. mckeller
Date:  Wed, Jul 29 1998 12:00 am
 
A sad Texan once prayed, “I wish it would rain - not so much for me, I’ve seen it—but for my seven year old.”

It’s so dry that the trees are whistlin’ for the dogs.
 
It’s so dry that the cows are giving evaporated milk.
 
A visitor to Texas once asked, “Does it ever rain out here? “Yes it does.” A rancher replied. “Do you remember that part in the bible when it rained for 40 days and 40 nights?”
“Yes i’m familar with Noah’s flood”, the visitor replied. “Well”, the rancher said, “We got two and a half inches during that spell.”
 
There’s an old Texas joke about a man who bet several of his friends that it would never rain again, and collected from two of them.