Origin of the term "eating your own dog food"

I'm trying to find the first usage of the term "eating your own dogfood", as a reference to companies, especially software companies, using their own products in house in order to more effectively troubleshoot. I'm fairly certain it originated with Microsoft, but I'd like to find out which year they first used it, if possible.

The Wikipedia article on the topic has a whole section on the origin of this phrase:

The editor of IEEE Software recounts that in the 1970s television advertisements for Alpo dog food, Lorne Greene pointed out that he fed Alpo to his own dogs. Another possible origin is the president of Kal Kan Pet Food, who was said to eat a can of his dog food at shareholders' meetings.

In 1988, Microsoft manager Paul Maritz sent Brian Valentine, test manager for Microsoft LAN Manager, an email titled "Eating our own Dogfood", challenging him to increase internal usage of the company's product. From there, the usage of the term spread through the company.

So the phrase didn't originate with Microsoft, but the first (known) use of the phrase in this context was indeed in 1988.

I seem to recall that tech companies were using "Eating our own cooking" prior to 1988. I also vaguely recall that some wag had changed that to "Eating your own dogfood", possibly as a jab at Microsoft. So it would be interesting to know if Maritz picked it up from the trade press or coined the phrase by morphing the Alpo claim with "Eating our own cooking". And in either case was "Eating our own dogfood" a bit of self-deprecating humor or was he oblivious to the negative implications of the transformed version?

By the way, Warren Buffett uses "Eating our own cooking" frequently. However, Buffett didn't meet Bill Gates until 1991. If Gates introduced this phrase into Microsoft, it was either because it was common marketing/sales jargon or he was reading Buffett shareholder letters.