Dick Kleis used a manure spreader to spell out "HAP B DAY LUV U" in a field visible from the living room of the couple's home.
Wife Carole was surprisingly happy with the gift of more than 50 tonnes of dung, reports the local Telegraph Herald.
"He's done weird things before for birthdays," she said. "But maybe not this weird."
Mr Kleis did also take his wife out for dinner and a movie to celebrate her 67th birthday, but wanted to do something unique.
"Someone said 'Do something you're good at', so I made a card with my 'pen'," he said. "I've just run out of stuff to do for her birthday."
Mr Kleis said it took about three hours and four loads of liquid manure to create the message.
"I was going to put a heart out there after the happy birthday, but I ran out of manure," he said.