Preston’s Dog Food is a company owned by Preston, a robotic dog designed to look after Wendolene, and may have been built to shear sheep as well. In A Close Shave, he began turning them into dog food instead. Wendolene got upset at this and ordered him to stop the sheep thefts. Instead, he locked her and Shaun up with the intention of turning her and the other sheep into dog meat as well.
Later, in the factory, Wallace, Wendolene, Gromit and the flock were almost turned into dog food until Shaun came to the rescue. In doing so, Preston ended up being minced himself as cogs, springs and pieces of metal filled up cans of the product. It’s unknown what happened to the company after Preston’s defeat and recreation, although near the end of the film, there was an article in the newspaper mentioning that Preston’s Dog Food is under investigation for the sheep rustling after Gromit was cleared of the charges.
In Episode 3 of Wallace & Gromit's Grand Adventures, a can of Preston's Dog Food can be seen, suggesting that the company is still in operation, presumably under new management.