Mars has said it does not know why a lorry load of faulty Skittles found spilled on a US road may have been on its way to become cattle feed.

The incident gained attention on social media after a sheriff posted on Facebook that "hundreds of thousands" of Skittles had been found spilled on a highway in Wisconsin.

Later, Dodge County Sheriff Dale Schmidt updated the post to say the sweets had fallen off a lorry had been expected to be used in animal feed.

As only red Skittles had spilled out, Schmidt also joked it would be difficult to "Taste the Rainbow."

Schmidt said one of his deputies came across the spill and sent him photographs, which he posted on online.

He said the Skittles, which had fallen out of their box and started to disintegrate in the rain, did not have the standard letter "S'' on them.

Mars spokeswoman Denise Young said the sweets were supposed to be destroyed because a power outage prevented the signature "S'' from being placed on the Skittles.

She said Mars planned to contact the sheriff's office and an unnamed farmer to find out more.

The company said the Skittles in question came from a factory in Illinois, which does not sell unused products for feed.

The other American plant that makes Skittles - in Texas - sells to a local processor that melts them down into syrup.

"We don't know how it ended up as it did and we are investigating," a Mars official said.

A variety of food byproducts are commonly used for animal feed, and the company said it has procedures for discarding foods for that purpose - all which follow Food and Drug Administration regulations.

Linda Kurtz, a corporate environmental manager at Mars, said they often sell unused sweets and ingredients to processors that incorporate them with other materials to make animal feed.

Josh Cribbs, a cattle nutritionist, added: "You might think, 'Oh my gosh, they might be eating a Skittle.

"In reality, that piece of candy is being broken down," he said.