Only twenty minutes later, they began to feel the effects of the other portion of the meal they had eaten.

At first, they started farting again. They stood up and farted in each others' faces, and, a minute later, their pee-soaked butts, even Rainbow Dash and Applejack.

Then, they felt a pressure in their bowels that was more than just a fart. They still let out a few more farts which were wetter, smellier, and, soon enough, started to stain their panties.

As the pressure grew, they each stood side by side, but facing opposite directions, so that each pony's head was close enough to the other pony's rear to get a good view.

"Wanna see how long we can hold it for best out of three?" said Rainbow Dash.

"You're on," said Applejack.

Only ten minutes went by. Then, Rarity felt another fart come, and, rather than let just it out, she pushed as hard as she could.

PPPPPPPPPBBBBBBBLLLLLLLLLLLRRRRRRRRRRT!

Rarity sighed as she released a wet brown mess into her purple panties. Fluttershy could see a big brown bulge on Rarity's butt.

Since Rarity messed herself, now it was her turn. Fluttershy closed her eyes tight and pushed out an equally wet, equally smelly mess into her green panties. She let out a sigh once she was done.

"Ooh, it's that time now?" said Pinkie, and she released a ton of hyperscat into her blue panties, to the point it overflowed and leaked down her legs.

"Wow, you messed yourself pretty bad," said Twilight.

"I ate a lot more than you," said Pinkie cheerfully.

"My turn now!" Twilight raised her rump in the air.

PPPPBBBBBBBBBBLLLLLSSSSSSSSHHHHRRRRRT!

Twilight's eyes went wide as she filled her panties with diarrhea. The mudslide flowed down her back legs while turning the butt of her panties brown, and what few particles were close to solid caused her underwear to bulge at the back. It smelled even worse than Pinkie's mess. When she was done soiling her panties, she looked as though she had sat in a big mud puddle.

"Looks like someone's got the trots!" said Pinkie, giggling, while Spike was laughing again.

Twilight also giggled. She hadn't been expecting it to happen this way, but she still liked it.

Ten more minutes went by while Rainbow Dash and Applejack continued to hold it, each trying to outlast the other.

Finally, Rainbow Dash couldn't hold it anymore. She groaned as she felt a log make its way out of her butt and into her panties.

Applejack had a clear view of the rapidly-expanding bulge on Rainbow Dash's butt. "Ah win again!" She wasted no time in letting her own logs of poop ooze out into her red panties.

Once Rainbow's first two logs were in her panties, she released the third wave, which was noisier and a lot wetter, much like Rarity, Fluttershy, and Pinkie. Although she had lost the contest, she could at least still enjoy the dirty sensation of messing her wet panties. It wasn't long before Rainbow's panties were overflowing with manure.

After they both finished, each pony rubbed her butt against her partner's butt, squishing each other's messes between them. As 40 minutes had gone by, they peed again, for the remainder of what they drank had now gone through them.

Then they sat around in a circle, letting out a collective sigh of satisfaction. They were very wet, dirty, and smelly, especially their heavily soiled panties, and they were sitting in Rainbow Dash and Applejack's puddle, and they all got a naughty pleasure out of every bit of it.

"That was awesome!" said Rainbow Dash.

"You can say that again," said Spike, who had enjoyed the show. Of course he was clear this was to be kept between the seven of them.

"We're gonna be having so much fun with these panties!" said Pinkie Pie.

"We'll need to wash them really good," said Twilight.

"It doesn't matter if they're still stained or if they still smell a little," said Rarity. "I made these panties specifically for using as a toilet on our sessions. We won't be wearing them anywhere else."

Twilight cleaned up the mess they had made in the room with her magic. She didn't bother using air freshener because the window was open so the room would air out soon now that it was clean. They also dumped the mess out of their panties and flushed it down the toilet.

Then the ponies took a long bubble bath, still wearing their dirty panties. With their usual pairings, they especially scrubbed each other's butts with extra soap until they were completely clean and didn't smell. Their panties still had a bit of a brown stain on the butt, but they didn't care - they knew they'd be wetting and messing them again for years to come!

Author's Note

I changed the last paragraph a bit to include the butt scrubbing in the bath.