Rice,lasts a long time and no funny odour.
Our bags are still super after 2yrs.
Just test for the time in micro oven +_ 2mins depends on bag size and oven. Wheat can burn and a fire hazard.
From Bev

Mine are wheat with a little lavender in them. They have lasted 6 years and going strong

Rice is the way to, i agree with bevgrift

I use rice and just fill the bag and when use it just warm in microwave. Works great for me. 100% cotton so it won't melt or burn in microwave.

I have used Rice AND dried Field Corn. The Corn stays hot LONGER than the rice.

Someone left a wheat bag in the microwave in our school staffroom and left the room, You could smell burning for weeks!

Cuties, I use fleece under the 100% cotton when I embroider on these, but now I am worried. Tested fleece with a lighter and it does not burn like cotton, it melts! Is this a problem, seeing that it is inside and the outside is cotton??!!

Wheat seemed to be used in the very early stages of these microwave heat packs then stories started coming about the wheat, after being heated too many times, being a fire risk or spontaneously combusting, so most turned to rice or cherry pips etc. Then there were stories about Lavender being toxic when heated, so it was decided it was better to add lavender in a sachet to an already heated pack, not micro it. I've also heard that a half a cup of water should be in the microwave when you heat the packs.

Rice in mine, wheat intolerances in this house. Never heard of using wheat before. From what I'm seeing others post wheats a bad idea and stick with cotton.

I use rice. But my daughter uses corn. From the feed store. Cheaper than rice. And it works. I just find that rice is easier for me to get.

I use rice in all of mine.

Depends a bit on where you live. Here mostly wheat (+lavender) is used, mine has lasted for ages although you can no longer smell the lavender.
I've filled some small ones with rice, which is more expensive here and corn is unavailable.
I do them in cotton and put them in an embroidered fleece outer covering afterwards.

I have heard of dried beans....but mine is rice and I can't wait to change it from the tube sock to a pretty bag one day. Man...where are the hours in a day?

I never heard about rice before :o)
Use wheat for years for smaller heat pads, for babies grape-pipes (lighter and smaller) and for bigger pads cherry-pipes (those you can handwash without problems).
Had one in the oven to test a new filling cause I made for a customer wings at one of my knuddies (currant-pipes; never seen something so small and fine with a better taste :o) and forgott it in the oven for about 30minutes; no problem! Temperature was still agreable (take 100°C Celsius top-/bottomheat) and even the wings with the currant-pipes made no trouble.
I never would put something dry like lavender inside cause I fear it could be a problem with the heat (most of all in the micro).
Every customer gets a manual from me how to use it with the temperature and the times and the most important for me: After using the micro for heating please toss the heat-pad cause they don't heat equally!
Greetings, Bettina

Just today there is another post about heat-pads and their filling ;o)
Greetings, Bettina

Filler Options
■Uncooked rice
■Wheat
■Feed corn
■Buckwheat hulls
■Barley
■Oatmeal
■Beans
■Flax seed
■Cherry pits
You can also add the following to the above for a soothing fragrant heating pad: Spices, herbs, essential oils.

■Ideas: lavender, rose petals, ground cloves, nutmeg, ginger, rosemary, cinnamon, peppermint oil, crushed mint
■If using: Mix herbs, spices and essential oil with choice heating pad filler (such as rice) and let sit in a sealed container for a few days (occasionally stirring). This will help set and distribute the fragrance a bit.

Don’t microwave anything other than cotton fabrics.

Copied and pasted from an article I saved to my compter.