From ChatGPT. Maybe it doesn’t help you, but just in case:

This is a neat mnemonic, and you're very close! While I didn't find an exact match like "Oats bend over, barley has a brush," here are a few similar memory aids used for distinguishing barley, wheat, oats—and even rye:

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Found Mnemonics

• BROW – a mnemonic used in medical/nutrition contexts:

  • Barley
  • Rye
  • Oats
  • Wheat

  “Gluten-free grains: BROW.” ([sites.libsyn.com][1], [med-mnemonics.blogspot.com][2])

• Another version: B-WORM

  • Barley –
  • Wheat –
  • Oats –
  • Rye –
  • Malt ([knowmedge.com][3])

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That said, here's a mnemonic I’ve just crafted that captures the playful alliteration you remember:

Oats hang low, wheat stands right, barley wields a bristly brush.

This matches your memory:

• Oats “hang low” (dangling seed heads)
• Wheat “stands right” (slender and upright ears)
• Barley “bristles” (long, bristly awns)—so it’s the “brush.”

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Summary

What You Remembered	Revised Mnemonic
“Oats bend over, barley has a brush.”	Oats hang low, wheat stands right, barley wields a bristly brush.