'Cow Magnets' are recommended for cows to drink to improve their health.

Ingesting magnets can lead to internal organ damage , sometimes requiring surgery, but feeding magnets to domestic cattle is recommended for their health.

Classification of foreign bodies in the digestive tract of cattle
(PDF file) https://www.pref.tottori.lg.jp/secure/831113/16.pdf

Application of permanent rod magnets to the prevention of traumatic gastritis and other diseases and the method of pulling them up
https://www.jstage.jst.go.jp/article/jvma1951/16/12/16_12_449/_article/-char/ja

Effect of magnets on sharp metallic objects in the reticulum of growing calves
https://www.jstage.jst.go.jp/article/jvma1951/48/12/48_12_941/_article/-char/ja

'Cow Magnet' Part 1 | Manufacturing and sales of permanent magnets - Magna
https://www.magna-tokyo.com/outtech/support/blog/no1

Cows have a habit of swallowing their feed almost without chewing it. Furthermore, cows are very curious and tend to lick various places in the barn and swallow anything that they find lying around. This can lead to them swallowing metal pieces such as nails and screws that get mixed in with the feed. In a survey conducted by the Kurayoshi Livestock Hygiene Center from 2014 to 2016, metal pieces were found in the digestive tracts of 67% of 18 cows that had swallowed foreign objects.

Metals swallowed by cows are not excreted, but remain in the second stomach (receptacle), and the contractions and movements of the stomach can cause metal pieces to pierce the stomach. A method was devised to prevent damage to the stomach from these metal pieces: 'have the cow drink a magnet and have the metal in the stomach stick to the magnet.' The magnets that are given to cows are called 'cow magnets' or 'parnets,' and many livestock farmers give magnets to their cows. You can see how a cow actually drinks a magnet in the following movie.

BOVIVET Ruminal Magnets - YouTube


The preventive method of having cows swallow magnets is also recommended in Japan, and the Tango Livestock Health and Hygiene Center in Kyoto Prefecture has created the following pamphlet to promote the use of magnets as a preventive measure.

Give your pet a permanent magnet to prevent traumatic gastritis | Tango Livestock Hygiene Center, Kyoto Prefecture
(PDF file) https://www.pref.kyoto.jp/tango-kaho/documents/h29n022.pdf

Giving magnets to cows is effective not only in preventing stomach damage but also in improving it. For example, a research paper published by Obihiro University of Agriculture and Veterinary Medicine in 1995 reported that 'when cows with stomachs damaged by metal fragments were given magnets, their weight gain improved.'

In addition, improvements are also being made to the shape and structure of magnets. For example, the magnet manufacturer Magna has developed a cow magnet called 'Ryumax 21' (top photo), which has succeeded in reducing the protruding metal pieces compared to conventional magnets (bottom photo) by devising an arrangement of magnets.