Japan's major islands aren't particularly spacious by continental standards, and one will not find major pasturage anywhere but on Hokkaidō. Why, then, would one of the 214 lexical markers (the "radicals") found in Japanese characters have "cow," or "ox" among them? Well, because the Japanese writing system...comes from China. Please don't forget that. The Japanese borrowed the language wholesale (this is only a slight exaggeration) from China in the sixth, seventh, and eighth centuries, and then worked the materials into a truly distinctive (indeed, symphonic) blending of native culture and foreign borrowing.
"Cow" came out in the wash, and has been a part of the language ever since. Even though Japan has never had major herds, it has become known for everything from Kobe beef to sukiyaki. Take a look at the etymology of the character.
Chinese (Mandarin): niu2
Chinese (Cantonese): ngau4
Japanese (On reading): go, gyū
Japanese (Kun readings): ushi
Korean:
Selections from The New Nelson Japanese-English Character Dictionary.
牛 Radical 93
Ushi cow. At left: ushi hen. Nickname: Cow.
牛皮 ぎゅうひ gyūhi cowhide (cow + skin)
牛羊 ぎゅうよう gyūyō cows and sheep (cow + sheep)
牛馬 ぎゅうば gyūba cows and horses (cow + horse)
牛肉 ぎゅうにく gyūniku beef (cow + meat)
牛車 ぎゅうしゃ gyūsha oxcart (cow + cart)
牛舎 ぎゅうしゃ gyūsha cow barn/pen (cow + shelter)
牛追 うしお(い) ushio(i) cattle herder (cow + drive)
牛使 うしつか(い) ushitsuka(i) cattle herder (cow + lead)
牛鍋 ぎゅうなべ gyūnabe sukiyaki (cow + pot)
牛糞 ぎゅうふん gyūfun cow manure (cow + excrement)
The standard combinations above (and they certainly have their own kind of intellectual adhesion, as they bind together like Velcro) give way to compounds that require at least a little bit of contemplation.
牛蒡 ごうぼう gobō burdock (cow + burdock)
牛痘 ぎゅうとう gyūtō cowpox, vaccine (cow + pox/smallpox)
牛疫 ぎゅうえき gyūeki rinderpest (cow + epidemic)
牛飲 ぎゅういん gyūin drink like a cow (cow + drink)
and, finally...gluttony:
牛飲馬食 ぎゅういんばしょく gyūinbashoku gluttony (cow + drink + horse + eat)
We'll conclude with a number of characters that have the element 牛 in them. Be sure to note the difference between the radical marker on the left side of many "ox/cow radical" characters and the normal form you have seen up until now. The left side element is called ushi hen, and should be clearly distinguished from the "hand" radical (number 64).
Compare these two characters: 捕 and 特. The first character has the "hand" radical, while the second has the "cow" radical. Learn to distinguish these.
牢 ロウ RŌ prison, jail, hardness
牧 ボク BOKU shepherd, care for, raise
物 ブツ, モツ, もの BUTSU, MOTSU, mono thing, object, matter
牲 セイ, いけにえ SEI, ikenie sacrifice, offering, gift
特 とく toku special, especially
牽 ケン, ひく KEN, hi(ku) draw, pull, haul, tug
