General synonyms:
* Farming
* Husbandry
* Cultivation
* Tilling
* Agribusiness
More specific synonyms:
* Crop production: Arable farming, horticulture, viticulture
* Livestock raising: Animal husbandry, ranching, dairying
* Forestry: Silviculture, arboriculture
* Fishery: Aquaculture, pisciculture
The best synonym for "agriculture" depends on the specific type of agriculture you're referring to.