This bronze-age culture undoubtedly extended across the Mekong to Laos, where the tin essential to form bronze (which is stronger than copper) alloys was being mined in the Nam Pa Taen valley. In the foothills east of the Mekong, it seems that a parallel culture was developing, based on horticulture and cultivation of taro and other root crops and dry-field rice. By the mid-first millennium BCE, farmers on the Khorat Plateau in northeast Thailand were using iron-tipped ploughs pulled by domesticated water buffaloes.