the brahmins who began with sacrificing many animals to the devas began to frown on this practice as selfish (reward-seeking) and violent, and began to prefer sacrifices of rice, flowers, leaves, water, things like that. in the bhagavad gita sacrifice is reframed as any action done for God, not as being reward-seeking, and the sacrifice of knowledge is seen as the highest sacrifice of all. any deed done reverently without selfishness becomes nishkama karma.