If "reap" means to harvest:
* Sow: To plant seeds, the opposite of harvesting.
* Plant: To put seeds or young plants in the ground.
* Cultivate: To prepare and care for land for planting.
If "reap" means to receive a reward or consequence:
* Give: To provide something to someone else.
* Donate: To give something to a good cause.
* Contribute: To give something to a common effort.
If "reap" means to gain something as a result of actions:
* Lose: To fail to gain something.
* Miss out: To fail to benefit from something.
* Suffer: To experience negative consequences.
The best opposite word for "reap" depends on the specific situation and the intended meaning.