Add this to your class that has @Mod.EventBusSubscriber
@SubscribeEvent
public static void onHarvest(BlockEvent.HarvestDropsEvent event)
{
if (event.getHarvester() != null)
{
List<ItemStack> drops = event.getDrops();
if(drops!=null && drops.size()>0) {
BlockPos pos = event.getPos();
IBlockState blockstate = event.getWorld().getBlockState(pos);
Block block = blockstate.getBlock();
if(block instanceof BlockGrass) {
drops.add(new ItemStack(Item.REGISTRY.getObject(new ResourceLocation("minecraft", "wheat_seeds"))));
}
}
}
}