hmmm - making a living off anything that relies on weather and market variations is a dodgy business.
I know nothing about the climate in Chaiyaphum - that's a good honest start.
Cassava has bombed this year - from a high of around 6b per kilo I think the current rate is about 1.70 to 1.80b per kilo. It is marginal whether harvesting it is even worthwhile this year, or just plough it in due to costs of labour, tractors etc. I am an observer and offer no advice (I don't know enough), my partner has land of her own, separate from main family farm, and the costs of land prep, ploughing, fertiliser, planting, periodical weed spraying . . . plus costs of harvest - tractor and labour she may just break even on 10 rai of cassava. They are furious (mildly!) that 2 weeks ago the govt re-opened the border to even cheaper Cambodian cassava.
Just my exp - eucalyptus are low return but low risk too (apart from fire), plant and forget for 4-5 years and sell (for paper pulp), the stumps are left to re-sprout and grow 2-3 stems which are harvested at 3-4 years, then stumps ripped out and start again. Returns will vary - soil and fertiliser make a difference Ive seen some pitifully skinny trees, sold by weight per tonne. 2yrs ago I planted a small trial plot on the farm, about 1km from main plantation; just 100 trees to see if they would do better with loving care and attention, they get fertilised every 3mths, been hand-pruned, and I even talk to them. Unsure what will happen long-term, now at 2yrs they are taller (up to 9.5m), better looking (kidding!) but not as sturdy as the farm-grown ones. Seems the extra effort and expense was wasted, they survive on the worst quality soil.
Again, my exp but the returns on rice are so low, hence their leasing the land out, it's low-lying and useless for anything else. Other land they lease out gets used for mushrooms and corn crops.
My partner's b-in-law manages the farm property, he's also a part-time farm advisor for local council and I trust his judgment, he has moved more towards sugar for 2012-14 but won't rely fully on one crop type, and will persist with some cassava and has several blocks of eucalyptus of various ages, of course if you can afford not to harvest them their value/tonnage will increase each year.
A few weeks ago you may have seen pineapples in the news, growers dumped 5 tonne on the highway in protest at low prices - there is a nationwide glut, market has dropped, they were getting 5b per kilo but now closer to zero as the canneries are oversupplied. Locally farmers are selling them from their trucks, roadside for 5 baht, or 5 for 20b. Desperate stuff they must be losing heavily.
Rubber - knowledge limited to their ideal sites/climate, and that they are labour intensive, need to find your own tappers, read stories of them ripping off landowners (one kg for you, one for me) - doesn't sound ideal to me unless you have exp family member who can be trusted. I think seven years till first latex is produced.
More reading - a
Korat Cassava Protestor and
Pineapples on the Highway and pictures and stories from my
year on the farm in Sa Kaeo province if that doesn't put you to sleep nothing will.
cheers.