Having a cow, horse, or other animal pull a plough has some advantages. Animals can make more of themselves, and can self-repair when injured. If you have spare fields for them to graze in, they'll mostly feed themselves. When they break down, you can sometimes turn them into meat and leather.
However, as many of my LJ friends can attest, large animals are not maintenance-free. Vet bills can be enormous and unexpected hits to your budget. Horses eat like ... well, a horse, which either means huge feed bills, or large areas of pasture that you can't use for growing crops. If you don't have enough land and workforce to breed, raise, and train your own animals, they can be quite expensive to buy. And you have to keep feeding and caring for them year-round, not just when you need them to work. All this work gives you the equivalent power of a one-horsepower engine, at best.
Tractors powered by internal combustion engines aren't always better. Fuel is expensive and flammable. Engines require regular maintenance by skilled personnel. And most importantly, unless your farm is large, you'll never make enough money from your crops to cover the payments on a modern tractor.
I think there is a place for small, inexpensive, solar-powered electric tractors.
A few people have already tried making such tractors.
Here's a video of an antique gasoline-powered tractor converted to solar-electric power, for about $10,000, giving 18 horsepower.
Here's one built from scratch as an electric tractor, for $15,000, giving 60 horsepower.
However, these are too expensive for a small subsistence farming family. They are also more powerful than necessary if you're tilling a small plot, and occasionally hauling produce to town.
Here's an electric all-terrain vehicle that retails for $1700, giving 4 horsepower. It can climb a 40-degree slope, has a range of over 60 km, and has low-maintenance brushless motors and sealed batteries. With a few modifications, this would be a good tractor. Make it wider (to prevent overturning when ploughing hills), make it taller (so the undercarriage clears crops and furrows), and reshape the exterior so it looks more like a traditional tractor (so banks will approve a loan for agricultural equipment). Add sturdy towing hitches to the front and back. Add solar panels on top (for recharging, and to shade the farmer). And now you've got something that might help improve the lives of small farmers in developing regions.