Combines came into Punjab in the 1970s and now cut most of its wheat and paddy. They are largely owned by contractors who move north with the harvest, working around the clock for a few weeks.
They cut fast and they cut high, leaving a stubble that a plough will not turn in easily. That single mechanical fact is the origin of the stubble burning problem.
They also ended harvest labour. Reaping was the largest employment of the agricultural year for landless households, done by hand with a sickle, paid in grain. That work is gone and nothing replaced it.
Straw management systems attached to the combine, and the Happy Seeder which sows through the residue, exist and are subsidised. Adoption is partial because the machines are expensive and the window is short.
A machine that solved one problem in 1975 created another that is now argued about in the Supreme Court every November.
The combine cuts high and leaves the stubble standing, which is the direct cause of the burning problem. A crop cut by hand left almost nothing in the field.
It also compressed the harvest into days rather than weeks, which is what made the double crop possible and what made the timing of paddy sowing so tight.
The machines are mostly owned by contractors who move north with the harvest, and a small farmer buys the service by the acre rather than owning the machine.