A movie shot in a country farm setting. Probably Texas or something similar. Farming family goes to a combine harvester demolition derby. I can't remember much more. I watched it in late 80's early 90's.
You are thinking of the 1987 television movie called "Race Against the Harvest" or "American Harvest".
It is about a pair of families that are contract harvesters in the Midwest (Texas) of the United States, who don't speak to each other because of an incident in the past. The film details the lives of contract harvesters as they battle weather and each other, and there is a scene where the rival families go at each other in their harvesters.