<Li> <P> Aircraft Pushback tractor </P> </Li> <P> Aircraft Pushback tractor </P> <Ul> <Li> Nebraska tractor tests: Tests, mandated by the Nebraska Tractor Test Law and administered by the University of Nebraska, that objectively test the performance of all brands of tractors, 40 horsepower or more, sold in Nebraska . In the 1910s and 1920s, an era of snake oil sales and advertising tactics, the Nebraska tests helped farmers throughout North America to see through marketing claims and make informed buying decisions . The tests continue today, making sure tractors fulfill the manufacturer's advertised claims . </Li> <Li> Tractor war (s), Great tractor war (s): A period of ruinous competition and price warring between tractor manufacturers in the 1920s, which led to a consolidation in the industry . </Li> </Ul> <Li> Nebraska tractor tests: Tests, mandated by the Nebraska Tractor Test Law and administered by the University of Nebraska, that objectively test the performance of all brands of tractors, 40 horsepower or more, sold in Nebraska . In the 1910s and 1920s, an era of snake oil sales and advertising tactics, the Nebraska tests helped farmers throughout North America to see through marketing claims and make informed buying decisions . The tests continue today, making sure tractors fulfill the manufacturer's advertised claims . </Li>

When did farm tractor attachments or implements become more widely versatile