2002 Fendt 411 tractor radiator pipe keeps sucking in . What the problem?

If the lower radiator hose is collapsing, you have a crappy hose (joking) or the real possibility is that your radiator cap isn't sealing or is bad. Once the the cooling system seals and pressurizes, there's no way the lower hose can collapse. So basically, the thermostat is opening and the strong water pump on that diesel is sucking like it should but the cooling system pressure cap is failing so the hose can collapse. Get a new pressure cap for the radiator or find out why it's not sealing and holding 15 PSI pressure.

Some lower hoses have a coil spring in them to retain the hose from collapsing, again, you will need your dealer or repair manual to know. If it was replaced, I wouldn't put it past some farm boy to pitch the old hose and not pull the spring out of it for reuse. Check on the lower hose stiffer spring but regardless, that hose can not collapse when the cooling system has 15 PSI pressure so I would go that route first. Being that I am an old army maintenance guru, I have seen a lot of dedicated and proprietary stuff. Those lower hose stiffener springs are fairly common on diesel equipment that has an Uber cooling system designed to run 100% duty cycle like large CAT gensets and such..

It's a rubber HOSE? Cars used to have a big metal spring in the lower hose. Contact the manufacturer for their solution.