Probably water in the hydraulic fluid. What happens is the water freezes and plugs up the hydraulic filter. I would do a crackle test and go from there. Crackle test is easy and it is a zero cost if you have some sort of hot plate. CRACKLE TEST. If it is not possible to change the fluid for a while and you still need the tractor then try to preheat the tractor in a garage or use a blower type heater.
It does not have to have water in the system to do that but it is a contributing factor. Cold oil is thick oil, therefore the viscosity of the trans/hyd oil is more critical as well as the cleanliness of the filter screen. It was because of poor cold weather performance that I switched to Dextron ATF. The hyd systen has become much more responsive in the winter and I do not get pump squeal (That can actually do some real damage to the pump--oil starvation). If you do not want to switch to the ATF still make sure you have the proper grade of oil in the trans and the filter screen is clean.
Mike
Water in the hydraulic system freezing?