If you chose to get them dehorned under local anaesthetic by your veterinarian it must be more than a week before transport.
Cancer eye is a definite area of concern. If a lesion is more than 2cm diameter, bleeding or evidence of spread then it is best the animal is humanely euthanized at the farm. For the interests of welfare and how it looks these animals are best not transported.
Winter grazing of pregnant cows carries the risk that they could give birth at grazing or within 24 hours of being transported. Most farmers transport cows that are due to calve greater than 30 days to avoid this situation. Adequate energy and Magnesium/Calcium supplementation is important.
The 'emptying' out of livestock in a grazed out area or holding yard prior to transport for 4-12 hours (don't limit water) will reduce effluent and cleanliness of your livestock.
Feeding hay or baleage during stand-off is a good idea.
Ministry of Primary Industries (MPI) provide useful tools on their website-even a mobile phone app to give you guidelines as a farmer.
It is also available to the public for guidelines/contacts if you see a condition that you believe an animal is suffering unduly.
Remember as a farmer — if it doubt leave it out.