Farmers must go into the barn or the pasture and visually observe each cow for the visible signs of heat. This has to happen 365 days a year, in the heat of the summer, the cold of winter, and during the rainiest of days. Even without missing a day, farmers still may only catch 50% of the cows in heat considering that cows can show heat signs at night when the farmers are not watching.
On larger farms with more staff, there can be discrepancies in the way different people track or view heat signs. And, each person must record their findings. Whether using a pen and paper or a spreadsheet, manual data entry always runs the risk of inadvertent mistakes and potential misinterpretations, all of which can lead to missed opportunities for catching the right moment for insemination. Finally, on a large farm with hundreds or thousands of cows, the sheer amount of time that must be spent on watching for heat signs takes away from time that could be spent on other necessary tasks.
The biggest downside of manual heat tracking is that it requires a lot of manpower hours and still results in missing up to 50% of cows in heat, reducing the reliability of this method.