Colostrum fed by bottle is preferable in that the suckling effect activates the esophageal groove and ensures the colostrum gets into the correct stomach being the abomasum and not the rumen.
When calves are stomach tubed the colostrum goes into the rumen which reduces the ability of the calf to absorb immunoglobulins. Therefor the second feed is required to achieve the desired amount of immunoglobulin absorption.
With a large number of calves being born on a daily basis in some of the large dairy units, a standard practice that seems to be put into place is the feeding of 4-5 liters of colostrum using a stomach tube as standard routine.
This may not be in best interest of the calf as insufficient immunoglobulin will be absorbed through the rumen. It may take more time to feed calves by using a bottle, but the desired long-term health consequences are for better by bottle feeding compared with tube feeding.