According to the story on page 28/682, in the old days, the master of the Royal Ploughing Ceremony was presided over by the monarch. His duty was to guide the plough behind the oxen within the guarded area.
Later on, the position has been changed to an appointee chosen by the government and approved by the king. That person would be a noble with high rank; ‘Praya’ or ‘Chao Praya’. The title of the master was named as ‘Praya Rakna’.
The reason that a royalty has been ruled out from taking the position of the master of the Ceremony is because the Ceremony of which the intention is to ensure good harvest but as weather cannot be predicted, the outcome is not always good.
If after the Ceremony is performed and it turns out that that year is a drought then, it is unavoidable that people must feel upset and blame the Ceremony especially the master who, for that year, happens to be a monarch that masters the Ceremony.
The king (don’t know who) did not want the royal family to be humiliated with such a non-existent matter so, he announced that since then on the royalty was to be ruled out from taking the position.
So, the risk falls completely on the appointee. There has always been a gossip that, each year, once the announcement of the master of the Ceremony is about to happen, each candidate prays and sometimes, even bribes divine entities to let him be a loser.
(The computer colored photo shows a Chao Praya who was appointed to be Praya Rakna, the master of the Royal Ploughing Ceremony, of the year (don't know what was the outcome!). This photo (B&W) was shot by King Rama V himself)
Note: Related story (page 42/1027), Praya Yuen Ching-Cha, the master of the Giant Swing Ceremony