I didn't ride in the beet truck last night after work....instead I went to the field that was being topped. I had ridden in plenty of grain-trucks in my time. I had never seen the topper work - and this was the last batch of the neighbors beets to be topped.
See the hidden top amongst the forage, thats the beet. Its been hiding and growing for the past several months accumulating sugar and hopefully making the farmer some money. After you've seen all the special equipment it takes to be a beet farmer - you understand the importance of sugar content and timing. The beet cannot be topped until just prior to being lifted. The farmer I rode with actually was warned by his field scout that they may have to abandon the field they were lifting on and go to a "fresher" field because removing the tops will cause the beet temperature to increase faster.
Basically, the topper is a street sweeper. The underside of the topper is pictured below. It moves along on top of the beet slicking the top right off.
If you have a good set of eyes, you can tell the difference in about 4 1/2 inches on the topped beets versus the untopped beats. Since they were concerned about the beet temperature, they were actually "temping" the beets throughout the day on the other field and were finally released to top this field last night about 4:30 p.m.
The topped beet rows look like this. These beets will probably stand uncovered like this hopefully no more than 24-30 hours before being lifted. The green forage will die down and you can see beets popping out of the ground.