Why Does Silo Have Holes?

Silo north of Bertrand on NN 0 W Granite Rd 12-05-2015Road Warriorette Shari and I cruised Scott and Mississippi counties looking for photos to illustrate my Bootheel project. Late in the afternoon, when everything took on a golden glow, we spotted this silo on CR NN near West Granite Road north or Bertrand.

The recent cold nights must have killed whatever foliage had attached itself to the structure.

My silo ignorance is showing

Silo north of Bertrand on NN 0 W Granite Rd 12-05-2015One of the problems with documenting an agricultural area is that I know practically nothing about farming.

Would someone explain why this silo (and  couple others we saw in the same area) have holes running up the side? Click on the photos to make them larger.