Sales tax laws get very ... persnickety.

Many years ago I worked for an office machine dealer that sold cash registers.  The county raised taxes to 7.25% but exempted groceries to only 1%.  A C-store customer needed his registers reprogrammed.  His accountant said a fountain drink was taxable at full rate because it was ready to consume.  But if there was a lid on it, it was not ready to consume so it was 1%.  If there was a straw in the cup it was ready to consume and was full rate.  But if the straw was not put in the cup it was not ready to consume so it was lower rate.

Farm exemption is like that.  For instance (I am told.  That's a warning) in Illinois if I buy sickle sections to cut wheat as a crop they are exempt.  If I buy them to cut hay for my cattle they are exempt.  But if I buy them to cut hay for my horses they are not exempt ... unless I buy them for hay for my horses I breed and sell as a farm business.

An example from https://tax.iowa.gov/farmers-guide-iowa-taxes#taxex

Example 4: Skidloader

Exempt if primarily used in agricultural production. A co-op uses a skidloader 60% of the time to clean the feedlot of its livestock operation. The remaining 40% is used to handle fertilizer and do utility work on the co-op premises. The skidloader is exempt because it is primarily (more than 50%) used in agricultural production.
Taxable if a co-op uses a skidloader 100% of the time to load bags of seed, fertilizer, and feed on customer’s trucks.