The 4th amendment covers unreasonable searches. Unreasonable can also be the frequency. The FDA has every legal right to inspect a farm. It only has a problem when those inspections become excessive. When the norm is once a year, and for this one farm its being done twice a month, that sure looks to be excessive. Unless they were getting information every 2 weeks that something was happening, its would fall under being unreasonable.
I have found no FDA document that talks about how frequently they are allowed to inspect, only that they say reasonable.
As for states enforcing federal laws, you should say it differently. The federal government does not need to say its okay. There is a big difference between the federal government saying its okay, vs saying what is not okay. What got AZ into trouble is that the federal government explicitly said that only they could enforce immigration. Had the federal government said nothing, then AZ would be in the clear. That is different.
There are problems here.
First, if the FDA has jurisdiction to inspect the facility, then it has jurisdiction.
A local county Sheriff doesn't get to decide what is or isn't "unreasonable", because the jurisdiction to inspect the facility is rooted in FDA regulations.
Now, if the Sheriff looked up the FDA regulations and found that the FDA is only allowed to inspect the facility once a month, then the Sheriff is acting as an advocate for his constituent. Fair enough. But just saying, "I think every two weeks is unreasonable under the 4th amendment" doesn't matter, because they're operating under FDA regulations regarding inspections.
And, we don't even know all of the background details. Has the farm been requested to show X, Y, and Z, and has continually refused? Has it been denying access to the farm, so that inspectors are forced to come out every two weeks, in order to perform their jobs? Who knows, based upon the source of the information that is clearly biased in favor of telling the Federal Government to kill itself and rot in hell.
You have a Sheriff opining that he thinks that it's "harassment", when his opinion means absolutely nothing regarding whether the inspections are OK under FDA regulations. And again, we're not really talking about 4th amendment privileges here, because a farm that is operating under any number of Federal laws and regulations isn't acting as a private entity that has 4th amendment privileges in regards to "searches", because it has availed itself to FDA regulations regarding inspections. In other words, the farm falls under FDA regulations regarding inspections, and the 4th amendment is no longer operable if the farm falls under FDA regulations regarding inspections, when it comes to the FDA doing inspections.
Now, the private farmer himself still has 4th amendment rights as a private individual, but his rights don't extend to the farm, and inspections of the farm under FDA regulations. As far as I know, the FDA wasn't trying to enter his private living quarters to search for private documents, but was trying to get onto a farm to inspect farm-related activities. Unless you can point out where the FDA is not allowed to do so, then the inspection seems to be lawful.
The Sheriff doesn't appear to be doing anything more than what Bundy-ites want, which is pretending that a county Sheriff is somehow as sovereign as the Federal government in regards to all issues of law.
The Sheriff is unable to legally arrest a Federal official performing Federal activities, just because the Sheriff don't like it. Again, this whole issue played out from 1861-1865, and the Sheriff's opinion on state v. federal sovereignty lost out...nevermind the fact that the articles of confederation had been tossed out 80 years before that.