Just be honest and tell them that you can not be away from the farm at this time.  I've been called twice and had to be excused both times due to other things going on.  The first I had a trip scheduled that I could not rearrange and I told them I would be gone during that time.  They told me I would be called for the next pool and I said no problem.  The second was during springs work and I'm a sales agronomist at the local elevator.  I explained that I could not be away at that time and once again I was told that I would have to serve during the next pool which again was no problem.