I did a little research of other states and oddly enough, that is the pretty much what the law has been changed to in your state.  As I understand it does not apply in all counties and don't know if it applies to your location.  If Iowa had this fence law or something like it, I think most of the concerns discussed here would become a nonissue. 

As it stands, Iowa's fence law allows one party to effectively take property from another without just compensation.  It's been an an eye opener for me to see the arguements made to try to defend the current Iowa fence law.  It takes a really solid case to get a law turned over in court on constitutional grounds, but it's been done in other states.  These things are better to be delt with in the legislature.  The only case brought to the Iowa Supreme court I've found on this issue involved small acreages and was not considered to be significant enough to overturn the law on these grounds. 

Other states had fence law nearly the same as Iowa, but have made some adaptations so that people are not forced to put up fence for neighbors when the fence is of no value to them.  The arguement that the fence is of value to them because it keeps the neighbors cows out of their field is really convoluted, but oddly enough had some merrit with the powers that be in Iowa.  Other states have determined the cattle are the responsibility of the owner and not the responsibility of the neighbor with no livestock.  The owner of the cattle is responsible for fencing them in and is liable for damages if they get out.  That being the case, the fence is not of value to the neighbor without cattle because the owner of the cattle is liable should the cattle damage the neighbor's crops anyway.  The owner of the cattle has a choice:  fence the cattle in, or don't build a fence and pay for damages if they damage a neighbor's property.  It's not the neighbor's responsibility to fence in someone else's cattle in some other states as it currently is in Iowa.