One of the most damaging pests of alfalfa is the pocket gopher. A recent study estimated that, when present, pocket gophers resulted in an 8.8% loss in revenue. The damage that pocket gophers cause to alfalfa can be quite varied but includes consumption of tap roots and above-ground vegetation that can result in reduced vigor and/or mortality of alfalfa plants, loss of irrigation water down burrow systems, and chewing on underground irrigation lines. Pocket gopher mounds can result in additional problems including serving as weed seed beds, burying of plants, and causing damage to farm equipment.
A number of options are currently available for controlling pocket gophers but most control centers on toxic baits, burrow fumigants, and trapping. Other control options are available as well, although their efficacy is either less clear or insufficient to recommend. Of the different control options, baiting is generally considered to be the easiest and quickest method for application. 1.8% strychnine is usually considered to be the most effective bait option. However, supplies of strychnine are currently quite low in theU.S., so the 1.8% formulation is not currently in production. Other bait options include 0.5% strychnine, 0.005% chlorophacinone, 0.005% diphacinone, and 2% zinc phosphide. Little comparative information is available on these materials, so I unfortunately have no real insight as to which alternative option to try. Perhaps you can try several and see which works best for you.
Although bait application is typically the quickest and easiest option for pocket gopher control, trapping and burrow fumigation with aluminum phosphide are generally considered more effective. Of these, aluminum phosphide is more effective and quicker to apply. However, aluminum phosphide is a highly restricted material for use; the user must know how to properly use it before applying. That being said, studies have shown >95% efficacy with aluminum phosphide, so it can definitely be very effective.
Trapping can also be very effective. It is more labor intensive than burrow fumigation, but it is not a restricted-use product, so no certification or reporting is required. As such, I like to use trapping when dealing with small populations of pocket gophers or when using it as a follow-up method to aluminum phosphide or baiting to clean out remaining pocket gophers. Several different styles of traps are available, but the Gophinator may be the most practical and effective (see Baldwin et al. 2013 for trap comparison study) for use in alfalfa fields.
Other control options are currently in use for controlling pocket gophers. However, these alternative control options are either insufficiently tested at this point to recommend (e.g., carbon monoxide producing machines), have proven ineffective (e.g., biocontrol, gas explosive device), or are not practical in many situations (e.g., flood irrigation). Greater detail on all of these pocket gopher control options can be found in Baldwin 2011.
Lastly, I strongly recommend that when managing pocket gopher populations, you should utilize an Integrated Pest Management approach. You will find that in most situations, you will be much more successful when you utilize multiple approaches rather than relying on any single control measure. Much detailed information can be found on developing such a plan in Baldwin 2011.