Jerry Bertoldo, DVM, Dairy Specialist, NWNY Dairy, Livestock & Field Crops Team
Lots of grass intended for making hay and haylage has been harvested with armyworms. The consensus is that for dry hay there is little concern. The armyworm caterpillars do not remain on dry forage, instead they seek succulent grass or corn to munch on. Armyworms are not known to contain any toxins, so accidental grazing or including of some in baled hay is not a problem.
Ensiled hay crop contaminated with armyworms may pose a different situation. Unfortunately, there is little information as to what can be expected when you chop up these pests and put them up in some form of haylage storage. It is known that haylage with high numbers of caterpillars becomes dark, slimy and smelly in a short period of time. Whether this “animal” protein leads to dangerous bacterial growth during storage is not certain. Similar ensiling of animal carcasses can lead to botulism risk. General clostridium growth in haylage is common to high protein legumes when put up too wet (<30% DM) and where the pH does not drop to 5.0 or lower. Haylage put up in air tight bags or upright silos at 40% DM or greater has less risk of this abnormal bacterial growth even though the pH does not drop to an ideal level.
The Kentucky abortion problem in horses some time ago has been tied to tent caterpillars, a very different species. Here the intestinal inflammatory reaction to the caterpillar hairs creates a compromise in the ability of the gut to keep bacteria and gut toxins out of the blood stream. This is what led to the mysterious abortion issues. Armyworm caterpillars are hairless hence no concern.
In summary, armyworms either accidently consumed with pasture or in dry hay are not a worry. Ensiled ones might be a concern. Shoot for the dry matter appropriate for the storage method used. If a haylage looks and smells bad, don’t feed it. Mark any baleage or AgBags that might have caterpillar contaminated haycrop included and check it before feed out. If you have layered a bunker silo with contaminated material, it will be more difficult to assess what you have. The amount of dangerous bacteria needed to cause health problems can be very small. That is known. How much risk is there for them to be present is the big question.
This article can be found online at: http://www.nwnyteam.org/MikesPestUpdate.htm.
