Grains infested with the Aspergillus flavus mold that produces aflatoxin will fluoresce a greenish-yellow color under black light. The color is not due to the aflatoxin itself, but a substance called kojic acid that the infected grains produce. To determine actual aflatoxin level, the grain has to be sent to a laboratory for more specific testing. Commercial hay-testing labs often offer fungal toxin testing, including aflatoxin, or you can contact your state’s Department of Agriculture and request testing.

False negatives can occur over time when grain is stored. Black light also will not reveal other types of fungal toxins. False positives are possible as well, but this includes a lot of things you wouldn’t want to find in your grain either, including Pseudomonas bacteria, some other types of mold and rodent urine.

Black light can also be used to "see" some bacterial and fungal skin diseases although, again, there can be both false positives and false negatives. Black light can make lice and their eggs (nits) much easier to see.

Think your feed room and hay storage areas are rodent proofed? You might think again if you check them with a black light. Rodent urine is strongly fluorescent.

Products

Black light products range in price from inexpensive penlight/flashlight types (like those used at night clubs after a fluorescing stamp is placed on the customer’s hand) to medical- and technical-grade units, like a Wood’s Lamp, that cost several hundred dollars. The most inexpensive lamps use regular light bulbs and a glass filter called Wood’s Glass that will block all but the long wavelength UV light. A compromise is commercial-grade units that produce light of only the longest UV wavelengths, UVA, and have enough wattage to generate a bright pattern. We’ve listed some representative products with moderate pricing in our chart.

Bottom Line

Black-light screening of what you feed your horse is far from foolproof, but it could prevent you from inadvertently feeding aflatoxin. If you feed corn, soybean or other seed meals, wheat/grain byproducts or any commercial feed containing them, a black light might be a reasonable investment.