Drought raises potential for cattle to be exposed to plant toxicities

Jul 01, 2007

Watch out for plant toxicity during drought conditions.

In some cases, plants become even more toxic to cattle during a drought, but, more than likely, cattle ingest toxic plants because of the lack of other feedstuffs.

This article examines some of the plants to watch out for and addresses additional factors that contribute to this problem.

Let's start with a list of reasons cattle fall ill to plant toxicity during dry conditions:

For cattle, differentiating between "good" and "bad" plants is a learned behavior. Consequently, toxicity is more likely to occur in young animals and those moved to a new location. Droughts often contribute to increased weed infestation of pastures, hay and crop fields, allowing greater access to these toxic plants. Cattle may be penned in corrals or drawn to low-lying areas — two more possible trouble spots. Cattle in poor body condition may have problems detoxifying plants.

For all these reasons, a grazing-management and supplemental-feeding plan are essential. Veterinarians and producers should be familiar with plants in their area that are toxic to cattle. The following discussion covers some of the plants and situations to watch out for in drought conditions. Keep in mind: There might be plants in some regions not covered in this report.

The nitrate, prussic-acid threat

Stressed plants accumulate nitrates and prussic acid (cyanide) more readily. Drought stress can cause both pasture forages and weeds to accumulate toxic levels of nitrates. Recently fertilized pastures are at higher risk, too. Remember, plants that have accumulated nitrates remain toxic after baling or ensiling. But forages can be tested for nitrates to prevent poisoning.

Prussic acid accumulates most often in sorghums, sudans and Johnson grasses. (These plants can accumulate nitrates, too.) While there isn't a test for prussic acid, it dissipates when plants are baled or ensiled. In other words, harvested forages are safe.

Prevention is critical when it comes to nitrate or prussic-acid poisoning, because mortality rates are high. Cattle with nitrate toxicity suffer from methemoglobinemia (brown blood) and cattle with prussic-acid toxicity have cyanohemoglobinemia (bright-, cherry-red blood). Nitrate and prussic acid both interfere with the oxygen-carrying capacity in the blood, so pregnant cattle surviving these poisonings often abort.

Coffeeweed and sicklepod

Two of the most toxic plants found in croplands and pastures are coffeeweed and sicklepod. Cattle generally will not graze the green plant unless other forages are scarce. They will readily eat the seed pods that dry after a frost. The plant remains toxic when harvested in hay, balage or silage. Coffeeweed and sicklepod toxicity in cattle cause weakness, diarrhea, dark urine and the inability to rise. There is no specific treatment or anecdote. Once animals are down, they rarely recover.

Pigweed or carelessweed is very common in areas where cattle congregate. Cattle will readily eat the young plants, but avoid the older plants unless forced to eat them. Most commonly, pigweed poisoning occurs when the plant is growing in the pen or corral, yet no hay or feed is provided. Redroot pigweed is more toxic than spiny-root pigweed, but is less common. Pigweed can accumulate nitrates, so sudden death is the most common outcome. It also contains oxalates, so renal failure also can occur.

Black nightshade is common in croplands and, like pigweed, grows in high-traffic areas. The green fruit is most toxic, so cattle should not have access to nightshade during this stage, and nightshade remains toxic in harvested forages. Nightshade affects an animal's nervous and gastrointestinal system, causing weakness, depression, diarrhea and muscle trembling among other signs. Bull nettle and horse nettle are in the same plant family as nightshade, but less toxic. Cattle usually avoid these plants unless other forages are not available.