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Weather favors algae toxicity

by Wyoming Livestock Roundup

As the weather becomes hotter and drier, livestock producers may want to visually check their ponds and dams for the presence of blue-green algae. The algae, also called cyanobacteria, is fatal to livestock. 

The algae occurs naturally in ponds and stock dams. There are several species of the algae, and not all are toxic. 

Dave Ollila, Extension sheep field specialist with South Dakota State University, said commonly, problems occur with the algae when it is hot and dry and the water becomes stagnant. Those conditions stimulate reproduction of the algae through a bloom. The blue-green algae starts out green in color but will turn blue after it dies and dries on the surface.

“Many times, the first sign of trouble is a trail of dead sheep leading away from the dam,” he explained. “The poison strikes them so quickly, they will die as they are walking away from the dam.”

Algae issues

Blue-green algae is poisonous to nearly all animals, including cattle, sheep, horses, pigs, dogs, ducks, fish and wild animals. The first sign of poisoning may be the presence of dead wildlife, like snakes and frogs, near the dam. 

According to an Extension Extra report issued by South Dakota State University, it is also toxic to humans, causing numbness of lips, tingling in fingers and toes and dizziness. Depending upon the species of blue-green algae, it can affect the nervous system or the liver. Animals may die immediately from drinking in the scum, or even hours or days later. Those that do survive will lose weight and perform poorly.

“Livestock affected with the nervous system toxins may show signs including muscle tremors, decreased movement and difficult breathing,” the report explained. “They will collapse and go into convulsions.” 

“Animals affected with the liver toxins may show weakness, pale colored mucous membranes, mental derangement, bloody diarrhea and ultimately death,” the report said.

Determining toxicity

Sandra Holcomb, a veterinarian in Harding County, South Dakota, said it can be hard to determine if the ponds and dams have toxic water caused by blue-green algae. 

“Many times, it depends upon the environmental conditions. The problem is if you have a big enough dam, parts can be good and other parts toxic,” Holcomb explained.

“I would encourage people to get their water tested before they turn their livestock out. It is not going to be a sure thing, but at least producers would have some idea whether the water is safe to drink or not – at least the day it is tested,” she said.

Typically, the wind blows all the algae to one side of the water and the toxins will become more concentrated making the water poisonous, Ollila explained. 

“A lot of times, it has to do with the wind,” Holcomb concurred. “The water could be bad one day and good the next. Several species of algae are responsible for the toxicity.”

Avoiding toxins

Ollila encouraged producers to regularly check ponds and stock dams during the summer. Although the water can be tested for the poison, Ollila said typically the bloom only lasts a few weeks, and the toxic period may be over by the time the water test comes back. 

Rain, heavy winds and cooler temperatures can break up the bloom and inhibit growth making it less toxic.

When blue-green algae toxicity does occur, producers may want to consider moving their livestock from that pasture, fence out areas where the toxicity occurs or use a different water source or pipeline. 

Copper sulfate can be added to the water as an algacide, if the site has a history of repeated blue-green algae blooms. 

“However, it can’t be used to treat the water if you have sheep,” Holcomb stated. “Copper is toxic to sheep.”

“The best thing to do is to fence the dam out or move livestock to another pasture until it rains or conditions are more favorable,” she said.

Ollila said although blue-green algae toxicity is something that occurs every year, it can be a more serious problem when there is a drought because the animals can’t find as many alternative water sources. 

“Blue-green algae toxicity isn’t something that is talked about a lot, although it can be a real problem,” he said. “People just seem to live with it.”

Gayle Smith is a correspondent for the Wyoming Livestock Roundup and can be reached at

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