Troxel says stagnant waters may contain excessive levels of blue-green algae, which may be toxic and result in death of cattle. Because of their stagnant, nutrient rich nature, small ponds and streams in late summer can have toxic algae blooms. Toxicity is most common following a rapid bloom in late summer when cattle are consuming a substantial amount of the algal surface scum. The problem is difficult to predict, and the first sign may be sudden animal death. Because of this, it is advisable to restrict cattle access to stagnant waters, especially when a substantial amount of algae scum is visible. Algae blooms can be controlled in ponds through the use of copper sulfate (blue stone), but the rapid dieoff of algae may result in a fish kill.
The best method to control algae is to eliminate the source of nutrients entering the pond. If copper sulfate is used, the recommended application rate to water depends on the alkalinity (total carbonates and bicarbonates) of the water. Copper ions can kill fish if the water’s total alkalinity is below 40 ppm. Copper sulfate treatment may be ineffective if alkalinity of the water is greater than 300 ppm. The maximum tolerable level of copper sulfate in water is 2.7 (sheep) and 6.8 (cattle) pounds of copper sulfate per acrefoot.
The formula to calculate the pounds of copper sulfate needed is: Total alkalinity (ppm) x 2.04 x acrefoot volume = 100 Do not exceed the application limits for livestock, especially sheep. Livestock (especially sheep) should not be watered for at least five days after the last visible evidence of the algae bloom. Care should be taken to avoid water that has algae cells, either from treatment with algicide or natural aging of the bloom, because most toxin is freed in the water only after breakdown of the intact algae cells.