What causes fescue toxicity in livestock grazing tall fescue?

Fescue toxicity in livestock grazing tall fescue is primarily caused by an endophyte fungus, which lives symbiotically within the grass. This fungus, known as Neotyphodium coenophialum, produces toxic compounds called alkaloids. These alkaloids can have various detrimental effects on livestock, including reduced feed intake, weight gain, and reproductive issues. The presence of the endophyte is often beneficial for the plant as it helps it resist pests and environmental stress, but it poses significant health risks to grazing animals.

The other options are associated with different kinds of toxicity or health issues in livestock but do not specifically pertain to tall fescue. For example, prussic acid is typically associated with sorghum species, while nitrates can accumulate in various forages, leading to toxicity under certain conditions. Nematodes are parasitic worms that affect plant health and productivity but are not responsible for the specific toxic effects linked to fescue. Thus, the endophyte fungus is the correct and most relevant cause of fescue toxicity in livestock.