What does a heat index above 150 signify for horses?

A heat index above 150 signifies that the environmental conditions are hazardous for horses, primarily indicating that the horse's ability to cool itself through sweating is compromised. At this level, the combination of heat and humidity makes it significantly harder for a horse to dissipate heat effectively. Horses rely heavily on sweating as a natural cooling mechanism, and when the heat index is elevated, they are at a greater risk for overheating or heat-related illnesses.

The other options do not accurately reflect the implications of a high heat index. Increased physical performance and optimal cooling conditions would suggest favorable situations for horses, which contradicts the risks associated with a heat index above 150. Improved hydration levels also do not directly correlate with high heat indexes; in fact, hydration can be challenged in extreme heat, as horses may sweat more and can lose fluids rapidly without adequate supplementation. Hence, option B accurately addresses the health risks presented at a high heat index for horses.