Pyroluria
Pyroluria in humans is said to be a genetically determined, familially clustered, biochemical-enzymatic disorder of haem metabolism. It is often also referred to as a "detoxification weakness". In this condition, increased amounts of pyrrole compounds bound to vitamin B6 and zinc are said to be excreted in the urine, thereby triggering symptoms of extreme vitamin B6 and zinc deficiency. This is not clearly proven even in humans, but is nowadays readily assigned to horses.
The "diagnosis" is usually accompanied by the recommendation to feed increased amounts of vitamin B6 and zinc and to ensure sufficient roughage. Among other things, warnings are given against feeding alfalfa or haylage. Sufficient scientific literature or studies cannot be found for any of this.
Naturally, the horse's own synthesis of B vitamins depends on a needs-based ration with a sufficient amount of roughage. Incorrect concentrate feeding or excessive amounts per meal can also significantly disrupt the gut biome. However, any resulting dysbioses in the gut can never lead solely to an isolated vitamin B6 deficiency, but at most to a deficiency of the entire vitamin B group. Likewise, zinc intake not adjusted to requirements should of course be avoided. However, a connection between the alleged condition pyroluria and the feeding errors mentioned is not substantiated and is questioned by scientific research.