Below is an excerpt from The Handbook of EQUINE PARASITE CONTROL by Dr. Craig R. Reinemeyer and Dr. Martin K. Nielsen that supports this.
Continuous Daily Treatment
This approach involves the daily administration of Pyrantel tartrate to selected horses, either perennially or throughout the grazing season. Daily administration of a dewormer should kill ingested infective larvae (L3) before they invade mucosal tissues, and would have therapeutic activity against recently emerged adults. Consequently, egg counts of horses receiving daily Pyrantel tartrate should remain low while receiving the regimen. Due to national differences in drug approvals and marketing strategies, daily Pyrantel tatrate formulations are only available in North America. Pyrantel pamoate / embonate resistance is much more prevalent in North America than anywhere else in the world: so it has been hypothesized that this may comprise cross-resistance, which was fostered by daily use of Pyrantel tartrate (Kaplan & Nielsen).
Due to the above statement and the fact that Horsemen’s Laboratory has found that many horses on the Pyrantel tartrate daily dewormer have positive fecal counts for strongyle eggs, I cannot recommend its use. If a horse owner decides to use a daily dewormer I would recommend to limit the time it is administered and the horse should be monitored closely by performing fecal egg counts to see if there is resistance or if a resistance is developing.