PZP is a risky pesticide. Will it ruin the treasured herd?
By Marybeth Devlin
The issue underpinning the use of PZP and the continuing cycle of removals of wild horses from the Pryor Mountain Wild Horse Range is: Whether there are excess wild horses. No, there aren’t. BLM creates the illusion of an overpopulation by administratively setting the maximum herd-size below minimum-viable population. The International Union for the Conservation of Nature determined that, if a herd were managed carefully per a stud-book, it could sustain itself genetically at a minimum of 500 individuals. Compare that number to BLM’s maximum: 120.
In fact, according to the latest genetic analysis, the Pryor Mountain herd is evidencing “a general trend for a decline in variations levels of the herd.” The recommendation was to “increase population size.” Yet, BLM stubbornly insists on its own failed approach of artificially limiting herd-size, declaring that it disagrees with the scientific “interpretation.”
But can the range accommodate more horses? Yes. By way of comparison, BLM allots 38 acres per cow or calf when setting the stocking-rate for livestock grazing. Thus, the 33,187 acres that compose the Pryor Mountain habitat can support 500 to 873 horses. When the WHR is restored to its original configuration, 44,920 acres, the high-bound can be increased to 1,182.
As for PZP, numerous independent studies have disproved the old theory that PZP merely blocks sperm attachment. In fact, PZP’s mechanism of action is to alter ovarian function, causing inflammation of the ovaries and cyst formation. PZP provokes an auto-immune response, wherein the pig-ovary-derived PZP antibodies attack the mares’ ovaries, resulting in dystrophy of those reproductive organs. Despite being hyped as a non-hormonal contraceptive, PZP causes “markedly depressed oestrogen secretion” in mares treated for just three consecutive years. The latter finding was disclosed by Dr. Kirkpatrick himself 23 years ago. PZP-use is associated with stillbirths, altered ovarian structure and cyclicity, interference with normal ovarian function, permanent ovarian damage, prolonged breeding season, and unusually-late birthing dates. A particularly troubling finding suggests that PZP can be selective against a certain genotype in a population.
PZP is touted as reversible; however, a recent study warned that just three years of treatment, or administration of the first PZP injection before puberty, may trigger infertility in some mares. Thus, only two PZP injections could be viewed as relatively safe, but it appears that even one injection is risky. The researchers warned that inducing sterility may have unintended consequences on population dynamics by, ironically, increasing longevity while eliminating the mares’ ability to contribute genetically.
Most pertinent to the Pryor Mountain herd is a longitudinal study on three herds treated with PZP — Little Book Cliffs, McCullough Peaks, and … Pryor Mountain. The researchers found that the birthing season lasted nearly year-round: 341 days. Out-of-season births put the life of the foals and the mares at risk. That same longitudinal study found that, following suspension of PZP injections, there was a delay in the mares’ recovery of fertility that lasted 411.3 days (1.13 years) per each year of PZP treatment. Thus, mares injected for four consecutive years (per BLM’s “prescription”) would be expected to take 1,645.2 days (4.51 years) to regain reproductive capacity. If disaster were to befall the Pryor Mountain horses, even if PZP were stopped immediately, it would take years for the herd to recover, if ever.
PZP has neither stopped nor slowed the roundups. Only lack of holding space has done that. Even the Pryor Mountain herd, injected for decades with PZP, is facing removals again this summer (per the usual three-year cycle) in addition to an intensified PZP “prescription” to be administered per an “equal opportunity program” eerily similar to Communist-China’s one-child policy. What’s ironic is that, for all the interference, BLM has achieved basically the same — or worse — record as has been attained the International Society for the Protection of Mustangs and Burros. ISPMB complies with the “hands-off” minimum-feasible management approach stipulated by the Wild and Free-Roaming Horses and Burros Act. ISPMB’s two wild herds grew 8.73 and 5.08 percent, respectively, without PZP and without removals. Pryor Mountain’s most recent report — reflecting management with PZP and with removals — grew by 8.26 percent.
BLM needs to get out of the way of Nature. Let the Pryor Mountain herd find its own appropriate population level.
(Note: Beware of petitions pushing PZP. Be sure to read everything you sign these days especially the fine print!)
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