During the winter of 1990/91 30% of the Whiskey mountain bighorn sheep herd was lost to a pneumonia outbreak. By 1997 the herd was half of what it had been. The geologist David Love reported seeing deformities in the sheep, that he had seen in animals with selenium poisoning. Later testing showed that the sheep were actually suffering from low selenium levels. The effects of either can sometimes be the same. Further study demonstrated that selenium deficiencies were indeed affecting the herd, and manifesting in the form of White Muscle Disease. Pat Hnilicka also observed “periodontal disease” in the herd. This has later been described to be very similar what has been observed as under bites, in animals in Utah, Montana, and Wyoming. This study also demonstrated that nitrate deposition was lowering forage selenium levels.

The selenium deficiencies seen in these sheep could not be accounted for by forage levels alone, although this was absolutely a complicating factor in these deficiencies. 2,4-D and Diacmba were known to be used in some of these applications. And based on the whole of the conditions documented it has been proposed that herbicide induced selenium deficiency and Post Partum thyroidistis is responsible for much of what we see play out following these herbicide applications including the selenium deficiencies that cause white muscle disease, failure to thrive, reduced fecundity and the propensity for pneumonia die offs. The supplementation of selenium in Postpartum thyroiditis has been shown to improve the condition, as was seen with selenium supplementation on Whiskey Mountain. These sheep were also deficient in other minerals that are associated with thyroid disease including cobalt and magnesium. Prior to the discovery of the selenium deficiency it was first believed to be a case of hypomagnesemia. This is laid out at the bottom of this page.

Selenium deficiencies have been seen in CA blacktail deer declines, Utah moose declines, WA elk declines, and many others across the West.

Torrey rim, at the foot of Whiskey Mountain, has been the site of habitat improvements for the Whiskey mountain bighorn sheep herd. Torrey rim is the winter sight for both the Arrow and Whiskey mountain herds. Sage brush cover on Torrey rim has been significantly reduced, and the entire area altered over the last 20+ years. Pesticide application in these habitat alterations occurred prior to every die off, beginning in the late 1980s. In the absence of pesticide applications prior to fall of 2014, the population began to rebound after 20 years of declines and suppression. Lamb numbers had come up to as high as 40 lambs to 100 ewes during this recovery.

Here is an excerpt from one proposal dated 2001 “Prescribed burns, herbicide control of mat forming cushion plants, range pitting, meadow restoration on BLM Ridge, Noon Rock, Whiskey Mountain, Sheep Ridge, Torrey Rim, Rim Draw, Trail Lake (HAs 9,10): 1170-1350 acres total” These applications predates one of the last large die offs.  Many of these treatments were intended to reduce sagebrush, and increase grass growth. The report for pre 2000 habitat enhancements is no longer available on line.

One of the “mat forming cushion plants” that was the target of herbicide applications is fringed sage brush. Wyoming Fish and Game has said that these are not palatable to bighorn sheep, yet when they hit their winter range on Torrey rim in September one of the first plants that they target is fringed sage brush. This has been observed by bighorn biologists. If these are not palatable plants to big horn sheep, then why do they target and consume them? One explanation is that the sheep target fringed sage brush during that small window when the first show up on Torrey rim for an essential oil in the sage brush that works like a dewormer ridding the sheep of lung worms. These lung worms have been shown to play a role in pneumonia outbreaks seen in these sheep.  Another explanation that has been proposed is that they are eating these plants because they have just been treated with pesticides. The application of pesticides on Torrey rim has occurred in August, just a month before the sheep arrive in September. Wildlife has been shown to be drawn into herbicide treated foliage to feed on it. Some herbicides like Tebuthiuron that is used for the removal of sage brush in wildlife habitat projects persists in the soil and plants for 10 years after application drawing wildlife into feed on treated areas for years after the initial application(1). 

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Herbicide exposure to selenium deficiencies to Postpartum thyroidititis and die offs:
Regarding the thyroid, selenium is a component of the enzyme that helps convert T4 to T3 peripherally, so deficiencies of selenium may impair thyroid function and promote hypothyroidism.”
 
A high TSH and low T4 and T3 indicate thyroid gland disease.
High T4 to T3 ratio (T7) or a high r-T3 (another rare test) are suggestive of peripheral cellular resistance as these levels indicate a decreased conversion of T4 to T3. Decreased conversion may also be due to selenium deficiency or mercury toxicity.”
 
Although clinical applications still need to be defined for Hashimoto’s disease, they are very interesting for pregnant women given that supplementation(selenium) significantly decreases the percentage of postpartum thyroiditis and definitive hypothyroidism.”
Environmental contaminants interfere with thyroid function including 60% of all herbicides, in particular 2,4-dichlorophenoxyacetic acid (2,4-D)”
“There was increased odds of hypothyroidism with ever use of the herbicides 2,4-D”
“It has also been reported that 2,4-D induced neurotoxicity may be partly due to generation of free radicals. When incubating rat cerebellar granule cells with 2,4-D in vitro, glutathione (GSH) levels and catalase activity were significantly reduced, whereas generation of reactive oxygen species (ROS) and activity of selenium-glutathione peroxidase (Se-GPx) were augmented” Citation for this: http://www.ncbi.nlm.nih.gov/pubmed/17449452?dopt=Abstract&holding=f1000,f1000m,isrctn
So you introduce 2,4-D into sheep winter range, they eat it. Se-GPx is activated to protect against the affects of 2,4-D, which depletes Se, along with glutathione.  And you then couple environmental conditions that deplete selenium feed levels, and everything falls apart. The resulting selenium deficiency is most likely responsible for the thyroid conditions, as selenium supplementation has been shown to decrease post partum thyroiditis. 

(1) http://www.jstor.org/stable/4043673?seq=1#page_scan_tab_contents