There are quite a few decent “Deer Minerals” out there. I Have been testing products for more than 20 years 
I do believe that These blocks probably don’t contain the important
Trace mineral package that A product like Nutra Deer has built into it. I would also be concerned with the high levels of selenium that some of these blocks contain. Just my 2 cents. 
“Selenium
At the other end of the scale is selenium. Selenium has a relatively small threshold for toxicity. In other words, toxicity can occur when the requirement is exceeded by a relatively small amount. In fact, the amount of selenium that can be used in a complete diet is government-regulated for many ruminant animals such as cattle, goats and sheep. In most cases, the maximum allowed in a complete diet is 0.3 ppm (milogram/kilogram = milograms of selenium per kilogram of total diet). Therefore, it is important when formulating a mineral/vitamin supplement to not have a selenium level that, when consumed at a typical amount, will cause the overall diet to have a selenium level exceeding 0.3 milograms of selenium per kilogram of total diet. In the side bar of this article, I have described an example using a hypothetical supplement containing 60 ppm of selenium, consumed at 2 ounces per deer per day, with the deer eating a total of 10 pounds (4.5 kilograms) of diet per day. In this example, we find that the resulting selenium consumption would be 0.75 ppm or milograms per kilogram of total diet. This is a little over double the regulated amount for most ruminants (for more details, see sidebar).
Will this supplement cause toxicity in deer? Technically, a selenium toxicity level in deer has not been established. However one thing is for sure: in our example, the supplement, based on a deer eating 4.5 kilograms of total diet, including 2 ounces of the mineral/vitamin supplement, will certainly cause the selenium in the diet to exceed the regulated amount of 0.3 ppm (milogram/kilogram). Keep in mind that this regulation was put in place due to the small threshold between selenium requirement and toxicity. It is important to note that the regulatory amount of 0.3 ppm is used for domesticated ruminants and is also commonly accepted by most deer nutritionists as the maximum level to be used in diets. Disregard for the safety factor imposed by the regulatory number in product formulation, if not considered dangerous, should at least be considered reckless.
One factor we did not consider in our example is additional selenium coming from natural forages. Certain soils have a high selenium content making the forage have a higher than normal level of selenium. If you combine a high selenium supplement with forages already high in selenium, toxicity concerns become even greater. Yet another factor to consider is that selenium has been shown to build up or be stored in body tissues. Prolonged over-consumption of selenium may also place a deer at risk of toxicity. In other words, problems may not occur right away, but over time, symptoms may begin to show up. In some cases, however, symptoms may not be very apparent, which is the case for animals suffering from chronic toxicity. In cases where acute toxicity occurs, symptoms can include emaciation, loss of hair, soreness and sloughing of hooves, anemia, blindness, staggering, paralysis and death. Again, we need to understand that selenium is a vital trace mineral in a deer supplement. However, selenium formulation should be based on the best information we have available in terms of maximum amounts of selenium that should be supplemented in a diet. As I mentioned before, disregard for the safety factors built in by commonly accepted maximum selenium levels is risky and a disservice to deer hunters and managers.”
