Fellow CT residents…does anyone know the general level of Selenium in the soil/hay in Connecticut? I’m not sure if my current boarding barn is feeding NY or CT hay, but I’m moving at the end of the month and I know the new barn feeds local CT hay. I have a horse that was switched to a forage only diet (alfalfa cubes and hay) recently and I’m trying to figure out the best supplement plan for him.
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This USGS site has it listed by county.
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I asked the vet when I moved here and was told that selenium supplementation is usually not required.
I’ve never supplemented in either CT or RI and the map linked above does support fairly high levels.
If you’re concerned with selenium, have your vet pull blood for testing toxicity or deficiency. The symptoms of both are pretty benign until they’re not.
I would be reluctant to supplement selenium based upon a regional map alone.
FYI: I’ve had my horse tested, and he was very low range of normal years ago. He was in Patterson, NY, right over the CT line by Sherman/ Danbury, and regularly ate CT & NY hay. I supplemented him for the duration of the tub-- 60 days? And his Se became firmly in the ‘normal’ range.
Agree that if there’s any concern, test the horse. This is one of the few nutrients where you need to test the horse, not the forage (or soil). The soil numbers are just averages for a general area. Other factors affect how much of which nutrients get into grass, and while we know a great deal about making interpretations of how much calcium and phosphorous and potassium and other nutrients get from the hay or grass into the horse, selenium is a different beast.
My county is a low Se area as per that map, but my horses are just fine in their selenium levels, between what’s provided in the hay and grass, and the 1-2mg they get from their additional feedstuffs.
That said, I have not seen any hay analysis that did not have issues that would benefit from supplementation. But the details depends on the specific analysis.
I’m in CT but just generally curious and new to in depth nutrition - how would one use the data from the map when making diet decisions?
You almost don’t 
Specific location soil testing is done to grow the best quality vegetation you want to grow (ie grass in this case).
Forage testing is done to “grow” the best horse.
Mineral content of the soil is only part of the equation. The type of grass, the season of growing, the pH of the soil, all factor into how much mineral uptake there is.
These soil maps are just generalizations. Selenium and iron are, IME, the most useful pieces to look at, but even then they are just maybe the “canaries in the mine” type of thing. If you’re on the NC coast with sandy soil that is low in iron, the odds of you needing to investigate adding copper and zinc to the diet are low. If your’e in an area with really high selenium, then you may want to keep a closer eye on your horse’s blood selenium levels so you don’t end up feeding too much.
Everything else pretty much starts with the forage analysis, and even then, it’s pretty iffy how well you can use grass analysis, since it changes pretty regularly and horses develop tastes for certain spots over others, and you may not get a very good representative sampling of all the different areas he likes to eat from.
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Also new to CT, and the map is not very useful.
If, say, the map showed very consistant, very low levels of Se in this entire region, then there would be a concern about getting enough in local pasture and hay, and testing the horse may be advised. If levels were consistently high, then it would be fairly safe to assume the horse would get enough.
But the levels are all over the map (hahahaha, I’m sorry) so it doesn’t really provide any direction.
(Also I don’t see the Se maps as providing direction on diet but direction on testing. Subtle difference, but important :yes:) 
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I’m in CT and have experience a lack of vit E problem so I supplement with Elevate- But SE is not necessary and can be toxic if given in excess
