Generally speaking you can apply potassium (K) any time. Again generally speaking low Mg in the forage is the result of very low Mg in the soil or very high K. In that case you might be better off to add Mg if low or not apply K if it is already there is a high level. The best way to tell is to do a soil test before applying any fertilizer. You will get better results for your buck by adding what is actually needed than by just guessing.
By the way I am not guessing, I have recent soil tests and pasture analysis which show what I need!
I have just heard various opionions on applying K (potassium) in the form of potash; - Potassium/Magnesium ratios thrown out of kilter etc. I run beef cattle and have heard it is not really too much of a concern for beef cattle(more of a concern with Dairy cows). But interested in to hear if anyone has experience with this.
If K and Mg are available in the soil in somewhat near the right ratio there is rarely a problem. In my area it is also often a problem for dairies. The reason for this is that they feed lots and lots of imported alfalfa which has been well fertilized with K. So they are bringing on lots of K to the farm. Not very much of this leaves in the milk so the rest goes out in the manure which gets spread on their land. K doesn't leach off or leave in any other way so it builds up over time. I have seen soil tests from dairies with up to 2,000 ppm of K. This leads to the grass luxury consuming the K. Which leads to health problems in the cattle (grass tetany, milk fever, retained placenta, etc).
This doesn't seem to be nearly as big a problem with beef cattle. I think this is because we aren't importing tons and tons of high quality alfalfa and the cows aren't pushed for production like they are at a dairy.