Many people, I expect, know that, when the grass starts growing vigorously in the spring, horses can sometimes have problems, ranging from mild footsoreness to full-blown laminitis and even founder with rotation of the coffin bone. A caveat - I am not a vet and this post is based on the information I have learned over time. The mechanisms that cause high levels of available sugars - particularly fructans - to trigger damage to the laminae that attach the hoof capsule to the inner structures of the hoof are not completely understood, but it appears to relate to a restriction of circulation followed by excess blood flow - think migraine headache and you've got the basic idea. This article gives a brief overview and includes some ways to avoid triggering laminitis in horses who are on grazing - they all involve restricting the amount of grazing. Ponies and certain breeds such as Morgans, and horses which tend to gain weight easily or who are "cresty" are at particular risk, and some of these horses may not be able to graze at all without risk. And horses who are insulin resistant (who may not have the typical grass-sensitive body type - many racing TBs have been dosed with lots of steroids and this can predispose them to later problems - Dawn falls in this category) or senior horses can also have problems. The most accurate indication of insulin resistance is a cortisol blood test repeated at different times during the day - blood glucose levels can be indicative but are not determanitive.
But the article also points out that there are other circumstances besides spring where horses may be at risk. Any time grass is rapidly growing - such as after a drought breaks - can be a problem. And then there's fall and frosty nights - it isn't the temperature of the grass (in itself) or the frost/moisture on the grass - it's the result of how grass grows. Grass, like any other plant, uses sunlight during the day to manufacture food for itself - and in the case of grass this includes carbohydrates like fructan. And there are factors that affect how much fructan the grass accumulates - for example, more will accumulate on a sunny than a cloudy day. (Also, different grass types are more or less prone to accumulating high levels of fructans, and many commercial horse feeds also have inappropriately high levels of sugars.) And then the grass uses the stored carbohydrates during the nighttime hours to grow - grass grows at night not during the day. In the fall, when nighttime temperatures are colder, the grass has less chance to grow and use up the accumulated fructans from the day before. When nighttime temperatures fall below 50F, I begin to restrict the grazing of my horses, bringing them into dry lot paddocks in the early afternoon - fructan levels rise throughout the day and are typically highest in the mid to late afternoon. I shorten things up even more as nighttime temperatures fall into the 40sF, bringing my horses off pasture in the late morning. Dawn is somewhat insulin resistant, and Pie's had one prior episode of spring laminitis, so I'm extra careful with them, and although Drifter has never had a problem, I keep him on the same schedule as it's easier for me and can't hurt him.
And then we get frosty nights - once that's happening, and particularly if the subsequent day is sunny and warmer, which often happens in our falls - it fact it's happening today - I make sure my horses are off pasture after only a few early-morning hours. Even a longer day in the big "dry" lot - where there are still plenty of grass nibbles although not a huge amount of grass - was enough to make Dawn somewhat sore-footed the day before yesterday. She was a little bit better yesterday after a much shorter grazing period, and is even a bit better today - there's no heat in any foot except a little bit on the inner side of the left front, and it's not bad, and she's never had strong digital pulses (Pie did last spring), which is good. I think she's going to be OK, but I'm keeping a very close eye on her. Once daytime and nightime temperatures are below freezing more consistently, and the grass stops growing, and rain and snow have leached the fructans from the standing grasses, sensitive horses are likely to have less trouble grazing.
A horse's being prone or not prone to being "footy" or worse from grass is fundamentally a metabolic/nutritional issue, not a hoof structure/trimming/shoeing issue, although a horse's metabolism does affect how its feet grow and perform. Both Dawn and Pie are on a custom magnesium/chromium/selenium/vitamin E to help with glucose metabolism. If a horse is in shoes, that can sometimes conceal a metabolic problem with grass until it's more advanced - it doesn't mean that the shoes have solved the problem just because a horse is footy without shoes and not footy with them. This is the first fall that Dawn's been completely barefoot, and I think that she might well have had these issues in the fall in the past but they were concealed by the front shoes. Any horse with abnormal hoof growth patterns or hoof "rings" should be suspected of having some metabolic issues. And it's important to have a feeling for what's normal for your horse - both in terms of how their feet are looking/growing, how warm/cool the hooves are depending on the time of day and what they've been doing (although cold hooves don't necessarily mean things are OK, since the first stage of laminitis is restriction of circulation - that comes before the excess circulation that produces hot hooves), and how they move on different surfaces. And it's important to learn how to take a digital pulse and to know what your horse's normal is - here's a video showing how to do it - the stronger the digital pulse the more likely there is to be a problem.
There's a good resource for horse owners out there - it's called safergrass.org. Grazing is good for horses . . . but then sometimes it isn't.