This is very interesting, I am always searching for nutritional values relating to hay and grass but they are difficult to find. I once found information about the ideal length of sword for optimal nutrition in cattle! Also, in relation to horses, I have read about the difference in sugar levels in (growing) grass in the morning compared to the afternoon and the significance of this for horses prone to laminitis. I wonder if the high sugar/laminitis connection for horses is the main reason your supplier has pulled the Timothy hay? Hopefully the increased sugar levels won't be such an issue for piggies. I agree a mix of hay is always best.
I mainly warn of the thing for guinea pigs because of unintentionally feeding a lot of sugar risks. Most people consider sugar content with regards to fruit or sweet veg, thinking of all "grass" or "hay" as the same and low risk. It's a balancing game with regards to the silica/ fibre levels of Timothy, and is going to depend a lot on the rest of your care but extra knowledge for informed decisions is always helpful.
I agree it's difficult though -
One of the reasons for the difficulties is that hay suppliers are only required to list the average nutrient profiles of the species of grass, and then to do maths on the predicted species makeup of the product, according to sowing data. You see this a lot with the bigger name horse feed companies (Emerald Green are the 'best known' for doing this). It's fine up until a point, but neighbouring fields on different farms can have very different soil composition and thus resulting nutrition in the hay/ grass. And of course as these are averages, it doesn't account for seasonal, topographical, and weather differences - it all feeds back. There are a few companies who get their actual sward tested for true nutrient values, and who only have fodder from a few specific farms go into their products (vs the open market), and it's one of these, Simple Systems, who have pulled their Timothy chop. It's general 'talk about town' in non-commercial large animal care this year though.
With regards to sugar levels in time of day, yes that's absolutely true. It's the biological details of photosynthesis and how and when the plants are doing what. Also relevant is what you were saying about how long the sward is in the first place, with shorter grass having a much higher sugar content. If you have a horse or any animal with metabolic health issues (laminitis is usually the main symtom of a wider issue, Equine Metabolic Syndrome) then you need to pay attention to this, as well as the soil temperature - particularly with regards to how big the difference is across 24hrs. I haven't contacted them directly to ask, but it's unlikely that they'd have pulled the whole crop just for 'increased risk to EMS horses' reasons (it's not the hay product with the highest sugar levels they sell). It's more likely that when they analysed the product that several other nutrient profiles were out of whack too. I can email them and ask if you'd be interested? I buy their stuff for my guinea pigs and also my sheep.