The following is mainly courtesy of Dr Eleanor Kellon and Dr Christina Fritz.

It seems it's never a good idea to feed straw to horses. First up, straw is the dead stalks of grain plants (which we should never feed to horses anyway), which have been sprayed with glyphosate to hasten their death and drying out.

The major reason most people feed straw rather than hay is to reduce the calories, but there really is not that much difference – 0.789 Mcal/lb on average versus 0.913 Mcal/lb for grass hay which is only a 14% drop (Dairy One Feed Composition Database). If you’re already feeding a mature hay or one chosen for low sugar and starch levels, the difference is even less since they typically run about 0.850 Mcal/lb with straw so then offering only a 7% reduction in calories.

Straw isn’t necessarily safe from a sugar and starch standpoint either. Sugar as high as 6.2% has been reported and starch up to 4.3%. Straws with a large amount of grain left in the seed heads will be even higher.

There are significant differences in the fibre fractions as well – not in a good way as straw is more difficult to ferment in the hindgut* which may result in bloated belly and diarrhea/faecal water syndrome, especially in older horses. Because straw is difficult to ferment this also means it provides poor prebiotic support for the microorganisms which would normally produce those beneficial B vitamins for the horse.

Protein is also severely deficient, averaging 5.3% in straw versus 10.9% in hay, necessitating protein supplementation. Mineral levels are similar, except for lower average phosphorus and magnesium, but may be less available because of binding to the higher fibre fractions.

To top it off, there is a higher risk of toxic nitrate levels in straw - https://hereford.org/wp-content/uploads/2017/02/issue-archive/0217_Nitrates.pdf

Finally, feeding straw can trigger a dangerous condition called hyperlipidemia, which is abnormally elevated levels of any or all lipids (fats) or lipoproteins in the blood; in humans one type of hyperlipidemia, hypercholesterolemia, means there’s seriously high levels of LDL (bad) cholesterol in the blood.

Obviously it depends on what ratio of straw is being fed; i.e. if a horse is fed only straw as their forage source, usually for supposed calorie-reduction. Straw is a wood fibre and can't be digested in the hindgut, so the hindgut fibre-fermenting microbes can’t produce the energy source for the horse (by way of three volatile fatty acids - proprionate, butyrate and acetate), so in essence despite the straw filling the horse’s stomach, it starves the horse of any energy.

This means the body will try to find other stored energy resources from protein and fat. However, horses can’t generate energy from fat, so this leads to hyperlipidemia, which brings on abnormally elevated levels of fats/lipoproteins in the blood, which can be fatal for the horse.

In a nutshell? There’s little reduction in calories but a much higher loss of protein, fermentability, and vitamin production compared to feeding hay. With all the supplementing you’ll end up having to do, it’ll cost you a bomb, plus analysing for nitrates is advisable. As Dr Kellon says, you’re better off double netting your hay, or investing in a slow-feeder system.

* Due to the ADF (acid detergent fibre) and NDF (*neutral detergent fibre), measures of feeds' cell walls/structural carbohydrate components.