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"I defy anyone to tell me they have a pasture with zero poisonous plants," said Dr.J.

"Then I reject your reality and substitute my own!" shouted Dr.[Redacted].

So, I wanted to have a more vintage feeling to cities in my setting, despite being around the technological levels of the 21st century. One of my ways to achieve that was to separate civilian methods of transportation into three main categories:

  • Between cities, people use cheap and fast maglev trains.
  • Sub-Atm underground "bullets" are used to transport materials and products. You can't use it to transport living creatures, though. These subterranean maglev pipes that have pressures of less than one atmosphere, though usually, it's not zero.
  • Horses and horse carriages are used to travel inside cities and on difficult terrains. I mean, there are only a few dragons and dragon riders, the rest of the park rangers will have to settle for an ass.

Now, these aren't just regular horses, but a special, genetically altered group of breeds, collectively referred to as D-Horses, which were the legacy of Dr.[Redacted].

City horses are smaller, around 145-150 cm at the shoulder. In winter, they grow fluffy coats to keep them warm. City horses are naturally capable of extra gaits that are pretty useful. They're also harder to spook, potty-trained, capable of understanding color-coded roadsigns and that they have to avoid trampling or running people over. Usually, they're more responsible than their drivers.

All-in-all they sound pretty good for short-distance travel. However, another key ingredient of making them more useful than cars is that D-Horses are way less susceptible to food poisoning than their non-altered cousins and can eat all kinds of plant matter and even eggshells. They still require some diversity in diet, but not as much as a horse. Since they're potty trained, we know their digestive system was altered, but I'm not sure how it should work.

Just a side note that guts also act as a kinda-sorta piston, helping push air out of the creature's lungs when running.

So, how should the D-Horses' digestive system maximalize its efficiency, as in extracting as much energy and usable material from food as feasible for the least amount of work?

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    $\begingroup$ I'm not sure you need any of this. Nothing will compete with cars, so horses (even great ones) are a choice. 21st century tech implies clean, controlled environments and no obnoxious foodstuffs available. With your horses so smart, simply make them refuse to eat anything but preselected, totally safe foods. $\endgroup$ – DWKraus Jul 30 '20 at 22:48
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    $\begingroup$ I totally miss the point of the quotes at the beginning of the post, unless your goal is just to confuse the reader $\endgroup$ – L.Dutch Jul 31 '20 at 2:50
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    $\begingroup$ I thought the bigger issue with horses was all the poop and horse corpses. Have you figured out your sanitation? $\endgroup$ – PoorCorrelation Jul 31 '20 at 2:51
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    $\begingroup$ @jamesqf Whether you measure a horse's height in hands or centimeters is irrelevant. $\endgroup$ – Mephistopheles Aug 1 '20 at 1:20
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    $\begingroup$ @jamesqf Low-effort measurements will be thrown out just as easily. It's just a tape, not a SAW, you won't break your back by carrying it. I always have at least two on me. As for the conversion, is it easier to have grains, ounces, pounds, some of which differ by region, or the SI system that's neat and logical, as all things should be? $\endgroup$ – Mephistopheles Aug 2 '20 at 0:51
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The actual volume of poisonous plants is negligible — the problem with them isn’t that they displace the plants that horses need to eat, it’s that the horse will be harmed if it eats them. So the simple solution is not to engineer the horse to be able to eat them, but rather to improve its (already pretty good) ability at avoiding them.

I keep horses in England, and they won’t touch ragwort in the field. We do remove it, because it becomes more palatable to them after it has died and dried out. Just give your horses the ability to identify and avoid dried ragwort (and other poisonous species, depending on your local flora) as poison, as well as fresh, and your problem is solved.

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I am pretty sure horses can eat eggs. As for the rest:

Toxin-specific commensals.

Horses eat a low quality high cellulose diet and depend on organisms in the gut which are capable of fermenting cellulose into molecules that mammalian digestion can handle. In your world, there are microbial supplements that are added to the gut biome of the horse. There is an array of supplements specific to each plant toxin the horse might encounter, and those caring for horses change the supplement mix each spring according to what toxic plants are expected that year. In some places it is the same ones year after year but in other areas they can differ, and one must keep up.

I could imagine engineering the microbes to handle toxic plants. But you could do this with primitive tech. Identify horses which can handle the toxic plants, or feed your test group toxic plants and see if some get less sick. Then supplement horses unused to the toxic plant with bacterial from the horses which can handle them: a fecal transplant. Once you establish a good microbe producer, you can keep that horse on a diet of the specific toxic plant and sell its bacteria (as dried feces) as a supplement.

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Ruminant digestive tracks wouldn't be too far of a stretch to slap in a horse. Ruminants such as deer and goats can extract nutrition from a very wide variety of plant materials often considered non-nutritive to most species. As for toxins, it is shown that some animals lick clays and minerals from the ground to bind to and counteract some toxins.

In addition, if you have all that extra bacteria due to a ruminant digestive track, you can come up with some strange science. "Some plants contain cyanogenic glycosides that can release hydrogen cyanide. Adapted bacteria can take the nitrogen in the cyanide and convert it to amino acids." Source

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