BCS is short for Body Condiion Score. It ranges from 1, "Poor", to 9, "Extremely Fat".
It's hard to tell whether and how much training actually affects a horse's BCS; feeding certainly does.
Foals are always born at 1 and increase to 3 to 5 by the time they are weaned (at six months). The game mercifully provides them with enough milk even if their dams are not present on the farm -- or even alive!. Still, it's a good idea to keep them in a pasture; they start eating grass early, and it's important to their development. The amount of milk diminishes over the first 5.5 months and stops at 6 months. Still-growing adolescents may drop 1 or 2 BCS points during growth spurts; feeding a supplement can really help then. Adults are more consistent.
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If you put a pregnant mare into a barn and don't assign her any food, or if she is in a very heavily grazed or mown pasture, she may lose her foal. This isn't supposed to happen. However, a friend, HorseLady72, told me she had put a mare in a barn, and forgotten to set up feeding for it (this was in 2015 or early 2016 when feeding was harder -- you had to go to a sub-menu; also, pasture
quality wasn't visible at all). She said the mare lost the foal after a few turns.
This could have been a human memory glitch, but please remember that computers and transmission lines are not perfect either. I don't know whether corrupted data could have caused the foal/pregnancy to disappear.
Any horse totally deprived of food will eventually lose condition down to 1
; how long it may survive in that state depends on factors including age. Horses over 18 are vulnerable. So far horses in this game only die when they reach a certain age, generally 20 or a little higher; this is regardless of BCS, level of training, etc.
There was a programming glitch which prevented mares from dying as long as they were pregnant OR if they gave birth and then were re-bred the same day. It has now been changed so that mares still carry the foal to full term and give birth. At that same turn, if they are 20 or older, they may die.
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If you compete your horse, 5 is best in general. 6 is just a little plump, still good enough for most non-racing shows. 4 is "moderately thin"; it's considered good racing trim by the computer (beneficial for Endurance, Harness Racing, Racing, Sprint Racing, and Steeplechasing) and accepted for most other performance (maybe just a little skinny for Saddleseat, Dressage, or Western Pleasure).
Low weight (1-3) and high weight (7-9) will be penalized in certain events, like Endurance or Cross Country. A thin or fat horse can't work as effectively.
There are some older explanations, in the Guides and replies to questions, about horses getting fat in pastures because they will always eat as much as they want and can get. (IRL many of them will self-regulate. I've even known totally indoor cats to do that!) Now that pastures can be mowed, it's possible to keep horses at or close to ideal weight/BCS if you have several pastures and the time and patience to mow them to various levels and monitor the amount of nourishment that results. Examples: Racers (this means high-metabolizing breeds*) will need rich pastures and easy keeping ponies do best on rather lean ones -- 30% or less.
Sometimes a horse's weight and condition will change or resist change. I have known closely monitored horses to fail to gain weight even when given generous amounts of food in barn or pasture. The best guess I can make is that the computer is simulating RL "gone off her/his food" incidents. ***Edited to add: Larissar did admit there is a "random factor" in the individual's response to feeding.*** Horses over 10 seem to be more susceptible to this. I've brought thin or thinning horses back to 4, 5, or 6 by giving them good pasturage (at least 80%) and supplemental food like Performance Mix or Sweet Feed. ***Edited to add: even tiny equines, ponies or minis measured in inches, need 100% of recommended. They are easily satisfied in lean pastures, though. Light horses and draft breeds need 100-105% of recommended.
*Examples of these breeds: Arabian, North African Barb, Turkmene, Akhal-Teke, Baladi, Thoroughbred, Standardbred; sometimes Hackney. ***Added: and Tchenarani, Deliboz, Russian Don, Anglo-Arabian, Norfolk Trotter ... YMMV especially with Part, Half, or Mix.***
If you think *this* is complicated
, be assured that taking care of real horses is far more so.