Cand is 14 (ok, 15 in spring) years old holsteiner. I guess she shows that TB blood few generations back - it's so hard to keep her weight on.

We only do trail rides, but I didn't ride her since October, due to her sacroiliac injury. She is kept out whole day and in at night. She also goes in stall for 3 meals.

Every winter she losts her weight. She goes from perfect weight to showing ribs. This year I blanket her and she is better, she doesn't show ribs, but I would like to see some more fat on her.

She has free choice hay, but she won't eat much when she's outside. She keeps grazing on that short, almost non-existent grass that has no nutrition. I think that's the biggest problem, that she doesn't want to eat enough hay. Even if I keep her in a dirt paddock, she won't eat as much hay as I would like to see. All other horses love this hay and no one else has problems with keeping weight just with hay (and they have limited quantity, because otherwise they would be too fat).

She also gets 1,5 liters (before soaking) of beet pulp and a cup of linseed.

Should I give her more beet pulp? What else can I do?

She has her teeth floated reguraly and doesn't show any signs of ulcers.

Some horses just can't sustain on hay only.
Try to change your measurement of feed to weight, liters is a volume, I'm not sure how to convert volume to calories.
Just as in humans she needs to intake more calories than what she expels per day to show a weight gain.
It can be done with many different feed stuffs & routines.
Weight across the topline is usually protein related, I have the majority of mine on differing amounts of alfalfa depending on each horses need.
3 meals or more a day is perfect.
Also keep in mind pain, you said a SI injury. Weight loss can also be a sign of pain.

I measured, she gets 1,2 kg of beet pulp (that's 2,6 lbs) a day, in 3 meals. Is that enough?

I asked for alfalfa in the shop and it is 30 euros for 20 days, if I feed minimal amount. It's ridicioulus! I'll see if I can get it somewhere cheaper, maybe some farmer will sell for less.

I don't think she losses her weight because of pain, she was like that every winter, when she wasn't injured.

There's plenty of room for feed increase.
Your feeding the very lowest end of the scale, there is a huge difference between what your currently feeding and maximum amount possible.
So if it's easy to feed beet pulp & she likes it increase your amount gradually.

Some guidelines to follow when adding beet pulp to supplement the regular feeding program (meaning that the horse is already receiving at least 0.5% of its body weight daily of a fortified horse feed) are:
1) feed no more than 3 pounds of dried beet pulp daily to a pony or growing horse less than one year of age,
2) feed no more than 5 pounds of dried beet pulp daily to an immature horse from 1 to 3 years of age, and
3) feed no more than 8 pounds of dried beet pulp daily to a mature horse (body weight of 1,200 pounds).
Of coarse if you need to feed a large amount it needs to be split into multiple meals, which you already know.

Remembering that it's soaked weight will be slightly more than 2x dry weight. Plus hay and other feedstuffs.
One kilogram of beet pulp contains 2.98 Mcal/kg, you can use that to judge against other feedstuffs,

Beet pulp does contain about 10 percent protein, 0.8 percent calcium and 0.5 percent phosphorus, making it a more "balanced" source of energy. It can be used to replace over 50% of the forage in horse's rations without adverse effects when fed with other balanced concentrates.
Although your mare is not currently on other concentrates, just hay & what grass she can find.


Typically a feeding rate of 5 pounds of beet pulp daily in addition to the regular ration for a mature horse that was maintaining its present body weight would result in a gain of about ½ pound per day, so you can adjust your feeding program accordingly when adding beet pulp to your horse’s diet.


If alfalfa is hard to find in your area, you can also try some other whole grains, barley can be beneficial [needs to be processed,just not fed whole] I also feed soybean meal & cracked corn to my hard keepers.

Thank you so much

So I can indeed feed her much more beet pulp. I gave her just a little more today and I will increase it slowly. If she doesn't start to get fatter in a month, I'll try soybean or corn

Thanks again.