In your question, you do not mention what the mare was fed prior to her journey to North America. Under the circumstances, it might be prudent to pinpoint what the mare ate when she was thriving in France. The logical place to start would be an attempt to emulate her European diet on American soil. With the variety of feedstuffs available in this country, it might be possible to do just that. Without that information, though, other options are available.

Focusing on forage first, good-quality grass might be the best thing for this mare. Few horses refuse fresh grass. If she has not been turned out recently, be sure to gradually increase her grazing time an hour or so every few days, beginning with a half-hour. If she is at all unsettled in the paddock or pasture by herself, provide a companion for her.

If regular turnout is not an option, give her as much hay as she will eat. At least once a day, remove the uneaten hay and give her fresh hay, making it as tempting as possible. Although she doesn’t like the taste of alfalfa, you could look for another energy-dense legume such as clover. She might find clover more palatable than alfalfa. Grain hays like barley or oat hay are common in France, and she may be accustomed to these. Ask your hay supplier if he has a way to source grain hay.

Chaff (chopped-up hay) is a popular feedstuff in Europe, and she might be familiar with this. It can be fed alone or mixed with a concentrate. Chaff is available in the United States. Your mare may have been fed haylage in France, and this would be much more difficult to obtain stateside.

In terms of a concentrated energy source, you might try high-quality oats. In France, it is common to feed horses straight grains, not textured or pelleted feeds.  Most common are rolled oats or steam-rolled barley and occasionally micronized corn.  If your mare is familiar with eating rolled grains, it may require some patience to get her to eat whole or crimped grains. Adding a slight amount of vinegar to the grains may stimulate her appetite.

Because the mare is not eating well, she may be becoming deficient in B vitamins, which are normally produced in the hindgut.  B-vitamin deficiency is known to affect appetite. A complete B-vitamin supplement is recommended if a horse is not eating normally, if a horse is heavily stressed, or if a horse is in heavy exercise.  Several different B vitamins work together in the body, so a supplement that contains a B-vitamin blend is suggested.

The mare might have other physical problems that are hindering her ability to gain weight. A thorough dental examination by a specialist will rule out any anomalies. An examination by a veterinarian to exclude any subtle pain that might be decreasing appetite would be a good idea, as would a review of the mare’s deworming schedule.

A relatively common problem among sporthorses, especially those in stressful situations, is subclinical acidosis. This syndrome is characterized by fluctuations in hindgut pH, which predispose horses to colonic ulcers. Inappetence with concomitant weight loss is one sign of subclinical acidosis, as is a general sourness in disposition and development of stereotypies (cribbing or stall-walking). The use of a hindgut buffer, designed to attenuate changes in hindgut pH, is useful in these situations.

Although you do not mention the mare’s normal demeanor, you might consider changing her environment if she seems sensitive to the goings-on around her. Instead of a front-and-center stall, choose for her a more secluded one, far removed from the hustle and bustle of a grooming stall or the feed room. Make sure she is still in the company of other horses, though.