Feed your horse several times a day and at regular times to aid in a healthy digestive system and adjustment to it's new diet. Try to avoid ground feeding whenever possible as it cuts waste and helps to stop parasite contamination. Make sure hay is kept dry to avoid losing nutritional value or leaving your hay open to mold produced substances. When needed make sure that your horse receives proper equine dental care also and have immunizations done regularly. Actually no they haven't they have been dying off for years.
Slaughter is only one of the causes.
Those that due survive would be considered to be in poor condition by the standards we use to judge the health of a domestic horse.