I know that there are a lot of people who feed rose hips to their horses. Before I learned about equine nutrition I did too, seemed like a good idea at the time when it was recommended to me. Rose hips is said to be high in vitamin C, antioxidants and other nutrients and it sure smells nice.
It's also renowned for its anti inflammatory properties, there are a number of human studies that show the effectiveness of rose hips as an anti inflammatory. All good? Yes! Now comes the BUT. Horses weigh hundreds of kilograms more than people and since needs are based on bodyweight, is rose hips an effective source of the antioxidants and nutrients for horses?
Rose hips is reported to contain between 1700 to 2000 mg of vitamin C per 100 grams in the dried product, wild rose hips is reported to contain a lot less, the NutritionData website has it at 426 mg per 100 grams. Using the upper level, a tablespoon of dried rose hips (~12 g) would provide ~240 mg vitamin C. This may be considered high in vitamin C for people but people weigh a lot less than horses. How much do horses need for an effective dose? It is well established that vitamin C can help with lung allergies. This study used 20 mg per kg in bodyweight per day, that would make it 10 grams or 10,000 mg for a 500 kg horse.
To feed 10 grams of vitamin C with dried rose hips you would have to use 500 grams or about 42 tablespoons per day. It is worth knowing that levels of 20 grams or so per day of vitamin C causes diarrhoea in many horses, not a good idea to overdo it with any source of vitamin C.
Rose hips contains vitamins A, D and E, essential fatty acids, antioxidant flavonoids (carotenes) and sugars. A horse gets plenty of A, D and carotenes from grass and/or hay and the essential omega fatty acid profile is the reverse of what we want; high in the pro inflammatory omega-6. A large amount would need to be fed to get anything close to a significant dose. It is known that rose hips contains an anti inflammatory active ingredient (scientists are trying to isolate it) but you have to feed well over a cup a day (300 g) to get any effect. And it does contain a lot of sugar which would be a concern for a horse predisposed to dietary laminitis. It should not be fed to any horses with insulin resistance (IR) or equine metabolic syndrome (EMS).
Does supplementation of vitamin C help horses, for example a horse with an allergy like Queensland Itch? I don't know but horses can manufacture their own vitamin C in the liver, they can't get scurvy like us. Vitamin C supplementation in horses comes with a price, it has been shown to increase the absorption of iron (pro inflammatory) and to be potentially prooxidant rather than antioxidant in the presence of high body iron stores. Many horses are iron overloaded as iron is one mineral that can't be easily excreted. Since horses can manufacture their own vitamin C I'd weigh up the pros and cons before deciding to use it. And if I did decide to supplement vitamin C I'd find a cheaper, more concentrated source without all that sugar.
Is rose hips a useful source of minerals? For example, using the data at NutritionData, one kilogram of wild rose hips contains 1 mg copper and 3 mg zinc. Since the National Research Council (NRC) recommends a base level (enough to prevent nutrient deficiency symptoms) of 100 mg copper and 400 mg zinc per day for a 500 kg horse in no to light work, clearly rose hips isn't going to be a great source. There are vastly better ways to supplement copper, zinc and the other minerals. I can advise on the best and most cost effective method for all nutrients.
For a non sugar sensitive or insulin resistant horse it appears that there is no harm in feeding rose hips but it's an expensive way to supplement minuscule amounts of nutrients.
Deaton CM, Marlin DJ, Smith NC, Roberts CA, Harris PA, Kelly FJ, Schroter RC (2003) Pulmonary bioavailability of ascorbic acid in an ascorbate-synthesising species, the horse. Free Radic Res. 2003 Apr;37(4):461-7
Jin F, Frohman C, Thannhauser TW, Welch RM, Glahn RP (2009) Effects of ascorbic acid, phytic acid and tannic acid on iron bioavailability from reconstituted ferritin measured by an in vitro digestion-Caco-2 cell model. Br J Nutr. 2009 Apr;101(7):972-81. Epub 2008 Aug 28
Nutrition Data analysis for wild (Northern Plains Indians) rose hips