Q. My horses live with beef cattle. Our pastures are very poor and we are feeding hay, but all the stock is losing weight and I think I need to supplement them with some other feed. My feed store sells several all-purpose feeds which claim to be appropriate to feed both the horses and cattle. Are these really okay to feed to horses?
A. The short answer to your question is that all-purpose feeds are at best “okay.” Not optimal, but probably okay. This assumes that they’re really designed and safe for both species.
Ionophore Toxicity in Horses
I would never give horses feed made solely for ruminant species. The reason? Cattle feeds have the great potential of containing some type of ionophore (an antibiotic used in ruminant feed that is toxic to horses).
Most feed manufacturers are aware of these risks and clean the lines between feeds containing ionophores and those designed for horses. Some not only they clean the lines but will run feed for other less sensitive species between a medicated feed run and feed made for horses. Of course some mills are more careful than others. In feeds sold as suitable for both cattle and horses there could still be that risk; in fact the risk exists in some feeds made solely for horses too, but hopefully manufacturers have taken care to remove possible sources of contamination before feed is milled for horses.
In a perfect situation all feeds fed to horses are milled through dedicated equine lines that never come in to contact with feed containing ionophores and other potentially harmful medications.
Assuming this mixed-use feed is ionophore free there are other factors that make it a less-than-optimal choice for horses. Cattle have a lower requirement for quality protein in their ration because they can utilize microbial protein. This is because in their digestive tracts, the source of microbial fermentation, precedes the site of protein absorption. In the horse the main site of protein digestion and absorption (the small intestine) occurs before the cecum and colon where the massive population of fiber utilizing microbes live. Horses, therefore, have a limited ability to utilize microbes as a source of protein unless they eat their feces. For this reason the dietary protein provided in an equine ration must be of better quality than that fed to ruminants such as cattle.
Most all-purpose stock feeds contain 10-12% crude protein. This isn’t an inappropriate amount of protein for most mature horses; most dedicated equine feeds provide a similar amount of crude protein but also guarantee lysine and methionine amounts. These are essential amino acids that the horse is unable to make and they must, therefore, be in the diet. Cattle and other ruminants can rely on their microbial protein as a source of these key amino acids, but for horses need them in the diet. Inadequate amounts of lysine and methionine have implications for coat and hoof quality as well as muscle production, immune function, hormone production, digestive enzymes, and many other processes.
Another key difference between all-purpose stock feeds and dedicated equine feeds is the level of mineral fortification. A good number of equine-specific feeds for horses at maintenance have higher mineral inclusion levels than those in general-purpose stock feeds. For example, the copper and zinc levels in an equine feed for adult horses at maintenance are around 40 parts per million (ppm) and 120 ppm respectively versus only 10 ppm and 50 ppm in a general stock feed. The level of copper will be particularly low if the feed is also for use in sheep as copper is toxic to sheep. In this case copper might be as low as 5 ppm.
Vitamin levels in all-purpose feeds might also be low, especially for vitamin E. Horses require more vitamin E as work increases due to the oxidation products generated in muscle tissue. Obviously this type of work is not a concern for cattle, and an all-stock feed may only provide 20 international units (IU) per pound versus the maintenance feed for adult horses that provides 80 IU of vitamin E per pound. For working horses the level of vitamin E will be higher, possibly upwards of 100 IU per pound.
Certainly, feeding a feed with some level of fortification is likely to provide horses some benefit. However, a feed designed to be fed to several different species is unlikely to do as good a job of meeting the specific needs of any one of those species as compared to species-specific feeds. For horses not in work there might be more leeway in feeding a less-than-optimal diet. In the long run, you’re better off using a feed specifically designed to meet equine dietary needs.