Energy, Feed Ingredients and the Horse

Energy is supplied to the horse via the diet but fundamentally energy is not a nutrient. Horses need energy to carry out their body’s essential daily functions, including the digestion and absorption of food, activity, growth and reproduction. The total energy contained in a feed is called gross energy (GE). The GE of a feedstuff is not a good indicator of the energy available to a horse from a feedstuff. Thus, with horses, we use digestible energy (DE) which is GE minus the energy contained in feces. Digestible energy is expressed in Mcal which is a multiple of the unit of energy calorie. Table 1 lists the DE of common feed ingredients in horses’ diets. In general, cereals provide more DE than hay. Vegetable oils contain more energy than cereals (~2.5 times as much DE as corn and three times as much as oats).

Energy is primarily derived from carbohydrates and fats/oils in the diet: Carbohydrates are commonly divided into non-structural carbohydrates (NSC) and structural carbohydrates. NSC’s consists of starch and simple sugars. Starch constitutes around 40% of the total weight of oats, 60% of the total weight of barley and 70% of the total weight of corn. The remainder of these grains are made up by structural carbohydrates, protein and some fat.

Starch is primarily digested in the small intestine of the horse by enzymes. The starch is broken up into single glucose units which are then absorbed by the horse and used as a source of energy. Any starch that is left undigested passes through the small intestine, rapidly fermented by bacteria in the hindgut and can potentially cause hindgut acidosis leading to digestive issues. Processing cereal grains involves changing the structure of the feed to enhance the efficiency with which starch is digested in the small intestine.

Processing includes cold processed (rolled, ground, cracked) or hot processed (crimped, pelleted, extruded, steam-flaking, and micronized). Rolling passes the grain between a closely fitted set of smooth rollers. Crimping uses rollers with corrugated surfaces. Steam-flaking subject’s grains to steam for various periods of time. Steam-flaked grains are usually less dusty than dry-rolled grains. Pelleting uses ingredients which are ground and then steam heated. Next the mash is pushed through a pellet die to obtain the desired size, cooled and dried to prevent mold growth. The ingredients for extruded feeds are grinded, mixed and the resulting mash is cooked using a combination of high temperature and pressure. Extrusion die nuggets or cubes are then cooled and bagged. Micronizing heats the grain using infrared heat until all moisture is vaporized. This ruptures the endosperm of the grain causing the starch to be reconfigured. Immediate flaking further reconfigures (gelatinizes) the starch which enhances the digestibility and nutritional value of the feed. The goal with all the types of processing is to increase the digestibility of the feed ingredient in the small intestine, thus lowering the amount of indigested starch entering the hindgut. Safety and energy value of the feed greatly increases with processing.

Figure 1. Processed pelleted horse feed

Structural carbohydrates are also referred to as fiber or roughage are found in feedstuffs such as hay, pasture, soybean hulls and sugar beet pulp. Components include cellulose, fructan, hemicellulose and pectin. Bacteria in the hindgut ferment structural carbohydrates producing volatile fatty acids which are absorbed from the hindgut of the horse and used as a source of energy, especially at rest. Crude fiber listed in Table 1 and is a measure of the quantity of cellulose, and other structural carbohydrates. All of the fibrous feedstuffs have higher crude fiber then the grains listed. Structural carbohydrates provide the bulk necessary for proper peristaltic action in the horse’s intestinal tract.

Feeding horses depends to a large extent on the relative proportions of the energy sources provided to the horse and the feedstuffs used. Energy required per day is influenced by body condition; physiological status (age, breed, production); duration and repetition of work; environment, metabolic efficiency and presence of any nutritionally related disease. 

Table 1. Nutrient Composition in Common Feed Ingredients Fed to Horses

Feedstuff	DE, Mcal/lb	Starch, %	NSC, %	Fat, %	Crude Fiber, %
Barley	1.66	54.26	59.47	2.43	5.80
Corn	1.76	69.44	73.13	4.17	2.36
Oats	1.52	43.96	48.68	6.24	9.37
Wheat Middlings	1.52	23.23	32.92	4.94	9.38
Beet Pulp	1.20	0.99	11.81	1.27	18.66
Soybean hulls	0.93	1.20	5.08	2.70	34.72
Grass Hay	0.91	1.65	12.85	2.55	31.40
Grass Pasture	1.03	2.08	13.07	3.45	32.53
Legume Hay	1.19	1.56	11.00	2.44	25.52
Mostly Grass Hay	0.94	1.85	12.28	2.63	24.61
Mostly Legume Hay	1.06	1.96	10.97	2.49	30.66

Values from Equi-Analytical