As the fiber content of a feed increases, its digestible energy, as well as its protein and dry matter digestibility, decrease. Because of this, a relatively good estimate of a feed’s digestible energy content may be obtained by using the digestible energy value given in Appendix Table 6 for a similar type of feed with a similar fiber content, or by using the information shown in Table 4-11, which gives the relationship between the crude fiber and digestible energy contents of horse feeds. Values obtained in this manner may be more accurate for the horse than are the values calculated for ruminants, which are what are used by many feed analysis laboratories. However, if utilization values for horses are used by the laboratory in calculating the feed’s energy content, that value should be used. The digestible energy content of the feeds used is needed to formulate and evaluate the diet, and to estimate the correct amount to feed. However, there is little correlation between the concentration of one nutrient and that of another nutrient in hay. Thus, inferences about the concentration of nutrient(s) based on the concentration of other nutrient(s) in a hay can be misleading. For example, a hay with a low fiber and high protein concentration may have a calcium or phosphorus content lower, or higher, than average for that type of hay. Thus, formulations and evaluations whenever possible should be based on actual analysis, rather than on averages obtained from feed tables. Analysis values outside of the normal range for that feed, as given in Appendix Table 6, should be evaluated closely, as errors in analysis do occur. If there is any question about a value, a re-analysis, preferably of a new sample, should be obtained.
Fig. 6-2(A, B). Taking a hay sample for nutrient analysis using a hay probe. After the probe is bored into the hay (A), the core of hay it contains is removed. Several cores of hay should be taken, put into a plastic bag (B), sealed, and sent to a feed analysis laboratory. Local agricultural extension agents or veterinarians usually know the location of the nearest laboratory. The minimum analysis required to formulate a feeding program for the horse is for crude protein, calcium, and phosphorus. Analysis for moisture, and either crude fiber, TDN, or energy, may also be necessary to assess the quality of the hay.
For vitamins and trace minerals of concern in the horse’s diet, it is generally less expensive and more practical to add them to the grain mix in a sufficient amount, or to use a grain mix that by itself contains a sufficient amount, to meet the horse’s requirements than it is to analyze the feeds for each of these nutrients and determine and add the exact amount needed to the diet. If the feeds being fed don’t already contain a sufficient amount of these nutrients to meet the horse’s requirements, that added is quite beneficial; if they do, although the additional amount added is of no benefit, it isn’t harmful. If the feeds contain a harmful excess, the amount added sufficient by itself to meet the horse’s requirements contributes little to the excess. This is because the horse’s requirement for these vitamins and minerals is a small percentage of the amount that is harmful; e.g., 2 to 5% for selenium, 1 to 3% for copper, 1 to 12% for zinc, and 10 to 20% for vitamin A. However, to determine if an excess, or a deficiency, of any nutrient is present—i.e., to evaluate the diet for any nutrient—all feeds consumed should be analyzed for that nutrient. In addition, you should determine the amount of that nutrient being received from sources other than the feeds that are being evaluated—e.g., the water, or nutrients being given orally or injected.
Feeds are analyzed by many different methods. The most common method used historically is the proximate or Weende analysis. As shown in Figure 6-3, the proximate analysis of a feed determines its content of moisture, crude fat or ether extract, total minerals or ash, crude fiber and nitrogen, and from it crude protein (which is sufficiently accurate only if all nitrogen in the feed is from protein). By subtracting the percentage of each of these from 100, the percent nonfiber or soluble carbohydrate in the feed, which is often referred to as the nitrogen free extract or NFE, is determined. From the values obtained, the digestible energy, the total digestible nutrient (TDN), content of the feed is frequently calculated. This calculation, however, may be based on values for ruminants, which for forages results in an erroneously high digestible energy content, and for grains and protein supplements an erroneously low value for horses. A major problem with the proximate analysis is that specific minerals or types of fiber aren’t determined. The type of fiber may be obtained by the detergent or Van Soest analysis. The higher a feed’s fiber content, the greater the benefit of this analysis in obtaining that feed’s digestible energy content. Thus, obtaining a feed’s acid or neutral detergent fiber content is of most benefit for forages and of little benefit for grain or protein supplements.
Near infrared reflectance spectroscopy (NIRS) is a newer method of feed analysis. It is based on predictive equations. The nutrients in the feed are not measured directly. Because of this, its accuracy is questionable for feeds outside the standard or normal range for that laboratory and, therefore, its results should not be relied on for analysis of exotic or nontraditional feeds. It is also reported to be unreliable for evaluating grain mix and protein supplements. Its advantages are that it is fast, relatively inexpensive, and usually reports results for acid and neutral detergent fiber, TDN or digestible energy, crude protein, calcium, phosphorus, magnesium, and potassium. However, like other analysis procedures, energy values are frequently those valid for ruminants and not horses.
Fig. 6-3. Proximate Analysis of Feeds
a Proteins contain 16 ± 2% nitrogen. Crude protein = nitrogen × 6.25, or nitrogen ÷ 0.16. Protein determined by this method will be erroneously high if the food contains nonprotein nitrogen such as urea or ammonia.
b It also consists of variable amounts of both soluble and nonsoluble fibers. It is frequently called nitrogen-free extract, or NFE, since it is the extract of acid and alkali digestion less the amount of protein determined from its nitrogen content. It is determined as the difference between 100% and the amount of everything else in the feed, i.e., 100% – % moisture – % crude protein – % crude fat – % crude fiber – % ash. Any errors in these analyses will also appear in the NFE value.
c Which is insoluble fibers, primarily cellulose and hemicellulose, not lost by acid and alkali digestion.
Methods of Expressing Feed Nutrient Content
Some laboratories report the nutrient content of a feed as the amount present in the feed dry matter; others report it as the amount present in the feed as fed, or as the laboratory received it. If it isn’t stated, it will generally be the amount present in the feed as fed. The amount given on the feed tag of commercially available feeds or supplements is the amount present in the feed as fed. The only way that a feed or diet can be accurately formulated, evaluated, or compared to other feeds or the animal’s requirements is if all are expressed on an identical moisture, or most accurately on an available calorie, basis. Concentrations on an available-calorie basis can be obtained by dividing the concentration in a feed by that feed’s available calories. For example: A feed provides 2 Mcal of digestible energy/kg and contains 1% calcium, i.e., 1 g of calcium/100 g of feed or 10 g of calcium/1000 g or/kg of feed. The feed would therefore contain the following:
However, the available calories provided by a feed or diet frequently aren’t known; therefore, most commonly the nutrient content of feeds, and animals’ requirements, are expressed on or converted to an equal-moisture basis. The best way to ensure that all are on an equal-moisture basis is to determine the amount of all nutrients in the feed or diet dry matter, i.e., the concentration of each nutrient that would be in the feed or diet if it did not contain any water or moisture.
Although for formulating and evaluating the diet it is necessary to have the amount of nutrients in all feeds and needed by the animal expressed on an equal-moisture basis, in preparing and feeding a diet the amount of all feeds must be on an as-fed basis. For example, you don’t feed 9 lbs of hay dry matter, you feed 10 lbs of hay, which in this example happens to contain 1 lb of, or 10%, moisture.
Thus, it is necessary to be able to convert back and forth from a dry matter to an as-fed basis. There are two rules that must be remembered to do this. These are:
Amount of Feed Needed by the Horse
The amount of feed needed depends on the horse’s energy needs and the energy content of the feeds consumed. The energy contents of horse feeds are given in Appendix Table 6. The horse’s energy needs depend on numerous factors, including its body weight, function (e.g., growth rate or stage of pregnancy or lactation), physical activity, and environment, and varies between individuals as discussed in the section on “Energy Needs” in Chapter 1. The average energy needs for the horse, based on its weight, function, and physical activity, can be obtained from Appendix Tables 4 and 5. To obtain this information, the weight of the horse must be known or determined.
Obtaining a Horse’s Weight
Obtaining the horse’s body weight is useful for many purposes, including:
Overfeeding or underfeeding, resulting in a gradual weight change, generally isn’t apparent until changes are severe. Inadequate or excessive growth rate also may not be readily apparent. Inadequate feed intake resulting in weight loss is one of the first indications of many illnesses. Horses, like other athletes, have an optional performance weight. Racehorses’ optimal racing weight has been reported to be within a 16-lb (7.3-kg) range; much less than can be detected from the horse’s appearance. Reproductive efficiency is also greatest within a certain body-weight range. Thus, there are many reasons for obtaining and monitoring the horse’s weight. The best way to do this is by using a walk-on scale. However, the horse’s weight can vary greatly with degree of hydration and gastrointestinal fill. Up to a 5% decrease in body weight due to dehydration cannot be detected visually, and the horse’s body weight may vary from 5 to 10% depending on the time since feeding and feed type. To alleviate errors in the body weight obtained, and to correct for changes in body weight due to variations in the degree of hydration and gastrointestinal fill, the horse should be weighed at a consistent time prior to feeding, watering, and physical activity, such as training or competing. If a scale isn’t available, a fairly accurate estimate of the horse’s weight can be obtained from accurate body measurements.
A girth measurement alone is fairly accurate, but accuracy can be improved by also measuring the horse’s length and by using the following formulas.
A fairly accurate estimate of the horse’s body weight can be calculated from this equation or from the nomogram derived from it and shown in Figure 6-4.
For light horse breed foals 1 to 6 weeks of age, a more accurate weight may be obtained from the following formulas.
The actual weight of light horse breed foals at 1, 2, 3 and 4 weeks of age was found to be within 5% of that calculated from this formula. However, to obtain this accuracy for foals at birth and at 12 weeks of age 17%, or 
, must be added to the weight obtained using this formula.
Fig. 6-4. Nomograms for estimating a horse’s weight from girth and length measurements. Girth is measured just behind the withers and elbows following respiratory expiration, and length is measured from the anterior point of the shoulders to the posterior point of the buttocks (tuber ischii) (illustrated in Fig. 6-5). Example: A horse’s girth is measured as 178 cm (70 in), which is plotted on the girth scale above as point A, and length is measured as 165 cm (65 in), which is plotted on the length scale as point B. Where a straight line drawn from point A to point B crosses the weight scale, indicated above as point C, is the horse’s weight, which in this example is 440 kg (968 lb).
Heart girth should be measured just behind the withers and elbows (where the cinch goes) following respiratory expiration; body length should be measured from the anterior point of the shoulder to the posterior point of the buttocks (tuber ischii) (Fig. 6-5). For accuracy in measuring length, two people are needed—one to hold each end of the tape.
A slightly less accurate but still good estimate of the horse’s weight can be obtained from the girth measurement alone and the information given in Table 6-1, or by using a horse-weight tape (Fig. 6-6). A horse-weight tape is used to measure the heart girth and is marked in pounds or kilograms of body weight corresponding to the girth measurement for the average horse. However, a heart girth measurement without the correction indicated in Table 6-1 is not accurate for pregnant mares. A heart girth measurement is also not sufficiently accurate to detect small changes in weight that influence physical performance, but it is quite adequate for estimating the amount of feed needed, and is much more accurate than a body-weight estimate based on visual examination.
Many horse people believe they can estimate fairly closely the horse’s weight just by looking at the horse. Perhaps some can, but most cannot, as shown by the following study. Seventy-seven farm managers and 62 veterinarians with an average of 17 and 21 years of professional experience with horses, respectively, from visual examination estimated the weight of five horses of varying size. Of the 695 estimates, 12.5% were over the horses’ actual weight by an average of 92 lbs (42 kg)/horse, and 87.5% were under by an average of 186 lbs (85 kg)/horse. Nearly 60% underestimated all five horses. There was no correlation between accuracy and years of professional experience with horses.
Fig. 6-5. Major external anatomic structures of the horse.
Determining Total Amount of Feed Needed by the Horse
The amount of feed needed can be calculated, as shown in the following examples, by dividing the horse’s dietary energy needs by the energy provided by the feed(s) consumed. When done correctly, the amount obtained will be quite accurate for the average horse, or group of horses, under similar conditions and fed feeds similar to that used in the calculation. However, the feed needed by the individual horse may be more or less than the amount determined because of (1) variations among horses in their dietary energy needs, (2) variations in the amount of feed consumed and the amount wasted or lost, (3) differences in environmental conditions, and (4) differences in the energy content of the feeds consumed. Always feed the amount needed to maintain the horse at optimum body weight and condition. To ensure that this is done, monitor each horse’s weight periodically and change the feeding program as needed.
Example 1: Idle Mature Horse.
Determine the amount of feed needed by a 900-lb (409 kg) idle mature horse. This value can be obtained by interpolating between the amounts needed by the 400- and 500-kg horse, as given in Appendix Tables 4 as 13.4 and 16.4 Mcal/day, respectively. The 409-kg horse therefore would need the following amount:
To determine the amount of feed needed to provide this amount of energy, divide the amount of energy needed by the energy that the feed provides. Thus, if early-bloom alfalfa hay, which provides 1.1 Mcal/lb of dry matter (DM) (as given in Appendix Table 6), were the only feed consumed, this horse would need to consume the following amount of it.
Table 6-1 Estimating a Horse’s Weight from Girth Measurement*
However, dry matter isn’t being fed. Hay is being fed, and, as given in Appendix Table 6 or from its analysis, it contains 90% dry matter. Therefore, to obtain 12.4 lbs of dry matter, the horse would need to consume the following amount of hay:
Generally, an additional 10 to 15% of hay must be fed to allow for that wasted and lost. Thus, in this example the following amount should be fed:
If small square bales weighing 60 to 65 lbs each are being fed, about 
of a bale/horse each morning and night would provide the amount of hay needed.
If 2 lbs of regular oats were fed daily, they would provide the following:
Feeding 2 lbs of oats/day would reduce the amount of hay needed by 3 lbs/day, as determined below.
Thus, if 2 lbs of oats/day were fed, you would also need to feed about 13 lbs of early-bloom alfalfa hay/day. This is instead of the 16 lbs/day that should be fed when no oats are fed.
Fig. 6-6. Estimating the horse’s weight from measurement of the girth. A weight tape marked in pounds of body weight (which for the average horse correlates well) is available from some feed stores and tack shops. A tape measure and the information given in Table 6-1 may be used instead.
Example 2: Working Horse.
A 1400-lb (636-kg) horse ridden at a slow trot for 3 hours/day for 3 days/week needs:
If the physical activity in this example is equivalent to light work, two other ways to estimate total energy needs for this 636-kg horse would be:
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