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Protein Needs
The amount of crude protein needed in the diet depends on: (1) the amount of that diet consumed, (2) the digestibility of the protein in that diet, (3) that individual animal’s need for protein, and (4) for the growing horse, but not the mature horse consuming feeds commonly fed to horses, the amino acid content or quality of the protein consumed.
The lower the digestibility of the protein ingested, the greater the amount needed in the diet. Protein digestibility varies with the source of the protein, the amount of protein and fiber in the diet, and the amount of heat produced in feed processing and storage. Horses are able to digest 45 to 85% of the protein in most commonly fed feeds, being on the upper end of this range for cereal grains and the lower end for mature grass forage. These values are considered in the indicated amount of crude protein needed. Protein digestibility does not, therefore, need to be considered unless it is reduced to below normal levels. This may occur if the feed has undergone excessive heating, as may occur with improper feed processing, or during storage because of inadequate drying prior to storage.
In addition to digestibility, the amount of a nutrient needed in a diet depends on the amount of that diet consumed. The amount of a diet consumed depends on the animal’s dietary energy needs and the energy content of the diet. The higher the energy content of the diet, the less of that diet the animal needs and will consume to meet its energy needs, and, therefore, the higher the concentration of all nutrients in that diet must be so that the animal will receive enough of them to meet its needs. Thus, a diet with an energy density of 0.9 Mcal/lb (2.0 Mcal/kg), as recommended for maintenance, should contain 8% protein, whereas one providing 1.3 Mcal/lb (2.85 Mcal/kg), as recommended for intense work, should contain 11.4% protein.
Although previously it was believed that physical activity had little or no effect on the horse’s protein needs, several studies indicate that the amount of protein needed increases with increased physical activity. The increased amount of protein is needed for: (1) increased muscle development and mass with increased physical condition, (2) perhaps increased muscle protein content, and (3) nitrogen lost in sweat. However, the increase in protein needed for physical activity is less than the increase in energy needed.
Besides water, the nutrient for which need increases the most as a result of physical activity is dietary energy, which the horse will consume if sufficient feed is available. As a result of the increased feed intake, more protein is consumed. This increase in protein consumption is more than that required to provide the additional protein needed for physical activity. Thus, although more protein is needed for physical activity, an increase in the protein percentage in the diet is not needed, unless the energy or caloric density of the diet is increased. In fact, if the energy density of the diet is not increased, the protein percentage in the diet can be decreased. However, most diets fed and those preferred for physical activity are higher in energy density than those fed the idle horse. This higher energy density is beneficial because it allows the physically active horse to obtain the increased energy needed without too large an increase in the amount of feed consumed. As a result of the lower feed intake because of the diet’s increased energy density, however, the additional protein needed for physical activity may not be consumed unless there is an equivalent increase in the percent protein in the diet. These are the reasons a higher percentage of protein, as well as a higher energy density, are recommended in the diet of the working horse and also of the stallion during breeding season.
During pregnancy, the mare’s protein and energy requirements are not increased until the last 3 months. A diet with an energy density of 1.0 to 1.1 (2.2 to 2.4 Mcal/kg), as recommended for the ninth month of pregnancy until foaling, should contain 10 to 11% crude protein.
During the first 3 months of lactation, both milk protein content and the amount of milk produced are at a peak. As a result, the amount of protein needed is greatest during this period. A diet with an energy density of 1.2 Mcal/lb (2.6 Mcal/kg), as recommended for the first 3 months of lactation, should contain 13% crude protein, and a diet with an energy density of 1.1 Mcal/lb (2.45 Mcal/kg), as recommended for lactation after the first 3 months, should contain 11% crude protein.
For growth, both the amount of protein and its quality or amino acid content are important. An energy density of 1.3 Mcal/lb (2.9 Mcal/kg), as recommended for weanlings, should contain 14.5% crude protein, and a diet with an energy density of 1.25 Mcal/lb (2.8 Mcal/kg), as recommended for yearlings, should contain 12.5% crude protein.
A greater amount of the essential amino acid lysine is needed by the young horse for growth than is available from microorganisms in its intestinal tract and than is present in many feeds. Two other essential amino acids, methionine and tryptophan, are also present in low quantities in cereal grains. However, the most, and generally only, limiting amino acid in the growing horse’s diet is lysine, although if the forage consumed is grass, threonine intake may also be marginal. The diet should provide at least 0.65% lysine and 0.50% threonine in its dry matter. The effects of inadequate lysine are well illustrated by the study results shown in Table 1–5 in which soybean meal, cottonseed meal, and cottonseed meal fortified with lysine were used as the protein supplements in the diet. Although all three diets had an identical protein content, when cottonseed meal without added lysine was fed, growth was slower and more feed was required.
To provide the amount of lysine needed by the horse for growth, all of the additional protein above that provided by the grain and forage consumed that is needed to meet the growing horse’s protein requirements should be provided by a protein supplement in which the protein contains at least 5 to 6% lysine, such as canola, soybean, fish or meat meals, or milk products. These protein supplements, when used to provide sufficient protein to meet the growing horse’s protein needs, will also provide adequate lysine. They are also relatively high in other essential amino acids needed by the horse for growth. If other protein supplements are used, i.e., those that do not contain at least 5 to 6% lysine in their protein, more of them than is needed to meet the horse’s protein requirements must be fed to provide adequate lysine. If not, growth rate and feed efficiency will be reduced.
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