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Nutrition Functions & Requirements

Feeding standards are based on the nutrient requirements of a specific species at a specific physiological state. In the U.S., the primary source of feeding standards is the National Research Council, commonly referred to as the NRC. The NRC regularly produces publications specifically stating the current recommended feeding standards for each of the domestic species. The NRC recommendations are a relatively accurate estimation of nutrient requirements. However, the NRC feeding standards are to be used as a guideline. As the standards do not always account for all variables potentially influencing the nutrient requirements, adjustments may be required.

The nutrient requirement for each nutrient is the total amount of each nutrient required to perform the various physiological functions. Dietary nutrients should be supplied at the required levels; both nutrient deficiencies and excesses have the potential to have detrimental effects on the animal. Nutrient requirements are either expressed as the total amount required each day or the concentration required in the diet. It is important to emphasize: the animal requires an amount of a specific nutrient versus a concentration of a specific nutrient. However, rations offered ad-libitum or free-choice are formulated based on nutrient concentration. Accurate ration formulation for ad-libitum, also referred to as ad-lib, intake requires an accurate estimation of intake to ensure the animal will consume the required amount of the nutrient. As an example, let us say an animal requires 150 grams of nutrient Z per day on a dry-matter basis. The animal’s estimated daily feed intake is 10 kilograms also on a dry-matter basis. Therefore, the required concentration in the diet is 15 grams per kilogram or 1.5% of the total diet also stated on a dry-matter basis. Additional terminology used to state requirements will be discussed in Module III.

The functions of nutrients are classified into five categories. The first category is nutrients required for maintenance. All animals have a maintenance requirement. Maintenance is defined as the state in which an animal is neither gaining nor losing nutrients. Formulating rations to include only the maintenance requirement is rare. Generally, rations also include nutrients for additional functions such as growth or lactation. Therefore, during ration formulation the maintenance requirements are used as an initial reference point and additional requirements are added depending on the animal’s specific physiological state. The second category is growth and development. In animals, growth and development is measured by determining the animal’s weight. Growth rate is determined by the weight change over time. Relative growth rates in life are represented by growth curves. Average growth curves are available for all species. Figure 2.12 illustrates an average growth curve for an animal with a mature body weight of 500 kg or approximately 1,200 pounds.

Similar to Figure 2.12, growth curves illustrate growth rate is the greatest from birth to puberty, intermediate from puberty to maturity, and lowest following maturity. Young animals have the greatest nutrient requirement per unit of body weight. Therefore, the diet of young animals must contain a relatively high quantity and quality of nutrients. Although it is essential to provide nutrients to achieve adequate growth rates, economics of growth is important to consider. Gross efficiency, defined as total production divided by total consumption, is greatest for young animals because a greater portion of the total nutrients are going towards productive growth versus maintenance. However, if maximal growth rate is not required to achieve a set growth point in a set period of time, it may be economically advantageous to feed a lower quantity and/or quality of nutrients over time and achieve the same total growth at a lower cost. Net efficiency, defined as total production divided by total nutrients over time, may not be significantly different and cost of feed will be decreased. The third category is lactation. Lactating animals with high milk production have an extremely high nutrient requirement. The primary nutrients in milk are water, lactose, lipids, protein, and a variety of minerals especially Ca and P. Nutrient requirement is directly related to level of production. As production increases, nutrient requirements increase. Milk production will vary between and within species. A lactating animal has two sources to fulfill her nutrient requirements, dietary nutrients and nutrients from tissue mobilization. The main source of nutrients such as water will come from the diet and limiting the dietary source will limit milk production. A lactating animal does have the ability to mobilize specific nutrients for a period of time without significantly limiting lactation. However, if the metabolized nutrients are not replenished from dietary sources, subsequent production will be limited. Figure 2.13 illustrates the relative relationships between milk production, dry matter intake, and body condition over a lactation.

In early lactation, a lactating cow cannot fulfill her nutrient requirements from the diet therefore the cow will metabolize body tissues resulting in the decrease in body condition score. As illustrated by the increase in body condition later in the lactation, the animal must replenish body reserves for the subsequent lactation. The fourth category of nutrient functions is reproduction. Nutrient requirements for reproduction can be divided into three time periods, prior to breeding, the initial two-thirds of gestation, and the final one-third of gestation. Nutrient balance prior to breeding can influence conception, establishment of pregnancy, and maintenance of pregnancy. The magnitude of the increase in nutrients is the greatest during the final one-third of gestation in comparison to the initial two-thirds of gestation. During the final one-third of pregnancy, in addition to supporting the growth of fetal tissues and associated fluids, the dam’s metabolic rate also increases. Limiting nutrients required during gestation will limit subsequent health and productivity of the dam and the offspring. Severe limitation of nutrients can result in death of the offspring and/or the dam. Pregnancy toxemia in sheep is an example of the effect of limiting energy required for reproductive functions. In severe cases of pregnancy toxemia, death is inevitable. The final function of nutrients is to perform work or physical exertion. The most common animals to perform work are horses. Figure 2.14 illustrates a horse performing work.

Performing work requires nutrients. The primary nutrients required are water, CHO (or an alternative form of energy), protein, vitamins, and minerals. Similar to lactation requirements, nutrients required for work are proportional to the level of work. Limiting nutrients required for work will decrease overall performance.

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