Swine Nutrition Guide: Nutrient Interactions

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Nutrient Interactions -- The absolute requirement for one nutrient can be influenced by the amounts of other nutrients in the diet. There will always be an excess concentration of some nutrients when using common ingredients. In some cases, excesses of one nutrient may cause an undesirable interaction with another nutrient.

Interactions can include mineral with mineral, mineral with vitamin, vitamin with amino acid, and amino acid with amino acid. Although there are many nutrient interactions, only a few are of practical importance when formulating swine diets with common ingredients.

However, others may be important when using nontraditional ingredients. Some of the more frequent nutrient interactions that can cause problems are discussed in this section.

Calcium and Phosphorus
Calcium is the most deficient mineral in diets formulated with cereal grains and oilseed meals. Phosphorus is also deficient in plant materials. Furthermore, much of the phosphorus in plants occurs as the organic complex phytate which renders it mostly unavailable to the pig. Thus, it is necessary to supplement diets with both calcium and phosphorus for satisfactory performance. Although the level of each nutrient is important, the ratio of calcium to phosphorus may be more important in certain situations.

The calcium:phosphorus ratio in grain and oilseed mealbased diets should normally be between 1:1 to 1.5:1, although wider ratios may be acceptable under certain circumstances. However, caution is necessary because high levels of calcium interfere with phosphorus absorption. At marginal levels of phosphorus, the ratio must be close to 1:1. As long as both calcium and phosphorus levels meet or exceed recommended levels, a ratio less than 1:1 is not detrimental, but usually results in more costly diets. At excess levels of phosphorus (implying considerable inorganic phosphorus is included) the calcium to phosphorus ratio may exceed 1.5:1. The total calcium to available phosphorus ratio in the diet needs to be close to 2:1.

Calcium and Zinc
The absorption of zinc is affected by the level of calcium in the diet. High levels of calcium included in diets with high levels of phytate cause zinc to be bound in a complex that renders both zinc and phosphorus unavailable to the pig. When formulation of diets results in high levels of calcium, zinc must be increased. The levels of zinc suggested in this publication assume reasonable levels of calcium.

Copper, Iron and Zinc
These three minerals are involved in interactions; however, the effects of increasing levels of one or more of these minerals in the diet are not consistent. Excess iron and zinc reduce copper availability. Extremely high levels of zinc can lead to a copper deficiency, which is characterized by anemia. Because of metabolic interactions, zinc sources with relatively low bioavailability (e.g., zinc oxide) might be superior to sources with high availability (e.g., zinc sulfate) when including zinc at high levels for nonnutritional purposes. High levels of copper (e.g., 250 ppm) are used as a growth promotant, and these levels are not toxic unless diets are deficient in iron and zinc (and high in calcium). When 500 ppm of copper has been fed there has been mixed success in lowering stored levels of copper by increasing zinc levels in the diet.

Vitamin E and Selenium
The interaction between vitamin E and selenium is related to the protection of tissues against the detrimental effects of peroxides. Vitamin E helps protect against peroxide damage by scavenging free radicals before they can attack cellular membranes and cause oxidative damage. Selenium is a component of glutathione peroxidase, an enzyme involved in the destruction of peroxides. Although vitamin E and selenium may not be substituted for one another, the interaction between the two nutrients results from the sparing effect of one on the need for the other. In addition, vitamin E plays an antioxidant role in feed. Trace minerals, such as copper, zinc, and iron increase oxidation and thus increase the destruction of vitamin E in stored feed. Other natural antioxidants, such as vitamin A, are also attacked and can spare vitamin E in this role. Factors that affect the amount of vitamin E and selenium to supplement are the level and type of dietary fat, presence of antioxidants in the feed, level of trace mineral inclusion and length and conditions of feed storage.

Amino Acids
Absolute requirements for individual amino acids can be determined assuming that all amino acids are provided in sufficient quantities without excesses (i.e., ideal protein ratios). However, when least-cost or best-cost diets are formulated, excesses of some amino acids are inevitable. The first limiting amino acid in these formulations (the amino acid for which the target level is last to be met as the amino acid source is increased in the diet) is usually lysine, but can be tryptophan, methionine, threonine, isoleucine or valine at certain growth phases and with certain combinations of ingredients.

The requirements for the essential amino acids methionine and phenylalanine depend on the level of the nonessential amino acids, cystine and tyrosine, respectively. Methionine can be converted to cystine, and up to 50% of the requirement for total sulfur amino acids (methionine + cystine) can be provided by cystine.

The same situation exists for phenylalanine and tyrosine (up to 50% of the requirement for total aromatic amino acids [phenylalanine and tyrosine] can be provided by tyrosine). However, neither cystine nor tyrosine can be converted to the essential amino acids methionine and phenylalanine.

Amino Acid Imbalance
This occurs when an essential amino acid other than the one that is first limiting is supplied in excess. It may occur as a result of adding a crystalline amino acid or a protein source high in that amino acid. The result is that the first limiting amino acid, which is supplied at a level that should be sufficient, now becomes deficient. Feed intake is reduced, and, as a result, there is a proportional reduction in pig gain. To correct the situation, the level of the excessive amino acid must be decreased or the level of the first-limiting amino acid must be increased.

Amino Acid Toxicity
This condition resembles an amino acid imbalance in that an amino acid other than the first limiting amino acid is supplied in excess quantity. However, an amino acid toxicity can not be corrected by adding higher levels of the first limiting amino acid. Toxicities invariably are caused by excess additions of crystalline amino acids and are corrected by reducing or eliminating the amino acid additions. While methionine and tryptophan are two amino acids that can cause toxicities, lysine and threonine rarely cause toxicity problems. Lower feed intake and pig gains can be expected as a result of amino acid toxicities.

Amino Acid Antagonism
This condition results from the excess of one amino acid that has a negative effect on a structurally similar amino acid. Because structurally similar amino acids compete for the same absorption and transport sites in the small intestine, high levels of one amino acid may create a metabolic deficiency of the other amino acid, even when that second amino acid is supplied at the required level in the diet. Lysine and arginine and leucine and isoleucine are examples of structurally similar amino acids that compete for absorption sites. An antagonism results in lower feed intake, lower pig gains and poorer feed efficiency. Antagonisms rarely are a problem in pigs fed grain and oilseed meal diets.

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