Dairy Research and Bovine Somatotropin

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History of somatotropin. Somatotropin (ST) is a protein hormone produced in the pituitary gland of all animals. Early in this century it was demonstrated that animal growth could be stimulated by administration of ST. In 1937 scientists observed that milk yield increased when BST was administered to lactating cows. From the '30s through the early '80s, knowledge on chemical structure, function and activity of ST from several animal species increased. Supplies of ST were limited during this period to what could be extracted from the pituitary glands of slaughtered animals. Recombinant DNA technology, similar to that used to produce insulin for humans, now provides amounts of ST sufficient for thorough testing and, for BST, commercialization.

ST is a protein hormone. Most plant and animal proteins, including ST, are produced by linking the twenty naturally occurring amino acids together (each amino acid is usually used more than once) in a particular sequence to form a chain. BST is a chain of 190 or 191 amino acids.

Differences in number of times each amino acid is used, which amino acids are used, and the sequence in which the amino acids are linked together in a protein determine the structure of proteins. Because the structure (3-dimensional shape) of a protein determines its function, proteins that differ in structure also differ in function. These structural differences explain why some proteins are hormones (insulin, ST) and others serve different functions (enzymes, cell structural components, muscle proteins, etc.). Structural differences also exist between human ST and BST and explain why BST has no effect on humans.

ST is not a steroid hormone. Steroid hormones (estrogen, progesterone, testosterone) are small (about the size of 4-5 amino acids) lipid structures, are not extensively digested, can be absorbed intact, and can be effective when taken orally (birth control pills). Protein hormones are usually much larger (BST has 190 or 191 amino acids), are extensively degraded, essentially can not be absorbed intact and have no effect when taken orally (insulin, ST). Although steroid hormones are identical among animal species, the structure of protein hormones frequently differs among animal species. This difference in structure can limit the ability of a protein hormone from one species to work in another species (see below).

Figure 1. How Bovine Somatotropin Works:
Figure 2. Effect of milk yield on proportion of net energy requirement (NEL) used for maintenance (shaded column area) and milk synthesis (clear column area). Data are from NRC for a 1430 lb. cow and indicate that as milk yield increases, the proportion of energy used for maintenance decreases. The high-producing cow is therefore more efficient and provides a greater return to the producer per quantity of feed consumed.
Figure 3. Proposed effect of management quality on milk yield response to dose of bovine somatotropin. (From Bauman DE: The Cornell experience. In Proceedings of the National Invitational Workshop on Bovine Somatotropin. Washington, DC, USDA Extension Service, 1987, p 46; with permission.)

BST is not active in humans. Protein hormones must bind to another type of protein (receptors) before they can cause a biological effect. This binding is similar to a lock and key. If the shape of the key does not fit the lock, the lock cannot be opened. If the structure (shape) of a hormone (key) does not fit the receptor (lock), it cannot bind to the receptor and no biological effect can occur.

The structure of BST and ST from other non-primate species is different from human ST. Although human dwarfism (the type caused by insufficient pituitary production of human ST) can be treated effectively with administration of human ST, BST and ST from other nonprimates are not effective in treating this deficiency. Only ST from primates (monkeys, etc.) has a structure sufficiently similar to human ST to allow binding to the human ST receptor.

BST coordinates the utilization of nutrients. BST directs nutrients toward growth in young cattle and toward lactation in milking cows. Administration of additional BST from pituitary extracts or from recombinant DNA technology complements the animal's natural ability to direct nutrients toward these functions.

Milk production responses of 10 to 15% are expected. Production responses of 10 to 15% during the treatment period (5 to 12% over an entire lactation) are expected. Variation in management skills and feeding practices contribute to variation in actual production response. Dairy producers should ensure their overall management practices are at least better than average before they use BST.

Production responses to BST administration are not magical. Cows that are adequately fed utilize a portion of their available feed nutrients to maintain themselves. The remainder of the available nutrients are used for production. Cows treated with BST must consume additional feed to support the greater milk production associated with BST administration. The amount of additional feed needed is directly related to the increase in milk produced. Because maintenance requirements are unchanged, a greater proportion of the additional feed intake is used to produce milk. This increases the efficiency of feed utilization (5 to 15%).

Composition of the diet fed to cows treated with BST does not need to change from that fed to cows producing large quantities of milk without BST administration. However, good nutritional management (a must for effective, sustained use of BST) dictates that diets for all cows be formulated to meet or exceed National Research Council recommendations in order to provide sufficient nutrients for the cow and the quantity of milk being produced.

Milk composition is not adversely affected by BST administration. Evidence demonstrates that the safe and wholesome nature of milk and milk products are not changed through the use of BST. Plant and animal products contain basic nutrients (proteins, fats, and carbohydrates) and a variety of hormones (both protein hormones and the fat-soluble, steroid hormones). Several factors such as stage of lactation, diet, and breed contribute to normal variations in nutrient and hormone composition of milk. Nutrient and hormone composition of milk from cows consuming adequate nutrients and receiving BST is not different from milk produced by cows under similar conditions without BST. Flavor and processing qualities of milk are not affected by BST treatment.

Food and Drug Administration has ruled that milk and meat from cows treated with BST are safe for human consumption. Review of scientific literature has allowed FDA to conclude that milk and meat from cows treated with BST, under conditions as authorized by FDA, is safe within the meaning of the Federal Food, Drug and Cosmetic Act. Therefore, FDA has placed no restriction on the commercial use of milk and meat from cows treated with BST. (Journal of the American Veterinary Medical Association. 1988. 192:1698). Information used by the FDA to evaluate food safety aspects of milk and meat from BST-treated cows has been summarized (Science 1990. 249:875). Very small amounts of BST are found in milk from untreated cows. When 12 times the proposed treatment dosage was administered to dairy cows, the amount (concentration) of BST in milk did not increase above that found in milk from untreated cows. BST is a protein and is digested like any other protein when it is consumed. Therefore, like insulin, another protein hormone, BST has no effect when administered orally or consumed as a natural component of milk. Growing rats that have had their pituitaries removed are deficient in ST and very sensitive to ST treatment. When these rats were fed BST at 400,000 times the amount found in a quart of cow's milk, no growth occurred. When BST was injected into the rats, growth increased.

Cows receiving BST are healthy. No adverse effects of BST on the health of treated cows have been detected in studies conducted in North America and Europe. Udder health, incidence of ketosis, displaced abomasum, milk fever, and feet and leg problems of BST-treated cows are similar to those of untreated high producing cows. Dairy records demonstrate an association between milk yield and incidence of mastitis. This association suggests that incidence of mastitis would increase in BST-treated cows because they produce more milk. This has been observed in some BST studies. However, BST does not appear to increase the incidence of mastitis above that expected from the greater milk yield.

Milk yield and antibiotic use. Although increased milk yield is associated with an increased incidence of mastitis, proper management practices can minimize the frequency and impact of mastitis. Not every mastitis case requires treatment of the cow with antibiotics. All milk is tested for the presence of antibiotics before it is marketed and dairy producers realize they must not ship milk contaminated with antibiotics. Producers incur significant financial penalties if they ship antibiotic-contaminated milk. Dairy producers have considerable incentive to follow established guidelines for antibiotic use. Use of BST will not increase the incidence of antibiotic contamination of the milk supply if producers use antibiotics as directed.

Reproduction. Investigation of reproductive performance of normal dairy cows has demonstrated that high-producing dairy cows tend to remain open longer and require more services per conception than lower producing cows. This translates into .005 more services and .3 more days open per every 100 pound increase in production per cow. Therefore, cows producing additional milk from administration of BST would be expected to have lowered reproductive performance compared with cows producing less milk. When the effects of increased milk yield are considered, results demonstrate no adverse effects of BST on reproductive performance.

Calves from BST-treated cows are normal. BST cannot cross the placental wall. Therefore, BST treatment of cows should not have an effect on their offspring. Several studies have demonstrated that cows treated with BST produce calves of normal size and that these calves are healthy and have normal growth rates. In addition, no unusual calving problems have been detected.

Long-term administration of BST does not appear to be detrimental to dairy cows. Many cows in the United States and Europe have completed more than two successive lactations in which they received BST. Several of these cows have also completed a subsequent lactation in which their productive performance was similar to what it was prior to treatment with BST. Data indicate that BST administration has no detrimental effects on performance and health of well-managed dairy cows. Producers with better than average management programs should experience little difficulty incorporating BST into their dairy program. Producers with less than adequate management programs will have difficulty maintaining proper body condition on their cows and will likely experience problems typically associated with thin, underconditioned cows.

Response per cow on small and large dairies should be equal. Other than economies of scale, large dairy operations have no apparent advantage over small dairy operations in the use of BST. Individual care and feeding of cows can provide improved overall management and may actually allow producers with small herds to achieve greater per-cow response from BST than obtained by producers with large herds. Since additional feed is necessary to support increased milk levels, Minnesota's edge in forage production and lower-than-average total feed cost provide another opportunity for Minnesota producers to benefit from techniques that increase milk yield per cow.

Dairy producers and companies producing BST should profit from BST use. Companies and dairy producers expect and deserve a reasonable profit from products they produce. Profitability of BST to dairy producers will ultimately determine product acceptance and use. As with any product or management technique, producers must evaluate the impact of BST on their overall farm finances. If use of BST does not increase profits for a dairy, the producer should either not use or discontinue the use of BST.

Should you use BST? Data from BST studies demonstrate that dairy management will play a major role in determining the benefit a producer can obtain from use of BST. This should not be surprising because experience has demonstrated that management techniques must improve as production per cow increases. Overall quality of management can be difficult to assess. However, producers who are successfully utilizing the management techniques listed below will likely obtain a greater, sustained response to BST than producers who are not using these techniques.

• Record-keeping system to monitor the herd and individual cow performance;
  
  
• Periodic forage analyses and routine ration balancing to meet lactational demands;
  
  
• Use of milk yield and body condition score to determine when diet shifts are necessary;
  
  
• Herd health program that focuses on prevention;
  
  
• Good milking practices;
  
  
• Good reproductive program.

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