A86
Bovine Somatotropin (BST)

BST is not a steroid hormone. Steroid hormones are small lipid structures that largely escape digestion and are readily absorbed into the bloodstream. The large protein molecules of BST cannot be absorbed intact, and once broken into smaller protein units, they lose any chemical activity. BST is a chain of 190 to 191 amino acids that differs structurally from human ST; therefore, it has no activity in humans at any concentration. Early efforts to treat dwarfism in humans with BST were unsuccessful for this reason. In addition, because protein compounds are rapidly degraded in the digestive system, they are rendered chemically inactive when taken orally. When injected with BST, experimental rats grow rapidly, but when given an oral dose 400,000 times the amount found in a quart of milk, no growth response occurred.

Milk from cows has always contained some BST, about 3 parts per billion. Supplementing cows with BST, however, does not increase the BST level in milk. Even in cows given 12 times the recommended dose of Posilac, there is no increase in the BST level in their milk.

In cattle, BST directs nutrients toward growth in young cattle and towards milk production in lactating cows. Supplementing a cow’s natural BST level with Posilac, repartitions available energy to the mammary gland, resulting in an 8-12 pound increase in milk production per head per day. An increase of 10 lbs. per day yields 3,050 lbs. more milk per cow per 305-day lactation. Posilac is given subcutaneously near the tailhead, neck, or shoulder once every 14 days. The first dose is not given until about 57-70 days after calving. Each dose delivers 500 mg of BST, the equivalent of 36 mg per cow per day.

The use of BST in cows does not come without some cost, mostly in terms of an increased level of management needed to support the cows at higher production levels. Nutritional support is especially critical. More nutrients are required to produce more milk. If the correct balance and amount of feed is not available, the results will be poor and may have negative effects on cow health. Rations must be formulated to supply enough energy and protein to maintain production, reproduction, and body condition. These requirements are not unique to cows on BST, but are similar to any cow producing at high levels. The following table shows the similar reproductive efficiency for BST and control cows at different levels of production.

Some small increases in mastitis cases were noted in BST trials. The following graph illustrates the relative importance of BST compared to other factors that contribute to new cases of mastitis. It shows, for example, that season of the year (in the Smith data) is 10 times more influential in causing mastitis than is BST use. Stage of lactation (days in milk) is 7 times more important.

The economic benefits of using BST will depend on the cost of the drug, the price of milk, the magnitude of response, the cost of labor and other overhead, and the price of feeds. The following worksheet compares a 7 lb. milk response to a 12 lb. response. Putting in the numbers for a specific dairy can help determine if BST use will be profitable.

BST will be most profitable for those dairies that have excellent management. BST should probably not be used when significant problems exist in the areas of reproduction, mastitis control, nutrition, disease control, or cow comfort. These problem areas should be corrected first, then BST use considered. Working closely with a veterinarian and nutritionist will help maximize opportunities for success. Other management options to increase milk production such as milking 3 times daily can produce a similar increase in milk yield and might be considered with or without BST use.