Invited review: Somatotropin and lactation biology.

Bovine somatotropin (bST) or popularly referred to as growth hormone, is a naturally produced hormone which is a key regulator of growth and milk production. Because of its marked influence and control of various functions in metabolism and growth, investigations have examined bST’s effect on animal-related factors such as nutrition, bioenergetics, metabolism, health, and well-being, and consumer-related issues such as product safety, milk quality, and manufacturing characteristics. Overall, bST orchestrates (both directly and indirectly) the coordination of key physiological processes involved in lactation. These benefits resulted in scientists developing recombinant bST (rbST) in the early 1980s to establish whether lactation and other performance benefits cannot be advanced and maintained over longer periods. Commercial rbST use in the United States began in 1994 and many other countries followed. Farmers benefitted substantially, but because of unwarranted pressure from consumer groups, many within the processing, grocery, and retailer industries began labelling and promoting “rbST-free” dairy products as a marketing strategy. Also, many Cooperatives worldwide had an aggressive strategy to market “rbST-free” milk to compete with “organic” milk and suggested producers would receive higher milk prices if they voluntarily stopped using rbST. In addition, many governments decided to ban the use of rbST.

The question is if there is any justification for the negative perceptions and banning of rBST. Therefore, the purpose of the review cited is to update the human and animal safety findings on rbST, as well as to discuss its effects on productivity and sustainability. The findings are reported under specific topic headings.

Milk production: The milk response to rbST was determined in a large field study in the nineties involving 340 herds and more than 80 000 cows over a 4-year period, and at plateau, the average daily response was 3.6 kg/cow more milk than control, with a large proportion of cows showing a response of about 5 kg/cow. This magnitude in response was confirmed in meta-analysis studies conducted later. Milk yield responses to rbST administration have been observed in cows varying in genetic merit, and across all breeds. The results furthermore showed that the nutrient requirements for maintenance and per unit of milk were unchanged and no special diets are required. Likewise, as observed for unsupplemented rbST cows, milk production in supplemented cows is affected by management quality, implying that little response should be expected if management or nutrition to support the intended increased milk yield is not adequate.

Milk composition: The effects of rbST on milk composition have been intensively studied. In brief, the nutritional components of milk and their manufacturing are not altered by rbST administration. Whereas rbST increases the total yield of milk protein (due to the increase in milk yield), the milk protein content and the pattern of specific milk proteins are unaltered. There are multiple factors that influence milk composition, and these amongst others, include genetics, diet, breed, and season; these factors affect milk composition identically in rbST-treated cows.

Udder control and response: The typical rapid increase in milk yield clearly indicates that nutrient uptake and utilization in the udder are enhanced in bST-treated cows. The change in the shape of the lactation curve suggests an initial increased rate of milk synthesis per cell and, during sustained rbST treatment, an increased maintenance of udder epithelial cells. The exact mechanism of how somatotropin stimulates milk production is not known. The current understanding is that it does not act directly on udder epithelial cells, but that the IGF system is indirectly mediating its anabolic effects. This hypothesis is advanced because by administering rbST to lactating cows, the blood circulating IGF-I and milk IGF-I levels will be increased, and because exogenous bST will increase the circulating IGF-I within 6 to 12 hours, maximum concentrations will occur at about 48 hours. The milk yield response is then apparent within 24 hours after the first rbST injection, and the maximum production response occurs within 4 to 6 days.

Animal welfare and reproduction:

Mastitis: Overall, the body of literature indicates that rbST poses no mastitis concern, and there is no indication that antibiotic use or milk antibiotic residues are increased by rbST. It is known that there is a small positive relationship between mastitis and milk yield irrespective of whether rbST is used or not, but changes in mastitis incidence related to milk yield are negligible when compared with other factors that contribute to mastitis, such as season, parity, stage of lactation, and interherd variation.

Body condition: There is no direct relationship between rbST and body condition, but the relationship between nutrition and milk yield as affected by the stage of lactation, as with non-supplementation, should be recognized. Collectively, both scientific reviews and reports from commercial dairy operations indicate that rbST-treated cows have an almost immediate increase in milk yield, followed by a more gradual increase in voluntary feed intake to maintain their body reserves and support nutrient needs for the increased milk yield. The importance of management in maintaining body condition of rbST-treated cows is that maximum economic benefit from use of bST will be achieved if (1) the cows are fed to maximize intake of diets with appropriate nutrient content and balance, (2) proper body condition of the cows is maintained, and (3) if health and reproduction management programs that prevent rather than cure problems are in place.

Reproduction: Reproductive performance of more than 800 cows in five separate studies demonstrated that milk production levels were a more significant factor affecting reproduction than rbST. Secondly, in a full lactation study involving 1 213 cows in 28 herds across the United States, the investigators reported that pregnancy rates, days open, twinning, cystic ovaries, or abortions were unaffected by treatment. Overall, data from most published studies support the concept that management of highly producing cows, including those achieving high production due to rbST treatment, requires a longer breeding interval to adjust to the metabolic demands of the higher milk yield. Extending the voluntary waiting period for high-yielding cows up to 120 days did not negatively affect key performance indicators of profitability.

Human safety: The following is of significance: (1) Even though some countries have not approved rbST for commercial use (e.g., Canada and the European Union), human safety of rbST has not been an issue; dairy products from rbST-supplemented cows are imported by these countries and marketed with no restrictions or special label requirements. (2) In a large study, investigators estimated that the risk of the number of human illnesses that could be caused by resistant bacteria due to rbST administration was one event per one billion people per year. (3) The JECFA (Joint Expert Committee on Food Additives) is an independent scientific committee which performs risk assessments and provides advice to the FAO, the WHO, and member countries of both organizations for the development of international food standards and guidelines. The JECFA at their 40th conference concluded that “the lack of oral activity of rbST and IGF-I and the low levels and non-toxic nature of the residues of these compounds, even at extremely high doses provide an unusually large margin of safety for humans consuming dairy products from rbST-treated cows”.

Sustainability: Aside from its proven efficacy and safety, the most compelling argument for using rbST in the current era of climate change is its effect on sustainability. Sustainability is an important consideration in agricultural production, with emphasis placed upon meeting human food requirements while mitigating the environmental effects. Because rbST enhances milk production, in theory it means that fewer cows will be needed, and therefore less impact on the environment, to feed a growing human population. As illustration, it was estimated that if the 2008 US milk supply was met by rbST use in just 15% of the US dairy herd, it would represent a reduction in the carbon footprint of the US dairy industry equivalent to removing 390 000 cars from the roads each year or planting about 290 million trees annually. Further, the beneficial environmental effect of the gains in feed efficiency would be equal to freeing up 540 000 acres of farmland currently used to produce dairy feedstuffs, a reduction in enough fossil fuel to heat more than 15,000 homes, and a reduction in water use sufficient to supply about 10 000 homes.

Concluding remarks: Worldwide, rbST is commercially used in about 20 countries, and global use has demonstrated manageable effects on cow health, reproductive performance, or herd longevity. On-farm management practices are key to effectively using rbST. Utilizing rbST consistently improves milk production and feed efficiency. Dairy products from cows receiving rbST are safe for human consumption, and importantly, utilizing rbST markedly reduces the environmental footprint of producing high-quality protein for humans. Thus, rbST remains a valuable tool for maximizing producer profitability while simultaneously improving the sustainability of the dairy sector.