14. Conclusions and Recommendations

Back to main page

14.1 Efficacy

14.1.1 Yield

The Panel concluded that rbST does increase production. In primiparous Holsteins the production increase averaged 3 kg or approximately 11.3%. In multiparous Holsteins the increase averaged 4.4 kg or approximately 15.6%. However, these are average values and actual responses varied from study to study.

The Panel concluded that the efficacy of the drug had been clearly established.

14.1.2 Composition

There was a very small increases in the butterfat content of milk produced and in the protein content of milk produced by multiparous cows. However, the magnitude of these increases were not of much consequence.

14.2 Animal Safety

The Panel reached a number of conclusions about the safety of the drug when used in animals.

14.2.1 Body Condition

Treatment with rbST reduced the body condition of cows and although dry matter intake was increased, this did not appear adequate to compensate for the increased energy output associated with the increased milk yield. This body condition score reduction appeared to carry over into the early portion of the next lactation. Over several lactations, this may result in an increased proportion of animals being below a level of body condition considered optimal for good health and production.

14.2.2 Mastitis

There was approximately a 25% increase in the risk of clinical mastitis in treated cows. It appeared as though there was also a slight increase in the prevalence of subclinical infections. However, the data relating to subclinical mastitis was limited. Furthermore, the Panel felt that current dairy health management practices would reduce but could not eliminate the increased risk of clinical mastitis that was associated with the use of rbST.

14.2.3 Antibiotic Residues

Given the relatively small expected increase in the number of cases of clinical mastitis, the current awareness amongst dairy producers of the problem of antibiotic residues in milk, and current programs for regular monitoring all milk shipments to dairy processors, the Panel felt that the probability of increased antibiotic residues in dairy products was very small.

14.2.4 Reproductive Effects

There were a number of effects on reproductive performance that were associated with the use of rbST. These included a substantial increase in the risk of non-pregnancy and a slight increase in days open in cows which do conceive. There was also inconclusive evidence of an increased risk of cystic ovaries and twinning. All of these adverse effects could be controlled by delaying use of drug until cows were confirmed pregnant. There was some limited evidence of an increased risk of retained placenta and abortion/fetal loss in treated cows but there were insufficient data to draw a firm conclusion about this possible effect.

14.2.5 Lameness

The Panel concluded that there was approximately a 50 % increase in the risk of clinical lameness associated with use of rbST. Many of the cases of lameness involved joints and dairy producers and veterinarians currently have a limited ability to control or eliminate this increased risk.

14.2.6 Other Health Effects

The Panel noted that there was not very much information about other potential health effects of rbST but did note that use of the product may reduce the risk of metabolic diseases (ketosis in particular) in the subsequent lactation.

14.2.7 Culling

In general, there was an increased risk of culling associated with the use of rbST, particularly in multiparous cows. When considered along with the increased risk of non-pregnancy, the Panel concluded that the use of rbST would likely reduce the lifespan of dairy cattle.

14.2.8 Animal Welfare

The Panel felt that there were a number of legitimate animal welfare concerns associated with the use of rbST. These included an increased risk of clinical mastitis and lameness, and a reduction in the lifespan of treated cows. Without better data on the frequency and severity of injection site reactions, the Panel could not determine if these represented a significant animal welfare concern.

14.3 Additional Information

The Panel recognized that rbST is one of the most extensively studied animal pharmaceuticals ever to be reviewed. In general, the Panel felt that it was able to make a reasonably informed assessment of the effect of the drug in terms of efficacy and animal safety.

However, a few specific areas where additional information would be beneficial, particularly if the drug is licenced and the adverse effects need to be managed are as follows.

• More information is needed with regard to the etiologic agents associated with the increased incidence of clinical mastitis cases and the increased prevalence of subclinical intramammry infections.
• The effect of rbST on the risk of cystic ovaries, twinning, retained placenta and abortion/fetal loss needs clarification.
• The frequency of injections site reactions needs clarification.
• More information about the effect of rbST on a variety of health conditions would be beneficial.

Most of the items identified above affect relatively few cows. Consequently, studies to provide the data identified would have to involve large numbers of animals in commercial dairy herds and would be expensive to conduct. Such studies should only be considered if it is felt that the information missing is pivotal to making a decision about whether or not to approve the product.