Report of the Canadian Veterinary Medical Association Expert Panel on rBST

Nutrition Update
Volume 9 No.4, February 1999

Editor's Note: Although this is not a nutrition issue, the evaluation process of rBST has been of considerable interest to the dairy industry and the general public. The following is a condensed version of the Executive Summary prepared by the CVMA Expert Panel on rBST and submitted to Health Canada in November, 1998 . The concerns of the panel led to Monsanto's request for licensing of their rBST product, Posilac, being denied.

The mandate of the CVMA Expert Panel on rBST, as approved by Health Canada, was to:

• Review the scientific data used by the Bureau of Veterinary Drugs to determine that Nutrilac (rBST) when used in accordance with its label directions will increase milk production without resulting in serious health problems which cannot be adequately controlled by current cattle management practices.

• Make observations and recommendations regarding the adequacy of the scientific data submitted by the manufacturer of Nutrilac (rBST) or existing elsewhere to make sound scientific assessments regarding the product efficacy and animal health risks associated with the use of Nutrilac in Canadian dairy cattle.

The Panel reviewed material provided by Health Canada from Monsanto's submission to have rBST approved for use in Canada and carried out an extensive review of the published literature on the subject. The effects of rBST were assessed in the following main areas: milk yield, milk composition, nutritional implications, body condition, udder health, reproduction, lameness, other health concerns, culling and animal welfare. If a detrimental effect was observed, the Panel discussed whether or not current dairy health management practices were adequate to control or eliminate the effect.

The Panel concluded that rBST does increase milk yield (3.5% FCM) by an average of 11.3% in primiparous cows and 15.6% in multiparous cows. There was considerable variation in the response between studies but all but one study reported a positive effect.

Treatment with rBST reduced the body condition of cows and although treated cows consistently increased their dry matter intake during the treatment period and on into the subsequent lactation, this did not appear adequate to offset the increased energy output associated with the higher yield. Consequently, treated cows started their next lactation in lower body condition than untreated cows.

Use of rBST increased the risk of clinical mastitis by approximately 25%. The Panel felt that while current dairy health management techniques could reduce this increased risk, they are not adequate to eliminate it. When the expected number of extra cases of mastitis was computed on a "per litre of milk shipped" basis, the increase was approximately 10%. Given this relatively small increase and the current programs for ensuring that antibiotic residues are not present in milk sold for human consumption, the Panel felt that the risk of increased antibiotic residues in dairy products was very small.

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 that do conceive. There was also inconclusive evidence of increased risks of cystic ovaries and twinning. These adverse effects could be controlled by delaying use of the 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 these potential effects.

Treated cows experienced approximately a 50% increase in the risk of clinical lameness. Many of the lameness cases involved fore and hind limb joints. The Panel felt that current health management practices were not able to eliminate this increased risk.

Use of rBST reduced the risk of ketosis and some other metabolic diseases in the postpartum period in the lactation following one in which rBST had been used. This was probably due to a combination of the reduced body condition of cows at calving at the start of the next lactation and the higher levels of dry matter intake in the subsequent postpartum period.

Treated cows were at higher risk of being culled. This was particularly true in multiparous cows. Most of the data on culling did not include removal for reproductive reasons so the increased risk of non-pregnancy would exacerbate this problem in commercial dairy herds.

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.

In general, the Panel felt that there were sufficient data available to make a reasonably informed assessment of the effects of rBST. There were four specific conditions (risk of cystic ovaries, twinning, retained placenta, and abortion/fetal loss) for which there appeared to be an effect associated with the use of the drug, but for which there was insufficient evidence to draw firm conclusions. There was also insufficient information to determine how frequently injection site reactions occur. If the product is approved for sale, more information will be required about the nature of the increased risk of mastitis and lameness in order to manage those problems as effectively as possible.

For More Information contact your Local Specialist or MAFRI GO Teams Office or Centre.