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Report of the 
Canadian Veterinary Medical Association Expert Panel 
on rbST 

(aka rBGH) 

Prepared for Health Canada Nov98
Full report available at http://www.hc-sc.gc.ca/english/archives/rbst/animals/ 

Executive Summary

At Health Canada’s request, the Canadian Veterinary Medical Association (CVMA) established an Expert Panel to review the issues of the efficacy and safety of recombinant bovine somatotropin (rbST).  The Panel was formed in March, 1998 and had expertise in epidemiology (Dr. Ian Dohoo - Chair), dairy health management (Dr. Luc DesCôteaux, Dr. Ken Leslie and Dr. Wayne Shewfelt), dairy nutrition (Dr. Alan Fredeen), livestock management and animal welfare (Dr. Allan Preston) and clinical pharmacology/large animal internal medicine (Dr. Patricia Dowling).  The Panel operated completely independently from Health Canada and the CVMA.

The Panel reviewed material provided by Health Canada from Monsanto’s submission to have rbST (sometribove) approved for use in Canada and carried out an extensive review of the published literature on the subject.  While studies based on Monsanto’s product and other companies’ products were all considered, emphasis was placed on the former.  The review process focused on studies which measured clinically relevant outcomes.  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.  Within each area, key measures of effect (eg. 3.5% fat-corrected-milk for milk yield) were identified and all data from the literature review were extracted.  These data were summarized in one or more meta-analyses to generate overall estimates of effect.  Other related, but less commonly reported,  measures of effect were also considered in a more subjective manner.  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.  Finally, the Panel discussed whether or not additional information was required in order to adequately assess the effects of rbST.

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.  There was evidence of a very small increase in the butterfat content (% fat) in the milk and in the protein content (% protein) in multiparous cows but the magnitude of the effects was too small to be of any consequence.

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%.  It appeared that there was also a slight increase in the prevalence of subclinical intramammary infections at the end of the treatment period.  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 (multiple births).  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.

7. Udder Health

The effects of rBST on udder health were divided into the effects on the frequency of clinical mastitis and the effects on subclinical mastitis (prevalence of intramammary infections).

7.1 Clinical Mastitis Rate and Risk

The panel examined evidence of the effect of rBST on two measures of clinical mastitis frequency.

• The incidence rate of clinical mastitis was computed by dividing the total number of clinical mastitis cases by the number of cow days at risk. In many studies, the total number of clinical mastitis cases was presented for each study group (treated and control) but the total number of cows days at risk was not presented. For these studies, the number of cow days at risk was estimated based on the duration of treatment and the assumption that lactation lengths in the two groups were equal. The incidence rate ratio (irr) is the ratio of the incidence rate in the treated group divided by the incidence rate in the control group.

• Clinical mastitis risk was computed by dividing the number of cows that were affected by one or more case of mastitis during the treatment period by the number of cows at risk. As with incidence rate data, the relative risk of clinical mastitis was often not presented, per se, but the number of cows affected in each group could be determined from the tables in the report. From these data the relative risk and its confidence interval were calculated.

7.1.1 Meta-analysis

Four meta-analyses were carried out to evaluate the effect of rBST on clinical mastitis frequency.

7.1.2 Comments and Conclusions.

The incidence rate ratios and the relative risk estimates from the four meta-analyses ranged from 1.24 to 1.29. This would correspond to a 24 to 29 % increase in the frequency of clinical mastitis. Most of the evidence about the effect of rBST on clinical mastitis frequency came from the PAMP study (12) and Monsanto's multi-location study (4) (i.e. these studies were assigned the greatest weight in the meta-analyses). One recent study was designed specifically to evaluate the effect of rBST on clinical mastitis (21). That study reported an overall incidence rate ratio (irr) of 1.22 which would agree quite closely with the results of the meta-analyses. In that study, 1 farm had a statistically significant increased frequency of clinical mastitis while 3 other farms had non-significant increases or decreases. Since the standard error of the incidence rate ratio could not be determined from the report, these results were not included in the meta-analyses. Overall, the panel concluded that current evidence suggests that rBST increases the frequency of clinical mastitis by approximately 25% during the treatment period.

7.1.3 Are the Effects Direct or Indirect? 

There has been some discussion in the literature as to whether the increased frequency of clinical mastitis associated with rBST is due simply to the indirect effects of increasing milk production or if there is a direct increased risk associated with use of the product. It has been argued that this point is academic in that, even if the effect is indirect (i.e. mediated through increased milk production), it still represents an effect of administration of the drug (22). However, very few studies attempted to address this question directly by carrying out separate analyses that control or do not control for level of milk production using multi-variable models. It is generally accepted that there is genetic antagonism between milk production and risk of mastitis. As cows are selected for higher milk producing ability the risk of mastitis increases. In a review of the genetics of disease resistance, Shook reported estimates of the genetic correlation between milk yield and clinical mastitis that ranged from -0.35 to +0.76 (23). Two simulation studies evaluating the potential impact of genetic selection for milk production assumed an average genetic correlation of 0.3. However, although milk production levels of cows are continually increasing over time the overall incidence of clinical mastitis does not appear to be increasing as rapidly. The lactation incidence risk reported for cows in Southern Ontario was 16.8 % in the early 1980's (24) and approximately 15 years later in the mid 1990's it was 19.8 % (25). Over the same period milk production has risen approximately 40% per cow. Although it was inevitable that there were some differences in definitions and recording procedures between the two studies the lack of a substantial difference may indicate either that the expected increase in the frequency of clinical mastitis associated with genetic progress in milk production is not really large or that improvements in management practices have kept pace with the increased risk through genetic selection.

7.1.4 Expected Increase In Cases of Clinical Mastitis.

However, it is important to note that an increased risk of 25% does not equate to an overall increase of 25% in the number of cases of clinical mastitis in the dairy industry for two reasons. First, the increase in risk is only observed during the treatment period which should commence at 56-70 days after calving according to the proposed label direction. Secondly, Canada has a supply management milk marketing system which endeavours to match milk supply with demand. If a producer reduced the number of cows milked to offset the increased production associated with the use of the product then the total number of cases would also be reduced. In order to obtain an estimate of the expected number of additional cases of mastitis some computations based on the distribution of mastitis and the distribution of milk production over a lactation were carried out. The details of the calculations and the assumptions used are included in Appendix 10. Results of the calculations suggest that rBST would produce an increase of approximately 19.4% in the total number of cases of mastitis per cow. If the producer reduced the herd size to keep total milk production constant (given that production per cow has increased) there will be approximately a 10.4% increase in the total number of cases of mastitis expected.

7.1.5 Antibiotic Residues

Based on the assumption of an approximate 10% increase in the risk of clinical mastitis cases per liter of milk produced, the Panel considered the potential for increased levels of antibiotic residues entering the food chain. There is now a great awareness amongst dairy producers of the problem of antibiotic residues in milk. In addition, there is rigid program for regular monitoring of all milk shipments to dairy processors (involving every tanker load of milk being tested for antibiotic residues). Consequently, the Panel felt that the probability of increased antibiotic residues in dairy products was very small.

7.1.6 Ability to Control/Eliminate Detrimental Effects

An assessment of whether or not udder health management practices available today are adequate to control or eliminate the increase risk of clinical mastitis is, at best, a subjective assessment. There are certainly new procedures available which may reduce levels of clinical mastitis in a herd and these include improved cow comfort and environmental management systems, the use of core antigen vaccines, pre-milk teat preparation, and improved monitoring programs for clinical mastitis. However, it was the view of the Panel that while these would reduce the risk of increased clinical mastitis, they would not eliminate it. It is also important to note that the availability of management practices to control clinical mastitis does not equate to the adoption of these practices. Whether or not advanced mastitis control practices would be adopted in herds using rBST is not known.

7.1.7 Additional Information Required

The Panel did not feel that additional information was required to determine whether or not rBST has an effect on the frequency of clinical mastitis. However, there was little information in the literature about the nature of the cases of clinical.mastitis observed. In particular, the distribution of etiologic agents was not determined in many studies. This information is important in evaluating whether newer udder health management programs will adequately control the problem. For example, one of the relatively new techniques, vaccination with a core antigen vaccine, is specifically for use against coliform infections. The Panel felt that additional information about the nature of the increased frequency of clinical mastitis would be required to do the best possible job of managing the problems in herds in which rBST was used.

7.2 Subclinical Mastitis 

The prevalence of subclinical mastitis in treated and non - treated cows was assessed by looking at two general parameters. First, somatic cell counts, which are a measure of inflamation in the udder and are an indirect indicator of subclinical infections were examined. Second the prevalence of intramammary pathogens as determined from cultures of milk samples were considered. Somatic cell count data were reported in various studies using any of the following scales: untransformed data, log2 transformed data, log10 transformed data, or natural log (loge ) transformed data . Previous research has shown that somatic cell count data should be log transformed for appropriate analysis. Consequently, papers which reported somatic cell counts as raw counts (untransformed data) were not included in the analyses. Most studies evaluated the effects of rBST on SCC throughout the treatment period through the regular collection of milk samples for analysis. While it is appropriate to evaluate the effect of rBST on milk production throughout the treatment period because the response to the drug is very rapid, it can be argued that any effect of rBST on the somatic cell count would be delayed in onset. This would likely arise from cows requiring a period of time on rBST before the prevalence of intramammary infections would rise and, in turn, result in an increased somatic cell count. Consequently, estimates of the effects of rBST on subclinical mastitis as measured by SCC may be biased towards the null (i.e. no effect) by the inclusion of data from the beginning of the treatment period. While several studies reported culture results from samples collected throughout the treatment period, only data from the last sample collection in which most of the cows were still milking were used in the meta-analyses. Consequently, the relative risk for the occurrence of a pathogen was based on only one sampling period per cow. This avoided the statistical problem of dealing with repeated measures since this problem could not be readily handled without the raw individual cow data being available for analysis.

7.2.1 Meta-analysis

Six meta-analyses were carried out to evaluate the effect of rBST on the prevalence of subclinical mastitis.

7.2.2 Comments and Conclusions

In general the meta-analyses of somatic cell count data did not show much evidence of an effect.of rBST. The fixed effect analysis of the SCC-linear score using data from all companies' products achieved marginal statistical significance (P = 0.055) but the result of this meta-analysis was substantially driven by a single small two lactation study involving 30 cows (26). The PAMP linear score data (12) was based only on linear scores determined between treatment days 110 and 200. The log10 SCC results were heavily influenced by a single, short term (12 week) study (14) in which no effect on SCC was observed but which apparently had very precise estimates of the average SCC. If this study was omitted from the meta-analysis, the overall effect increased to 0.049 (P=0.095). Overall, the Panel concluded that although their was an apparent trend toward slightly increased SCC during the treatment period, no firm conclusion that such an effect was present could be drawn. Even if the effect were present, it was relatively small. An increase of 0.05 units in the log10 SCC over the baseline in the control cows would only correspond to an increase from 38,900 cells/ml to 43,600 cells/ml. When the prevalence data were examined it was evident that the point estimates of the relative risk for the prevalence of subclinical mastitis varied quite widely, especially when results from all companies' studies were included. Consequently, the Panel focused on studies based on Monsanto's products even though results were only available from 4 groups of cows in 2 studies. The results were also based on all organisms combined since there were too few isolates of individual organisms to support meaningful analyses. In all 4 groups, the point estimate of the relative risk was greater than 1 and the pooled estimate was 1.62 (P=0.048) suggesting a 62% increase in the prevalence of intramammary infections. However, the confidence interval for this estimate was very wide (1.004, 2.624). The only other direct measure of intramammary infections recorded in the database was an estimate of the new infection rate determined in each year of a 2 year study (26) using Cyanamid's product. That study reported relative risks of new infections greater than 1 in each of the two years although neither was statistically significant. The Panel concluded that there was an increase in the prevalence of subclinical mastitis. However, it should be noted that although the point estimate of the relative risk was 1.62 (equivalent to a 62% increase in the prevalence of intramammary pathogens) this estimate had a very wide 95% confidence interval of 1.004 to 2.62. When the evidence from the analyses of somatic cell counts and milk sample cultures were taken together, the data available did not allow the Panel to draw strong conclusions about the potential effects of rBST on subclinical mastitis. In general, subclinical mastitis is difficult to quantify and it is even more difficult to get a good evaluation of the etiological agents involved. Most of the trials conducted were directed at evaluating the effect of the drug on milk production. These studies were not designed to delve into the potential problem of subclinical mastitis in any depth. There appeared to be a discrepancy between the somatic cell count data and the prevalence data with the former suggesting little effect and the latter identifying a increased prevalence of.subclinical mastitis. Much of this difference could be attributed to the fact that the cell count data were accumulated over the whole treatment period while the prevalence of infection data were selected from the end of the treatment period. Overall, the Panel concluded that there probably was an increased prevalence of subclinical intramammary infections in rBST treated cows, but that it was difficult to quantify the magnitude of the increase.

7.2.3 Ability to Control/Eliminate Detrimental Effects

In general, increased levels of subclinical mastitis may be more amenable to control than an increased frequency of clinical mastitis. Dry cow antibiotic therapy at the end of lactation could be expected to eliminate many of these subclinical infections. However, the use of dry cow therapy is variable across herds. The Panel did recognize a concern if there was an increased prevalence of Staph. aureus infections. These are often difficult to eliminate and represent a considerable biosecurity risk for spread of infection to other cows. However, in general the Panel felt that the effect on rBST on subclinical mastitis was manageable.

7.2.4 Additional Information Required

In general, there was relatively little information about the effects of rBST on subclinical mastitis. More information about the nature of udder health problems, and in particular the etiologic agents involved would be required to better assess the effects of rBST and to deal effectively with any problems that arose from use of the product.

8. Reproduction

The panel evaluated a number of measures of reproductive health and performance. Parameters that either affect or reflect the breeding performance will be presented first. These include:

Subsequently three parameters that reflect the state of the cow during her gestation period and at the subsequent calving were evaluated and these included:

8.1 Incidence of Cystic Ovaries

Most studies reported the incidence of cystic ovaries in terms of the risk of this condition (i.e. the number of cows affected divided by the number of cows at risk).

8.1.1 Meta-analysis

Two meta-analyses were carried out.

8.1.2 Comments and Conclusions

With the exception of the multiparous cows in the PAMP study (12), all studies reported an increased risk of cystic ovaries associated with rBST treatment, although only one of the individual relative risk estimates was statistically significant. This one significant result was derived from a study in which rBST had been administered intramuscularly (5). Overall, it appeared that treatment increased the risk by approximately 20% although this apparent increase was not statistically significance (P = 0.11). Two of the papers reviewed, evaluated the mechanism by which rBST may affect ovarian performance and it was found to have an affect on the development and size of ovarian follicles (27,28). This would be consistent with a possible increase in the frequency of cystic ovaries. The Panel concluded that although there appeared to be an increased risk of cystic ovaries in treated cows, most of the evidence for this effect came from a study in which rBST was administered intramuscularly. The Panel concluded that there were insufficient data to draw a firm conclusion about the effect of rBST on cystic ovaries.

8.2 Services per Conception (SPC)

Services per conception reflects the number of times that cows which ultimately conceived had to be bred in order to conceive. The parameter does not take into account cows which were bred but which did not conceive.

8.2.1 Meta-analysis

Two meta-analyses were carried out to evaluate the effects of rBST on the number of services per conception.

• Effects of rBST on the number of services per conception based on studies using all companies' products.

• Effects of rBST on the number of services per conception based on studies using Monsanto's product.

8.2.2 Comments and Conclusions

The Panel concluded that there was no effect of rBST on the number of services per conception required in cows which did conceive.

8.3 Days Open (DO)

Days open is the number of days from calving until a cow is rebred and conceives. It can only be computed for cows which have a confirmed pregnancy.

8.3.1 Meta - Analysis Two meta-analyses were carried out to evaluate the effects of rBST on the number of days open.

• Effects of rBST on the number of days open based on studies using all companies' products.

• Effects of rBST on the number of days open based on studies using Monsanto's product.

8.3.2 Comments and Conclusions

When the data from 18 groups studied (involving all companies products) were evaluated there was a small but statistically significant (P=0.01) increase (approximately 5 days) in average days open. When only studies based on Monsanto's data were evaluated, a similar effect was observed although it was not quite statistically significant (P=0.066). The Panel concluded that there was evidence that the average days open would be slightly increased by the use of rBST. This effect was small and amounted only to approximately 5 extra days. However, as with services per conception, days open was only computed for cows which conceived.

8.4 Twinning (Multiple Births)

Twinning is the birth of two calves. In the context of this review, it signifies the birth of two calves at the parturition following the lactation with rBST administration. In general, twin births are considered undesirable because they are much more likely than single births to be followed by complications.

8.4.1 Meta-Analysis

Two meta-analyses were carried out to evaluate the effects of rBST on the risk of twinning

• Effects of rBST on risk of twinning based on studies using all companies' products.

• Effects of rBST on risk of twinning based on studies using Monsanto's product.

8.4.2 Comments and Conclusions.

Most of the evidence for, or against, an increased risk of twinning came from the PAMP study (12). The results from that study are split. There appeared to be a decreased risk in primiparous cows and an increased risk of twinning in multiparous cows (although neither were statistically significant). One other study (5) reported a large increases in risk of twinning associated with rBST (relative risks of 7.1 and 11.7 in primiparous and multiparous cows respectively although only the latter was statistically significant). However, cows in this latter study were injected intramuscularly and Monsanto suggests that IM injections resulted in a higher incidence of reproductive problems. The Panel concluded that there may be an increased risk of twinning but no firm conclusions could be drawn.

8.4.3 Additional Information Required

The problem with assessing the impact of rBST on twinning was the limited number of studies which followed cows through to calving following treatment with the drug. Although the two main studies providing data on the risk of twinning (5,12) had data from a total of 791 cows, one would require data from 2000 cows (1000 cows in each treatment group) to be relatively certain of detecting a doubling of the risk (from 2.5% to 5%). A recent study published in the Journal of Dairy Science (29) has reported a general increased risk of twinning in Holstein Friesian cows with a rise in the incidence from 1.4% of lactations in1983 to 2.4% in 1993. This paper also identified increased milk production and increased frequency of cystic ovarian diseases as risk factors for increasing the number of twins born. Since rBST increases milk production and appears to increase the risk of cystic ovarian disease (P= 0.11) these may both have contributed to the apparent increased risk of twinning which was observed in most of the studies reported.

8.5 Non-Pregnancy Risk

Many studies reported the proportion of cows which ultimately conceived during the treatment period (reported as a pregnancy rate). In order to be consistent with other health outcomes, the overall effect of rBST on pregnancy has been evaluated in this report as the risk of the cow failing to conceive (i.e. non-pregnancy). One difficulty in analyzing these data was the problem of identifying which pregnancies occurred before the onset of treatment and which ones occurred afterwards. Whenever possible, data from the two time periods were separated and only those from the treatment period were used in the analysis.

8.5.1 Meta-analysis

Two meta-analyses were carried out to evaluate the effects of rBST on the non-pregnancy risk ..• Effects of rBST on the non-pregnancy risk based on studies using all companies' products.

• Effects of rBST on the non-pregnancy risk based on studies using Monsanto's product.

8. 5.2 Comments and Conclusions

Although the point estimates of the relative risk of non-pregnancy varied widely across studies, they were quite consistently greater than 1. Overall, the relative risk of non-pregnancy was approximately 1.4 (equivalent to a 40% increase in the risk of non-pregnancy). One study (30) reported conception data in terms of the hazard ratio for pregnancy (which estimates the risk of a treated cow getting pregnant at a given point in time compared to the risk of a control cow). That study reported a significantly reduced hazard ratios (0.38) which indicates that treated cows were less likely to conceive. The Panel concluded that the use of rBST in non-pregnant cows increased the risk of the cow not becoming pregnant by approximately 40%. In commercial dairy operations, failure to conceive would normally result in the cow being culled. (See Section 8.8 - Overall Assessment of Reproductive Effects)

8.6 Risk of Abortion

8.6.1 Meta-analysis

Two meta-analysis were carried out to evaluate the effect of rBST on the risk of abortion.

• Effects of rBST on the risk of abortion based on studies using all companies' products.

• Effects of rBST on the risk of abortion based on studies using Monsanto's product.

8. 6.2 Comments and Conclusions.

The definition of abortion and how it was determined varied considerably across studies. However, most of the evidence about the effect of rBST on the risk of abortion was derived from the PAMP study (12) in which abortion was simply defined as "abortion indicated by dairyman". When all studies were evaluated, there appeared to be no evidence of an increased risk of abortion. However, when only studies based on Monsanto's product were examined, the estimate of the relative risk was 1.67 (P=0.078). "Fetal loss" was also reported in two Monsanto studies: PAMP and "Multi-location IM study" (4,12). Although not clearly defined, this was presumably based on the loss of rectally confirmed pregnancies. Relative risk of 1.2, 1.11 and 1.78 were reported for PAMP - primiparous, PAMP -multiparous and "Multi-location IM" respectively, but none of the individual estimates were significantly greater than 1. The overall conclusion of the Panel was that there was some evidence of an increased risk of abortion / fetal loss associated with use of the product but there were inadequate data to draw a firm conclusion.

8.6.3 Additional Information Required

As noted above, there were inadequate data to allow the Panel to come to a firm conclusion about the effect of rBST on the risk of abortion/fetal loss. Studies to provide those data would have to have a consistent process for defining and recording abortions and fetal losses.

8.7 Gestational Length

Gestation length is the time (number of days) from the breeding of conception to the subsequent calving.

8.7.1 Meta-analysis

Two meta-analyses were carried out to evaluate the effects of rBST on gestation length.

• Effects of rBST on gestation length based on studies using all companies' products.

• Effects of rBST on gestation length based on studies using Monsanto's product.

8.7.2 Comments and Conclusions.

The Panel concluded that there was no consistent evidence of an effect of rBST on gestation length.

8.8 Retained Placenta

The proposed label for the product also refers to a possible increased risk of retained placenta in the carry-over period in treated cows. One study (4) which recorded the frequency of retained placenta following treatment with rBST reported a relative risk of 1.6 (P=0.1). The Panel concluded that while there appeared to be some evidence of increased risk of retained placenta, there were insufficient data on which to base a firm conclusion.

8.9Overall Assessment of Reproductive Effects

The Panel concluded that the use of rBST has some negative effects on reproduction in dairy cows. Treatment was associated with a substantially increased risk of non-pregnancy and a small increase in days open in cows which did conceive. The Panel concluded that although there was some evidence of increased risks of cystic ovaries, twinning, retained placenta and abortion/fetal loss, a lack of data precluded firm conclusions being drawn about these four outcomes. Studies which employed a consistent approach to defining and recording these reproductive events would be required to provide such data. There did not appear to be any effect of rBST on the number of services required for cows which did conceive or the length of the subsequent gestation.

8.10 Ability to Control/Eliminate Detrimental Effects

Given that most of the cows in the studies reported were on well managed reproductive programs, the Panel did not feel that current dairy health management practices would be able to control or eliminate the apparent detrimental effects of rBST on reproductive performance if treatment started at approximately day 60 post-calving when most cows are not pregnant. However, since the detrimental effects were primarily related to the breeding of cows, an obvious solution to avoid these problems would be to delay use of the product until cows were confirmed pregnant. Delaying use of the product until after pregnancy has been confirmed would not deal with the potential increased risk of retained placenta or abortion/fetal loss, if those increased risks are confirmed.

8.11 Additional Information Required

In general the panel felt that there was good information available for the two most important measures of reproductive performance: days open and non-pregnancy rate. Unfortunately, there were insufficient data to confirm or rule out possible increased risks of cystic ovaries, twinning, retained placenta, and abortion/fetal loss. Delaying use of rBSt until after cows conceive would remove any concern about the possible effects of the drug on cystic ovaries and twinning rates. Additional data from studies with a consistent approach to detecting abortion/fetal loss will be required before firm conclusions about the possible effects of rBST on the loss of pregnancy can be drawn.

9. Feet and Legs

In this section, virtually all causes of clinical lameness were combined and the overall effect of the drug on the risk of clinical lameness was examined. The increased risk of lameness was measured in terms of relative risk. There was considerable variation across studies in how lameness was defined, diagnosed and recorded.

9.1 Meta-analysis

Two meta-analyses were carried out to evaluate the effect of rBST on the risk of clinical lameness.

9.2 Conclusions and Comments.

The Panel concluded that the risk of clinical lameness was increased approximately 50% in cows treated with rBST.

9.3 Are the Effects Direct or Indirect?

While it is difficult to make biological sense of a direct effect of rBST on lameness, the Panel speculated that the mechanism may be through the increased milk production response to rBST forcing changes in the nutritional management of those cows. Higher producing cows require a diet with very high energy and protein density. Cows fed close to a 60% concentrate-forage ratio (C:F) or a high starch-low fibre diet, are more susceptible to lameness problems, especially laminitis (31). Laminitis is a multi-factorial disease with many predisposing factors, including: grain overload, poorly balanced rations, management (stall design and comfort) and behaviour (32,33). Acute laminitis is evident by reluctance to rise or to walk. The clinically apparent effects on the structure of the hoof include solar hemorrhage, claw abnormalities, white line hemorrhage and/or abscesses and sole ulceration (34). Consequently, cows with laminitis potentially spend more time lying down and consequently may have an increased risk of carpal, tarsal and fetlock lesions when floor surface is hard with little bedding materials to support their weight. High producing dairy herds attempting to maximize energy intake are continually confronted with subclinical acidosis and laminitis. While management of feeding and husbandry practices can be implemented to reduce the incidence of the disease (35) the control of lameness problems in dairy herds remains problematic.

9.4 Ability to Control/Eliminate Detrimental Effects

The two main studies contributing data to the meta-analyses were one by Wells et. al (36) and the PAMP study (12). In the former, the most common causes of lameness in treated cows were lesions of the carpus and tarsus, followed by interdigital swelling. In the latter, lesions of the fetlock and hoof were most commonly reported but lesions of the hock contributed the most to the number of days on treatment. While the Panel recognized that recent improvements in stall and stall surface design may help reduce some of these problems in newer dairy facilities, in general, the Panel did not feel that current dairy cattle management techniques would be able to control or eliminate the increased risk of lameness. Additional studies would be required to better define the nature of the clinical lamenesses observed and to determine possible mechanisms by which those lamenesses had occurred. Two specific concerns associated with increased risk of lameness were noted. The first is that lameness may have a detrimental effect on reproductive performance in that cows may be unwilling to stand for mounting. These cows would be less likely detected in heat. The second relates to the definite need for increased dry matter intake in rBST treated cows. It is important that treated cows be able to walk to and compete effectively for feed provided to the herd in free stall barns. Lame cows may have difficulty in achieving the necessary dry matter intake, resulting in further loss of body condition.

12. Animal Welfare

Any assessment of the impact of a product on animal welfare is inherently subjective in nature. However, the Panel did discuss four specific animal welfare issues.

First, the Panel addressed the question as to whether regular subcutaneous injections were likely to be an animal welfare concern. Such injections would be a category "B" procedure according to the Canadian Council on Animal Care (see Appendix 11). Tthe Panel did not feel repeating it every 14 days constituted an animal welfare concern.

Secondly, the Panel discussed the possible effects of injections site reactions. The equivocal data on injection site reactions raised concern about the possible occurrence of reactions. However, without more substantial evidence about the frequency and severity of those reactions, the Panel could not draw strong conclusions about this potential animal welfare concern.

Third, the Panel discussed the animal welfare effects of an increase in the frequency of clinical diseases (mastitis and lameness). One of the fundamental principles of animal welfare is that animals should be maintained free of disease to as great an extent as possible. The recommended code of practice for the care and handling of dairy cattle (1990) states.

"Nearly all husbandry systems impose restrictions on the stock, some of which can cause an unacceptable degree of discomfort or distress by preventing the animals from fulfilling their basic needs. Meeting these needs, and others that must be considered, includes providing the following:

In general, the Panel felt that current health management practices for dairy cattle were inadequate to eliminate the increased risk of clinical mastitis and lameness associated with the use of rBST and consequently there is a legitimate animal welfare concern. On the other hand, rBST does appear to reduce the risk of metabolic disease in subsequent lactations when used in over-conditioned dairy cattle.

Finally, there is evidence that cows treated with rBST have a reduced life span. Both the increased risk of culling and the increased risk of non- pregnancy would contribute to a reduction in the lifespan of treated cattle.

Overall, the Panel felt that there were animal welfare concerns that were associated with the use of rBST.

source: http://www.hc-sc.gc.ca/english/archives/rbst/animals/

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