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Expert opinions of strategies for milk fever control
Preventive Veterinary Medicine 55 (2002) 69–78
Expert opinions of strategies for milk fever control Jan Tind Sørensena,c,*, Søren Østergaarda,c, Hans Houeb,c, Jens Hindhedea,c a
Department of Animal Health and Welfare Foulum, Danish Institute of Agricultural Sciences, P.O. Box 50, Tjele DK-8830, Denmark b Department of Animal Science and Animal Health, Royal Veterinary and Agricultural University, Grønnega˚rdsvej 8, DK-1870 Frederiksberg C, Denmark c Research Centre for the Management of Animal Production and Health, DK-1870 Frederiksberg C, Denmark Received 20 June 2001; accepted 3 May 2002
Abstract Dairy cows have several risk factors for milk fever. The most-suitable strategy to control milk fever in a specific herd will depend on herd-specific circumstances such as the attitude and skills of the farmer, the opportunities available in the production system and the economic consequences of a certain strategy. To develop feasible strategies in a modern loose-housing dairy herd, we carried out an expert opinion study. Animal husbandry advisors and veterinary practitioners were used as experts. The experts quantified the effect on milk fever and time needed by the farmer for two preselected options: Ca-gel fed orally peripartum and a low dietary cation–anion-difference in the dry-cow ration. The estimated mean relative risk (compared to no strategy) was 0.45 and 0.42, respectively, and the median time needed by the farmer was 7 and 33 min, respectively, per cow year. The experts also suggested 12 control strategies. The two control strategies, which were predicted to be the most, relevant were Ca-gel fed orally peripartum used alone and used in combination with a low Ca dry-cow diet. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Milk fever; Dairy cattle; Control strategies; Expert opinion
1. Introduction The incidence of milk fever in dairy cattle is typically 5–10% per lactation; however, the between-herd variation is considerable (Houe et al., 2001). Increased herd size, new * Corresponding author. Present address: Department of Animal Health and Welfare Foulum, Danish Institute of Agricultural Sciences, P.O. Box 50, Tjele DK-8830, Denmark. Tel.: þ45-8999-1343; fax: þ45-8999-1500. E-mail address: [email protected] (J.T. Sørensen).
0167-5877/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 8 7 7 ( 0 2 ) 0 0 0 6 8 - 5
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feeding principles like the TMR system and new housing and milking systems enhance the need for development of new strategies for controlling milk fever at herd level. Several options for controlling milk fever in the dairy herd are available (Horst et al., 1997). These options are often complex to use and can require special production facilities or management skills. Furthermore, feasible combinations of options (strategies) might vary between herds. Knowledge on relevant production strategies for controlling milk fever in modern milk production is scarce, however. There is increasing interest among epidemiologists in using expert opinions in a systematic manner for quantification of parameters which are difficult to estimate directly from empirical data. Expert panels have been used to quantify risk factors for contagious diseases such as swine fever and foot-and-mouth disease (Horst, 1998), for bovine respiratory disease (Van der Fels-Klerx et al., 2000) and ovine caseous lymphadenitis (Binns et al., 2000). Another possible use for expert opinion studies is evaluation of the relevance of various alternative control options and control strategies applied in a livestock herd. Such evaluations are important for the relevance of different control strategies but are hardly known (other than to field experts). Our aim was to present the strategies to control milk fever in a dairy cattle herd; strategies were developed through an expert opinion study.
2. Material and methods Twenty advisors were selected (12 animal husbandry advisors and 8 veterinary practitioners) as experts. The advisors were all experienced advisors for dairy cattle farmers. Their identity was kept secret and they did not know the answers of the other experts at any time. Three questionnaires were completed during February and August 2000. Eighteen experts completed all 3 questionnaires and 19 experts completed questionnaires 2 and 3. The expert opinion procedure is illustrated in Fig. 1. The results from the first questionnaire were used to formulate the second questionnaire and the results from the second questionnaire were used in the third questionnaire. 2.1. First questionnaire The effects of two control options (Ca-gel fed orally to the cow peripartum and a low dietary cation–anion-difference (DCAD) in the dry-cow ration) on milk fever risk are well
Fig. 1. Diagram showing the procedure for the expert opinion study using three questionnaires.
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documented in the literature (Thilsing-Hansen et al., 2002). The experts were asked to specify a most likely value, a minimum value, and a maximum value of the relative risk of milk fever when using Ca-gel orally compared to not using it. For DCAD, the experts then were asked to specify these three estimates for each of three shifts in DCAD. These were defined according to typical range of DCAD in a dry-cow ration and the fact that relative risk seems to change significantly around 0 meq/kg dry matter (DM) (Thilsing-Hansen et al., 2002). The experts were asked to specify their minimum and maximum values to be as close as possible to each other such that they were 90% confident that the true value would fall between them. These three changes were: (a) a change from 150 to 100 meq/kg DM; (b) a change from 150 to 0 meq/kg DM; (c) a change from 400 to 150 meq/kg DM. The experts were also asked to suggest any other options (to control milk fever) relevant for use on typical Danish dairy farms. 2.2. Second questionnaire The experts were asked about their opinion on the relevance of each option suggested in the first questionnaire and further to suggest strategies for controlling milk fever. A strategy was defined as either a combination of options or a single option, which could be recommended to be used alone in a dairy herd. The experts scored the relevance of each control option on a scale from 1 to 5 (1: not relevant; 5: highly relevant). Relevance was explained to be a general evaluation including biological, economical and practical aspects in the dairy herds. 2.3. Third questionnaire In the third questionnaire, the experts were asked about their opinion on every strategy suggested by at least two experts in the second questionnaire. Each expert was asked to distribute 100 points among the strategies according to the relative relevance of each strategy given the set of strategies defined in the second questionnaire. The experts were also asked to give an estimate on the expected effect on time needed by the farmer for the strategy and the effect of the strategy in terms of reducing the milk fever risk. They used a scale from 1 to 5 (1: very little effect; 5: very large effect) on both issues. Quantitative assessments of probability distributions for expert opinion on effects and time needed for selected control options were expressed by density functions. Density functions were formulated using Betapert functions for each expert and combining the functions as described by Vose (2000). Differences between opinions on relative relevance of suggested control strategies were analysed using a generalised linear model with a Poisson distribution and a log-link function The full model included the explanatory variables: 12 categories of control strategy (1–12), 2 categories of expert group (‘‘advisors’’ and ‘‘vets’’), scale value of time needed (1–5), scale value of effect on milk fever risk (1–5) and the two-term interactions among these. Model reductions were done by stepwise deleting the non-significant ðP > 0:05Þ term with the highest P-value
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computed by likelihood-ratio statistics. The term ‘‘control strategy’’ was forced to stay in model.
3. Results 3.1. First questionnaire The control options suggested by the experts in the first questionnaire are listed in Table 1. In total, 13 control options were suggested. The control option Ca-gel fed orally peripartum was specified as administration of Cagel twice before calving and twice after calving. Sixteen experts provided all three values (most likely, minimum, and maximum) on the relative risk of milk fever. For each of the 16 experts a Betapert distribution was estimated and an aggregated density function was made (Fig. 2). The average relative risk for the aggregated density function was 0.45. Fourteen experts provided the needed three values (minimum, most likely and maximum) for all three changes in DCAD. The aggregated density functions for their expected Table 1 Control (for bovine milk fever) suggested by 18 experts in the first questionnaire Ca-gel fed orally Low DCAD during dry period Culling of high-risk cows Body-condition control Reduced milking post-partum Milking to calving Low Ca in dry-cow ration Short dry period length Mg-level control peripartum P-level control peripartum Low carbohydrate peripartum High D-vitamin level prepartum Induced calving
Fig. 2. Aggregated probability density function (PDF) for expert opinion on effect of Ca-gel fed orally peripartum on the relative risk of milk fever (RR). Based on the opinion from 16 experts.
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Fig. 3. Aggregated PDFs for expert opinion on effect of DCAD in the dry-cow ration on relative risk of milk fever (RR) when moving from 150 to 100, 150 to 0, and 400 to 150 meq/kg DM. Based on the opinion from 14 experts.
Fig. 4. Aggregated PDF for expert opinion on the time needed by the farmer for administration of the Ca-gel orally to a cow. Based on the opinion from 17 experts.
effects on relative risk of milk fever is shown in Fig. 3. The median estimated relative risks for the three changes in DCAD were: 0.42, 0.64 and 0.71 for 150 to 100, 150 to 0, and 400 to 150 meq/kg DM, respectively. The time needed for Ca-gel fed orally was for administration of four doses to one cow. The aggregated density function for time needed to feed Ca-gel orally peripartum was calculated from the three point estimates from 17 experts (Fig. 4). Expert opinion produced a skewed trimodal density function with a peak around 10 min (median 17 min per cow treated). Assuming this option applied 0.40 times per cow year (approximately all the cows at third or later calving), then the annual time needed would be calculated to be 7 min per cow year.
Fig. 5. Aggregated PDF for expert opinion on the time needed per day to reduce the DCAD in the dry-cow ration by adding anions to the feed ration. Based on the opinion from 17 experts.
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Fig. 6. Expert opinion on the relevance of 13 options to control milk fever ranked as medians for animal husbandry advisors and veterinary practitioners respectively. Based on the opinion from 19 experts.
The time needed for the option of using a low DCAD in the dry-cow ration depends on how the low DCAD is produced (Fig. 5). The density function has a peak at 5 min and has a median of 9 min/day for a 100-cow dairy herd. Thus, the median time used per cow year would be 33 min. 3.2. Second questionnaire The average score for each control option from the second questionnaire shows that the median score for eight options were 3 or higher, indicating a general opinion that they are relevant (Fig. 6).
Table 2 Strategies to control milk fever suggested by 19 experts in the second questionnaire Ca-gel fed orally Low Ca in dry-cow ration Body-condition control Low DCAD during dry period Reduced milking post-partum Culling of high-risk cow A combination of Ca-gel fed orally and low Ca in dry-cow ration A combination of Ca-gel fed orally and body-condition control A combination of Ca-gel fed orally and low DCAD during dry period A combination of Ca-gel fed orally and reduced milking post-partum A combination of low Ca in dry-cow ration and body-condition control A combination of low Ca in dry-cow ration and low DCAD during dry period
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Fig. 7. Expert opinion on the relative relevance of the 12 milk fever strategies suggested by at least two experts. The opinion is expressed as overall average and average for animal husbandry advisors and veterinary practitioners respectively. Based on the opinion from 19 experts.
In total, 12 strategies each were suggested by at least two experts (Table 2). The experts suggested six options relevant to be used alone and six combinations. There was general agreement between the two groups of experts. 3.3. Third questionnaire The relative relevance of the strategies is shown in Fig. 7 as the median for the two groups of experts. There seems to be a high degree of agreement between the two groups of
Fig. 8. Expert opinion on the efficiency of the suggested 12 milk fever control strategies expressed as medians for animal husbandry advisors and veterinary practitioners respectively. The order of the strategies is similar to the order in Fig. 7. Based on the opinion from 17 experts.
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Fig. 9. Expert opinion on the time needed by the farmer to apply on each of the 12 milk fever control strategies expressed as medians for animal husbandry advisors and veterinary practitioners respectively. The order of the strategies is similar to the order in Fig. 7. Based on the opinion from 17 experts.
experts on the two most-relevant strategies, i.e. Ca-gel fed orally peripartum alone or in combination with a low Ca level in the dry-cow ration. The experts were also asked about their opinion on the effect of the suggested control strategies in terms of reducing the milk fever risk and their opinion on the time needed by the farmer applying the strategy in question (Figs. 8 and 9). The control strategies are in the same order in Figs. 8 and 9 as in Fig. 7. It appears that there is not complete coherence between experts’ average opinion on relative relevance and their average opinion on efficiency or time needed. The statistical analysis (available upon request from the first author) showed that relative relevance of a control strategy was related to expected effect on milk fever risk whereas relative relevance was not significantly related to time needed or the expert group (vets or advisors).
4. Discussion For two well-documented control options, the experts were asked to provide an estimate on the effect on milk fever risk. The effects assumed by the experts for Ca-gel fed orally peripartum and a low DCAD in the dry-cow ration were quite similar to what is found in the literature (Thilsing-Hansen and Jørgensen, 2001). The effect of the option to keep a low Ca level in the dry-cow ration shows inconsistent results in the literature except for those keeping the Ca level very low in the diet (below 20 g/day) (Horst et al., 1997; ThilsingHansen and Jørgensen, 2001). The experts did not specify what they meant by a low Ca level in the dry-cow ration. However, it is very difficult in commercial milk production to compose a feed ration with <20 g of Ca per cow per day. Addition of a Ca-binder to the feed has been described (Jørgensen et al., 2001; Thilsing-Hansen and Jørgensen, 2001). Bodycondition control to avoid fat cows at calving has not been documented directly. It is,
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however, supported by epidemiological studies on post-partum risk of diseases (Østergaard and Gro¨ hn, 1999; Heuer et al., 1999). The option to reduce milking during the first milking after calving has not been documented in the literature. Generally there was high coherence between expert opinion on effect on milk fever risk and expert opinion on relative relevance of the suggested control strategies. It was interesting to us to note that the strategy of combining a low DCAD in the dry-cow ration and Ca-gel fed orally peripartum was not seen as particularly relevant although it was assumed to have a large effect on milk fever risk. It might be that the advisors expect this strategy to be relatively time consuming. Vose (2000) advises use of expert opinion only in situations where data have never been collected or data are too expensive to obtain. The relevance of strategies for controlling diseases at farm level cannot be obtained directly from empirical data with high precision and therefore are a valid area for expert opinion studies. The economic consequences of strategies can be approached by simulation studies (Østergaard et al., 2000). Advisors have experience with the problems related to implementation of control strategies on farms. It is, however, important that their assumption on effects on production is in agreement with scientific documentation. In our case developing milk fever control strategies this seems to be the case.
Acknowledgements We thank all participating experts for their time and input and also Anne Kudahl’s comments on the manuscript. References Binns, S.H., Green, L.E., Smith, J.Q., 2000. Economic and risk analysis of possible methods for control of ovine caseous lymphadenitis in the UK. In: Proceedings of the Ninth Symposium of the International Society on Veterinary Epidemiology and Economics, pp. 43–45. Heuer, C., Shukken, Y.H., Dobbelaar, P., 1999. Post partum body condition score and results from first test day milk as predictors of disease, fertility yield and culling in commercial dairy herds. J. Dairy Sci. 82, 295–304. Horst, H.S., 1998. Risk and economic consequences of contagious animal disease introduction. Ph.D. Thesis. Department of Economics and Management, Wageningen Agricultural University, 147 pp. Horst, R.L., Goff, J.P., Reinhardt, T.A., Buxton, D.R., 1997. Strategies for preventing milk fever in dairy cattle. J. Dairy Sci. 80, 1269–1280. Houe, H., Østergaard, S., Thilsing-Hansen, T., Jørgensen, R.J., Larsen, T., Sørensen, J.T., Agger, J.F., Blom, J.Y., 2001. Milk fever and subclinical hypocalcaemia. An evaluation of parameters on diagnosis, risk factors and biological effects as input for a decision support system for disease control. Acta Vet. Scand. 42, 1–29. Jørgensen, R.J., Hansen, T., Jensen, M.L., Thilsing-Hansen, T., 2001. Effect of oral drenching with zinc oxide or synthetic zeolote A on total blood calcium in dairy cows. J. Dairy Sci. 84, 609–613. Østergaard, S., Gro¨ hn, Y.T., 1999. Effect of diseases on test day milk yield and body weight of dairy cows from Danish research herds. J. Dairy Sci. 82, 1188–1201. Østergaard, S., Sørensen, J.T., Kristensen, A.R., 2000. A stochastic model simulating the feeding-healthproduction complex in a dairy herd. J. Dairy Sci. 83, 721–733. Thilsing-Hansen, T., Jørgensen, R.J., 2001. Hot topic: prevention of parturient paresis and subclinical hypocalcaemia in dairy cows by zeolite. Administration in the dry period. J. Dairy Sci. 84, 691–693.
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Thilsing-Hansen, T., Jørgensen, R.J., Østergaard, S., 2002. Milk fever control principles reviewed. Acta Vet. Scand. 43, 1–19. Van der Fels-Klerx, H.J., Horst, H.S., Dijkhuizen, A.A., 2000. Risk factors for bovine respiratory diseases in dairy young stock in the Netherlands: the perception of experts. Livest. Prod. Sci. 66, 35–46. Vose, D., 2000. Risk analysis. A Quantitative Guide, second ed. Wiley, New York, 418 pp.
Expert opinions of strategies for milk fever control Jan Tind Sørensena,c,*, Søren Østergaarda,c, Hans Houeb,c, Jens Hindhedea,c a
Department of Animal Health and Welfare Foulum, Danish Institute of Agricultural Sciences, P.O. Box 50, Tjele DK-8830, Denmark b Department of Animal Science and Animal Health, Royal Veterinary and Agricultural University, Grønnega˚rdsvej 8, DK-1870 Frederiksberg C, Denmark c Research Centre for the Management of Animal Production and Health, DK-1870 Frederiksberg C, Denmark Received 20 June 2001; accepted 3 May 2002
Abstract Dairy cows have several risk factors for milk fever. The most-suitable strategy to control milk fever in a specific herd will depend on herd-specific circumstances such as the attitude and skills of the farmer, the opportunities available in the production system and the economic consequences of a certain strategy. To develop feasible strategies in a modern loose-housing dairy herd, we carried out an expert opinion study. Animal husbandry advisors and veterinary practitioners were used as experts. The experts quantified the effect on milk fever and time needed by the farmer for two preselected options: Ca-gel fed orally peripartum and a low dietary cation–anion-difference in the dry-cow ration. The estimated mean relative risk (compared to no strategy) was 0.45 and 0.42, respectively, and the median time needed by the farmer was 7 and 33 min, respectively, per cow year. The experts also suggested 12 control strategies. The two control strategies, which were predicted to be the most, relevant were Ca-gel fed orally peripartum used alone and used in combination with a low Ca dry-cow diet. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Milk fever; Dairy cattle; Control strategies; Expert opinion
1. Introduction The incidence of milk fever in dairy cattle is typically 5–10% per lactation; however, the between-herd variation is considerable (Houe et al., 2001). Increased herd size, new * Corresponding author. Present address: Department of Animal Health and Welfare Foulum, Danish Institute of Agricultural Sciences, P.O. Box 50, Tjele DK-8830, Denmark. Tel.: þ45-8999-1343; fax: þ45-8999-1500. E-mail address: [email protected] (J.T. Sørensen).
0167-5877/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 8 7 7 ( 0 2 ) 0 0 0 6 8 - 5
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feeding principles like the TMR system and new housing and milking systems enhance the need for development of new strategies for controlling milk fever at herd level. Several options for controlling milk fever in the dairy herd are available (Horst et al., 1997). These options are often complex to use and can require special production facilities or management skills. Furthermore, feasible combinations of options (strategies) might vary between herds. Knowledge on relevant production strategies for controlling milk fever in modern milk production is scarce, however. There is increasing interest among epidemiologists in using expert opinions in a systematic manner for quantification of parameters which are difficult to estimate directly from empirical data. Expert panels have been used to quantify risk factors for contagious diseases such as swine fever and foot-and-mouth disease (Horst, 1998), for bovine respiratory disease (Van der Fels-Klerx et al., 2000) and ovine caseous lymphadenitis (Binns et al., 2000). Another possible use for expert opinion studies is evaluation of the relevance of various alternative control options and control strategies applied in a livestock herd. Such evaluations are important for the relevance of different control strategies but are hardly known (other than to field experts). Our aim was to present the strategies to control milk fever in a dairy cattle herd; strategies were developed through an expert opinion study.
2. Material and methods Twenty advisors were selected (12 animal husbandry advisors and 8 veterinary practitioners) as experts. The advisors were all experienced advisors for dairy cattle farmers. Their identity was kept secret and they did not know the answers of the other experts at any time. Three questionnaires were completed during February and August 2000. Eighteen experts completed all 3 questionnaires and 19 experts completed questionnaires 2 and 3. The expert opinion procedure is illustrated in Fig. 1. The results from the first questionnaire were used to formulate the second questionnaire and the results from the second questionnaire were used in the third questionnaire. 2.1. First questionnaire The effects of two control options (Ca-gel fed orally to the cow peripartum and a low dietary cation–anion-difference (DCAD) in the dry-cow ration) on milk fever risk are well
Fig. 1. Diagram showing the procedure for the expert opinion study using three questionnaires.
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documented in the literature (Thilsing-Hansen et al., 2002). The experts were asked to specify a most likely value, a minimum value, and a maximum value of the relative risk of milk fever when using Ca-gel orally compared to not using it. For DCAD, the experts then were asked to specify these three estimates for each of three shifts in DCAD. These were defined according to typical range of DCAD in a dry-cow ration and the fact that relative risk seems to change significantly around 0 meq/kg dry matter (DM) (Thilsing-Hansen et al., 2002). The experts were asked to specify their minimum and maximum values to be as close as possible to each other such that they were 90% confident that the true value would fall between them. These three changes were: (a) a change from 150 to 100 meq/kg DM; (b) a change from 150 to 0 meq/kg DM; (c) a change from 400 to 150 meq/kg DM. The experts were also asked to suggest any other options (to control milk fever) relevant for use on typical Danish dairy farms. 2.2. Second questionnaire The experts were asked about their opinion on the relevance of each option suggested in the first questionnaire and further to suggest strategies for controlling milk fever. A strategy was defined as either a combination of options or a single option, which could be recommended to be used alone in a dairy herd. The experts scored the relevance of each control option on a scale from 1 to 5 (1: not relevant; 5: highly relevant). Relevance was explained to be a general evaluation including biological, economical and practical aspects in the dairy herds. 2.3. Third questionnaire In the third questionnaire, the experts were asked about their opinion on every strategy suggested by at least two experts in the second questionnaire. Each expert was asked to distribute 100 points among the strategies according to the relative relevance of each strategy given the set of strategies defined in the second questionnaire. The experts were also asked to give an estimate on the expected effect on time needed by the farmer for the strategy and the effect of the strategy in terms of reducing the milk fever risk. They used a scale from 1 to 5 (1: very little effect; 5: very large effect) on both issues. Quantitative assessments of probability distributions for expert opinion on effects and time needed for selected control options were expressed by density functions. Density functions were formulated using Betapert functions for each expert and combining the functions as described by Vose (2000). Differences between opinions on relative relevance of suggested control strategies were analysed using a generalised linear model with a Poisson distribution and a log-link function The full model included the explanatory variables: 12 categories of control strategy (1–12), 2 categories of expert group (‘‘advisors’’ and ‘‘vets’’), scale value of time needed (1–5), scale value of effect on milk fever risk (1–5) and the two-term interactions among these. Model reductions were done by stepwise deleting the non-significant ðP > 0:05Þ term with the highest P-value
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computed by likelihood-ratio statistics. The term ‘‘control strategy’’ was forced to stay in model.
3. Results 3.1. First questionnaire The control options suggested by the experts in the first questionnaire are listed in Table 1. In total, 13 control options were suggested. The control option Ca-gel fed orally peripartum was specified as administration of Cagel twice before calving and twice after calving. Sixteen experts provided all three values (most likely, minimum, and maximum) on the relative risk of milk fever. For each of the 16 experts a Betapert distribution was estimated and an aggregated density function was made (Fig. 2). The average relative risk for the aggregated density function was 0.45. Fourteen experts provided the needed three values (minimum, most likely and maximum) for all three changes in DCAD. The aggregated density functions for their expected Table 1 Control (for bovine milk fever) suggested by 18 experts in the first questionnaire Ca-gel fed orally Low DCAD during dry period Culling of high-risk cows Body-condition control Reduced milking post-partum Milking to calving Low Ca in dry-cow ration Short dry period length Mg-level control peripartum P-level control peripartum Low carbohydrate peripartum High D-vitamin level prepartum Induced calving
Fig. 2. Aggregated probability density function (PDF) for expert opinion on effect of Ca-gel fed orally peripartum on the relative risk of milk fever (RR). Based on the opinion from 16 experts.
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Fig. 3. Aggregated PDFs for expert opinion on effect of DCAD in the dry-cow ration on relative risk of milk fever (RR) when moving from 150 to 100, 150 to 0, and 400 to 150 meq/kg DM. Based on the opinion from 14 experts.
Fig. 4. Aggregated PDF for expert opinion on the time needed by the farmer for administration of the Ca-gel orally to a cow. Based on the opinion from 17 experts.
effects on relative risk of milk fever is shown in Fig. 3. The median estimated relative risks for the three changes in DCAD were: 0.42, 0.64 and 0.71 for 150 to 100, 150 to 0, and 400 to 150 meq/kg DM, respectively. The time needed for Ca-gel fed orally was for administration of four doses to one cow. The aggregated density function for time needed to feed Ca-gel orally peripartum was calculated from the three point estimates from 17 experts (Fig. 4). Expert opinion produced a skewed trimodal density function with a peak around 10 min (median 17 min per cow treated). Assuming this option applied 0.40 times per cow year (approximately all the cows at third or later calving), then the annual time needed would be calculated to be 7 min per cow year.
Fig. 5. Aggregated PDF for expert opinion on the time needed per day to reduce the DCAD in the dry-cow ration by adding anions to the feed ration. Based on the opinion from 17 experts.
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Fig. 6. Expert opinion on the relevance of 13 options to control milk fever ranked as medians for animal husbandry advisors and veterinary practitioners respectively. Based on the opinion from 19 experts.
The time needed for the option of using a low DCAD in the dry-cow ration depends on how the low DCAD is produced (Fig. 5). The density function has a peak at 5 min and has a median of 9 min/day for a 100-cow dairy herd. Thus, the median time used per cow year would be 33 min. 3.2. Second questionnaire The average score for each control option from the second questionnaire shows that the median score for eight options were 3 or higher, indicating a general opinion that they are relevant (Fig. 6).
Table 2 Strategies to control milk fever suggested by 19 experts in the second questionnaire Ca-gel fed orally Low Ca in dry-cow ration Body-condition control Low DCAD during dry period Reduced milking post-partum Culling of high-risk cow A combination of Ca-gel fed orally and low Ca in dry-cow ration A combination of Ca-gel fed orally and body-condition control A combination of Ca-gel fed orally and low DCAD during dry period A combination of Ca-gel fed orally and reduced milking post-partum A combination of low Ca in dry-cow ration and body-condition control A combination of low Ca in dry-cow ration and low DCAD during dry period
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Fig. 7. Expert opinion on the relative relevance of the 12 milk fever strategies suggested by at least two experts. The opinion is expressed as overall average and average for animal husbandry advisors and veterinary practitioners respectively. Based on the opinion from 19 experts.
In total, 12 strategies each were suggested by at least two experts (Table 2). The experts suggested six options relevant to be used alone and six combinations. There was general agreement between the two groups of experts. 3.3. Third questionnaire The relative relevance of the strategies is shown in Fig. 7 as the median for the two groups of experts. There seems to be a high degree of agreement between the two groups of
Fig. 8. Expert opinion on the efficiency of the suggested 12 milk fever control strategies expressed as medians for animal husbandry advisors and veterinary practitioners respectively. The order of the strategies is similar to the order in Fig. 7. Based on the opinion from 17 experts.
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Fig. 9. Expert opinion on the time needed by the farmer to apply on each of the 12 milk fever control strategies expressed as medians for animal husbandry advisors and veterinary practitioners respectively. The order of the strategies is similar to the order in Fig. 7. Based on the opinion from 17 experts.
experts on the two most-relevant strategies, i.e. Ca-gel fed orally peripartum alone or in combination with a low Ca level in the dry-cow ration. The experts were also asked about their opinion on the effect of the suggested control strategies in terms of reducing the milk fever risk and their opinion on the time needed by the farmer applying the strategy in question (Figs. 8 and 9). The control strategies are in the same order in Figs. 8 and 9 as in Fig. 7. It appears that there is not complete coherence between experts’ average opinion on relative relevance and their average opinion on efficiency or time needed. The statistical analysis (available upon request from the first author) showed that relative relevance of a control strategy was related to expected effect on milk fever risk whereas relative relevance was not significantly related to time needed or the expert group (vets or advisors).
4. Discussion For two well-documented control options, the experts were asked to provide an estimate on the effect on milk fever risk. The effects assumed by the experts for Ca-gel fed orally peripartum and a low DCAD in the dry-cow ration were quite similar to what is found in the literature (Thilsing-Hansen and Jørgensen, 2001). The effect of the option to keep a low Ca level in the dry-cow ration shows inconsistent results in the literature except for those keeping the Ca level very low in the diet (below 20 g/day) (Horst et al., 1997; ThilsingHansen and Jørgensen, 2001). The experts did not specify what they meant by a low Ca level in the dry-cow ration. However, it is very difficult in commercial milk production to compose a feed ration with <20 g of Ca per cow per day. Addition of a Ca-binder to the feed has been described (Jørgensen et al., 2001; Thilsing-Hansen and Jørgensen, 2001). Bodycondition control to avoid fat cows at calving has not been documented directly. It is,
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however, supported by epidemiological studies on post-partum risk of diseases (Østergaard and Gro¨ hn, 1999; Heuer et al., 1999). The option to reduce milking during the first milking after calving has not been documented in the literature. Generally there was high coherence between expert opinion on effect on milk fever risk and expert opinion on relative relevance of the suggested control strategies. It was interesting to us to note that the strategy of combining a low DCAD in the dry-cow ration and Ca-gel fed orally peripartum was not seen as particularly relevant although it was assumed to have a large effect on milk fever risk. It might be that the advisors expect this strategy to be relatively time consuming. Vose (2000) advises use of expert opinion only in situations where data have never been collected or data are too expensive to obtain. The relevance of strategies for controlling diseases at farm level cannot be obtained directly from empirical data with high precision and therefore are a valid area for expert opinion studies. The economic consequences of strategies can be approached by simulation studies (Østergaard et al., 2000). Advisors have experience with the problems related to implementation of control strategies on farms. It is, however, important that their assumption on effects on production is in agreement with scientific documentation. In our case developing milk fever control strategies this seems to be the case.
Acknowledgements We thank all participating experts for their time and input and also Anne Kudahl’s comments on the manuscript. References Binns, S.H., Green, L.E., Smith, J.Q., 2000. Economic and risk analysis of possible methods for control of ovine caseous lymphadenitis in the UK. In: Proceedings of the Ninth Symposium of the International Society on Veterinary Epidemiology and Economics, pp. 43–45. Heuer, C., Shukken, Y.H., Dobbelaar, P., 1999. Post partum body condition score and results from first test day milk as predictors of disease, fertility yield and culling in commercial dairy herds. J. Dairy Sci. 82, 295–304. Horst, H.S., 1998. Risk and economic consequences of contagious animal disease introduction. Ph.D. Thesis. Department of Economics and Management, Wageningen Agricultural University, 147 pp. Horst, R.L., Goff, J.P., Reinhardt, T.A., Buxton, D.R., 1997. Strategies for preventing milk fever in dairy cattle. J. Dairy Sci. 80, 1269–1280. Houe, H., Østergaard, S., Thilsing-Hansen, T., Jørgensen, R.J., Larsen, T., Sørensen, J.T., Agger, J.F., Blom, J.Y., 2001. Milk fever and subclinical hypocalcaemia. An evaluation of parameters on diagnosis, risk factors and biological effects as input for a decision support system for disease control. Acta Vet. Scand. 42, 1–29. Jørgensen, R.J., Hansen, T., Jensen, M.L., Thilsing-Hansen, T., 2001. Effect of oral drenching with zinc oxide or synthetic zeolote A on total blood calcium in dairy cows. J. Dairy Sci. 84, 609–613. Østergaard, S., Gro¨ hn, Y.T., 1999. Effect of diseases on test day milk yield and body weight of dairy cows from Danish research herds. J. Dairy Sci. 82, 1188–1201. Østergaard, S., Sørensen, J.T., Kristensen, A.R., 2000. A stochastic model simulating the feeding-healthproduction complex in a dairy herd. J. Dairy Sci. 83, 721–733. Thilsing-Hansen, T., Jørgensen, R.J., 2001. Hot topic: prevention of parturient paresis and subclinical hypocalcaemia in dairy cows by zeolite. Administration in the dry period. J. Dairy Sci. 84, 691–693.
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