Epidemiological and economic study of the repeat breeder syndrome in Michigan dairy cattle. II. Economic modeling

Preventive Veterinary Medicine, 14 (1992) 99-114

99

Elsevier Science Publishers B.V., Amsterdam

Epidemiological and economic study of the repeat breeder syndrome in Michigan dairy cattle. II. Economic modeling S.Q. Lafi a, J.B. Kaneene a, J.R. Blackb and J.W. Lloyda'b aPopulation Medicine Center, A 109 Veterinary Medical Center, Michigan State University, East Lansing, M148824-1314, USA bDepartment of Agricultural Economics, Michigan State University, East Lansing, M148824-1314, USA (Accepted 13 March 1992 )

ABSTRACT Lafi, S.Q., Kaneene, J.B,, Black, J.R. and Lloyd, J.W., 1992. Epidemiological and economic study of the repeat breeder syndrome in Michigan dairy cattle. II. Economic modeling. Prey. Vet. Med., 14: 99-114. Data from 96 dairy herds randomly selected in the State of Michigan were used to: ( 1 ) estimate the economic impact of repeat breeder cow syndrome (RBS) and related risk factors and (2) evaluate the economic effects of changing during the dry period some nutritional risk factors that influence RBS. The economic analyses were based on an epidemiological model which considered both the direct and the indirect effects of the significant risk factors. The mean direct costs of milk fever, dystocia, and repeat breeder syndrome for an 86-cow herd per year were $ 231, $ 529, and $ 994, respectively. Farms that fed 100% dry hay or haylage to dry cows had an increase in the cumulative incidence (CI) rate of milk fever and a reduction in the CI rate ofdystocia in comparison with farms that fed a ration containing corn silage. Increasing the annual CI rate of milk fever in a herd by 1% was associated with an increased total (direct and indirect ) cost per herd per year of $133. Increasing the annual CI rate ofdystocia in a herd by 1% was associated with an increased total cost per herd per year of $ 140. These results suggest that proper nutrition during the dry period to prevent milk fever and dystocia is essential for reducing the incidence and cost of repeat breeder syndrome.

INTRODUCTION

There is growing concern among producers and veterinarians regarding reduction in productive efficiency caused by diseases. In the literature there are few articles which assess specifically the economic effect of the repeat breeder syndrome (RBS) in dairy cattle (independent of other reproductive disorCorrespondence to: J.B. Kaneene, Population Medicine Center, A 109 Veterinary Medical Center, Michigan State University, East Lansing, MI 48824-1314, USA.

© 1992 Elsevier Science Publishers B.V. All rights reserved 0167-5877/92/$05.00

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ders). The costs arising from RBS and other reproductive diseases (including a larger number of days open, extra veterinary treatments, labor, fewer calves born, and loss due to culling and replacement) have been assessed by several investigators (Lineweaver, 1975; James and Esslemont, 1979; Olds et al., 1979; Oltenacu et al., 1980, 1981; Britt, 1981; Steele et al., 1981; Thompson et al., 1983; Holmann et al., 1984). Several investigators attempted to assess the economic effects of reproductive diseases and culling in dairy cattle using the partial budgeting technique (Olds et al., 1979; Oltenacu et al., 1981; Holmann et al., 1984; Dijkhuizen et al., 1985; Bartlett et al., 1986; Jansen et al., 1987). However, these studies focused on the economic effects of only one disease, and they did not consider indirect risk factors. It is known that reproductive diseases are interrelated and do not occur as independent events. The indirect effects among reproductive diseases are biologically important and in some instances exceeded the direct effects (Erb and Martin, 1980; Erb et al., 1981, 1985; Curtis et al., 1983, 1985; Salman et al., 1984; Erb, 1987; Salman and Meyer, 1987). The hypothesis presented in this study is that the interrelationships among reproductive diseases are significant economically, and that an economic approach which considers both the direct and the indirect effects should be used when financial consequences of diseases are estimated. Using the epidemiological model that was developed previously (Lafl and Kaneene, 1992 ) as the basis for these economic analyses, the objectives of this paper were ( 1 ) to estimate the economic impact of RBS and related risk factors and (2) to evaluate the economic effects of changing some dry period nutritional risk factors on RBS and its risk factors. MATERIALS AND METHODS

Study population Data for this study were obtained from the National Animal Health Monitoring System (NAHMS) in Michigan. The methods of data collection, herd selection, study design, and calculation of incidence rates are described elsewhere (Kaneene and Hurd, 1990). Briefly, the State of Michigan joined NAHMS in the fall of 1986. The objective of NAHMS is to provide statistically valid data on animal and poultry health-related events. The data are used to estimate the incidence, prevalence and cost of these events. This study was conducted in two 12-month phases: 1987-1988 and 1988-1989. The procedures and methods that were used for the study design, herd selection, data collection, and calculation of the incidence rates of the various diseases involved were the same in both phases. The annual average herd size was 86 _+59 cows (mean +_SD). The annual

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mean cumulative incidence (CI) rate o f RBS was 6.9 _+7.3 cases per 100 cow years. The annual mean cumulative incidence rates o f milk fever ( M I L F ) and dystocia ( D Y S T ) were 4.4 _+2.2 and 4.9 + 7.5, respectively. Twenty-six cows were diagnosed with both M I L F and DYST. Six cows were diagnosed with both D Y S T and RBS. F o u r cows were diagnosed with M I L F and RBS, and none of the cows were diagnosed with all three diseases.

Epidemiological model Figure 1 depicts the path model o f RBS identified by Lafi and Kaneene ( 1992 ). This model was the basis for the economic analyses o f this study. For simplicity and practical purposes, the average age o f the herd described in the epidemiological model was not considered in the economic analyses.

Milk production and price of milk Milk production included only the milk sold to milk processors each month, not milk fed to calves or disposed of. The average milk production per cow per year was 14 400 + 3377 lb, and the average milk price was $ 0.112 l b - l .

.

"3.01

-, /

Y

\. (021)*

=

Fig. 1. Final path model of RBS using the significant risk factors obtained from the principle components regression analysis (data derived from 96 herds participating in the National Animal Health Monitoring System in Michigan 1986-1989). Values in parentheses represent the standardized regression coefficients. Values not in parentheses are non-standardized regression coefficients. *P< 0.5; **P< 0.01; ***P<0.001.

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ECONOMIC MODELING

After the significant risk factors have been identified, and the indirect effects of the diseases identified and quantified, the next step is to attach monetary values to the quantified impacts. In order to satisfy Objective 1, the partial budgeting procedure was used to estimate the economic effects of MILF, DYST, and RBS. (A more general approach would take explicit account of the impact of disease on the replacement decision. The approximate approach used here is easier to understand, is consistent with the availability of information, and is a good approximation. ) Partial budgeting is a description of the economic consequences of a specific change in farm procedure on an annual basis and is typically made up of four sections: ( l ) additional revenue realized from the change; (2) reduced cost resulting from the change; (3) revenue foregone as a consequence of the change; (4) extra cost incurred owing to the implementation of the change (see e.g. Harsh et al., 1981 ). In this context, the change may be a disease prevention program. As some data that are very critical for calculating the expenses and the revenues associated with the occurrence of each of the three diseases considered here were not available in our data base, some assumptions were necessary. Further, these estimates should be considered to be expectations and hence fractional animals are included. Assumptions Feed cost Some participating farms changed their feed according to the season of the year, and many of the participating farms that produced their own feed did not know the exact amount of feed that was used. In addition, many farms did not analyze their feed. Consequently, it was not possible to estimate the exact feed intake for cow maintenance a n d / o r milk production using our data base. Thus, a hypothetical feed ration (Table 1 ) was formulated based on the recommendations of the National Research Council (NRC, 1988). It was assumed that all participating farms used this hypothetical feed ration for all lactating cows during their participation in the study. The cost of this ration ($ 3.77 per 100 lb of ration dry matter) was calculated using the average feed prices that were reported in Michigan Agricultural Statistics of 1987 and 1988 (Michigan Department of Agriculture, 1987 and 1988 ). Herd size It was assumed that the number of milking cows in each herd was fixed during their participation in the study. This assumption does not agree with the real situation at the farm, but it simplified the number of assumptions needed for calculating the economic effects of the diseases involved. For ex-

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TABLE 1 Hypothetical feed ration used in the analysis of this study (source: NRC, 1988)

Proportion (dry matter basis) NEro I (Mcal lb-t ) Crude protein (proportion) Price (per 100 lb)

Corn

Soybean

Alfalfa

Corn silage

Total

0.375 0.890 0.100 4.517

0.050 0.870 0.490 10.389

0.287 0.610 0.180 2.710

0.287 0.770 0.080 0.026

1.000 0.728 0.166 3.7722

~Net energy for maintenance. 2Prices obtained from Michigan Department of Agriculture ( 1987 and 1988 ).

ample, based on this assumption some variable costs (such as utilities and bedding) and fixed costs (such as labor, interest, insurance of livestock, equipment, building, and management costs) were constant. In addition, this assumption simplified the estimation of the value of extra heifers. To maintain constant herd size, these extra animals were assumed to be sold at birth. This added revenue should be considered a proxy for the more realistic value that would be obtained by calculating the net benefit associated with raising these animals and increasing future sales of bred heifers a n d / o r cull cows.

Days open The time period of each phase was 12 months, and none of the participating herds in the first phase were included in the second phase. Therefore, following all cows until the end of their lactation was not possible. We assumed a calving interval of 365 days for a herd free of disease (RBS, MILF or DYST) (305 milking days and a 60-day dry period). A repeat breeder cow was defined as a cow or a heifer that failed to become pregnant following three or more breedings or inseminations. DeKruif (1976) and Bulman and L a m m i n g (1978) reported that 60% of the repeat breeder cows conceived by the fourth insemination and about 16-25% conceived by the fifth insemination. Therefore, we assumed that each case of RBS was associated with 60 extra days open and 2.5 extra inseminations. Several studies that were conducted on an individual cow basis revealed that MILF did not increase the days open or the n u m b e r of inseminations per conception (Dohoo and Martin, 1984; Erb et al., 1985). Based on these reports, we assumed that cases of MILF were not associated with extra days open. T h o m p s o n et al. (1983) reported that difficult calving (DYST) was associated with a significant increase of 20.4 days open and slightly decreased milk production immediately post-calving, but there was only a small negative effect of 165 kg mature equivalent for cows averaging 8574 kg of milk

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carried through the lactation. This difference in milk production was not statistically significant. On the basis of the results reported by Thompson et al. ( 1983 ), it was decided to consider that each case of DYST is associated with an extra 20 days open. As it was assumed that MILF does not affect the number of days open nor the number of inseminations per conception, cows that had both MILF and DYST, or both MILF and RBS were considered as having two independent diseases and the cost components of each disease were estimated separately. The two diseases, DYST and RBS, also were considered as independent events. However, as a small number of cows (only six) experienced both diseases, an adjustment for the days open due to dystocia in these six cows was ignored. For estimating the net effect of each disease involved, two examples will be demonstrated for each of the three diseases. These examples will involve a comparison between two hypothesized herds. The first herd will have the incidence rate of the disease, and the second herd will be a disease-free herd. The two herds will have the same average age (2.7 lactations), a herd size of 86 cows, and an average milk production of 14 400 lb per cow per year. These data were obtained from the study and represent the descriptive statistics of an average herd in the State of Michigan (Lafi and Kaneene, 1992).

Linearity It was assumed that the economic impact of the diseases considered was linear with respect to incidence rate. While this assumption may not be entirely realistic, evidence to the contrary could not be found in the literature, and the data base was not sufficiently complete to support or refute this independently. PARTIAL B U D G E T I N G OF RBS

Revenues associated with RBS In general the revenues on a dairy farm come from milk production, calves sold and culling.

Milk production A model was derived based on Wood's (1969) model to forecast the average daily milk yield for each month within a lactation. Herd with a 6.9% CI rate of RBS. In this herd, 5.9 cows (86X6.9%) experienced RBS and 80.1 cows did not experience RBS during the year. Thus, the 80.1 cows had a calving interval of 365 days and each of the 5.9 cows had a calving interval of 425 days (365 + 6 0 ) . Because of the 5.9 repeat breeder cows, the calving interval of this herd was equal to 369 days (i.e. [ (5.9/86)

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X 425 ] + [ (80.1/86 ) × 365 ] ). Using Wood's ( 1969 ) equation and Shultz's (1974) standardized values for the parameters involved in Wood's equation, the extra total amount of milk produced per lactation from each RBS cow was 1190 lb. The amount of milk produced per day from cows without RBS was equal to ( 14 400)<80.1 )/365 =3160 The amount of milk produced from cows with RBS (lb day-~ ) was equal to (15 590X 5.9)/425 =216 The total amount of milk produced by this herd (lb year-~ ) was equal to (3160+216)×365 = 1 232 240 The annual revenue ($) obtained from milk production was equal to 1 230 240X0.I 12 =138 011 Herd with a zero CI rate o f RBS. The calving interval of this herd (86 milking

cows) was 365 days (305 milking days and a 60-day dry period). Consequently, the annual revenue obtained from milk production was equal to $138 701 (14 4001byear-~X86X0.112 $ / l b - l ) . Calves sold Herd with a 6.9% CI rate per I00 cow years. The annual mortality rate of

calves obtained from this study was 13 per 100 calvings. This mortality rate included all stillbirths and all neonatal deaths. We assumed that 50% of the born calves are males, that all male calves are sold at birth for $ 60 each, and that females are kept for replacement. The revenues obtained from the sold calves (assuming none of the calves died because of RBS) in this herd were calculated as follows. ( 1 ) For cows without RBS 86X ( 1 - 0 . 0 6 9 ) X ( 1 - 0 . 1 3 ) = 69.7 calves year(2) For cows with RBS

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86X 6.9%X ( 1 - 0 . 1 3 ) X 365/425 = 4.4 calves year- 1 Total n u m b e r of surviving calves= 69.7 + 4.4 = 74.1 calves yearTotal revenues from sold calves ($) = (74.1 X0.5) × 6 0 =2223 Herd with a zero CI rate o f RBS. Using the same herd size, the annual total surviving calves was equal to

86×(1-0.13) =74.8 In order to keep the herd size the same, some of the female calves must be sold. Assuming that the extra female calves were sold at birth for the same price as male calves, the total n u m b e r of calves sold in this herd was equal to 7 4 . 8 - (74.1 × 0 . 5 ) =37.8 The total revenues obtained (for an 86-cow herd) from sold calves ($) was equal to 37.8×60 =2268 Culling and death losses The overall death rate for cows was 3.6% ( 301 / 8256 ) and the culling rate among the repeat breeder cows was 60.8% (404/664). According to the annual summary of production records published by the Dairy Herd Improvem e n t Association ( D H I A ) in Michigan for 1987 and 1988, the average culling rate in dairy farms was 33% of which 7.4% (2.44% of the herd) were culled for dairy purposes. The total culling and death rate was equal to 36.6% ( 3 3 % + 3.6%). The value of these rates will be used for estimating the cost associated with culling throughout the analyses of RBS, DYST and MILF. Herd with a 6.9% CI rate o f RBS. Using the herd size and overall death rate, the expected n u m b e r of dead cows was 3.1 ( 86 × 3.6% ). Using the DHIA culling rate, the total n u m b e r of culled cows (including those culled for RBS, DYST, and MILF ) was 28.4 (86 × 33%). Of those animals, 2.1 (28.4 X 7.4%) were culled for dairy purposes and 26.3 ( 2 8 . 4 - 2 . 1 ) were sold for beef. The average price for a cow sold for dairy purposes was $ 1 1 0 0 and a cow sold for beef was worth $ 350. The disposal cost of a dead cow was $ 22.

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Annual revenues ( $ ) = (2.1X 1100) + (26.3 X 350) - (22X3.1) =11 447

Herd with a zero CI rate of RBS. Using the same example, 3.6 cows (60.8% X 5.9 ) with RBS were culled for beef. In order to keep the culling and the death rate fixed (same) in the farm (with or without the presence of the disease), the 3.6 cows need to be allocated proportionally between cows culled for dairy and beef; the total n u m b e r of cows sold for dairy was equal to 2 . 4 ( 2 . 1 + ( 3 . 6 X 2 . 1 / 2 4 . 8 ) , where (2.1/24.8) is the proportion of culling without RBS sold for dairy purposes). Similarly, the total n u m b e r of cows sold for beef would be reduced by 0.30 cows (3.6X2.1/24.8) to 26 (26.3-0.3). Annual revenue ($) = (2.4X 1100) + (26X 350) - (3.1 × 2 2 ) =11 672 Expenses associated with RBS The yearly total cost of RBS in the herd with a 6.8% CI rate of RBS was the sum of: veterinary services including veterinary treatments, cost of drugs used by farmer, discarded milk, cost of extra breedings or inseminations, and cost of feed (saving) associated with lower milk production due to RBS. The cost of veterinary services was based on the receipts received from the veterinarian. The cost of drugs was estimated by the farmer based on the a m o u n t and the price of the drug used. These costs were collected on a monthly basis by the Veterinary Medical Officers (VMOs). Because some assumptions were made regarding discarded milk, and costs of semen and feed, a brief summary of calculating the cost of these items will be presented.

Milk discarded because of RBS treatment The a m o u n t of milk discarded because of treatment was estimated by the farmer and in a few instances included milk production lost owing to an acute disease. In such cases the loss was estimated as the difference between what the cow was producing before the illness and that produced during the illness. Of the milk discarded because of treatment, 60% was fed to calves instead of milk replacer (the price of reconstituted milk replacer, approximately $ 7.00 cwt- 1, was calculated based on a sample (n = 4) of Michigan feed suppliers, October 1987 ) which was thus considered a decreased expense; the truly discarded milk was considered a decreased revenue. However, in the analysis of RBS, these values disappeared when rounding to the nearest dollar. Expenses associated with breeding or insemination Typically, when cows were rebred several times (RBS), cheaper semen from lower quality AI sires or from a clean-up bull was used; thus, offspring with

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lower genetic potential were produced. The income foregone because of this was not considered. According to the Michigan Animal Breeder's Cooperative (MABC, 1988), the price of semen for 1987-1989 ranged between $ 4 and $ 90 with an average of $10.70 per dose. An estimate of$ 7 for each extra breeding or insemination associated with RBS was used. In the example used, each of the 5.9 cows received 2.5 extra inseminations.

Feed cost According to the N R C ( 1988 ), each p o u n d of 3.5% fat milk produced requires 0.32 NEm. The a m o u n t of milk not produced owing to RBS was 6160 lb year- ~. Each p o u n d of the hypothetical feed ration was enough to produce 2.3 lb of milk. Thus, 2678 lb year- 1 of feed were saved because of RBS. The net cost of the saved feed was equal to $101 ( 3 . 7 7 × 2 6 7 8 ) / 1 0 0 ) . PARTIAL B U D G E T I N G OF DYST AND MILF

The methods used for computing revenues and expenses for DYST and MILF were identical to those presented under RBS. For DYST, the following values were employed: a CI rate of 4.9 per 100 calvings; calving interval extended by 20 days; extra milk produced per DYST lactation of 411 lb; a calf mortality rate of 39%; culling and death rates due to DYST of 2.8% and 9.6%, respectively. For MILF, the following values were employed: a CI rate of 4.4 per 100 calves; culling and death rates due to MILF of 2.3% and 7.7%, respectively.

Sensitivity analysis In order to achieve Objective 2, a sensitivity analysis was conducted to show how changes in the proportion of hay (PHAY) in the ration would affect the cost of MILF, DYST and RBS. The PHAY value, calculated as the ratio of hay a n d / o r haylage dry matter (FHAY) to F H A Y + c o r n silage dry matter (COSD), ranged between 0.469 and 1%. Three values of PHAY within this range ( m a x i m u m , mean, and m i n i m u m values) were chosen to illustrate the sensitivityanalysis. These values were 1.00, 0.565 and 0.469, respectively. READING THE PATH COEFFICIENTS

The interpretation of path coefficients in the path model is similar to the interpretation of the regression coefficients. The total effects of DYST on RBS are equal only to the direct effect. The total effects of MILF or PHAY were equal to their indirect effects, e.g. the indirect effect of PHAY (when equal to 1 ) on RBS was equal to the sum of the indirect effect through DYST ( - 7 . 0 0 4 × 0 . 2 2 ) and the indirect effect through MILF and DYST

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( 1.688 X 0.57 X 0.22 ). The same method was used to calculate the total effects of MILF on RBS.

RESULTS

The partial budget analyses of RBS, DYST and MILF are shown in Table 2. The cost associated with RBS, DYST and MILF per case and per 1% incidence rate is shown in Table 3. The sensitivity analysis results are shown in Table 4. TABLE2 Annual partial budget of repeat breeder cow syndrome in Michigan (data derived from 96 dairy herds participating in the National Animal Health Monitoring System in Michigan 1986-1989) Item

Repeat breeder syndrome

Dystocia ($)

Milk Fever ($)

- 690 -45 - 225 0

- 159 - 108 - 178 - 4

0 0 - 133 - 5

- 22 - 10 - 103 0 + 101 - 994

- 96 - 7

- 61 - 39

0

+ 7

0 + 23 - 529

0

($) Revenues Milk production Calves sold Culling and death losses Discarded milk

Expenses Veterinary services Drugs used by the farmer Cost of extra semen Discarded milk fed to calves Feed saved from less milk produced Total annual cost per herd

- 231

TABLE3 Annual cost associated with repeat breeder syndrome, dystocia and milk fever per case and per 1% annual incidence rate (data derived from 96 dairy herds participating in the National Animal Health Monitoring System in Michigan 1986-1989) Disease

Direct cost per case ($)

Direct cost per 1% incidence rate ($)

Total (direct and indirect ) cost per 1% incidence rate ($)

Repeat breeder syndrome Dystocia Milk fever

168 126 61

144 108 53

144 140 133

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TABLE 4 Sensitivity analysis on proportion of hay (PHAY) with other variables held constant (data derived from 96 dairy herds participating in the National Animal Health Monitoring System in Michigan 1986-1989) PHAY

1.000 0.562 0.469

Cost ($) Milk fever

Dystocia

Repeat breedel syndrome

87.03 48.91 40.82

-661.39 -371.70 -310.19

- 189.53 - 106.51 -88.89

Calculation of direct and indirect costs Increasing the annual CI rate of MILF (independently from the ration) from 0 to 1% had an associated cost of $ 53 year-1 because of MILF, and a $ 62 ( 1 0 8 . 5 9 × 0 . 5 7 ) cost per year due to DYST and indirectly would increase the cost of RBS per year by $ 1 8 (0.22X0.57X 144). Therefore, increasing the annual CI rate of MILF by 1% in a herd would cost a total of $ 1 3 3 ( 5 3 + 6 2 + 18) per year. Increasing the annual CI rate of DYST by 1% in a herd would cost $ 1 0 8 per year due to DYST and would cost $ 32 (144X0.22) per year due to RBS (total of $ 1 4 0 ) . DISCUSSION

Milk production The average milk production per cow per year was 14 400 lb. This average is very close to the average milk production per cow per year reported in Michigan by the National Agricultural Statistical Service (NASS) for the years of 1986 and 1987. The NASS results were based on a sample of 10% of the total herds in the State of Michigan. Results of this study were based on a sample of 1% of the total herds population indicating that the sample used in this study represented the total herd population in the State of Michigan.

Assumptions Many assumptions were made in the analyses of this study to demonstrate the methodology used and not the specific results. Examples of these include the fixed number of cows in the herd, the hypothetical feed ration, and culling rate of the DHIA. Some of these assumptions might have introduced biases

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into the calculation of the costs and the income associated with the diseases involved.

Repeat breeder cow syndrome About 79% of the net cost of RBS was due to milk loss and about 23% was due to culling. The net cost of RBS was $ 1 6 8 per case. Earlier studies in Michigan (Bartlett et al., 1986) estimated losses of $ 385, $ 429, $ 612 per case associated with repeat breeder cows that had three, four, and five inseminations, respectively. In their analyses, most costs were due to days open (they assumed that each day open cost $ 2.50). In our analyses, a different methodology was used for calculating the costs and the revenues of RBS.Therefore, it is not surprising that there are cost differences between the two studies. Because the current m e t h o d employs actual values for feed costs and milk production levels, it can be expected to be more accurate than use of a standard dollar loss per day open (Lloyd et al., 1987).

Dystocia The largest cost component associated with DYST was due to reduced value of the culled cows (four). About 20% of the total cost associated with DYST was due to reduction in the n u m b e r of calves sold, and about 30% of the cost was associated with the reduced milk yield. In the literature, there are some conflicting reports about the effects of DYST on milk production. Many investigators reported that DYST did not affect milk production (Philipsson, 1976a,b,c; Erb and Martin, 1980; Erb et al., 1981; Martin et al, 1982; Dohoo and Martin, 1984; Erb et al., 1985 ). However, Schneider et al. ( 1981 ) found that cows unaffected by dystocia, retained placenta, or endometritis had greater milk yields within 1 m o n t h postpartum.

Milk fever On average, 58% (129.63/226.86) of the cost associated with MILF was due to culled and dead cows. About 17% of the annual total cost was due to drugs used by farmers. Treatment of acute hypocalcemia is highly successful in that the clinical condition of the cow usually improves dramatically. This may cause some kind of false security for farmers by making them believe that MILF can be treated with no significant cost if they closely watch their cows around calving. However, our results indicate that a total annual cost ot $ 1 3 3 is associated with each 1% increase of the herd's CI rate of MILF, suggesting that more attention is needed to prevent and reduce the incidence rate of this disease.

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Prevention

The a m o u n t of money that can be spent to prevent the occurrence of each disease must be less than the cost associated with its occurrence if there is to be a net economic gain. Practically, a zero incidence rate of these diseases in a given herd is not feasible. Therefore, more research is needed to identify the o p t i m u m incidence rates of these diseases. SUMMARY

The economic analyses of RBS, DYST, and MILF were based on an epidemiological model which considered both the direct and the indirect effects. The average cost per herd per year ofRBS, DYST, and MILF was $ 994, $ 529, and $ 231, respectively. Farms that fed 100% hay or haylage to dry cows had an increase in the CI rate of MILF and a reduction in the CI rate of DYST in comparison with cows that were fed corn silage. For management purposes, the indirect cost of MILF on RBS and DYST was greater than the cost associated with MILF alone. Also, the indirect cost of DYST on RBS was more than 25% of the cost associated with DYST alone. These results suggest that measures are needed to prevent MILF and DYST in order to reduce the cost and the incidence of RBS. ACKNOWLEDGMENTS

This project was supported in part by U S D A / A P H I S / V S Grant No. 12-1693-229 and Michigan State University (MSU) College of Veterinary Medicine. We thank the state, federal, and MSU Veterinary Medical Officers for their essential role in collecting the data. We thank the faculty and staff of the Division of Epidemiology and the Department of Agricultural Economics for their input. Special thanks go to RoseAnn Miller for managing the data.

REFERENCES Bartlett, P.C., Kirk, J.H. and Mather, E.C., 1986. Repeated insemination in Michigan HolsteinFriesian cattle: incidence, descriptive epidemiology and estimated economic impact. Theriogenology, 26: 309-322. Britt, J.H. (Editor), 1981. Dairy Cattle: Fertility and Sterility. W.D. Hoard & Sons, Ft. Atkinson, WI, pp. 24-49. Bulman, D.C. and Lamming, G.E., 1978. Milk progesterone levels in relation to conception, repeat breeding and factors influencing acyclicity in dairy cows. J. Reprod. Fertil., 54: 447458. Curtis, C.R., Erb, H.N., Sniffen, C.J., Smith, R.D., Powers, P.A., Smith, M.C., White, M.E., Hillman, R.B. and Pearson, E.J., 1983. Association of parturient hypocalcemia with eight periparturient disorders in Holstein cows. J. Am. Vet. Med. Assoc., 183:559-561.

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