Metritis complex in Michigan Holstein-Friesian cattle: incidence, descriptive epidemiology and estimated economic impact

Preventive Veterinary Medicine, 4 (1986) 235--248 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

235

M E T R I T I S C O M P L E X IN M I C H I G A N H O L S T E I N - F R I E S I A N C A T T L E : INCIDENCE, DESCRIPTIVE EPIDEMIOLOGY AND ESTIMATED ECONOMIC IMPACT

PAUL C. BARTLETT, JOHN H. KIRK, MARGARET A. WILKE, JOHN B. KANEENE and EDWARD C. MATHER Department of Large Animal Clinical Sciences, College of Veterinary Medicine, Michigan State University, East Lansing, MI 48824 (U.S.A.) (Accepted for publication 28 May 1986)

ABSTRACT

Bartlett, P.C., Kirk, J.H., Wilke, M.A., Kaneene, J.B. and Mather, E.C., 1986. Metritis complex in Michigan Holstein-Friesian cattle: incidence, descriptive epidemiology and estimated economic impact. Prey. Vet. Med., 4: 235--248. Twenty-two Michigan dairy herds participating in a computerized herd health program were studied to determine the incidence and epidemiologic characteristics of metritis. Of 3773 lactations studied, 18% of the cows developed metritis at least once during the lactation. Metritis was reported most frequently immediately following calving. No clear pattern of association with age or season was demonstrated. An estimate was made of the extra cost associated with a cow treated for metritis. After including the effect on reproductive efficiency, milk production, cost of medication and losses due to culling, the total cost estimate was $106.00 for a lactation with metritis.

INTRODUCTION Several studies have s h o w n t h a t a p p r o x i m a t e l y 90% o f cows have bacterial uterine i n f e c t i o n during the first 10 days post p a r t u m (Rasbech, 1 9 5 0 ; De Bois, 1 9 6 1 ; Elliott et al., 1 9 6 8 ; Morrow, 1980). Most c o w s soon eliminate this bacterial flora, b u t others experience d e l a y e d uterine i n v o l u t i o n and c o n t i n u e d infection. Specific diseases such as brucellosis, vibriosis and leptospirosis can cause metritis (MET) resulting in e m b r y o n i c d e a t h (Archibald, 1 9 7 6 ; M o r r o w , 1 9 8 0 ) . More c o m m o n , however, is n o n s p e c i f i c MET w h i c h can result f r o m a variety of organisms t h a t gain access to the r e p r o d u c t i v e t r a c t via the cervical canal. Fertility can be impaired t h r o u g h adverse effects o n s p e r m a t o z o a and an a b n o r m a l uterine e n v i r o n m e n t leading to e m b r y o n i c death. R e t a i n e d p l a c e n t a (RP) in the c o w is a major predisposing cause of MET. T h e principal deleterious effect o f RP o n the d a i r y c o w is impaired fertility b y delaying i n v o l u t i o n of the uterus, t h e r e b y facilitating uterine i n f e c t i o n (Sandals et al., 1 9 7 9 ) . 0167-5877/86/$03.50

© 1986 Elsevier Science Publishers B.V.

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Knowledge of the economic impact of MET is the first step toward eventual determination of the cost effectiveness of preventive and disease controi techniques. In addition, an understanding of the economic impact of MET is needed to direct disease control and research programs on a state and national level. The objectives of this paper are to describe the incidence of MET among Michigan dairy herds participating in our study, and to estimate the mean economic impact of this condition. MATERIALS AND METHODS The data used for this study were collected from 22 herds using the FAHRMX dairy herd health computer network described in a previous publication (Bartlett et al., 1985). In brief, the network utilized microcomputers in local veterinary clinics to produce weekly management reports for each participating herd. A central computer was used to produce m o n t h l y herd health reports which compared the performance of each herd to a standard goal, and to the average of all herds using the system. The value of the reports to the farmers and the participating practitioners provided an incentive to record the necessary data regarding insemination, calvings, culls, disease occurrences, reproductive examinations, etc. In addition, a questionnaire was completed for each herd pertaining to physical facilities, management techniques and economic parameters. The FAHRMX data base was surveyed to identify cows with completed lactations. A lactation, by definition, began with a calving and ended with a cull or death at least 30 days later, a calving not more than 600 days later, or the end of the research period (1 June 1985) at least 150 days later. At least one insemination and one reproductive examination were required during the course of the lactation, except when the lactation ended with a death or cull. The rationale and data processing involved in identifying valid lactations for study have been previously described (Bartlett et al., 1986a). All herds participating in the project received scheduled reproductive visits from their veterinarian one or two times a month. Each week, the program reminded the farmers and the veterinarians as to which animals needed post partum examination. The number of days post partum of this reminder was designated for each herd by the veterinarian, and ranged from 10 to 30 days post partum, with a mean of 17 days post partum. The number of days post partum of each individual cow was displayed on the computer o u t p u t for the veterinarian while he or she was performing the reproductive exam. MET was diagnosed by rectal examination, based on the clinical evaluation of the uterine size, the thickness of the uterine wall and the fluid content of the uterine cavity in relation to the number of days post partum of the cow. Reports of antibiotic treatments were not necessarily considered indicative of metritis unless metritis was explicitly specified. Any placenta not passed within 12 h post calving was considered to be retained, and was reported by the herdsman. Analysis of data was performed with a data base manager and the SPSS statistical program (Nie et al., 1975).

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Incidence The lactational incidence rate is defined as the proport i on (or percentage) of lactations for which a disease was reported. For each age group, the percentage of lactations with MET was com puted. The effect of age on RP was analyzed in a similar manner. The number of days post calving (stage of lactation) at which MET was first diagnosed was analyzed by dividing the n u mb er of cows first diagnosed with MET in a given time interval, by the number o f open (non-pregnant) cow-days at risk during that time interval. Once a cow developed MET she was no longer considered at risk for the remainder o f the lactation. Lactational incidence rates for each m o n t h were c o m p u t e d as the number of cows first diagnosed with MET in each month, divided by the number of open (non-pregnant) cow-days at risk in each month. The Walter and Elwood test was employed to test for any significant seasonal effects (Walter and Elwood, 1975).

Estimation of mean economic impact o f M E T Estimation of the mean cost associated with a cow developing MET and receiving tr eatme nt includes the following cost c o m p o n e n t s : increased interval from calving to successful insemination (CSI), increased num ber of inseminations, increased n u m b e r of reproductive examinations, cost of medication, loss of milk withheld following use of medication, and the loss due to culling.

Effect on reproduction The m e t h o d of calculating CSI has been previously described elsewhere (Bartlett et al., 1986b). For lactations terminating with a calving, the insemination closest to 279 days (the gestation period) before the calving which ended the lactation was considered to be the successful insemination. If the project was terminated before the lactation ended, the last insemination before a pregnancy report was taken as the successful insemination. If the project ended before the cow was called pregnant, CSI could not be calculated for that lactation. Cows culled when t hey were pregnant were assumed to have conceived on their last insemination. To adjust for differences between herds, the deviation from herd average CSI (CSI-D) was c o m p u t e d for each lactation and used to compare lactations with MET to lactations without MET. This comparison was next adjusted for the effect o f confounding factors by the use of the Multiple Classification Analysis available with the ANOVA subroutine of the SPSS c o m p u t e r program (Nie et al., 1975). Because there were no significant interactions, multiple classification analysis was used to display the results of the analysis of variance model. This m e t h o d showed the net effect of each variable

238 after controlling for the effects of the other variables in the model. The dependent variable was the CSI-D for the lactation. MET (2 intervals), number of inseminations (4 intervals, 1,2,3 and > 3 ) and season (4 intervals, starting with D e c e m b e r - - F e b r u a r y ) were i n d e p e n d e n t variables, and milk p r o d u ctio n and age were covariates. Because the cost of delayed conception, long lactations and/ or dry periods is not incurred until very late in the lactation, it is rarely incurred by cows which were culled during the lactation. For this reason, the above described analysis of covariance model used to estimate the impact on CSI-D was p e r f o r m e d after excluding all lactations which ended with a cull. The cost of culled cows is considered in a different cost c o m p o n e n t . The data base was searched to obtain the num ber of reproductive examinations (rectal palpations) and inseminations received by each cow during her lactation. The annual questionnaire was used to determine the average price charged per reproductive examination, the average value of adult cattle and the average per cow slaughter value. The data base was also searched to determine which sires were used, so that the mean price per unit of semen could be calculated. The cost of prolonged CSI used for this study was $2.50 for each day a cow remained nonpregnant b e y o n d its herd average for lactations without metritis. This estimate was derived from the work of several investigators (Lineweaver, 1975; Lawson, 1978; Olds et al., 1979; Ferris, 1985).

Effect on milk production All of the 22 herds studied participated in the Michigan Dairy Herd Imp r o v e m e n t Association (DHIA) for the duration of the study. As such, we were able to obtain the p r o d u c t i o n records for each cow on computer tape and enter them into our data base manager. Data obtained from DHIA in this manner included the m o n t h l y milk weights, the last (lactation final) estimate of 305 mature equivalent (305 ME) milk product i on and the 305 ME milk p r o d u c t i o n from the previous lactation. To adjust for herd differences, a deviation from herd average was calculated for the 305 ME milk p r o d u c t i o n variable (305 ME-D). 305 ME in the previous lactation was compared between lactations with MET and lactations w i t h o u t MET by use of St ude nt 's t-test. Analysis of covariance was used to assess the possible c o n f o u n d i n g effects of lactation length, CSI-D, age and season in determining the effect of MET on 305 ME-D.

Effect of culling Calculation o f the attributable risk (Schwabe et al., 1977) associated with MET provided a m e t h o d of estimating how many culls could be attributed to MET. Because older cattle could have been at higher risk of both being culled and o f developing MET, the cull rates for MET and non-MET cattle

239

were dard cows herd

age-adjusted using seven age groups and all 3773 lactations as the stanpopulation (.Schwabe et al., I977). The mean market value for milking and the mean slaughter value of milking cows were obtained for each on the annual questionnaire.

Association with RP Because the FAHRMX data were recorded daily as the events occurred, the relative risk and attributable risk of RP as a contributing factor for MET could be calculated directly. Use of the odds ratio as an estimate of the relative risk was not deemed necessary (Fox et al., 1970; Schwabe et al., 1977). Medication and milk witholding costs Twenty-one different medications used to treat MET were studied with regard to dosage and recommended milk withholding time. The 1980 retail price list of the Veterinary Clinic Center, Michigan State University was used to determine the cost of each medication. The recommended number of days of milk to be discarded following administration of each of the medications was multiplied by the nearest DHIA daily kg of milk production for the cow which was treated, providing an estimate of the amount of milk which was withheld from sale. We estimated that approximately half of the value of this milk was salvaged by feeding it to calves. RESULTS

Of 4069 lactations in the FAHRMX data base, 296 (7.3%) were excluded because of error regarding inseminations (158), or lactation length (138). This 70 63

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left 3773 lactations for study. The lactational incidence rate of MET was 18% ( 6 7 9 / 3 7 7 3 ) and for RP was 7.1% ( 266/ 3 773). The lactational incidence rate for MET varied widely from 3% to 45% among the 22 herds studied. The lactational incidence rate for RP also varied widely, from 2% to 18%. MET was diagnosed most f r e que nt l y between 11 and 20 days post partum. It was less c o m m o n during the summer m ont hs (Figs. 1 and 2), although this distribution was not significant (Walter and Elwood, 1975). There was no clear pattern of association between age and occurrence of MET, but older cows did have a higher incidence of RP (P = 0.0001, by Chi square) (Table I). TABLE I F r e q u e n c y o f metritis and r e t a i n e d p l a c e n t a diagnosis by age Age*

Number of lactations

Lactations with metritis

Metritis lactational i n c i d e n c e (%)

< 3 3 4 5 6 7 8

1411 883 610 364 216 139 150

276 136 93 81 39 28 26

19.6 15.4 15.2 22.3 18.1 20.1 17.3

59 71 47 28 16 23 22

4.2 8.0 7.7 7.7 7.4 16.5 14.7

Total

3773

679

18.0

266

7.1

*Age in years at the start o f t h e lactation. * * S i g n i f i c a n t at P = 0.02, by Chi-Square with 6 df. * * * S i g n i f i c a n t at P = 0 . 0 0 0 1 , by Chi-Square with 6 df.

Lactations w i t h RP

RP lactational incidence (%)

241 TABLE II Economic impact of metritis complex Cost component

Lactations with metritis

Lactations with- Total out metritis loss

Number of lactations 679 3094 Culling Unadjusted cull rate 26.4% 20.5% Age-adjusted cull rate 26.6% 20.5% Relative risk, culling 1.3 (26.6/20.5) Attributable risk, culling 6,1 (26.6-20.5) Treatment Medication costs Discarded milk (half price) Reproduction Mean reproductive exams 4.3 3.1 Mean inseminations 2.0 1.8 Mean CSI* 146 128 Mean CSI, dev, from +16 - 3 herd mean Mean CSI, dev. from herd +12.6 - 2.5 mean, culled cows excluded Mean CSI, dev. from +5.8 -1.2 herd mean, adjusted** (7 days × $2.50 x 500 lactations) Total cost associated with metritis (679 lactations)

Loss/affected lactation

$41 400

$60.97

$ 1864 $16 194

$ 2.74 $23.85

$2037 $2037

$3.00 $3.00

$8750

$12.89

$72 282

$106

*Number of days from calving to successful insemination. **Adjusted for confounding effects of numbers of inseminations, season, milk production and age by use of analysis of covariance. Culled cows excluded.

Impact on reproductive efficiency (Table II) The n u m b e r o f d a y s post p a r t u m w h e n the first r e p r o d u c t i v e e x a m i n a t i o n was actually p e r f o r m e d ranged f r o m 21 to 46 days, with a m e a n of 32 days. The CSI-D was available for 96% ( 3 6 0 8 / 3 7 7 3 ) o f the lactations. The analysis o f c o v a r i a n c e m o d e l used CSI-D as the d e p e n d e n t variable and MET, season and n u m b e r o f inseminations as the i n d e p e n d e n t variables. Age and 305 ME-D were covariates (Table III). All main effects were significant (P < 0 . 0 0 4 ) with little evidence t h a t interactions were m e a n i n g f u l (P > 0.05). Each covariate was also significant (P < 0.002). The effect o f M E T (P = 0 . 0 0 1 ) on CSI-D was r e d u c e d f r o m 15.2 t o 7.0 days after adjusting for the effects o f the o t h e r variables in the m o d e l b y the multiple classification analysis t e c h n i q u e . An R 2 value of 0.47 indicated t h a t a sizable p o r t i o n o f the variability in the d e p e n d e n t variable c o u l d be explained by o u r statistical m o d e l .

242 TABLE III Results of analysis of covariance. Dependent variable = CSI-D. Source of variation Covariates Age 305 ME-dev. from herd ave.

Mean square

Sig. of F

16 720 399 956

0.002 0.001

18 761 7545 1 190 000

0.001 0.004 0.001

2-way interactions 3-way interactions

2347 1125

0.138 0.734

Residual

1673

Main effects Metritis Season Number of inseminations

Multiple R" = 0.47 On the basis o f the q u e s t i o n n a r i e c o m p l e t e d b y the f a r m e r s , it was determ i n e d t h a t t h e m e a n charge per c o w for a v e t e r i n a r y r e p r o d u c t i v e e x a m i n a t i o n was $2.50. T h e m e a n cost f o r an a m p u l e or s t r a w o f s e m e n was $15.00. In T a b l e II, l a c t a t i o n s with M E T are c o m p a r e d to l a c t a t i o n s w i t h o u t M E T o n t h e basis o f t h e n u m b e r of r e p r o d u c t i v e e x a m i n a t i o n s given a n d t h e n u m b e r o f i n s e m i n a t i o n s received.

Impact on production (Table II) T h e 305 ME was available for 91% o f the p r e v i o u s l a c t a t i o n s for c o w s in their s e c o n d or greater l a c t a t i o n . F o r these p r e v i o u s lactations, c o w s w i t h M E T in t h e c u r r e n t l a c t a t i o n did n o t d i f f e r significantly in 305 ME-D f r o m cows w h i c h did n o t have M E T (P = 0.15, b y S t u d e n t ' s t-test). T h e 3 0 5 ME was available f r o m t h e Michigan D a i r y H e r d I m p r o v e m e n t A s s o c i a t i o n ( D H I A ) f o r 92% ( 3 4 7 4 / 3 7 7 3 ) o f t h e lactations. L a c t a t i o n s with M E T h a d a m e a n 305 ME-D o f - 1 2 0 . 6 kg ( - 2 6 8 lb), c o m p a r e d to 26.6 kg (59 lb) f o r l a c t a t i o n s w i t h o u t M E T (P = 0.02, b y S t u d e n t ' s t-test). An analysis o f c o v a r i a n c e m o d e l was d e v e l o p e d w i t h 305 ME-D as t h e d e p e n d e n t variable and with age (7 intervals), season (4 intervals) a n d M E T (2 intervals) as t h e i n d e p e n d e n t variables (Table IV). L a c t a t i o n length a n d CSI-D were covariates. Each c o v a r i a t e was significant at P = 0.001. T h e m a i n effects o f age a n d season w e r e also significant at P = 0 . 0 0 4 and P = 0 . 0 0 1 , respectively. T h e m a i n e f f e c t s for M E T w e r e n o t significant (P = 0.09). T h e r e was little evid e n c e t h a t i n t e r a c t i o n s w e r e m e a n i n g f u l (P > 0.1). An R 2 value o f 0.08 ind i c a t e d t h a t a small p o r t i o n o f t h e v a r i a b i l i t y in t h e d e p e n d e n t variable c o u l d be e x p l a i n e d b y o u r statistical m o d e l .

243 TABLE IV Results of analysis of covariance. Dependent variable = 305 ME, dev. from herd average Source of variation Covariates CSI-D Lactation length

Mean square

Sig. of F

130 000 000 1 840 000 000

0.001 0.001

30 500 000 55 100 000 26 900 000

0.004 0.001 0.091

2-way interactions 3-way interactions

7 520 000 7 330 000

0.756 0.726

Residual

9 410 000

Main effects Age Season Metritis

Multiple R 2 = 0.08

Culling (Table H) E i g h t h u n d r e d a n d t h i r t e e n (21.5%) o f the 3 7 7 3 l a c t a t i o n s which we studied w e r e t e r m i n a t e d b y a cull, o c c u r r i n g at a m e a n o f 220 + 103 d a y s p o s t p a r t u m . L a c t a t i o n s with M E T w h i c h w e r e culled were t e r m i n a t e d at a m e a n o f 229 ± 105 d a y s p o s t p a r t u m . T h e age-adjusted cull r a t e was 26.6% for l a c t a t i o n s w i t h M E T , and 20.5% for l a c t a t i o n s w i t h o u t M E T . T h e relative risk o f being culled was t h e r e f o r e 1.3 (26.6 + 20.5) for lactations with MET. An a t t r i b u t a b l e risk of 6.1% (26.6 - 20.5) indicates the p e r c e n t a g e o f culled l a c t a t i o n s w h i c h c o u l d be a t t r i b u t e d to M E T . T h e r e f o r e , 41.4 (0.061 X 6 7 9 ) culls c o u l d be a t t r i b u t e d to M E T . Because the c o s t of the average c o w was $ 1 0 0 0 ( m a r k e t value m i n u s slaughter price), the average M E T l a c t a t i o n was a s s o c i a t e d w i t h a $ 6 1 . 0 0 (41.4 X 1 0 0 0 + 6 7 9 ) loss d u e to culling.

Medication and milk withholding costs T h e t o t a l c o s t o f m e d i c a t i o n used to t r e a t M E T was $1864. This c o m e s to $ 2 . 7 5 p e r a f f e c t e d lactation. A m e a n o f 102 kg (227 lb) o f m i l k was disc a r d e d f o r each r e p o r t o f MET, r e p r e s e n t i n g 3.8 d a y s o f d i s c a r d e d milk. This was a m e a n cost o f 168 kg (374 lb) o f m i l k lost for each l a c t a t i o n a f f e c t e d w i t h M E T . Using a price f o r m i l k o f $ 2 7 . 8 0 p e r 100 kg, this r e p r e s e n t s an e c o n o m i c loss o f $32 3 8 8 or $ 4 7 . 7 0 per l a c t a t i o n a f f e c t e d with MET. If a b o u t o n e h a l f o f t h e value o f this m i l k is salvaged b y feeding it to calves, the t o t a l e s t i m a t e d loss is a p p r o x i m a t e l y $16 194.

Association with RP F o r t y - f o u r p e r c e n t ( 1 1 7 / 2 6 6 ) o f l a c t a t i o n s w h i c h b e g a n with a RP even-

244 tually developed MET during the lactation. In contrast, only 16% of the lactations without RP developed MET. The relative risk for developing MET of cows with RP is therefore 2.75 (44/16), and the attributable risk is 28% (P < 0.0001, by Chi square). DISCUSSION The lactational incidence rate of MET in Ontario has been reported by Dohoo et al. (1983), Martin et al. (1982), and Erb and Martin, (1980a) to be 18.2, 11.1 and 13.8% for MET, and 8.6, 8.0 and 7.1% for RP, respectively. These rates were obtained from a nearby geographical location in Ontario and did not differ substantially from our findings in Michigan. It is recognized that all investigators studying MET, including ourselves, must accept a very subjective case definition of MET which is susceptible to differences among examining veterinarians. Development of a simple and usable standard case definition for MET is difficult because of the necessarily subjective nature of uterine palpation and the need to evaluate all findings in relation to the number of days post partum. Our findings agree with others which showed higher rates of RP with increasing age (Withers, 1957; Erb et al., 1958; Muller and Owens, 1974; Erb and Martin, 1980b; Segerson et al., 1981; Martin et al., 1982; Dohoo and Martin, 1984a). Some investigators have found increasing rates of MET in older age groups (Erb and Martin, 1980b; Martin et al., 1982). Others, like ourselves, found no clear pattern between MET and age (Erb and Martin, 1978; D o h o o and Martin, 1984a). Our study showed no clear seasonal pattern, other than non-significantly decreased rates in the summer. Much discrepancy exists in the literature regarding the effect of season on RP and MET. RP has been reported to be highest in the spring (Wetherill, 1965), summer and autumn (Erb et al., 1958) and winter (Muller and Owens, 1974). D o h o o and Martin (1984a), Erb and Martin (1978), Erb and Martin (1980b) and Roine and Saloniemi (1978) detected no significant seasonal effects, as did our study. MET was reported to be more likely in early autumn (Erb and Martin, 1978, 1980b) while Dohoo and Martin (1984a) found no seasonal pattern. There is general agreement that MET is most frequently diagnosed immediately post partum (Dohoo et al., 1983; Erb et al., 1984). The number of days post partum of first MET diagnosis depends on the number of days post partum at which the veterinarian routinely performs post partum reproductive examinations, and the subjective decision regarding when normal uterine involution should have occurred. It is therefore most reasonable to assume that MET incidence is highest immediately after calving, and declines steadily thereafter. A second peak of MET occurrence at > 180 days post partum was found by D o h o o et al. (1983) and possibly represents intensive follow-up of cows with severe reproductive problems.

245

Association with other diseases The role of RP in predisposing the bovine uterus to MET is well documented (Muller and Owens, 1974; Sandals et al., 1979; Erb et al., 1981a, b); Dohoo and Martin, 1984b) and was confirmed in this study.

Reproductive effects The 19-day difference in CSI-D between lactations with MET and those without MET is somewhat less than has been previously reported. Sandals et al. (1979) found a difference of 48 days, and Erb e t a l . (1981b) found a difference of 28 days. Dohoo and Martin (1984c) were unable to demonstrate a significant difference in CSI for MET with onset before 20 days post partum, but f o u n d differences of 26 days and 61 days for lactations with MET onset at 22--60 and > 60 days post partum, respectively.

Estimated reproductive economic impact For purposes of estimating the total economic impact of MET, it was necessary to exclude cows which were culled before comparing CSI in lactations with MET to lactations without MET. This was done for two reasons: (1) The cost of excessive CSI is realized by low production late in the lactation, or occasionally, through an excessively long dry period. Because the mean time of culling for cattle was at 229 days post partum, this cost was largely never incurred for cattle which were culled. (2) Cows which the farmer has already decided to cull are less apt to be examined by the veterinarian for MET and are less likely to be bred and diagnosed pregnant. These potential biases make it difficult to assess both the assignment into case and control group, as well as the assessment of CSI. When culled lactations were excluded as described above and adjusted for the confounding effects of number of inseminations, season, age and production, the difference in CSI-D between lactations with MET and lactations without MET dropped to 7 days.

Production economic impact The impact of MET on lactation final milk production appears to be either small or nonexistent. Our analysis of covariance model indicated that the initial finding of a decrease in 305 ME-D of 147 kg (327 lb) appeared to have been largely due to confounding with age, season, and lactation length. MET became an insignificant predictor of production when these confounding variables were included in our statistical model. As such, no effects on lactation final milk production were included when estimating the economic impact of MET (Table II). Erb et al. (1981a,b) were not able to demonstrate a significant detrimental effect of MET on production. Dohoo

246 and Martin ( 1 9 8 4 c ) f o u n d no d e t r i m e n t a l e f f e c t for l a c t a t i o n s w i t h M E T b e f o r e 21 d a y s p o s t p a r t u m , b u t did find a small d e t r i m e n t a l e f f e c t o f 2 . 0 - 4.6% on kg per d a y o f m i l k p r o d u c t i o n o c c u r r i n g later in the l a c t a t i o n . H o w e v e r , t h e y r e p o r t e d an overall increase in 305 d a y p r o d u c t i o n o f 1.2-2.9% associated w i t h MET. T h e value o f $ 2 . 5 0 f o r e x t r a d a y s o p e n incorp o r a t e s the r e d u c t i o n in m i l k p r o d u c t i o n w h e n m e a s u r e d o n a p e r d a y o f life basis. It w o u l d be r e a s o n a b l e to e x p e c t t h a t o n e o f the e f f e c t s o f M E T w o u l d be to increase 305 ME-D t h r o u g h a d e l a y e d c o n c e p t i o n . This i m p a c t on p r o d u c t i o n is i n c o r p o r a t e d into the $ 2 . 5 0 figure. As such, the finding of no e f f e c t on 305 ME-D was s o m e w h a t p r e d i c t a b l e .

Culling economic impact O u r overall culling rate o f 2 1 . 5 % agreed closely w i t h t h e 20% culling rate f o u n d b y D o h o o a n d Martin ( 1 9 8 4 d ) . T h e results o f Martin et al. ( 1 9 8 2 ) a n d C o b o - A b r e a u et al. ( 1 9 7 9 ) were similar to o u r s in t h a t t h e y also f o u n d a positive a s s o c i a t i o n b e t w e e n M E T a n d culling. D o h o o a n d Martin ( 1 9 8 4 d ) , h o w e v e r , did n o t find this a s s o c i a t i o n , a n d suggested t h a t this was b e c a u s e c o w s w h i c h t h e f a r m e r had a l r e a d y d e c i d e d to cull were never e x a m i n e d f o r MET, t h u s biasing t h e d a t a to a p p e a r as t h o u g h culled c o w s had less M E T t h a n t h e y a c t u a l l y did. O u r e s t i m a t e d value o f t h e d a i r y c o w s t h a t we s t u d i e d m a y be s o m e w h a t high d u e to t h e p r e p o n d e r a n c e o f registered c o w s in t h e herds using F A H R M X . T h e r e a d e r m a y wish to a d j u s t this e s t i m a t e to reflect less valuable livestock. ACKNOWLEDGEMENT This p r o j e c t was s u p p o r t e d , in part, b y a g r a n t f r o m t h e W.K. I<_ellogg F o u n d a t i o n o f B a t t l e Creek, Michigan.

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