Title	:	Effect of RBST in Commercial DairyHerds
Source	:	Monsanto Dairy Group
Author	:	Rueg, Fabellar,Hintz
Date	:	1998
Content	:

Effect of the Use of Bovine Somatotropinon Culling Practices in Thirty-two Dairy Herdsin Indiana, Michigan, and Ohio

P. L. RUEGG,*,1 A. FABELLAR,† and R. L. HINTZ†
*Monsanto Dairy Business,
2692 Blue Haven Ct., East Lansing, MI 48823
†Monsanto Dairy Business,
800 N. Lindbergh Blvd., St. Louis, MO 63167

ABSTRACT

Data from 5468 Holstein dairy cows in 32 herdswere used to determine the effect of the use of bovinesomatotropin (bST) on culling practices over a13-mo period. After an initial herd inventory, monthlyinformation regarding cow entry and exit from theherds was obtained by seven participating veterinarians.Culling was coded by farmers for the followingreasons: low production, reproduction, somatic cellcount, mastitis, sickness, dairy purposes, lameness,death, and other. In the control herds, cows were nottreated with bST during the trial. Adopter herds weredefined as herds that utilized supplemental bST for?25% of the cow-days during the trial. Mean herd useof bST in adopter herds was 38.6%. No difference inthe number of cows culled per cow-day at risk wasdetected between control and adopter herds (0.09 and0.11%, respectively). Amount of in-herd use of bSTwas unrelated to culling. No significant differenceswere determined between adopter and control herdsin the percentage of cows that were culled for any ofthe nine possible culling reasons. The results of thisstudy suggest that culling patterns in herds that usebST are unaffected for at least the first year afteradoption.

INTRODUCTION

The decision to cull dairy cows is complex and isaffected by many herd factors, including age of thecow, production, reproductive performance, milkprice, feed costs, market value of culled cows, andhealth of the individual cows (8). Culling is generallycharacterized as either voluntary (for low milkproduction or dairy sales) or involuntary (all otherreasons). A high percentage of involuntary cullingcan have a significant negative impact on net revenueper cow per year (14). Therefore, the impact of herdhealth and management decisions on culling hasreceived considerable attention (3, 9, 11).

The occurrence of many common dairy diseasesduring lactation increases the risk of culling (9, 11).Conversely, survival in dairy herds has been shown tobe positively related to milk production (9). Managementdecisions that increase the efficiency of milkproduction without adversely impacting cow healthshould enhance the opportunities for voluntary cullsand increased genetic gains.

The use of bST technology is common on manydairy farms. The ability of bST to enchance productionand the safety of the product are well established(2). Most evaluations of production and health effectsin cows receiving bST over multiple lactations havebeen single-herd studies with a limited number ofcows located in controlled research environments (4,6, 13). The approval of bST (Posilac?; Monsanto Co.,St. Louis, MO) in 1993 increased the opportunity forbroader field-based epidemiologic studies. The objectiveof this study was to determine the effect of theuse of commercial bST on culling practices in dairyherds.

MATERIALS AND METHODS

Data and Data Collection

Data from 32 commercial Holstein dairy herdswere used in this observational study. Herds werelocated in Indiana (n = 4), Michigan (n = 13), andOhio (n = 15). Dairy herds used in this study wereselected from farms involved in herd health programswith the cooperation of the private dairy veterinariansparticipating in this study. Eight veterinariansinitially agreed to participate in the study. Oneveterinarian relocated during the course of the studyand discontinued data collection from his herds.Veterinarians were asked to nominate adopter herds(herds that were using supplemental bST) and controlherds (herds that were not using bST) for participationin the study. Herds included in the study wererequired to contain >50 milking Holstein dairy cowsenrolled in the DHIA and were required to have arolling herd average milk production at the initiationof the study that was >7257 kg. Additional inclusioncriteria included the use of AI as the primary breedingprogram, no plans for major herd expansion, andno involvement in disease eradication programs basedon culling. Farms with seasonal calving programs orintensive rotational grazing programs were excludedfrom this study. Initially, 45 herds (23 adopter herdsand 22 control herds) were enrolled in the study.Seven herds (2 adopter herds and 5 control herds)did not have complete data available at the conclusionof the study and were not included in the final dataanalysis. Four of the excluded herds were assigned tothe veterinarian who relocated during the course ofthe study. Six additional herds changed their usagepattern of bST during the course of the study andwere excluded from the study (2 control herds beganbST treatment, and 4 adopter herds either discontinuedbST use or restricted the use of bST).

In July 1994, each herd was visited by a technician,and a baseline management survey was conducted.An initial inventory of lactating and dry cowswas completed, and the participating veterinarianreceived data collection forms. Beginning in August1994, veterinarians were requested to fill out amonthly form for each herd with the following information:number of cows (lactating and dry) at thebeginning and end of each month, a list of cows thatleft the herd each month and reasons for cow removal,a list of cows that entered the herd, and number ofcows that received bST. Data collection continuedthrough August 1995.

Usage patterns of bST on the study farms weredetermined by each farmer. Control herds (n = 13)contained cows that did not use bST during the trialperiod. Adopter herds (n = 19) utilized supplementalbST for ?25% of the cow-days during the trial. Cowdayswere calculated as the mean number of cows(milking and dry) each month multiplied by thenumber of days in the month (regardless of eligibilityfor bST use). Cows that were not eligible for bSTtreatment included nonlactating cows, cows prior to63 d of lactation, and cows that were approaching thedry period.

In-herd use ( IHU) of bST was calculated as thenumber of cows that was injected with bST divided bythe mean herd size per month or year (regardless oflactation status or eligibility for bST). Culling density( CD) was calculated as the number of cows(lactating and dry) culled per cow-day at risk forculling multiplied by 100. The CD was calculated foreach herd. Culling rate ( CR) was calculated as thetotal number of cows culled divided by the sum of thebeginning cow inventory and total cows added. Forcows that were culled, codes were assigned as to thereason for culling: production, reproduction, SCC,mastitis, sickness, dairy purposes, lameness, death,or other.

Data Analysis

Descriptive statistical analyses were conducted usingStatgraphics Plus? (16). Least squares analyseswere performed using PROC GLM of SAS (15). Datawith one record per herd were analyzed using a onewayanalysis of variance to test the effect of treatment.The level of significance used was P < 0.05.Power of the test for the proportion of cows culled foreach reason was calculated using SAS.

RESULTS AND DISCUSSION

Descriptive Results

The number of herds (n = 32) enrolled in thisstudy was comparable with previous culling studies[n = 35 in the study of Barkema et al. (1), n = 32 inthe study of Dohoo and Martin (9), and n = 34 in thestudy of Milian-Suazo et al. (11)]. At the initiation ofthe study, 5303 milking and dry cows were enrolled(1716 cows in control herds and 3587 cows in adopterherds); 2156 cows were added, and 1991 cows wereremoved from the herds for a net gain of 165 cows inthe 32 participating dairy farms. Of 7459 total lactations1991 lactations ended in culling (26.7%). Thisrate was slightly higher than CR from previousreports (9, 11). Dohoo and Martin ( 9 ) reported a19.3% CR in 32 Ontario dairies, and Milian-Suazo etal. (11) reported a 18.7% CR in New York dairies.Both of those studies (9, 11) were performed in smallherds, and Dohoo and Martin ( 9 ) commented thatthe CR reported in their study was most likely underestimatedbecause not all of the lactations initiatedwere followed to culling or to the subsequent calving.A recent survey that represented 83.1% of US dairyoperations was performed by the National AnimalHealth Monitoring System; a 23.5% CR was reported(12).

In our study, the final herd inventory consisted of5468 lactating and dry cows. The number of cowsadded per herd by treatment group was 4.2 for adopterherds and 6.5 for control herds. No differences (P= 0.69) in cow inventory changes were detected betweentreatment groups.

Herd selection criteria resulted in reasonably comparabletreatment groups that were representative ofwell-managed dairies (Table 1). At the initiation ofthe trial, no significant differences were detected betweentreatment groups in herd size, mean herd DIM,SCC, number of days dry, or days of lactation at AI(Table 1). At the beginning of the study, the rollingherd average for milk production for adopter herdswas higher than that for control herds (Table 1).This result was expected because most adopter herdswere utilizing bST prior to enrollment in the trial.Adopter herds also demonstrated some evidence ofsuperior replacement management. Age at first calvingwas 1 mo earlier for cows in adopter herds thanfor cows in control herds, and this difference approachedsignificance. At the completion of the study,rolling herd average milk production for adopterherds was higher than that for control herds (Table1).

As expected, a difference ( P < 0.05) in IHU of bSTbetween treatment groups was detected. Mean IHU ofbST was 0 for control herds and 38.6% (SE = 0.03)for adopter herds. The theoretical maximum IHU ofbST for treatment herds was estimated to be about65% because cows that were not eligible for bST wereincluded in the mean herd size calculation.

CR, CD, and bST Use

CD and treatment group. Various methods areused to calculate CR for dairy herds (1, 5, 7, 10, 17),making it difficult to compare CR among studies. Thenumerator is quite consistent and is generally thenumber of cows exiting the herd over a defined period.Denominators for CR vary considerably. A commonmethod of calculating CR is to divide the number ofcows culled over a defined period by the number ofcalvings during that period (1). Using this method,the numerator is not necessarily included in thedenominator. To account for changes in herd size overtime, the denominator is sometimes calculated as thenumber of cows currently in the herd minus one-halfof the number of cows sold (5). The mean milkingherd size for the defined period (17) or the number ofcows present on the test day is also sometimes usedas the denominator (7). Finally, the denominator forCR in some DHI reports is the number of cows currentlyin the herd plus the number of cows leavingthe herd in the last 365 d (10). Depending on themethod used, seasonal changes in herd size and herdexpansion may also affect the interpretation of CR.

In this study, when CR was defined as the numberof cows culled divided by the sum of the beginningherd inventory and the number of cows added to theherd, the CR were 24.6 and 27.2% for control andadopter herds, respectively. These differences werenot significant ( P > 0.10; Table 2). Culling rates inthis study were comparable with similarly calculatedrates reported in other studies (1, 5).

Culling density was 0.09 and 0.11% for controlherds and adopter herds, respectively ( P = 0.07; Table2). The CD was calculated rather than using themore traditional CR because CD more sensitivelyreflects current changes in herd inventory. The CDmeasures the number of cows culled per cow-day atrisk and is unaffected by herd expansion or seasonalchanges in cow numbers. Culling densities of 0.08 to0.12% correspond with approximately 30 to 45% CRin herds with stable cow populations. In this study,CD was normally distributed within a range between0.05 and 0.15% across all study herds (Figure 1). TheCD was consistent with expected culling practices forwell-managed herds and was unaffected by the use of Posilac?.

TABLE 2. Effect of bST use on herd culling density (CD) and culling rate (CR).1,2

Herd	CD		CR
	(%)
	LSM	SE	LSM	SE
CONTROL	0.09	0.005	24.6	1.19
ADOPTOR	.11	0.004	27.2	.99

1. Adopter herds (n = 19) were defined as herds that utilized supplemental bST for ?25% of the cow-days during the trial. Control herds (n = 13) were defined as herds that did not use bST during the trial. The CD was calculated as the number of cows (lactating and dry) that were culled per cow-day at risk for culling multiplied by 100. The CR was calculated as the total number of cows that were culled divided by the sum of the beginning cow inventory plus total cows added.

2. No differences were detected between treatment groups (P > 0.05).

CD and IHU of bST. No relationship existed between CD and IHU of bST ( P > 0.10; Table 3). In this observational study, no attempt was made to influence patterns of use of bST in any of the herds. The IHU of bST in adopter herds ranged from 25 to 64%. The wide range of values for IHU of bST was representative of the diversity of usage patterns on the study farms. In this study, there was no indication that herds that used bST extensively culled more cows than did herds that used bST less extensively or not at all.

TABLE 3. Culling density (CD) according to in-herd use (IHU) of bST.1

IHU of bST		CD<VAR>2</VAR>
		(%)
	(no.)	LSM	SE
0%	13	0.09	0.005
25 to 32%	7	0.11	0.007
33 to 48%	7	0.11	0.007
>48%	5	0.10	0.008

1. No differences were detected between treatment groups (P > 0.10).

2. Calculated as the number of cows (lactating and dry) that were culled per cow-day at risk for culling multiplied by 100.

Reasons for Culling

No significant differences were detected between adopter and control herds in the proportion of cows that were culled for any of the nine identified culling reasons (Table 4). A number of studies (1, 9, 11, 12) have analyzed reasons for culling. A comparison of reason-specific CR among studies is difficult because each study has identified different categories of reasons for culling. Given these differences, the reported reasons for culling in our study appeared to be reasonably consistent with past studies. The categories of reproduction (fertility) and low production were included in several studies. Although no statistical comparison was attempted, the proportion of cows that were culled for reproduction (23% of culled cows in both treatment groups) in our study appeared to be consistent with past reports [17% for Dohoo and Martin (9), 26% for Milian-Suazo et al. (11), and 27% for the National Animal Health Monitoring System (12)]. The percentages of cows that were culled for low production in previous studies [20% (9), 21% (11), and 22% (12)] were slightly higher than those observed in this study.

No significant differences in reasons for culling were identified, and the proportion of cows culled in the adopter and control herds was virtually identical for several of the reasons listed in this study. For example, the difference in the percentage of cows that were culled for reproduction, SCC, and lameness was <1.0% between adopter and control herds. The possibility of Type II errors (declaring no difference between groups when a difference does exist) must be considered whenever small numbers of herds are used as the experimental unit. In this study, the likelihood of detecting treatment effects was limited by the number of herds that were enrolled in the study and the small differences in culling percentages between groups. For data on the proportion of cows that were culled for each reason, the calculated power (likelihood of detecting a true treatment effect) ranged from 0.05 to 0.23. Because of the small differences between groups in the percentage of cows that were culled, an increase in the power to detect differences would require a very large study.

CONCLUSIONS

Herd selection criteria used in this study resulted in comparable treatment groups that were representative of well-managed dairy farms. Significant differences in CD and CR were not detected between adopter and control herds. The CD was not related to IHU of bST, and no significant differences were found between adopter and control herds in the proportion of cows that were culled for the nine identified culling reasons. This study suggests that culling patterns in herds that adopt bST are unaffected for at least the 1st yr after adoption of bST.

ACKNOWLEDGMENTS

The authors thank Byron Ramos for his diligence in performing the initial farm surveys and the participating veterinarians who supplied data for this study: Dick Wiley, Richard Meiring, Vern Lambright, Kevin Jeffers, Dale McKenzie, Paul Busman, and John Arbaugh.

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Source: Monsanto Dairy Group
Author: Rueg, Fabellar, Hintz