Differences in Semen Values Between Summarized and Sampling Sires1

Differences in Semen Values Between Summarized and Sampling Sires 1 R. B. T H O R N T O N , G. L. H A R G R O V E , and L. W. SPECHT Department of Dairy and Animal Science The Pennsylvania State University University Park 16802

ABSTRACT

INTRODUCTION

Holstein cows in 19 Pennsylvania state-owned herds were used to estimate the additional value (premium) of semen from sires with progeny summaries versus the value of semen from pedigree-selected sampling sires available through artificial insemination. Within each herd, original cows were divided into two breeding groups; one group was bred to summarized sires a n d the other to sampling sires. An average of 8.4 inseminations was used to produce each milking daughter, based on an average of 5.39 services per live heifer calf and 64.3% survival of heifers from birth to enrollment on production testing. There were 457 first lactations of daughters of summarized bulls and 407 first lactations of daughters of sampling bulls. Differences of milk yield favored the summarized group in each of the first three lactations. Survival rates of cows from first to second and third lactations were 71.6 and 50.8%. Milk was priced at $30.65 with a $.35/.1% fat differential per 100 kg with 50% of gross returns assigned to cover costs of production. The premium for semen from summarized versus sampling sires should be at least $3.75. This premium is a minimum because investment in semen plus interest compounded at 5% per annum was required to be recovered in three lactations and no value was given for type, pedigree, or contribution to future generations.

Two groups of sires are available through artific;al insemination (AI) studs. The groups are 1) summarized bulls with United States Department of Agriculture (USDA) estimates of their transmitting abilities for production based on daughter performance, and 2) sampling sires, pedigree-selected for milk yield, which just have entered progeny testing. Because of their demonstrated transmitting abilities, summarized sires usually are selected by dairymen. However, sampling sires need to be included in breeding programs to accomplish the progeny testing that will provide summarized sires for the future. Because of the reluctance of dairymen to use sampling sires, financial rewards may be necessary to accomplish effective progeny testing. One suggested plan is to use the semen-pricing structure of AI studs. To develop an equitable semen-pricing plan, the economic difference in values of semen from summarized and sampling sires must be measured. Studies have investigated the value of semen (8, 14, 23, 32). Profitable investment in semen depends upon number of services per conception, number of live births per conception, sex ratio, and survival from birth to first calving. Return from this investment is dependent upon value of milk production of daughters, productive life of daughters, effect on production of future generations, and increased feed, labor, and veterinary costs to produce additional milk. In addition, invested capital should be discounted. Methods of comparing values of semen among summarized bulls have been proposed (5, 9, 13, 23). However, estimates of the difference in value of semen from summarized and sampling sires are unavailable. The purpose of the study was to determine the difference in value of semen from summarized and sampling sires as experienced by dairymen who use sampling sires to breed a portion of their herds. This difference in semen value will be referred

Received July 27, 1981. 1Journal No. 6260, Pennsylvania Agricultural Experiment Station, The Pennsylvania State University, University Park 16802. 1982 J Dairy Sci 65:1588-1595

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SEMEN VALUES to as the additional value (premium) that economically can be paid for semen from summarized sires versus the price of semen from sampling sires. DATA AND METHODS Project Design

A breeding project was started in 19 Pennsylvania state-owned Holstein herds to compare summarized and sampling sires for differences in values of their semen. All sires were available through two Pennsylvania-based AI units. The AI units retain the best of summarized sires on performance of their daug~ ters whereas sampling sires enter AI service on tl~eir pedigree merit. Within each herd, original females were divided into two breeding groups of equal size; one group was bred to summarized bulls and the other to sampling bulls. Bathgate et al. (4), in checking the division of cows, reported no significant differences between the two groups of cows for lactation milk and fat yields. Heifers in dam at project initiation were assigned to one of the two groups at first breeding. When a female was assigned to a breeding group, she and all of her project female descendants remained in that grOup. Thus, there were two breeding groups in each herd. Project breedings were started in the cooperating herds between October 15, 1972, and March 15, 1972. Summarized sires were selected on preferences of dairymen and reflect breeding choices of these cooperators independent of the research project. Young sires were used during a 2- to 6-mo sampling period. In addition, dairymen were encouraged to limit to six the number of cows pregnant to any one young sire. This resulted in young sires being used nearly at random. Breeding, calving, and disposal information was reported monthly. Premium

The following procedures were used in calculating the additional value of semen from summarized sires. Project data were used in estimating parameters for the first three factors (items 1 - 3 ) . Economic conditions (item 4) were specified by us.

Average Number of Inseminations to Produce a Milking Daughter. Number of services was obtained from the product of two figures,

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number of services per heifer calf living a~cbirth and reciprocal of the survival rate of heifers. Number of services per live heifer calf at birth was determined by counting all breedings of service periods started before January 1, 1979. A service period consisted of all inseminations from first breeding to subsequent calving or disposal. A similar period, ending 9 mo past the date for inseminations, was used in counting the number of live heifer calves at birth. Survival rate of heifers was comparison of number of live heifer calves at birth with the opportunity to enter production to number of those heifers enrolled on Dairy Herd Improvement Association (DHIA).

Difference in Milk Yield Between First Generation Daughters of Summarized and Sampling Sires for First, Second, and Third Lactations. Differences between breeding groups of first generation daughters for milk, fat, and protein yields were calculated f r o m lactation records acquired from Pennsylvania DHIA. Records were started before January 1, 1980. However, for daughters with records-in-progress on January 1, 1980, additional yield in that lactation produced between January 1, 1980, and the June, 1980, test date was included. Records initiated by abortion were not included. All lactation records were extended to 305 days and corrected for age by factors developed at the Pennsylvania State University (6) for comparison during editing of records and estimation of yield differences. However, after yield differences between groups were estimated, October factors of .769, .870, and .943 for milk and .775, .877, and .952 for fat were used to convert the first, second, and third lactation 305-2×-ME differences to actual production. The factors were based on average age at calving of 26, 39, and 52 m o for first,, second, and third lactations. Outlying records were eliminated by a two-step procedure. First, records were checked for yields that deviated by more than three standard deviations from the mean of the data set. Second, any record outside three standard deviations was eliminated from the data set if a) yield deviated by more than three standard deviations from the herd-group mean and days in milk were less than 100 or b) yield deviated by more than four standard deviations from the herd-group mean and days in milk were greater Journal of Dairy Science Vol. 65, No. 8, 1982

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THORNTON ET AL.

than 99. Records were rejected if the criteria were failed on milk, fat, or protein. The model for yield traits included herd, year-of-calving nested within herd, breeding group, and error. All effects, except error, were fixed. First lactation records were eliminated if either of the breeding groups was not represented in a subclass of herd and year of calving. Survival Rates o f Cows f r o m First to Second and Third Lactation. Survival rate of daughters from first to second lactation was determined by matching first lactations of daughters calving prior to November 1, 1978, with a second lactation file. Records were required to be initiated before November 1, 1978, to allow daughters an opportunity to calve the second time. The same procedure was used to determine the proportion of cows surviving from second to third lactation. The product of the two survival rates was the percentage of daughters surviving from first to third lactation. Economic Conditions. Based upon the November, 1980, blend price in Pennsylvania, milk was priced at $30.65/100 kg of milk testing 3.5% fat (20). A differential of $.35/.1% fat was used also. Fifty percent of gross returns from the sale of milk were assigned to cover costs of increased production (25). Smith (24) has argued that interest rates should be in the range of 3 to 5% to reflect a real interest rate that is free of the inflationary hedge built into current lending rates. The investment in semen, plus interest compounded at 5% per annum, was required to be recovered in three lactations. First payment was to be made upon completion of the first lactation (4 yr after purchase of semen). All available data were used in estimating parameters; therefore, number of observations varies from trait to trait. Thus, traits measured early in life had more observations than traits measured late in life. Several of the institutional herds were disbanded after the beginning of the project. When this occurred, animals either were dispersed to the public or were transferred to other project herds. In addition, some project animals were transferred among operating herds. In a supplementary study, conception rates on first service of summarized and sampling group bulls were 46.1 and 46.5%. These rates were on 3460 and 3095 first services to sumJournal of Dairy Science Vol. 65, No. 8, 1982

marized and sampling sires. Therefore, with no significant difference between sire groups for fertility, data from both breeding groups were pooled in estimating number of services per live heifer calf and survival rate of heifers. Data were pooled to avoid any potential bias from selection against females in the young sire group. In addition, to avoid the same possible bias, pooled data from both breeding groups were used in determining the survival rate of cows between lactations. The equation used to calculate the extra value of semen from the summarized sires was: ([(PR× ND)(I+I) 4 --P1](I+I)--P2) (1 + I) -- P3 = 0 where PR = premium per dose of semen that can be recovered from increased income based on milk production, ND = number of doses of semen to produce each milking daughter; I = interest charged per annum on remaining debt (.05); P1, P2, P3 = payments to be made at end of first, second, and third lactations based on extra value of milk above production costs and considering the proportion of daughters surviving to the respective lactation (1.00, .716, and .508).

RESULTS Number of Inseminations to Produce a Milking Daughter

Number of inseminations to produce each milking daughter was 8.4 (Table 1). This number was the product of 5.39 services per live heifer calf at birth and the reciprocal of a 64.3% heifer survival rate. The range by herd for the number of services per live heifer calf was 3.55 to 8.19, and the range by herd for heifer survival was 42.4 to 80.1%. Yield Differences

Least squares means for 305-2×-ME lactation yields of milk, fat, and protein are by breeding groups in Table 2. Differences in milk yield between breeding groups also are listed. Differences of milk and protein were significant in first lactation (P<.05) and in second lactation (P<.10). All other differences, pooled over years, were not significant (P>.10).

SEMEN VALUES TABLE 1. Number of inseminations to produce milking daughters. Number of inseminations per live heifer calf at birth Services Live heifers at birth Services per live heifer calf

15,181 2,817 5.39

Survival rate of heifers from birth to enrollment on DHIA Available heifersa Heifers with first lactation records Survival rate (%)

1,837

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In m o s t years, c o w s o f the s u m m a r i z e d group had greater yields t h a n c o w s o f t h e sampling group (Table 2). However, first lactation r e c o r d s initiated in 1975 and 1 9 7 9 b y s u m m a r i z e d d a u g h t e r s were l o w e r in milk yield t h a n t h e c o m p a r a b l e first l a c t a t i o n r e c o r d s o f sampling daughters. During these years, n u m b e r s o f o b s e r v a t i o n s w e r e small. T h e same t r e n d o c c u r r e d f o r t h e 1st y r t h a t s e c o n d (1976) and t h i r d l a c t a t i o n (1977) r e c o r d s w e r e available.

1,182 64.3 Survival Rate of Cows Between Lactations

Number of inseminations used to produce a milking daughter

8.4

aAvailable heifers were heifers that had the opportunity to enter production (birth dates before December 1, 1977). Number of available heifers differs from number of live heifers at birth because counts cover different periods.

Percentages o f first g e n e r a t i o n daughters surviving f r o m first t o s e c o n d and f r o m s e c o n d t o t h i r d l a c t a t i o n s are in Table 3. O f the 729 c o w s with the o p p o r t u n i t y to calve a s e c o n d t i m e (first calving b e f o r e N o v e m b e r 1, 1978), 522 or 71.6% had a s e c o n d lactation. T h e rate

TABLE 2. Least squares yield means and differences of first generation daughters of summarized and sampling group bulls by lactation number and year-of-calving,a

Year

N

Summarized group Milk Fat

Protein

1975

1979

18 149 b 144 95 51

8196 8371 8712 8579 7793

310 310 313 314 291

263 264 279 264 244

Pooled

457 b

8413

310

265

1976 1977 1978 1979

5 101 108 70

8574 9040 8853 8650

363 318 326 315

292 294 280 275

Pooled

284

8882

325

286

Sampling group Milk Fat

Protein

13 141 c 116 c 83 54

8497 8171 8396 8414 7936

317 304 301 316 305

265 259 269 260 245

--301 200 316 e 165 --143

407 d

8244

307

260

169 e

5 97 c 91 42

9415 8763 8491 8658

330 319 312 326

306 287 270 276

-841 277 362 e -8

235 c

8649

321

280

233 f

N

Milk yield difference

Firstlactation 1976

1977 1978

Secondlactation

Thirdlactation 1977 1978 1979 Pooled

3 59 68

7888 8985 8456

316 337 304

267 280 266

3 59 62

8775 8811 8378

324 335 301

278 276 266

-887 174 78

130

8682

321

273

124

8585

319

271

97

aMeans and differences in lactation yield are 305-2×-ME. Difference equals summarized group means minus sampling group. bN for protein is three less than indicated because of missing protein data. CN for protein is one less than indicated because of missing protein data. dN for protein is two less than indicated because of missing protein data.

ep<.05. fP<.lO. Journal of Dairy Science Vol. 65, No. 8, 1982

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THORNTON ET AL.

TABLE 3. Survival rates of cows from first to second and from second to third lactations. Number availablea

Number with later lactation

Survival (%)

First lactation to second lactation

729

522

71.6

Second lactation to third lactation

371

263

70.9

acows were required to calve before November 1, 1978, to allow the potential for a later calving.

f r o m second to third lactation was 70.9%. The c o m b i n e d product, percentage of cows surviving f r o m first to third lactation, was 50.8%. Percentage of cows in each breeding group remained stable f r o m lactation to lactation. Percentages of summarized daughters were 52.9, 54.7, and 51.2% for first, second, and third lactations. Premium

With the use of estimates of parameters and previously defined procedures, the additional value of a unit of semen f r o m summarized sires c o m p a r e d with the value of a unit f r o m sampling sires was $3.75. This value is based on 5% per annum interest. Intermediate statistics used to determine the p r e m i u m are in Table 4.

DISCUSSION

Interpretation of the p r e m i u m requires knowledge of m e r i t for milk yield of sires in the summarized group. This knowledge is useful in assessing the average merit of summarized sires to which y o u n g sires were compared. Merit of sires was estimated by tabulating U S D A Predicted Differences for milk yield (PDM). F r o m m o s t recent available sire summaries, July, 1980, the average PDM of sires of the first lactation daughters in the summarized group was 430 kg (Table 5). The average PDM of all active AI Holstein sires is available for the latter years of the project (21) and in Table 5 for comparison. A p p r o x i m a t e l y 3 y r elapse b e t w e e n semen usage and subsequent calving dates of

TABLE 4. Values pertinent to the calculation of first, second, and third payments, a

Milk (kg)

Summarized group Fat Value % Fat (kg) ($)

Milk (kg)

Sampling group Fat Value % Fat (kg) ($)

Payment b (S)

First lactation Mature equivalent Actual

8,413 6,470

3.71

310 240

2,030.61

8,244 6,340

3.75

307 238

1,998.69

15.96

Second lactation Mature equivalent Actual

8,882 7,727

3.69

325 285

2,419.71

8,649 7,525

3.74

321 282

2,369.62

17.93

Third lactation Mature equivalent Actual

8,682 8.187

3.73

321 306

2,575.22

8,585 8,096

3.75

319 304

2,552.26

5.83

aActual production is obtained from the mature equivalent figures by dividing by the appropriate age-monthof-calving factors. Values ($) are based on $30.65/100 kg milk with $.35/.1% fat differential. bpayments ($) are the value ($) differences between the group multiplied by .50 and multiplied by the survival rate for the respective lactations. Journal of Dairy Science Vol. 65, No. 8, 1982

SEMEN VALUES

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TABLE 5. Average Predicted Difference Milk of sires of first lactation summarized and sampling group daughters by year of daughter's calving,a

Year 1975 1976 1977 1978 1979

Pooled

Active AI siresb PDM

N~

.... .... 167 189 198

12 30 36 26 20

18 149 144 95 51

310 431 435 415 482

10 41 43 41 32

13 141 116 83 54

193 196 257 258 299

117 235 178 157 183

61

457

430

123

407

240

190

Summarized group N~ PDM

N~

Sampling group N2 PDM

PDM difference

aMeans and differences in average PDM (kg). PDM difference equals summarized group minus sampling group. N1 is the number of sires. N2 is the number of daughters. bActive AI sires are Holstein sires from Table 1 of (21). Values are from the first available summary of the year 3 yr prior to the year of calving (first column).

resulting daughters. Thus, 1977 calving corresponds to 1974 sire selection. Comparison of PDM of sires of summarized-group daughters in this project and PDM of all active AI sires reveals an average selection superiority of more than 250 kg. Apparently cooperating dairymen were applying substantial selection pressure on PDM in choices of summarized sires. By July, 1980, USDA sire summaries were available for all sampling sires. Therefore, the average PDM of summarized and sampling sires could be compared to quantify the competition between sire groups. The average PDM of the sires of the first lactation sampling group daughters was 240 kg. The difference in average PDM between the sires of summarized and sampling groups was 190 kg. The average PDM of sampling bulls compared to all active AI sires, for those years where both figures are listed (Table 5), indicates sampling sires were superior. The distribution of number of sires and number of daughters per sire differed between the two breeding groups. The 457 first lactation summarized group cows were sired by 61 bulls. The range was from 1 to 56 daughters per sire for first lactations of summarized cows. These compared with 123 sires of 407 first lactations of sampling daughters with a range of 1 to 13 daughters per sire. No one young sire was used excessively in the project. Services occurred from late 1972 through early 1977. Differences between daughter groups of 169,

233, and 97 kg in first, second, and third lactation milk yield are generally larger than in the literature. In Norway, Syrstad (30) reported that the difference of milk yield per lactation between 53,029 daughters of progeny-tested sires and 24,672 daughters of sampling sires was 36 kg. McCraw et al. (12) studied 109,589 DHIA first lactation records from 1,928 Holstein herds to compare summarized, sampling, and natural-service sires. Daughters of young sires averaged 84 kg less milk per lactation than daughters of summarized sires but produced 103 kg more milk per lactation than daughters of natural-service sires. However, Legates and Myers (11) in 1970 found that daughters of sampling sires produced 99 kg more milk in first lactation than their herdmates. The 8.4 inseminations used to produce a milking daughter is higher than earlier estimates. Everett (9), Pearson (18), and Shook and Sendelbach (23) used six services per milking daughter in their determinations of the value of semen. However, the number of services used is dependent upon, among other factors, conception rates and survival of heifers from birth to enrollment of DHIA. In the supplementary study on first service conception rates, the rates of 46.1 and 46.5% for summarized and sampling sires were within the usual range of 45 to 60% (7, 19, 22, 26). These results indicate that other factors are the cause of the high number of inseminations per milking daughter. The project heifer survival rate of 64.3% is Journal of Dairy Science Vol. 65, No. 8, 1982

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THORNTON ET AL.

comparable to the 68.8% reported by Moore and Richardson (16). Numerous other reports of survival are available (1, 2, 10, 15, 17, 27). However, cited references involved a specific segment of the animal's life span, such as birth to 3 mo, and are not directly comparable to our study. In general, other reports indicate that survival rates of less than 70% are low. Like heifer survival, survival rates of 71.6 and 50.8% for cows from first to second and third lactations are below other reports. Percents range from 91 to 78 and from 86 to 65 for survival of cows from first to second and third lactations (1, 15, 28, 29). As herds merged, complete assimilation of all cows may not have occurred, and voluntary culling of cows m a y have increased because of overcrowding of facilities. Herd management also affects the survival rate of cows between lactations. In a preliminary analysis of the data (31) the premium was $4.65. There are three reasons for the change of the premium. First, an additional 80 first lactation records were added to the data set that was analyzed in the preliminary study. These records were initiated in 1979. Second, in the preliminary study, the difference of first lactation yield was assumed to apply to differences of second and third lactations. However, differences of second and third lactations of 233 and 97 kg differ from first lactations. Third, milk prices in the preliminary study valued milk only and ignored any difference in test between the two groups. Daughters of sampling sires had a higher percentage fat than those of summarized sires. Thus, the premium dropped to $3.75 as additional observations and estimates were included in calculations. The premium is a minimum; several factors contribute to this condition. First, the investment in semen was required to be recovered in three lactations. If differences of milk yield existed in fourth and later lactations, the premium would increase as additional returns were received from the sale of milk. However, the average cow remains in the milking herd less than 4 yr (3). Second, no value was given to the effect of sires on milk yield of future generations. Some percentage of superiority of summarized sires should appear in future generations. Thus, the return from investment in semen from summarized sires would increase in comparison to the return from investment in Journal of Dairy Science Vol. 65, No. 8, 1982

semen from sampling bulls. However, this return occurs several years after initial investment. Third, any potential type, pedigree, or merchandizing differences between daughters of summarized and sampling sires were not measured. Only returns from differences in lactation milk and fat yields were included in calculating the premium. Differences in protein yield were significant in first and second lactation but were not included as most dairymen are not paid for protein. The premium will vary among individual dairymen. Dairymen operate under their own economic conditions and goals. F o r example, the relationship between milk prices and production costs differ from dairyman to dairyman. Herd management influences survival rates of both heifers and cows. In addition, differences exist among dairymen as to the relative importance of type characteristics, pedigree, and sales potential. These preferences influence sire selection procedures. However, for dairymen with herd management and sire selection standards similar to those of cooperating herds, the premium is appropriate under the assigned economic conditions. Because dairymen often use summarized sires as their standard for comparison, a financial incentive for using sampling sires may be necessary. The additional value of semen from summarized sires is also the value of the incentive that should be given to dairymen to encourage them to participate in progeny testing programs. Therefore, a reward of $3.75 per unit of semen from young sires should be offered to dairymen who participate in sampling AI young sires.

SUMMARY

Based on a comparison of daughters sired by AI summarized and sampling bulls, the additional value of a unit of semen from summarized bulls compared to a unit from sampling bulls was $3.75. The average PDM of the sires of first lactation summarized group daughters was 430 kg (services were between 1972 and 1977). The summarized sires in this project were more than 250 kg above the average PDM of all active AI sires. Estimates from the breeding project were 8.4 inseminations per milking daughter; first, second, and third lactation milk yield differences o f 169, 233, and 97 kg; and

SEMEN VALUES survival rates o f 71.6 and 50.8% f o r d a u g h t e r s f r o m first to s e c o n d a n d t h i r d lactations. The i n v e s t m e n t in semen, plus i n t e r e s t c o m p o u n d e d at 5% per a n n u m , was r e q u i r e d to be r e c o v e r e d in t h r e e lactations. With the e s t a b l i s h m e n t o f the e c o n o m i c d i f f e r e n c e in t h e value o f s e m e n f r o m summ a r i z e d and sampling sires, s e m e n - p r i c i n g within t h e AI i n d u s t r y can be used to help a c c o m p l i s h p r o g e n y testing o f y o u n g bulls. A financial i n c e n t i v e o f $3.75 per u n i t o f s e m e n should be given t o d a i r y m e n t o e n c o u r a g e t h e m t o use AI sampling sires.

REFERENCES

1 Allaire, F. R., H. E. Sterwerf, and T. M. Ludwick. 1977. Variations in removal reasons and culling rates with age for dairy females. J. Dairy Sci. 60:254. 2 Appleman, R. D., and F. G. Owen. 1975. Breeding, housing, and feeding management. J. Dairy Sci. 58:447. 3 Asdell, S. A. 1951. Variation in amount of culling from DHIA herds. J. Dairy Sci. 34:529. 4 Bathgate, J. M., G. L. Hargrove, J. M. Buckalew, and L. W. Specht. 1976. First service conception rates of proven and sampling Holstein sires. Paper 71st Annu. Mtg. Am. Dairy Sci. Assoc., Raleigh, NC. 5 Cole, T. C., and J. M. Rakes. 1979. Use this formula for determining value of dairy semen. Hoard's Dairyman 124: 396. 6 Cooper, J. B. 1980. Age and month-of-calving adjustments and extension factors for milk, fat, and protein yields. M. S. thesis, The Pennsylvania State Univ. 7 Davidson, J. N., and T. B. Farver. 1980. Conception rates of Holstein bulls for artificial insemination on a California dairy. J. Dairy Sci. 63:621. 8 Everett, R. W. 1975. Income over investment in semen. J. Dairy Sci. 58:1717, 9 Everett, R. W. 1977. Profitable sire selection starts with economic analysis. Dairy Herd Manage. 14(1):14. 10 Hartman, D. A., R. W. Everett, S. T. Slack, and R. G. Warner. 1974. Calf mortality. J. Dairy Sci. 57:576. 11 Legates, J. E., and R. M. Myers. 1970. Results from young sire sampling. J. Dairy Sci. 53:383. (Abstr.) 12 McCraw, R. L., K. R. Butcher, and B. T. McDaniel. 1980. Progeny tested sires compared with pedigree selected young sires. J. Dairy Sci. 63 : 1342. 13 McGilliard, M. L. 1978. Guidelines for purchasing semen. J. Dairy Sci. 61:1680.

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14 McGilliard, M. L. 1978. Net returns from using genetically superior sires. J. Dairy Sci. 61:250, 15 Meek, A. M . 196i. Survivorship in dairy cows. Ph.D. thesis, Iowa State Univ., Univ. Microfilms, Ann Arbor, MI. 16 Moore, E. D., and D. O. Richardson. 1976. Breeding and management research at the Lewisburg Dairy Experiment Station. Jersey J. 23(16):29. 17 Oxender, W. D., L. E. Newman, and D. A. Morrow. 1973. Factors influencing dairy calf mortality in Michigan. J. Am. Vet. Med. Assoc. 162:458. 18 Pearson, R. E. 1976. Managing herd breeding. Page 137 in Natl. Workshop Genet. Improvement of Dairy Cattle. 19 Pelissier, C. L. 1970. Factors contributing to low breeding efficiency in dairy herds. J. Dairy Sci. 53:684. (Abstr.) 20 Pennsylvania Crop Reporting Service. 1981. Special dairy report. Pennsylvania Dept. Agric., Harrisburg, PA 81 : 1. 21 Powell, R. L., and G. R. Wiggans. 1981. Averages of predicted differences by summary date, breed, and bull status. US Dept. Agric. Dairy Herd Improvement Lett. 51(1):5. 22 Rounsaville, T. R., P. A. Oltenacu, R. A. Milligan, and R. H. Foote. 1979. Effects of heat detection, conception rate, and culling policy on reproductive performance in dairy herds. J. Dairy Sci. 62:1435; 23 Shook, G. A., and A. G. Sendelbach. 1975. How to determine the best semen buys. Hoard's Dairyman. 120:1002. 24 Smith, C. 1978. The effect of inflation and form of investment on the estimated value of genetic improvement in farm livestock. Anita. Prod. 26:101. 25 Smith, N. E. 1976. Maximizing income over feed costs: Evaluation of production response relationships. J. Dairy Sci. 59:1193. 26 Spalding, R. W., R. W. Everett, and R. H. Foote. 1975. Fertility in New York artificially inseminated Holstein herds in Dairy Herd Improvement. J. Dairy Sci. 58:718. 27 Speicher, J. A., and R. E. Hepp. 1973. Factors associated with calf mortality in Michigan dairy herds. J. Am. Vet. Med. Assoc. 162:463. 28 Stewart, H. M., E. B. Burnside, and W. C. Pfeiffer. 1978. Optimal culling strategies for dairy cows of different breeds. J. Dairy Sci. 61:1605. 29 Stewart, H. M., E. B. Burnside, J. M. Wilton, and W. C. Pfeiffer. 1977. A dynamic programming approach to culling decisions in commercial dairy herds. J. Dairy Sci. 60:602. 30 Syrstad, O. 1973. There is little difference between daughters of young bulls and those of progeny tested bulls. Anim. Breed. Abstr. 41:182. 31 Thornton, R.'B., G. L. Hargrove, and L. W. Specht. 1981. Additional value of semen from summarized bulls. J. Dairy Sci. 64(Suppl. 1):75. (Abstr.) 32 Young, C. W. 1972. Are top PD sires your best buy? Hoard's Dairyman 117:981.

Journal of Dairy Science Vol. 65, No. 8, 1982