Effects of culling for male fertility in a dairy cattle population

ELSEVIER

Livestock Production Science 47 (1997) 211-219

Effects of culling for male fertility in a dairy cattle population Eva-Marie Stilhammar

*, Erling Strandberg, Jan Philipsson

Department of Animal Breeding and Genetics, Swedish University of Agricultural Sciences, P.O. Box 7023, S-750 07 Uppsala, Sweden

Accepted 23 September 1996

Abstract The objective was to study the effectiveness of various culling strategies for male fertility population and the consequences for the bull testing program. Nine different culling strategies were combinations of three levels of culling for both semen quality (sperm motility after non-return rate (NRR). The correlation between MOT and NRR used was 0.2 or 0.5. The culling strategies varied between 0.2 and 3.5%-units NRR increase in the current generation.

on the fertility in a dairy cattle were studied. These strategies freezing/thawing, MOT) and effect on NRR of the various

With the objective of progeny testing a fixed number of bulls for dairy traits every year, the more realistic strategies required testing 7-25% more bulls for male fertility and gave an increase in NRR of up to I%-unit. These strategies emphasized direct culling on NRR. A fertility test of 500 doses per bull based on a 56-day non-return rate was suggested. This will give a 10% more efficient use of the testing capacity than will using 1,500 doses per bull selected solely for semen quality without any other fertility test. 0 1997 Elsevier Science B.V. Keywords: Dairy cattle; AI; Male fertility; Non-return

rate; Sperm motility; Culling strategies;

1. Introduction Culling for bull fertility can be based on semen characteristics, such as sperm morphology and sperm motility, on the anatomy of the sexual organs, on libido or on fertility results, recorded as non-return rate of inseminated cows. Sperm motility after freezing/thawing and non-return rate have been used most commonly to measure indirectly fertility in bulls. Sperm motility can give a qualitative description of the freezability of an ejaculate and serve as a guarantee of minimum quality, if correlated to the total number of sperm cells inseminated (Sijderquist, 1991). Non-return rate is an indirect measure of the actual fertility, but has proved its value for monitor-

* Corresponding

author

Bull

fertility

ing bull fertility in artificial insemination (AI) services (Oltenacu and Foote, 1976; Sullivan, 1988; Killian, 1992; Schaeffer, 1993). Improved fertility among bulls can be achieved by (1) discarding individual semen ejaculates with inferior sperm characteristics within bull, (2) culling bulls due to generally poor semen quality, and/or (3) culling bulls with a low non-return rate. For the first two approaches the relationship between the semen characteristic used and non-return rate is a key quantity. Swedish data show that estimates of this relationship are weak when based on AI bulls already preselected for sperm motility (Stilhammar et al., 1994b). According to Linford et al. (1976) laboratory tests at that time were of little value for predicting fertility, but could be used for setting limits below which poor semen samples should be discarded. Amann (1989) asserted that there is a

0301-6226/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PI2 SO301-6226(96)01407-S

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relationship between different laboratory tests and fertility, but that it is rather dependent on the use of accurate and specific laboratory tests and fertility data. There are indications in the literature that the sperm population, on reaching the oviductal isthmus and ampullary-isthmus junction, the site of conception, is enriched in both viability and the proportion showing normal morphology, compared with the ejaculate evaluated in the laboratory (Saacke et al., 1994). Thus, non-return rate will better reflect the actual fertility result than will the assessed semen quality of the ejaculate to be used for insemination. Traditionally in Sweden, quite a large proportion of the bulls have been discarded based on semen characteristics but with a large variation between bull studs. In some AI studs 35-40% of the bulls which had passed the growth rate test were culled on semen quality, whereas in other studs only 5-10% were culled (Bemes, 1981; Stilhammar, 1984). The trend in Sweden has been towards accepting lower motility values in favour of monitoring the bulls for their non-return rate and culling of the worst bulls (H&d, 1994). If bulls are kept alive whilst being progeny tested, it is then possible to use the best progeny tested bulls intensively later. Consequently, it is important that these bulls have good fertility. For an efficient AI service it is important to apply such strategies that account for the consequences of culling for male fertility in an optimal way. The two most critical variables in the culling of bulls are the proportion of bulls culled due to the results only of laboratory semen tests and the proportion of bulls culled based on a non-return rate actually obtained. For several reasons, this investigation has been restricted to the study of these two variables. A third option could be, as previously mentioned, to discard only individual ejaculates showing poor semen quality but not cull the bulls. Bull studs, at least in Sweden, show no interest in this procedure (H&d, 1994; Sijderquist, 1994). Due to space limitations and costs they do not want to keep bulls alive if a high rate of culling of ejaculates for semen quality is anticipated. Practical experience and studies have also shown that, with a repeatability of 0.5-0.6 between ejaculates within bulls (StHlhammar et al., 1989), it is only if something extraordinary happens to the bull (e.g. a disease) that the quality will change drastically from one ejaculate to another.

Production Science 47 (1997) 211-219

It was also shown by Oltenacu et al. (19801, that culling of bulls would give rise to a smaller increase in the number of bulls for sampling than would culling of ejaculates within bull, given the same expected increase in fertility. The general practice is therefore to monitor semen quality after thawing as a guidance for early culling of the worst bulls and then discard semen only with temporarily impaired sperm quality and anticipated lowered fertility. Thus, the objective of this study was to establish the efficiency of various culling strategies among bulls for male fertility, based on sperm motility, chosen as a possible semen characteristic recorded, and non-return rate as a measure of fertility achieved. The consequences as regards overall fertility and involuntary culling of cows in a dairy cattle population and the requirements for performance testing of bulls were also to be examined.

2. Methods The various stages of culling are illustrated in Fig. 1. In the first stage, bulls undergoing performance testing are selected for conformation, growth rate and development of sexual organs. In Sweden, between 25% and 40% of the performance tested bulls, depending on the breed, are culled at this stage (B&strBm, 1994). Bulls proceeding to stage two are culled for low libido and inferior semen characteristics, mainly motility before and after freezing. According to Hitid (19941, lo-15% of the bulls are culled for these reasons. Generally speaking, semen characteristics corresponding to a motility of at least 45-50% are required. No specific fertility test is currently applied, but the bulls’ non-return rates are continuously checked. However, in the present study, the bulls kept in stage 2 were assumed to be tested and culled specifically for NRR in stage 3. This test is assumed to be based on 500 inseminations, giving an accuracy (the correlation between true and estimated real fertilizing ability) of about 0.85 in predicting the bull’s true fertility (Sdlhammar et al., 1994a). The selected bulls are then assumed to be used with a further 1,000 doses to form the first complete daughter group for progeny testing for production traits and other economically important traits and to confirm the bull’s fertility.

E.-M. Sdlhammar et al. /Liuestock Production Science 47 (1997) 211-219

I mST

FRRTILITY

213

I

OP

LIBIDO

SELECTION

STaOg

STAGE

STMR

Growth rate Sexual organs Conformation

Sperm quality Includins 52 of the bulls independently culled for sperm motility below 41%, poor libido and/or pathological sperm defects.

Fig. 1. Flow-chart

2

3

NRR

showing the different culling stages.

Motility (MOT) was assumed to have a mean of 53% and a standard deviation (SD) of 6% (St%lhammar et al., 1989) and non-return rate (NRR) was assumed to have a mean of 55% and SD of 4.0% (Stilhammar et al., 1994b). The correlation between MOT and NRR (rMOT,nRR) was assumed to be 0.2. This relationship was based on the results from analyses of Swedish data (Stahammar et al., 1994b) and corresponds to a regression of NRR on MOT, b NRR,‘MOT = 0.13. As an alternative, a stronger correlation, rMoT,NRR= 0.5 (bNRR,MOT = 0.33), was also studied. Nine alternatives combining three minimum levels of approval for motility and NRR, respectively (mean, mean minus 1 phenotypic SD and mean minus 2 phenotypic SD) were studied. In the present study it was assumed that 5% of the bulls were culled, having sperm cells with obvious pathological defects or a motility below 41% (X - 2SD). The former culling reason was assumed to be independent of motility. Furthermore, it was assumed that the fertility value of each bull remains the same throughout its time of use in AI. The overall effect on NRR among selected bulls following culling based on motility (indirect) and on NRR (direct) was studied, as was how many young bulls must be performance tested and tested for fertility in each alternative when the objective is to progeny test 100 bulls for dairy traits each year.

Under Swedish field conditions, with a mean of 55% NRR, 10 doses of semen are needed to obtain one informative daughter in milk production (Bkstrom, 1994). This assumption includes effects of sex ratio, mortality , animals sold, inseminations/service period, percentage recorded females, percentage heifers with a first lactation in recorded herds, and inseminated females/actual number of cows. The desired number of informative daughters (NID) per bull was set to 150 and defined as daughters being born in recorded herds and having produced at least one lactation. This number is determined primarily by the desire to achieve accurate sire proofs for stillbirths, female fertility and mastitis resistance. To calculate the number of doses per bull needed for the progeny testing (TDOS) at different levels of NRR, the equation used was: TDOS = (1501: 10*0.55)/NRR

(1)

The results of the various strategies were applied in a population totalling 250,000 cows, of which 80% were recorded. Using the different culling alternatives, the proportion of young bull semen (YB) used was calculated as: YB = [TDOS *

loo] +

[NBULL * SELMOT *

X ( 1 - SELNRR) * 500]/450,000

(2)

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E.-M. Stiilhammar et al./Livestock

where: TDOS is the total number of doses per each of the 100 young bulls selected on the basis of NRR. NBULL is the number of bulls needed for the test of male fertility (motility and NRR), equalling lOO/(SELMOT * SELNRR), because 100 bulls are to be progeny tested each year. SELMOT is the proportion of bulls kept (selected) in stage two. The culling in this stage included culling of bulls with motility below 41% and independent culling of bulls due to pathological sperm defects. The total proportion culled for those reasons was 590. SELNRR is the proportion of bulls kept (selected) in stage three. 500 is the number of doses per bull used for fertility testing on which they are selected in stage three. 450,000 is the total number of inseminations needed in the population, calculated as number of cows times 1.25 (25% more females than the actual number of cows are inseminated to cover heifer matings and the involuntary culling) times 1.8 (inseminations/service period). The first term of the numerator of Eq. (2) corresponds to the 100 bulls actually progeny tested, the second term to those bulls only being tested for fertility but culled subsequently. The extent to which variation in male fertility affects involuntary culling among cows was calculated. It was assumed that the females had an equal prospect of five possible heat cycles and that the heat detection rate was 0.65 (Oltenacu et al., 1986). This corresponds to an average of at least three possible inseminations before they were culled, and the use of bulls on an average female population with 55% NRR. Furthermore it was assumed that 90% of the females conceived within the five cycles, if the male fertility was complete (100%). The involuntary culling among cows due to variation in bull fertility was calculated as percent females returning to heat after five possible heat cycles.

3. Results Results discussed are from using rMoT,NRR = 0.2 unless otherwise stated. There was an indirect effect

Production Science 47 (1997) 211-219

on NRR after the second stage of culling, due to culling of bulls with poor sperm motility (Table 1). The indirect phenotypic effect ranged from zero to 0.7% NRR (for culling limit being equal to the mean MOT, not shown in Table l), depending on the intensity of culling for motility. The corresponding maximum indirect effect was 1.6% NRR for = 0.5. Direct culling on the phenotypic ‘MOT,NRR value for NRR showed, as expected, a greater effect on NRR than did culling based on motility, resulting in up to 3.1% increase in NRR on average in the selected group of bulls (Table 1). The overall effect of culling for fertility amounted to up to 3.3% (3.5% when ‘MOT,NRR = 0.5), which was close to 1 SD (SD = 4.0%) (Fig. 2). With the objective of progeny testing 100 young bulls per year, the number of bulls needed for fertility selection (NBULL) varied from 107 to 361, depending on the selection intensity in different stages (Table 1, Fig. 2). The proportion of bulls selected for male overall fertility varied between 28% and 93%. At the highest selection intensity, e.g. when 361 bulls were tested for motility and NRR in order to select 100 bulls for progeny testing, the need for station testing capacity, the proportion of females used with young bull semen, was much greater than is realistic in a population of this size. Therefore, these alternatives are not shown in Table 1. In the more realistic alternatives, 81% and 95% of the young bulls, respectively, were selected on motility only, and altogether 80-93% of the bulls were selected for both motility and NRR. These strategies would result in up to 1% increased NRR in the population as a whole. An increase in NRR of about 1% can be achieved with several culling strategies, resulting in different total numbers of selected bulls, depending on the balance between culling in stages two and three (Fig. 2). Selecting primarily on motility (limit for motility = mean), about 220 bulls will be needed for fertility testing, whereas only about 140 or 125 (limits for motility being mean - SD and mean - 2SD, respectively) if more bulls are tested directly on NRR. For the alternative with the motility limit being mean of 0.5 hardly affected the 2SD, a stronger rMOT,NRR results at all (Fig. 2). For the motility limits being mean and mean - SD, a certain effect in NRR was achieved with a lower number of bulls.

E.-M. Stiilhammar et al./Liuestock

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Production Science 47 (1997) 211-219

Table 1 Indirect and direct effect on non-return rate (NRR, %I when culling was based on motility (MOT) (culling stage 21 and NRR (culling stage 31, respectively, in a population with MOT and NRR mean (SD) of 53% (6%) and 55% (4%), respectively. The correlation between MOT and NRR was 0.2 or 0.5. With the objective of progeny testing 100 bulls for milk production each year, the total number of bulls needed is given when culling for male fertility is considered. Results for the culling limit equal to the mean MOT and NRR are not shown

72.2 80.5 80.9 93.4

139 124 124 107

51 (2 - SD) 47(x--2SDl 51 (X-SD) 47 (X - 2SD)

85.6 98.1 84.6 97.9

56.2 55.4 56.1 55.2

0.2 0.2 0.0 0.0

1.0 0.2 1.1 0.2

51 47 51 47

88.6 98.8 85.1 98.2

56.4 55.7 56.1 55.2

0.5 0.5 0.1 0.1

0.9 0.2 1.0 0.1

95.1

55.0

47(?-SD)

81.5

55.5

41 (2 - 2SD)

95.1

55.1

2SD)

=

1.4 0.7 1.1 0.2

(%P)

55.2

‘MOT.NRR

143 125 124 107

total

Selected bulls in stage 3

o.2

47(x-SD) 41 (?-

69.8 79.9 80.4 93.1

direct

81.5

=

1.2 0.4 1.1 0.2

indirect

(%l a

‘MOT,NRR

Total number of bulls needed for test of semen and fertility

Effect on NRR

Limit for NRR culling

Selected bulls in stage 2

Bulls kept % d

NRR mean2 ’ of kept bulls

NRR mean1 b of kept bulls

Limit for motility culling

o.5

a Including independent culling (5%) b Mean1 = the mean after culling for ’ Mean2 = the mean after culling for d After culling for semen quality and

(X-SD) (X - 2SD) (X-SD) (X - 2SD)

of bulls with pathological MOT. MOT and NRR. fertility.

defects or motility levels below 41%.

2.5 --

1 -Motilitylevelfor

rl#0T*RR=O.2 +lltOel-l rmor,uRn=o.J --•-.mean

0.5 --

+

mean-SD

--o--mean-SD

-4-meart-2SD ..

. -mean-25D

I

0, 1OD

l

150

200

250

300

350

400

Number of bulls needed for fertility selection

Fig. 2. Change in non-return rate &RR, o/o) and number of bulls needed (NBULL) to obtain 100 bulls for progeny testing for nine different culling strategies. Points with the same limit for culling for low motility (MOT) are connected with a line. Unbroken lines are for rMOT,NRR= 0.2, broken lines for rMOT,NRR= 0.5. For each line, the points are for the subsequent NRR culling limits mean - 2SD, mean - SD, and mean, from left to right, respectively.

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E.-M. Stiilhamnmr et al./Livestock

Table 2 Number of doses per young bull (TDOS) and involuntary culling among cows (%) after five possible heat cycles in relation to bull fertility (NRR, %) NRR

47

51

55

59

63

61

TDOS % culled cows

1,755 10.9

1,617 8.1

1,500 5.9

1,398 4.1

1,309 2.7

1,231 1.7

The population mean is about 56% NRR for the most realistic strategies compared above, but the selected bulls will of course, depending on the culling strategy, show individual variations in NRR. In each culling strategy there will still be a range of 12-20% in NRR among the selected bulls, although the overall mean difference between the strategies amounts to only 3%. The number of doses per bull needed to achieve the desired number of informative daughters of 150 will depend on the bull’s NRR (Table 2). A bull with poor fertility (47% NRR) will need 1755 doses in order to produce 150 informative daughters. There was a difference of 200-300 doses per bull between bulls with medium and high fertility in achieving the same number of daughters. The different culling strategies resulted in an average use of young bull semen that varied between 33% and 43% with the achieved average number of doses per young bull (Table 3). However, for the more realistic alternatives shown in Table 1, the level of use of young bull semen will be around

Table 3 Young bull semen (YB, %) and number of doses per young bull (TDOS) on average as a result of different culling strategies aiming at 100 progeny tested bulls per year Alternatives

for culling strategy NRR

YB

TDOS

z

x f-SD X -2SD

40 34 33

1415 1460 1478

Z-SD

I f-SD i-2SD

42 34 33

1420 1468 1489

E -2SD

x X-SD f-2SD

43 35 33

1420 1471 1495

Motility

Production Science 47 (1997) 211-219

35%. Although the differences in proportions of selected bulls were quite large in the culling strategies presented, the use of young bulls differed very little. When 90% of the females are capable of conceiving and are given the same opportunity to do so, average fertile bulls (mean) caused 5% lower and 4% higher involuntary culling respectively, compared with bulls with low (47%) and high (67%) NRR values (Table 2). The average difference in involuntary culling of cows in the different culling strategies amounted to 1.4%.

4. Discussion In Sweden, sperm motility is currently one of the quality criteria applied to frozen/thawed semen (Sijderquist, 1991). There are other potential traits besides motility, such as concentration and morphology of the sperm cells. Sperm cell viability and percentage sperm cells having a normal conformation are generally interrelated (Siiderquist, 1991). Thus, a large number of abnormalities involving the sperm tail are associated with reduced sperm motility. The frequency of sperm abnormalities, such as abnormal heads, nuclear pouches and proximal cytoplasmic droplets, is low but significantly correlated with the 56-day non-return rate of frozen semen (SGderquist et al., 1991). Motility is often regarded as a subjectively assessed trait, but it can readily be registered for every ejaculate. Computer assisted systems for objective assessment of sperm motility are available but not generally found justified in a routine laboratory test (O’Connor et al., 1981; Kupferschmied et al., 1993). The motility data analysed by Stllhammar et al. (1994b) were based on subjective evaluations of sperm motility. The veterinarians making these assessments are usually very experienced and can be assumed to keep the scale consistent when assessing the semen over longer periods. However, it might very well be that different veterinarians do not apply exactly the same scale. The choice of sperm motility as a culling criterion for semen quality in this study should be seen as only one of several possible criteria that have a fairly good relationship with fertility (NRR). If better laboratory criteria can be

E.-M. Stillhammar et al. /Livestock

Production Science 47 (1997) 211-219

217

identified that give a closer relationship than the one presently found with motility, the value of the laboratory tests for the final result will improve. However, markedly improved semen quality criteria need to be found, as the NRR variation in the currently used semen, selected on all possible criteria available, is quite large. Nevertheless, motility values set as limits must be viewed in relation to the defined mean for this study. The relations between MOT and NRR used in this study were chosen based on the estimates made by St%lhammar et al. (1994b) and could represent either 56-day or 168-day non-return rate. However, some of those estimates did not deviate significantly from zero, while the highest coefficients of regression were about 0.2 compared with the 0.13 and 0.33 chosen in this study. In the future, semen traits assessed at the laboratory level and with a better predictive value of sperm quality may be discovered and the relation to actual fertility may be improved. Saacke et al. (1991) suggested other components worth attention (besides viability and morphology) as qualitative semen traits, in order to measure the functionality of sperm cells. Some of these traits have undoubtedly been accounted for by the classical criteria, yet the correlation between these semen traits and fertility leave a considerable variation in male fertility still unexplained.

The culling possibilities are closely dependent on the assumed relationship between motility and NRR. In this study the relationship was based on AI bulls preselected for motility. The variation in NRR among bulls seemed to be the same throughout the range of motility present, 40-70%. With the maintained increase in NRR in the population as a whole of l%, a stronger correlation (0.5) between motility and NRR gave a very similar result in number of bulls needed to be able to progeny test 100 bulls per year. However. the difference between the two alternatives using the motility culling limits of mean - 2SD and mean - SD became smaller with increasing value of rMOT,NRR(Fig. 2). Nevertheless, it still was better to base the main part of culling for fertility on NRR. The increase in NRR in the dairy population as a whole is assumed to be of the same magnitude as in the selected group of bulls. The value of improved fertility was shown to be about 25 SEK per NRR %-unit (Stalhammar, 1995). For the current model population with 250,000 cows this would mean an increase net return per year of 6.25 million SEK. The underlying assumption is that there is a perfect relationship between the results as a young bull and as on old bull. If the relationship is weaker, 0.5 say, then the increase in NRR in the population would be correspondingly lower as the economic benefits would be.

4.1. Change in NRR due to culling and its economic importance

4.2. Optimal use of testing capacity

A difference of l-3% increase in NRR was achieved between the calculated strategies. The same increase in NRR can be achieved with quite different strategies and the one which results in fewer bulls to cull for a given NRR improvement should generally be chosen. The overall intensity of selection can be regarded as an indirect measure of the cost of culling based on male fertility, because a high intensity requires that more bulls are tested, which involves more bull-dams, etc. Today, probably 5-10% of all bulls are culled because of poor motility. If not only motility is considered, but also semen having a low concentration, more bulls would be culled in stage two (H&&d, 1994). Earlier, a stronger culling was performed most likely based on freezability of the sperm cells (Berries, 1981; Stilhammar, 1984).

A possible culling strategy is illustrated with the stated objective of progeny testing 100 bulls and the results from Table 1. Culling for overall male fertility is assumed to be about 20%, combining both motility and NRR in a culling strategy, with culling based primarily on NRR. For this alternative, 124 bulls are needed and of these, 6 bulls are culled for not achieving a motility over 40%, for weak libido, or sperm cells showing pathological defects, plus 18 bulls because of poor NRR. The bulls being progeny tested will be used for 1,500 doses each at a mean NRR of 55%. Culling 18 bulls based on NRR will save a testing capacity of 18 X 1,000 and allow testing another 12-14 bulls. Using 500 doses per bull in a fertility test with the objective of progeny testing 100 bulls each year for milk, the testing capacity needed is 159,200 inseminations with young

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bull semen. With the same testing capacity, but using 1,500 doses per bull and no fertility test, the objective will not be achieved, but only about 90 bulls can be progeny tested. The advantage of a fertility test based on 500 doses per bull in this example is that 100 usable bulls are produced after progeny testing rather than 90 that fulfill the fertility requirements. An effect of selecting for NRR based on 500 doses per bull is that the doses from the selected bulls are more spread over time, which may cause a slight extension of the period for final evaluation of the daughters. However, the bulls’ first daughters will come as early as in the case with 1,500 doses. The group as a whole will cover several seasons better, which might improve the accuracy in the evaluation for other traits. If 500 doses per bull are used for a fertility test, these will also provide enough daughters to preliminarily evaluate milk traits. The time lag from testing on non-return rate to receipt of the fertility result is dependent on the non-return rate interval chosen. Considering this time lag it might be better to use the 56-day non-return rate, though the accuracy of 500 doses would then be somewhat lower (0.751, than by using 168-day nonreturn rate. Today, the 56-day non-return rate is used rather than 168-day non-return rate to monitor bull fertility in AI services in Sweden. A special fertility test is not used at present. The difference between 1,500 doses per bull evaluated and culled on the 56-day non-return rate as today, and a fertility test with 500 doses per bull based on NRR will be a better utilization of the test capacity by lo-15% in the latter case.

5. Conclusions The best way to improve the fertility results of a dairy population appears to be to emphasize direct culling based on non-return rate after a small amount of culling based on semen quality and libido. If markedly better predictors of fertility from laboratory tests are found, this would affect the balance between indirect and direct culling somewhat. Culling of young bulls based directly on NRR using 500 doses per bull in a special fertility test with an accuracy of 0.75-0.85 is recommended.

Production Science 47 (1997) 211-219

Applying such a policy gives about 10% more efficient use of the testing capacity. From a practical point of view it would be better to select on a 56-day non-return rate with slightly lower accuracy rather than on 168-day non-return rate. An increase in overall fertility in the population of about 1% NRR will be achieved in the most realistic culling strategies, which should require not more than about 125 bulls for fertility testing (motility and NRR) in order to achieve 100 bulls for progeny testing. By and large, the different culling strategies lead to fairly similar levels of involuntary culling of cows, although bulls with low NRR can cause 4-9% more involuntary culling than the best ones depending on strategy chosen. By keeping the culling based on semen quality at a low level, fewer bulls will be needed to achieve a desired number of bulls for progeny testing for milk production without reducing the NRR in the population as a whole.

Acknowledgements The authors wish to express their appreciation to DVM Magnus H&d and VMD Lennart Soderquist for valuable discussions and comments on the manuscript.

References Amann, R.P., 1989. Can the fertility potential of a seminal sample be predicted accurately? J. Androl., 10: 89-98. Bemes, G., 1981. Utslagsorsaker och fertilitet hos ungtjurar i semin. Examensarbete Nr. 95. Swedish University of Agricultural Sciences, Department of Animal Breeding and Genetics, Uppsala, 31 pp. (in Swedish). B&tram, L.-O., 1994. Svensk Avel. Personal communication. H&d, M., 1994. Svensk Avel. Personal communication. Killian, G.J., 1992. Fertility factors in seminal plasma. Proc. 14th Tech. Conf. on Artificial Insemination and Reproduction, Milwaukee, WI, April 24-25, pp. 33-38. Kupferschmied, H.U., Wiedennann, J., Kiipfer, U., Gaillard, C. and Stranzinger, G., 1993. Semen quality and fertility in cattle: Facts and new results of applied research. 5th Meet. Nazionale, Studio della efficienza riproduttiva degli animali di interesse zootecnico, Bergamo, 30 April, pp. 9-18. Linford, E., Glover, F.A., Bishop, C. and Stewart, D.L., 1976. The relationship between semen evaluation methods and fertility in the bull. J. Reprod. Fertil., 47: 283-291.

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