Effects of Selection for Scrotal Circumference in Limousin Bulls on Reproductive and Growth Traits of Progeny 1,2

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1 Effects of Selection for Scrotl Circumference in Limousin Bulls on Reproductive nd Growth Trits of Progeny 1,2 D. W. Moser 3, J. K. Bertrnd, L. L. Benyshek, M. A. McCnn, nd T. E. Kiser Animl nd Diry Science Deprtment, University of Georgi, Athens ABSTRACT: Nine pirs of Limousin bulls from nine contemporry groups were cquired, with ech pir consisting of one lrge scrotl circumference (SC) bull nd one smll SC bull. Averge djusted yerling scrotl circumferences were 36.3 cm (SD 1.6 cm) nd 28.5 cm (SD.9 cm) for lrge SC (LP) nd smll SC (SP) bulls, respectively. In ddition to the phenotypic grouping, non-prent SC EPD were used to group bulls into high (HE, >.53 cm), verge (AE), nd low (LE, <.61 cm) lines. Ech bull ws mted to rndomly ssigned group of 15 to 20 Brngus Hereford cows ech yr for 1 to 3 yr. Birth weights, wening nd yerling weights nd heights, nd ultrsound mesurements for bckft nd ribeye mesurements were tken on 407 progeny. Blood smples were collected on 210 heifer progeny when they verged 11, 13, nd 15 mo of ge to determine whether they hd reched puberty. When subjected to breeding soundness exm (BSE), LP bulls scored higher ( P <.01) for motility s well s totl BSE score. The LP clves hd hevier birth weights ( P <.05) nd greter testiculr mss t wening ( P <.01) thn SP clves. The HE nd AE bull clves hd greter ( P <.02) testiculr mss thn did the LE bull clves. A greter ( P <.05) percentge of HE heifers hd reched puberty by the 11- nd 13-mo mesurements thn either the AE or LE heifers. The HE heifers reched puberty t younger ge thn AE ( P <.01) or LE (P <.001) heifers. Selection using SC EPD ws more effective thn phenotypic selection in reducing ge t puberty in dughters. Key Words: Beef Cttle, Heifers, Predicted Difference, Puberty, Scrotum, Selection Responses J. Anim. Sci : Introduction Reproductive trits such s ge t puberty hve significnt effect on the profitbility of cow-clf enterprise (Werth et l., 1991; Rege nd Fmul, 1993; Melton, 1995). However, direct selection for reproductive chrcteristics in beef cttle is difficult. Femle reproductive trits re generlly low in heritbility, difficult to mesure under commercil conditions, nd prticulrly influenced by mngement prctices such s fixed clving sesons. Severl studies hve shown fvorble reltionships between scrotl circumference ( SC) in bulls nd reproductive trits in femles. Correltions between SC in bulls nd ge t puberty or ge t first breeding of hlf-sib heifers hve been shown to be fvorble in purebred popultions (Brinks et l., 1978; Toelle nd Robison, 1985). In nother study of purebred beef cttle, Smith et l. (1989b) found fvorble but nonsignificnt regression of ge of puberty on sire SC. However, Gregory et l. (1991) showed heterosis to hve significnt effects on both SC in bulls nd ge t puberty in heifers. Morris et l. (1992) found fvorble genetic correltion between hlf-sibs for 13-mo SC nd ge t puberty in popultion of both purebred nd crossbred cttle. The gol of this study ws to determine the impct of selecting sires on phenotypic SC or SC EPD for use in crossbreeding system. 1 This reserch ws funded by the Georgi Agric. Exp. St. nd the North Americn Limousin Foundtion nd ws prt of Htch project H Apprecition is expressed to the personnel of the Centrl Brnch Experiment Sttion, Etonton, GA, for mngement of subject nimls nd to M. M. Rollosson, M. C. Kelley, nd A. H. Nelson for their ssistnce with dt collection nd nlysis. 3 To whom correspondence should be ddressed. Received Jnury 9, Accepted April 14, Mterils nd Methods Generl. Nine pirs of yerling Limousin bulls born in 1987 or 1988 were cquired from seven purebred herds locted in the southestern nd midwestern United Sttes. Ech pir of bulls originted from the sme yerling contemporry group nd hd similr EPD for wening weight nd yerling weight. Pirs 2052

2 SELECTION FOR SCROTAL CIRCUMFERENCE IN CATTLE 2053 were selected to hve n 8-cm difference in yerling SC, djusted to 365 d ccording to formul proposed by Lunstr et l. (1988). Breeding soundness exmintions ( BSE; Bll et l., 1983) were performed on ll bulls, nd dt from semen tests were recorded. Bulls were required to produce vible semen before being turned out to breeding pstures. In ech of 3 yr, four or six pirs of bulls were individully mted to between 15 nd 20 Brngus Hereford cows t the Centrl Georgi Brnch Sttion, Etonton. Femles were rndomly ssigned to sires ech yer, but the number of cows nd number of heifers mted ws the sme for ech sire. During the first 2 yr of the project, four pirs of bulls were used. Five new pirs of bulls entered the project in yr 3, nd one of the four originl pirs remined. Cows were exposed to bulls for 75 d beginning in erly April ech yer. Actul birth weights were recorded on ll clves. At wening, weights nd hip heights were tken, long with live niml ultrsound mesurements for bckft nd ribeye re tken between the 12th nd 13th ribs. The ultrsound mesurements were determined by trined technicin using rel-time, B-mode ultrsound scnner with 10.7-cm, 3.5-MHz liner rry trnsducer (Alok 210 DX, Corometrics Medicl Systems, Wllingford, CT). The imges were recorded nd lter nlyzed using n Alok 633 ultrsound imge nlysis unit. Also t wening, bull clves were mesured for SC, nd cstrted, nd testes weights were recorded. At 1 yr of ge, the progeny were gin weighed, mesured for hip height nd pelvic re, nd ultrsoniclly scnned for bckft nd ribeye re. After wening, heifers were developed on diet of 45% ground corn, 10% soyben mel, nd 45% soyben hulls, fed t 1% of the heifer s body weight dily (sfed bsis). Heifers consumed bermudgrss hy nd minerl on n d libitum bsis. Beginning pproximtely December 1 ech yer, heifers grzed whet nd ryegrss pstures every other dy, nd the mount of the corn, soy mel, nd soy hull rtion ws reduced to.5% of body weight dily (s-fed bsis). Blood smples (pproximtely 7 ml) were collected by coccygel or jugulr venipuncture on ll heifer progeny weekly for three consecutive weeks when the group verged 11, 13, nd 15 mo of ge, for totl of nine blood smples for ech heifer. Blood smples were refrigerted overnight, nd ser were collected the next dy nd frozen until ssyed for progesterone ( P 4 ) ccording to the method of Dunlp et l. (1981). Heifers were judged to hve ttined puberty when P 4 concentrtion exceeded 1 ng/ml of serum for two of the three consecutive weeks. Dte of puberty ws ssigned to be the dte of the first smple with P 4 concentrtion >1 ng/ml. Groups of 15 to 20 heifers were exposed to yerling Angus bulls in single-sire pstures for 50 d, beginning when the group verged 410 d of ge. Heifers were rectlly plpted to dignose pregnncy 60 d following removl of bulls. Pregnncy percentge ws defined s number of pregnnt femles divided by number of femles exposed to bulls. Sttisticl Anlyses. Sires were grouped for nlysis by two methods. Phenotypic yerling SC ws used to group lrge SC ( LP) nd smll SC ( SP) lines. Dt were lso nlyzed by grouping sires by non-prent SC EPD. When the experiment ws designed, SC EPD were not vilble for the Limousin breed. Non-prent SC EPD on the sires were clculted in the Jnury 1995 North Americn Limousin Foundtion ( NALF) ntionl cttle evlution (NALF, 1995). Accordingly, sires were ssigned to SC EPD groups fter dt hd been collected. Four sires with SC EPD greter thn.53 cm were ctegorized s the high SC EPD ( HE) group (rnge of.74 to.99 cm). The low SC EPD ( LE) group consisted of five sires with SC EPD less thn.61 cm (rnge of.71 to 1.63 cm). The remining nine sires, with SC EPD rnging from.58 to.49 cm, mde up the verge SC EPD ( AE) group. The verge SC EPD of ll Limousin nimls born in 1987 nd 1988 ws.039 cm, with stndrd devition of.228 cm. Therefore, bulls ssigned to the HE nd LE SC EPD lines hd SC EPD tht were 2.5 stndrd devitions greter thn nd less thn the breed verge, respectively. All HE sires were lso LP sires, nd ll LE sires were SP sires. Averge EPD, djusted yerling SC, nd number of progeny of phenotypic nd EPD line sires re listed in Tble 1. Breeding soundness dt of the sires were nlyzed by nlysis of vrince, with yer of birth nd line s independent vribles (SAS, 1988). Yer of birth ws not significnt vrible for ny BSE trits nd ws removed from the model. Percentge of sires meeting BSE criteri for certin recommendtions ws nlyzed s binry (0, 1) trit. Progeny dt were nlyzed using the MIXED procedure of SAS, version 6.07 (SAS, 1992). The model used for nlyses of ll trits mesured in progeny included fixed effects for birth yer of clf, phenotypic SC line or SC EPD line of sire, interction between yer nd line, ge of dm clss (2-, 3-, 4- or 5-yr of ge), nd sex (for trits mesured in both sexes). Age of clf ws dded to the model s covrite if the trit nlyzed did not hve stndrd djustment procedures for these fctors. Birth, wening, or yerling EPD of sire were lso included in the model s covrite, depending on whether the trit being nlyzed ws mesured t birth, wening, or t 1 yr of ge. This ws done to ccount for differences in growth between sire groups, prticulrly for the SC EPD groups. Sire, nested within yer nd line, ws included s rndom effect. Percentge estrous cycling t given mesurement nd pregnncy sttus were nlyzed s binry trits.

3 2054 MOSER ET AL. Tble 1. Phenotypic scrotl circumference (SC) nd SC expected progeny difference (EPD) line weighted verge EPD nd djusted SC SC EPD Line Lrge SD Smll SD High SD Avg SD Low SD BA SD No. sires No. progeny BW b EPD, kg WW c EPD, kg YW d EPD, kg MA e EPD, kg SC EPD, cm Adj. SC, cm b Breed verge, ll nimls born in 1987 nd 1988 (NALF, 1995). Birth weight. Wening weight. Yerling weight. Milking bility. Results nd Discussion Overll mens for 192 mle nd 215 femle offspring re listed in Tble 2. Yer of birth ws highly significnt for ll trits. Yer line interctions were not significnt nd were removed from the model. Anlyses by Phenotypic Scrotl Circumference Lines. Lest squres mens for breeding soundness trits of the 18 bulls re listed by line in Tble 3. Scrotl circumference score, motility score, nd totl BSE score were higher for LP bulls ( P <.01). Our findings of fewer bnormlities nd greter motility for the lrge SC bulls gree with Knights et l. (1984). They reported phenotypic correltion between SC nd semen score of.51 nd phenotypic correltion between SC nd semen motility score of.52, using scoring system in which more fvorble results received lower score (1 = excellent, 2 = good+, 3 = good, 4 = good, 5 = fir, etc.). Neither primry nor secondry bnormlities were different between the two lines, but totl bnormlities were lower for LP bulls ( P <.10). A greter ( P <.01) percentge of LP sires received recommendtion of stisfctory thn SP sires. These dt ffirm the recommendtions for BSE recently dopted by the Society for Theriogenology (Chenoweth et l., 1992) tht require minimum SC of 30 cm for bulls 15 mo of ge or younger to be clssified s stisfctory potentil breeders. Bulls with yerling SC less thn 30 cm (SP line) scored significntly lower on the BSE in our study. Lest squres mens for growth trits of progeny re listed by line in Tble 4. Birth weight ws greter ( P <.05) for progeny of LP sires, despite these sires hving similr birth weight EPD. Bourdon nd Brinks (1986) found similr reltionship, reporting genetic correltion between birth weight nd yerling SC of.18. Wening growth nd composition mesurements were nerly identicl between groups, s expected, becuse the lines were selected to hve similr growth EPD. Likewise, no significnt differences were found in these trits t 1 yr of ge. Lest squres mens for reproductive trits re listed by line in Tble 5. Wening testiculr weight ws greter ( P <.01) for the LP line. Wening SC ws not significntly greter for the LP line, lthough the correltion between wening testiculr weight nd wening SC ws.85. This my due to the reltively lower precision of SC mesurement s predictor of testiculr development in comprison to weighing the excised testicles. The similrity in SC cross groups my lso be due to the young ge t which the clves were mesured. If mesurements hd been tken t 1 yr of ge, greter difference might hve been observed. Neely et l. (1982) estimted the heritbility of wening SC to be.08 ±.20, in comprison to n estimte of.44 ±.24 for yerling SC. In contrst, Morris et l. (1992) found lower estimte of heritbility for 13-mo SC thn for 8-mo SC (.29 ±.10 vs.50 ±.11). However, their estimte of genetic correltion with ge t first estrus ws of greter mgnitude for 13-mo SC thn for 8-mo SC (.60 ±.24 vs.11 ±.22). No significnt differences between phenotypic lines were found for ny of the femle reproductive trits mesured in the progeny. A 6.9-d dvntge in ge t puberty for the LP line ws observed, but this difference ws not sttisticlly significnt. Although greter percentge of LP heifers ttined puberty by the 13-mo nd 15-mo mesurement points, these differences were not significnt. Pregnncy percentge for the two groups ws identicl. There ws significnt vrition mong the sires within lines for most femle reproductive trits, including ge t puberty. Anlyses by Scrotl Circumference EPD Lines. Lest squres mens for breeding soundness trits of sires re listed by SC EPD line in Tble 3. No significnt differences were observed mong SC EPD lines for most breeding soundness trits of sires. The SC score ws higher ( P <.05) for HE sires thn for LE sires.

4 SELECTION FOR SCROTAL CIRCUMFERENCE IN CATTLE 2055 Tble 2. Phenotypic mens by sex Mles Femles Men SD Men SD No. clves Birth wt, kg Adj. wening wt, kg Wening height, cm Wening bckft, cm Wening ribeye re, cm Adj. yerling wt, kg Yerling height, cm Yerling bckft, cm Yerling ribeye re, cm Yerling pelvic re, cm Wening SC, cm Wening testiculr wt, g Percentge cycling t 11-mo msmt b 10% 30% Percentge cycling t 13-mo msmt 23% 42% Percentge cycling t 15-mo msmt 84% 37% Age t puberty, d Pregnncy percentge 86% 35% b Scrotl circumference. Mesurement. Abnormlities tended to be lower nd scores tended to be higher s SC EPD incresed. Selection for high SC EPD does not eliminte the need to evlute potentil sires for breeding soundness. Lest squres mens for growth trits re listed by line in Tble 4. Wening ribeye re ws greter ( P <.02) for AE clves thn for LE clves. No other differences in growth or composition trits were found mong the SC EPD lines. Although SC is fvorbly correlted with growth trits (Bourdon nd Brinks, 1986), no dditionl effect of SC EPD line ws found, so wening nd yerling weight EPD ccounted for differences in progeny growth mong sires. Lest squres mens for reproductive trits by SC EPD line re listed in Tble 5. Wening testiculr weight ws greter ( P <.02) for HE nd AE clves thn for LE clves. At both the 11-mo nd 13-mo mesurements, greter ( P <.05) percentge of HE heifers hd reched puberty thn AE nd LE heifers. If heifers re to clve by 2 yr of ge, they must conceive by 15 mo of ge. Fertility of puberl estrus in beef heifers is lower thn third estrus (Byerley et l., 1987). It follows tht heifers sired by bulls with high SC EPD would hve more opportunities to conceive in time to clve by 2 yr Tble 3. Lest squres mens for breeding soundness trits of sires by phenotypic scrotl circumference (SC) nd SC expected progeny difference (EPD) lines SC EPD Line Lrge Smll High Averge Low Primry bnormlities, % 6.67 ± ± ± ± ± 1.89 Secondry bnormlities, % ± ± ± ± ± 3.56 Totl bnormlities, % ± 2.28 e ± 2.42 f ± ± ± 3.37 SC score ± 2.58 g ± 2.58 h ± 4.70 i ± 4.70 ij ± 4.21 j Morphology score b ± ± ± ± ± 4.42 Motility score c ± 1.46 g ± 1.46 h ± ± ± 2.46 Totl BSE d score ± 5.69 g ± 5.69 h ± ± ± 9.38 Percentge rted stisfctory 100% ± 12% g 44% ± 12% h 100% ± 23% 67% ± 15% 60% ± 21% Percentge rted stisfctory or questionble 100% ± 8% 89% ± 8% 100% ± 12% 89% ± 8% 100% ± 11% b >34 cm yerling SC = score of 40, cm = 24, <30 cm = 10 (Bll et l., 1983). <25% totl bnormlities (TA) = score of 40, 26 39% TA = 24, 40 59% TA = 10, >59% TA = 3 (Bll et l., 1983). Rpid liner = score of 20, moderte liner = 12, slow liner to errtic = 10, very slow nd errtic = 3 (Bll et l., 1983). Breeding Soundness Exm (Bll et l., 1983). Superscripted mens within row nd group of lines (SC or SC EPD) lcking common superscript differ ( P <.10). P <.01. P <.05.

5 2056 MOSER ET AL. Tble 4. Lest squres mens for growth trits of progeny by sire phenotypic scrotl circumference (SC) nd SC expected progeny difference (EPD) lines SC EPD line Lrge Smll High Averge Low Birth wt, kg ± ± 1.12 b ± ± ± 1.23 Adj. wening wt, kg ± ± ± ± ± 3.43 Wening height, cm ± ± ± ± ± 1.07 Wening bckft, cm.29 ± ± ± ± ±.02 Wening ribeye re, cm ± ± ± 1.76 cd ± 1.70 c ± 1.73 d Adj. yerling wt, kg ± ± ± ± ± 5.03 Yerling height, cm ± ± ± ± ± 1.49 Yerling bckft, cm.27 ± ± ± ± ±.04 Yerling ribeye re, cm ± ± ± ± ± 3.60,b c,d Superscripted mens within row nd group of lines (SC or SC EPD) lcking common superscript differ ( P <.05). P <.02. of ge. If producers breed heifers erlier thn mture cows, it is especilly importnt tht they rech puberty by 13 mo of ge. By the 15-mo mesurement, more AE heifers thn LE heifers hd reched puberty ( P <.10). The HE heifers reched puberty t younger ge thn either AE ( P <.01) or LE ( P <.001) heifers. The relibility of using SC EPD in selection for erlier puberty in heifers will be enhnced with greter ccurcy of SC EPD. Our findings of erlier puberty for dughters of high SC EPD bulls concur with Brinks et l. (1978) nd Toelle nd Robison (1985). Both studies found fvorble genetic correltions between yerling scrotl circumference in bulls nd ge t puberty in hlf-sib heifers in popultions of purebred nimls. However, Gregory et l. (1991) found heterosis to hve significnt effect on the percentge of heifers reching puberty by 368 d, 410 d, nd 452 d, with the gretest effect t the younger ge. Our study shows tht even within crossbred popultion, selection on SC EPD cn reduce ge t puberty. When sires re selected for high SC EPD within crossbreeding system, lrge percentge of heifers should rech puberty erly enough to clve t 2 yr of ge or younger. With the fvorble correltions between ge t puberty nd other femle fertility trits reported by Smith et l. (1989), dughters of higher SC EPD bulls my be expected to produce their first nd second clves t n erlier ge s well. Despite the dvntge in ge t puberty for the HE group, pregnncy percentge for the groups ws similr. In this study, sire selection using SC EPD ws shown to be more effective in reducing ge t puberty of dughters thn ws selection for phenotypic SC. Significnt differences were found mong SC EPD groups for dughter ge t puberty, but not between phenotypic groups. Hough et l. (1985) nd Mhrt et l. (1990) hve demonstrted the effectiveness of direct selection using EPD to improve growth trits in beef cttle. This study shows tht EPD re n effective Tble 5. Lest squres mens for reproductive trits of progeny by sire phenotypic scrotl circumference (SC) nd SC EPD lines SC EPD line Lrge Smll High Averge Low Wening scrotl circumference, cm ± ± ± ± ±.86 Wening testiculr wt, g ± b ± c ± d ± d ± e Percentge cycling t 11-mo msmt 12 ± 4 12 ± 5 23 ± 5 f 8 ± 4 g 8 ± 5 g Percentge cycling t 13-mo msmt 32 ± 7 22 ± 8 44 ± 6 f 25 ± 5 g 16 ± 6 g Percentge cycling t 15-mo msmt 92 ± 4 88 ± 4 88 ± 5 94 ± 4 h 84 ± 5 Age t puberty, d ± ± ± 5.2 i ± ± 5.1 Pregnncy percentge 89 ± 4 89 ± 5 87 ± 6 90 ± 5 89 ± 6 Yerling pelvic re, cm 2 (heifers) ± ± ± ± ± 1.72 b,c Mesurement. Superscripted mens within row nd group of lines (SC or SC EPD) lcking common superscript differ ( P <.01). P <.02. P <.05. Averge more thn Low ( P <.10). High younger thn Averge ( P <.01) nd Low ( P <.001).

6 SELECTION FOR SCROTAL CIRCUMFERENCE IN CATTLE 2057 tool to improve reproductive trits, even when selection is on correlted trits, rther thn directly on the trit of interest. Implictions Bulls with higher scrotl circumference (SC) expected progeny differences (EPD) cn be expected to produce dughters reching puberty t significntly erlier ges, even when used in crossbreeding. Greter percentges of heifers estrous cycling erly in the breeding seson cn lso be expected. Selection using non-prent SC EPD is more effective thn using phenotypic SC mesurements to reduce ge t puberty. Beef breed ssocitions tht include SC dt in their ntionl cttle evlution progrms provide producers with useful informtion to improve femle reproductive trits in their herds. Commercil producers should combine selection of high SC EPD sires with well-designed crossbreeding systems to reduce ge of puberty of heifers. Literture Cited Bll, L., R. S. Ott, R. G. Mortimer, nd J. C. Simons Mnul for breeding soundness exmintion of bulls. J. Soc. for Theriogenology 12:1. Bourdon, R. M., nd J. S. Brinks Scrotl circumference in yerling Hereford bulls: Adjustment fctors, heritbilities, nd genetic, environmentl nd phenotypic reltionships with growth trits. J. Anim. Sci. 62:958. Brinks, J. S., M. J. McInerney, nd P. J. Chenoweth Reltionship of ge t puberty in heifers to reproductive trits in young bulls. Proc. West. Sec. Amer. Soc. Anim. Sci. 29:28. Byerley, D. J., R. B. Stigmiller, J. G. Berrdinelli, nd R. E. Short Pregnncy rtes of beef heifers bred either on puberl or third estrus. J. Anim. Sci. 65:645. Chenoweth, P. J., J. C. Spitzer, nd F. M. Hopkins A new bull breeding soundness evlution form. Proc. Annu. Mtg. Soc. for Theriogenology. p 63. Dunlp, S. E., T. E. Kiser, G. B. Rmpcek, R. R. Kreling, nd F. N. Thompson Effect of suckling on cortisol, progesterone nd luteinizing hormone in postprtum beef cows. Theriogenology 16:185. Gregory, K. E., D. D. Lunstr, L. V. Cundiff, nd R. M. Koch Breed effects nd heterosis in dvnced genertions of composite popultions for puberty nd scrotl trits of beef cttle. J. Anim. Sci. 69:2795. Hough, J. D., L. L. Benyshek, nd J. W. Mbry Direct nd correlted response to yerling weight selection in Hereford cttle using ntionlly evluted sires. J. Anim. Sci. 61:1335. Knights, S. A., R. L. Bker, D. Ginol, nd J. B. Gibb Estimtes of heritbilities nd of genetic nd phenotypic correltions mong growth nd reproductive trits in yerling Angus bulls. J. Anim. Sci. 58:887. Lunstr, D. D., K. E. Gregory, nd L. V. Cundiff Heritbility estimtes nd djustment fctors for the effects of bull ge nd ge of dm on yerling testiculr size in breeds of bulls. Theriogenology 30:127. Mhrt, G. S., D. R. Notter, W. E. Bel, W. H. McClure, nd L. G. Bettison Growth of crossbred progeny of Polled Hereford sires divergently selected for yerling weight nd mternl bility. J. Anim. Sci. 68:1889. Melton, B. E Profiting from chnge in the U.S. beef industry: Genetic blnce for economic gins. Technicl Report of the Ntionl Cttlemen s Assocition, Englewood, CO. Morris, C. A., R. L. Bker, nd N. G. Cullen Genetic correltions between pubertl trits in bulls nd heifers. Livest. Prod. Sci. 31:221. NALF Ntionl Limousin Genetic Evlution Mnul. (Spring Ed.). North Americn Limousin Foundtion, Englewood, CO. Neely, J. D., B. H. Johnson, E. U. Dillrd, nd O. W. Robison Genetic prmeters for testes size nd sperm number in Hereford bulls. J. Anim. Sci. 55:1033. Rege, J.E.O., nd T. R. Fmul Fctors ffecting clving dte nd its reltionship with production trits of Hereford dms. Anim. Prod. 57:385. SAS SAS/STAT User s Guide (Relese 6.03). SAS Inst. Inc., Cry, NC. SAS The MIXED procedure. In: SAS Technicl Report P- 229, SAS/STAT Softwre: Chnges nd Enhncements (Relese 6.07). p 287. SAS Inst. Inc., Cry, NC. Smith, B. A., J. S. Brinks, nd G. V. Richrdson Estimtion of genetic prmeters mong reproductive nd growth trits in yerling heifers. J. Anim. Sci. 67:2886. Smith, B. A., J. S. Brinks, nd G. V. Richrdson. 1989b. Reltionships of sire scrotl circumference to offspring reproduction nd growth. J. Anim. Sci. 67:2881. Toelle, V. D., nd O. W. Robison Estimtes of genetic correltions between testiculr mesurements nd femle reproductive trits in cttle. J. Anim. Sci. 60:89. Werth, L. A., S. M. Pritchrd, S. M. Azzm, D. A. Fiske, G. H. Pfeiffer, M. K. Neilsen, nd J. E. Kinder Evluting net income from different durtions of breeding sesons in beef production using deterministic simultion model. Agric. Syst. 37:275.