SELECTION IN HEREFORD CATTLE. EXPECTED AND REALIZED RESPONSE

Transcription

1 . SELECTION IN HEREFORD CATTLE. EXPECTED AND REALIED RESPONSE L. N. Nwakalr 1, J. S. Brinks 2 and G. V. Richardsn s Clrad State University, Frt Cllins ABSTRACT Data frm 14 inbred lines and 14 linecrss grups f Herefrd cattle at the San Juan Basin Research Center, Hesperus, were used t evaluate expected and realied respnse in birth and weaning traits and pstweaning traits in males and females ver a 28-yr perid. There were large differences in the means and variances f the perfrmance traits amng the inbreds and linecrsses, with the inbreds shwing inbreeding depressin and greater variability amng lines, while the linecrsses manifested within-breed hetersis. Except fr gain frm weaning t 12 m, in females, genetic prgress was expected in all traits studied, mainly due t sire selectin. Regressins f annual trait means n years indicated psitive phentypic trends in the inbreds fr heart girth circumference at birth, adjusted weaning weight (adjusted fr inbreeding), weaning scre, final weight, feed cnsumptin and the yearling weights and gains in females. Changes were negative fr ther traits. In the linecrss grup phentypic trends were psitive in all traits except heart girth circumference, weaning age, initial test weight and feed efficiency. Estimated genetic prgress per generatin due t within-line selectin was negative in mst f the traits in the inbreds but was cnsiderably psitive fr the linecrsses fr mst f the traits. As expected, between-line selectin yielded greater genetic imprvement in the inbred than in the linecrss ppulatin. The different patterns f respnse in the tw ppulatins are attributed t high rates and levels f inbreeding. Althugh variable, the actual prgress was belw predictin in mst f the traits studied. (Key Wrds: Herefrd, Cattle, Selectin, Genetic Gain.) Intrductin Experience has shwn that mst traits f ecnmic imprtance in beef cattle have mderate t high heritabilities and can be imprved by selectin. In practical, as well as experimental, animal breeding it is desirable t knw what genetic prgress is being made and whether this is in agreement with the expected level based n the knwledge f heritability and selectin applied. Hwever, previus investigatins pint ut that selectin prgress frequently falls shrt f expectatin (Lush, 1951; Pirchner, 1969). Evidence frm the literature has shwn that when selectin is practiced, inbreeding fllwed by crssing f the lines in a large ppulatin 1Dept. f Anim. Sci., Univ. f Nigeria, Nsukka, Nigeria. 2Prfessr, Dept. f Anim. Sci., Clrad State Univ., Frt Cllins Bimetrician, ARS, USDA, Frt Cllins, CO Received April 1, Accepted Octber 30, can be a ptent means f changing the ppulatin means (Flwer et al., 1963; Harwin, 1963; Brinks et al., 1967, 1971, 1972; Urick et al., 1968; Ray et ai., 1970; Lasley, 1972; Nwakalr et al., 1976). In the first paper f this series (Nwakalr et al., 1986), the selectin applied and the generatin intervals were studied in inbred and linecrss ppulatins f a Herefrd cattle herd. The purpse f this paper is t evaluate the expected genetic prgress, as well as the genetic respnse actually realied, in the same herd. Agreement between expected and actual respnse is examined. Materials and Methds The data were the same as thse used in the first paper f this series (Nwakalr et al., 1986) t study the intensity f selectin; 14 inbred lines and 14 crrespnding (by line f inbred sire) linecrss grups f Herefrd cattle at San Juan Basin Research Center, Hesperus, Clrad. The birth and weaning traits studied were birth weight (BW), heart girth circumference (HGC) at birth, weaning weight (WW), weaning 937 J. Anim. Sci :

2 938 NWAKALOR ET AL. scre (WS) and weaning age (WA). Weaning scre was measured n a scale f 6 pints in which 6 represents prime; 5, chice; 4, gd; 3, medium; 2, cmmn and 1, cull. The pstweaning perfrmance characteristics f males studied were initial test weight (IW), final weight ff test (FW), feed cnsumed (FC), feed efficiency (FE) and average daily gain (ADG) ver the test perid. Pstweaning traits f females included gain frm weaning t 12 m, 12-m weight (12W), gain frm 12 t 18 t, 18-m weight (18W) and mature cw weights in spring (SPW), summer (SUW) and fall (FAW). Adjustments f data were as given previusly (Nwakalr et al., 1986). Expected Genetic Prgress. Genetic prgress (AG i) in each f the traits cnsidered in the index was calculated directly frm the relative weights in standard measure (aj), the genetic crrelatins (rgig j) and square rts f heritability (gi) as: AG i =.~ (aj)(rgigj)(gi)(gj) l (Harvey and Bearden, 1962). This is equivalent t the matrix multiplicatin A'G, where A' is the transpse f A (the rw vectr f the relative weights in standard measure) and G is the genetic variance-cvariance matrix in standard measure. In the G matrix, the diagnal elements amunt t the heritabilities (genetic variances) and the ff-diagnal elements are the prducts f rgigj'gig j multiplicatins (genetic cvariances). The genetic crrelatin and square rts f heritability values were taken frm previus studies f Armstrng (1964), Brinks et al. (1964), Petty and Cartwright (1966) and Prestn and Willis (1974). The AG values were btained fr inbred male, inbred female and Iinecrss female selectin. The expected genetic prgress per generatin in standard measure resulting frm selectin f bulls was cnverted int actual units by multiplying by the respective standard deviatins (table 1) and then averaged with that frm selectin amng females r halved, as the case may be, t estimate the verall genetic prgress expected frm selectin. Imprvements expected in IW and FW f bulls were calculated in actual units frm WW and ADG infrmatin. In female selectin, the expected genetic prgress in actual units in the gain perids was btained as the difference between the tw weights cncerned. Realied Genetic Prgress. The actual genetic prgress resulting frm selectin was estimated by within-line regressin f ffspring deviatins frm annual trait means n generatin numbers, after making the necessary adjustments fr knwn causes f envirnmental variatins. Generatin numbers were calculated fr all animals as G = (G s + G d)/2 + 1, where G is the generatin f ffspring; Gs, the generatin f sire; and Gd, the generatin f dam (Brinks et al., 1961). The generatin numbers measure the average number f segregatins back t fundatin animals (Kch et al., 1974). All fundatin animals and all parents that came frm the cntrl line in the herd were assigned generatin er. As Kch et al. (1974) nted, this prcedure prvides a direct estimate f respnse per generatin that is independent f maternal effects r selectin bias. Estimates f genetic change resulting frm within-line selectin were calculated fr the inbred and linecrss ppulatins as the average f regressins by line f inbred sire. In additin, apprximate estimates f genetic change due t between-line selectin as a result f culling f six f the riginal 14 lines and their crrespnding linecrss grups were btained as the difference between the pled phentypic regressins f perfrmance recrds n years and the average f within-line phentypic regressins n years. Results and Discussin Means and Variances. The verall means and variances f perfrmance characteristics are shwn in table 1. The means manifest hetersis fr the linecrsses and inbreeding depressin fr the inbreds, where the average inbreeding had risen t 36.3% by Hwever, because lwer feed cnsumptin per unit f gain is desirable, slightly higher values fr the linecrss ppulatin f 6.71 and 6.75 fr unadjusted and adjusted FE as cmpared with crrespnding values f 6.51 and 6.46 fr the inbred grup suggest slightly mre efficient feed cnversin f inbreds. Linecrss bulls, n the ther hand, gained faster and cnsumed mre feed. Kch et al. (1963) pinted ut that increases in feed cnsumptin may nt result in prprtinal increase in gain; a curvilinear trend culd result frm increased lsses thrugh heat increment f feeding and(r) changes in cmpsitin f gain at the higher levels f intake.

3 GENETIC PROGRESS IN HEREFORD CATTLE 939 C~ [.. [- r,. ~ ' ~ ~ 0 u r~ X M M [- ~'~ 0 u

4 940 NWAKALOR ET AL. The pled, within-line standard deviatins f the traits shw that pprtunity was cnsiderable fr selectin. Higher variances in the inbred ppulatin result frm an increase in genetic variance between individual lines. Expected Genetic Prgress. The genetic crrelatins and square rts f heritability used t estimate expected genetic prgress are given in table 2 (Armstrng, 1964; Brinks et al., 1964; Petty and Cartwright, 1966; Prestn and Willis, 1974). Using the matrix multiplicatin prcedure described befre and setting: G= / I, L / the expected genetic prgress frm sire selectin (AG s) was btained as: and with WW adjusted fr inbreeding, A'G = [( )] G with WW unadjusted fr inbreeding, A'G = [( )] G. The results f these multiplicatins (table 3) shw that genetic prgress was expected frm sire selectin fr all traits under study except gain frm weaning t 12 m in females. Mst f the expected genetic prgress in WW f 4.07 and 4.02 kg resulted frm sire selectin. Expected genetic prgress using WW adjusted and unadjusted fr inbreeding, respectively, amunted t 4.62 and 4.43 kg fr IW and and kg fr FW. Prgress expected per generatin in actual units was fairly large fr ADG (.040 kg) but smaller fr FE (--.018). Even thugh selectin pressure was relatively mild n pstweaning traits in females, a fair amunt f additinal imprvement shuld be expected in IW, FW, FE and ADG frm selectin amng females. Therefre, expected prgress indicated fr these traits (table 3) are likely t be biased smewhat dwnward. Expected genetic prgress per generatin in standard measure resulting frm female selectin (AG d) was btained by matrix multiplicatins as shwn, setting: TABLE 2. GENETIC CORRELATIONS AND SQUARE ROOTS OF HERITABIL1TY (gi) USED TO ESTIMATE EXPECTED GENETIC PROGRESS Inbred males Trait WW FE ADG (gi) Weaning wt (WW) a.58 c.55 d Feed efficiency (FE) a.60 d Avg daily gain (ADG) a Females Trait WW 12W 18W SPW FAW (gi) Weaning wt (WW) b.67 c.59 b.51 b.55 d 12-m wt (12W) b.66 b.62 b.64 b 18-m wt (18W) b.74 b.71 b Mature spring wt (SPW) b.72 b Mature fall wt (FAW) b aarmstrng (1964). bbrinks et al. (1964). Cpetty and Cartwright (1966). dprestn and Willis (1970).

5 GENETIC PROGRESS IN HEREFORD CATTLE 941 ~ inbred females with WW adjusted fr inbreeding, G = A'G = [( )]G; inbred females with WW unadjusted fr inbreeding, m A'G = [( )]G; TABLE 3. EXPECTED GENETIC PROGRESS PER GENERATION IN STANDARD AND ACTUAL UNITS FROM SELECTION AMONG MALES AND FEMALES Prgress in standard measure Prgress in actual measure a WW adjusted WW unadjusted WW adjusted WW unadjusted fr Fx b fr Fx b fr Fx b fr Fx b Trait AG s AG s AG s Avg c AG s Avg c Weaning wt (WW) Initial test wt d Final wt ff test e Feed efficiency Avg daily gain (ADG) Males AG d AG d AG s AGd Avg c AG s AG d Avg c Inbred females Weaning wt (WW) Gain, wean t 12 mf m wt , Gain, 12 t 18 mf m wt Mature spring wt Mature fall wt Linecrss females Weaning wt (ww)g Gain, wean t 12 m f m wt Gain, 12 t 18 m f ran wt Mature spring wt Mature fall wt aunits in kg except fr feed efficiency. blnbreeding effects. CAvg = [(AG s + AGd)/2] fr weaning weight in males and pstweaning traits in females, = 1/2AG s fr pstweaning traits in males, where AG s and AG d are expected genetic prgress frm selectin amng bulls and females, respectively. dprgress = Prgress in WW + 20 (Prgress in ADG). eprgress = Prgress in WW (Prgress in ADG). fprgress = Difference between the tw weights cncerned. gadjusted weaning weight.

6 942 NWAKALOR ET AL. linecrss females, A'G = [( )] G. The results in standard and actual units shw that except fr WW, negative genetic prgress was expected in all traits frm selectin f inbred females (table 3). On the cntrary, psitive prgress was expected frm linecrss female selectin in WW, SPW and FAW. Fr 12W and 18W, genetic prgress expected frm linecrss female selectin was als negative. As demnstrated earlier (Nwakalr et al., 1986) mst f the selectin pressure fr increased weights and gains was n the sires. Therefre, sire cntributin t prgress in the pstweaning traits f females has t be taken int accunt. Eighteen-mnth weight in heifers and FW in bulls describe abut the same stage f develpment and bth traits appear t have nearly the same estimate f heritability and indicate similar genetic crrelatins with weights and gains recrded earlier in life. Therefre, 18W in heifers was cnsidered nearly equivalent t FW in bulls and ne standard deviatin f selectin pressure in FW f bulls was equated t ne standard deviatin f selectin pressure in 18W f heifers (Brinks et al., 1965). Cnsidering that the genetic crrelatin between weights f bulls and heifers is nt unity, the expected genetic imprvement btained is prbably biased upward but represents a reasnable maximum estimate. The standardied selectin differential f.46/generatin fr FW f bulls was set equal t.460 f 18W f heifers, and the expected direct respnse in 18W calculated. The crrelated respnses in ther pstweaning traits in females were then cmputed. The results in actual units frm selectin amng sires and females fr pstweaning traits in females tgether with their averages (table 3) shw that genetic imprvement was expected in WW, and all pstweaning traits in females studied except gain frm weaning t 12 t. Because the phentypic crrelatins and the genetic parameters used t estimate expected genetic prgress were nt derived frm the data presently being studied, the results are likely t be biased. The expected genetic prgress per generatin in standard measure in WW frm selectin amng sires f abut.270 is lwer than.360 reprted by Brinks et al. (1965). Averaged ver males and females, the expected prgress fr WW wuld be.15/generatin, which is slightly lwer than the results f Buchanan (1982) fr three lines studied. The expected prgress in ADG frm the present study f abut.420/ generatin is similar t values reprted fr the same trait by Buchanan (1982). Realied Respnse t Selectin Pbentypic Time Trends. The phentypic regressins f the means f perfrmance traits n years, tgether with the averages f withinline regressins fr the inbred and linecrss ppulatins, are shwn in table 4. Estimate f the ttal phentypic change was btained by multiplying the pled regressin by the number f years fr each trait. The regressins cntain bth the genetic and the envirnmental trends; the genetic trend is due t bth withinand between-line selectin because six f the 14 lines develped were culled frm the herd because f lw perfrmance during the selectin perid. The averages f within-line regressins cntain the envirnmental trend and the genetic change due t selectin withut culling f pr lines. The average phentypic trend in BW was negative fr bth inbred and linecrss calves, resulting in a decline f abut.4 kg ver 5 yr in each ppulatin. Hwever, t avid prblems f dystcia and mrtality, large increases in BW are nt desirable. Heart girth circumference at birth increased by 3.8 and 8.1 cm ver the years in the tw ppulatins. Frm the regressin n years, adjusted WW (unadjusted fr inbreeding) shwed a decrease f 9.8 kg ver 28 yr in the inbred grup. After crrecting the data fr inbreeding effects, a psitive trend was btained fr bth the inbreds and linecrsses, resulting in phentypic imprvements f 9.2 and 12.1 kg, respectively. There were increases f and pints in WS ver the years fr the tw grups. T the extent that part f this phentypic change is genetic, an imprvement wuld be expected in bdy cnfrmatin at weaning due t selectin. Weaning age declined by 10.3 and.3 d ver the years fr the inbred and linecrss calves. The difference in trend prbably indicates a tendency fr the linecrss animals t calve earlier in the seasn than the inbreds. Phentypic changes in pstweaning traits f bulls were negative fr IW, amunting t declines f 9.1 and 9.8 kg in 25 yr, respectively,

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8 944 NWAKALOR ET AL. fr inbreds and linecrsses. The regressins fr FW and FG appear extremely large and prbably verestimate the actual genetic imprvement, due t abrupt envirnmental changes. Ttal increases were 46.5 and 68.4 kg ver 17 yr fr FW and 371 and 422 kg in 23 yr fr FC, respectively, in inbred and linecrss ppulatins. Regressins fr FE were psitive and amunted t and units ver 23 yr, respectively, fr inbred and linecrss bulls fr unadjusted data. The crrespnding values were x 10-2 and units fr adjusted data. These changes represent average phentypic declines in FE. Linecrss bulls appear t have been less adversely affected than their inbred cntempraries. Whereas 140-d ADG decreased by x 10-2 kg in 25 yr fr inbred bulls, it increased by 3.9 x 10-2 kg fr linecrss bulls. The regressins f pstweaning traits f females n years indicate substantial phentypic imprvements fr yearling weights and gains; ttal increases ver the years were, respectively, fr inbreds and linecrsses: 9.2 and 3.9 kg fr gain frm weaning t 12 m, 14.5 and 20.9 kg fr 12W, 4.2 and 7.7 kg fr 12- t 18-m gain, and 21.1 and 36.2 kg fr 18W. Regressins were negative fr the inbreds and psitive fr the linecrsses fr all three mature cw weights. Over 27 yr, these amunted, respectively, t decreases and increases f and 34.1 kg fr SPW,-15.9 and 36.9 kg fr SUW and and 42.9 kg fr FAW. Armstrng (1964) reprted negative phentypic trends fr IW, FW, ADG and FE fr inbred bulls in the same herd ver the years. Brinks et al. (1965) and Brinks and Knapp (1975) demnstrated that in additin t a marked detrimental effect n birth and weaning traits, inbreeding f calf and dam als had a sieable detrimental effect n pstweaning gain and FW in bulls and a large detrimental effect n pstweaning weights and gains in females. Genetic Prgress. The estimated genetic changes per generatin in the inbred and the linecrss ppulatins due t within-line selectin were btained as the average f within-line regressins f ffspring deviatins frm annual means f perfrmance traits n generatin numbers (table 5). Estimates f the ttal genetic changes ver the years are derived by multiplying the genetic change per generatin by (n - 1), where n is the number f genera- tins. Results shw that by 1973, 5.5 and 5.4 generatins f selectin had been practiced, respectively, in the inbred and linecrss ppulatins. Inspectin f generatin numbers by line and year shwed lack f variability in early years, which made intra-year regressin n generatin numbers statistically impssible. Pssibly, sme cnfunded year effects and insufficient verlap f generatins culd bias upward the estimated genetic changes. Hwever, the use f within-line ffspring deviatin frm annual means f traits shuld remve a large part f the year-t-year envirnmental variatins. Estimates f genetic changes (table 5) shw that except fr adjusted WW (adjusted fr inbreeding) all birth and weaning traits studied declined genetically ver the years in the inbred ppulatin. Over the years, there were declines f -.7 kg in BW, cm in HGC at birth, -.5 kg fr adjusted WW (unadjusted fr inbreeding) and pints fr WS. Substantial genetic imprvement f 15.6 kg appears t have been made ver the perid in adjusted WW (adjusted fr inbreeding). The ttal genetic changes f -.5 and 15.6 kg in WW f inbred calves are greater than the ttal phentypic changes f -9.8 and 9.2 kg derived frm table 4, suggesting that the envirnmental trend was negative. In the linecrss ppulatin, genetic regressin per generatin yielded ttal genetic increase f 1.2 kg in BW ver the perid. Genetic change amunted essentially t er fr HGC at birth and shwed a slight decrease f pints fr WS during the selectin perid. Abut equal phentypic and genetic changes f.432 and.423 kg/yr in WW wuld imply that the envirnment was fairly stable s that virtually all the phentypic imprvement was genetic; the results btained fr the inbreds and thse frm previus study (Nwakalr et al., 1976) make this seem unlikely. The genetic regressins fr pstweaning traits f males shw that inbred bulls declined by -1.3 and -8.7 kg in IW and FW ver the years. These are equivalent t decreases f -.04 and Hwever, genetic prgress was indicated fr the bulls in FC, FE (adjusted and unadjusted) and ADG, amunting t imprvements f 12.3 kg,.668 and.590 units and.08 kg, respectively, ver the perid. In linecrss bulls, there were genetic imprvements f 17.9, 2.5 and 32.6 kg in IW, FW and FC, respectively. Althugh the linecrsses increased mre, genetically, in FC than the inbreds, they declined

9 GENETIC PROGRESS IN HEREFORD CATTLE 945 TABLE 5. ESTIMATES OF GENETIC CHANGES IN PERFORMANCE TRAITS PER GENERATION AND YEAR FOR THE OVERALL INBRED AND LINECROSS POPULATIONS Trait a lnbreds Linecrsses bg/ bg/ genera- bg/ genera- bg/ N tin yr N tin yr Calves Birth wt, kg HGC at birth, cm 1, , Adjusted WW (unadjusted fr Fx), kg 1, Adjusted WW, kg 1, , Weaning scre, 10-2 pints 1, , Bulls Initial test wt, kg Final wt, kg Feed cnsumed, kg FEU, 10-2 units FEA, 10-2 units d ADG, 10-2 kg Females Gain (wean t 12 m), kg m wt, kg Gain (12 t 18 m), kg t wt, kg Mature spring wt, kg Mature summer wt, kg Mature fall wt, kg ahgc = heart girth circumference, WW = weaning weight, Fx = inbreeding effects, FEU = unadjusted feed efficiency, FEA = adjusted feed efficiency, ADG = pstweaning average daily gain..169 and.112 units in FE ver the years fr adjusted and unadjusted data. The ttal imprvement f.05 kg in ADG fr the linecrss bulls was less than that in the inbred ppulatin. The lss in FE despite genetic imprvement in FC amng linecrss bulls may be attributed t increased lsses thrugh heat increment f feeding and(r) changes in cmpsitin f gain at the higher levels f intake (Kch et al., 1963). In pstweaning traits inbred females shwed negative genetic changes fr gain frm weaning t 12 m, 12- t 18-t gain, 18W f heifers, SPW, SUW and FAW f cws, yielding verall declines f -1.6, -7.0, -7.4, -4.0, and kg, respectively, ver the generatins. Regressin cefficient fr 12W suggests imprvement f 4.4 kg in genetic merit during the same perid. In linecrss females, there was psitive respnse t selectin in all yearling weights and gains and mature weights. Ttal genetic increases were 6.8, 27.9, 3.6 and 19.2 kg fr gain frm weaning t 12 m, 12W, gain frm 12 t 18 m and 18W; in standard measure, these crrespnd t.67, 1.1,.28 and.67. Genetic imprvement amunted t 2.0, 23.0 and 13.0 kg fr SPW, SUW and FAW; the equivalent imprvements in standard measure were.05,.52 and.29. As pinted ut by Kch et al. (1974), intrayear regressin f ffspring n generatin cefficients is subject t sampling errr due t the rather small variatin f number f generatins present within a given year and t sampiing variatin in rate f increase in cumulative selectin per generatin fr selected sires and dams. Whereas respnse t selectin was cnfunded with cumulative inbreeding f dam and ffspring in the inbred lines, inbreeding effect was apparently negligible in the linecrsses, where linecrss females were used as dams. Inbreeding als culd have cntributed errr frm randm genetic drift amng different lines subjected t the same type f selectin

10 946 NWAKALOR ET AL. (Dickersn et al., 1974). It seems plausible t pstulate that high rates and levels f inbreeding are respnsible fr the different patterns f genetic respnse bserved fr the inbred and the linecrss grups. Armstrng (1964) reached a similar cnclusin in his study in which selectin in inbred lines generally failed t imprve genetic merit in all traits studied. Genetic Prgress frm Between-Line Selectin. The bjective f the breeding system was nt s much t take advantage f inbreeding per se as t increase hmygsity by restricting breeding and selectin within individual lines f sires. The ultimate gal f this selectin strategy was t create lines distinct frm ne anther, which n crssing wuld result in maximum heterygsity and within-breed hetersis. The culling f inferir lines frm the herd (between line selectin) ver the years prvided additinal pprtunity fr genetic imprvement. Of the 14 lines and their linecrss grups that were develped ver the years, six have been discarded and eight still remain in the herd. Lines 6, 7 and 8 were culled in 1955 and lines 1 and 15 in 1958 and 1967 fr lw prductivity. Line 2 was discntinued in 1964 because f dwarfism, which als had been bserved in line 8. The line culling prvided pprtunity t enlarge mre prmising lines and start new nes and als shuld lead t faster genetic prgress in the verall herd. Previus analysis f data demnstrated that the discntinued lines had, in fact, belw-average genetic merit (Nwakalr et al., 1976). The pled phentypic regressins given in table 4 cntain the envirnmental change and all the genetic change. Under the assumptin that there was n migratin in the herd and barring mutatin and randm drift, the genetic change is due t selectin effects. Because selectin was practiced within and between lines the pled regressins represent the envirnmental change and the genetic changes due t within and between line selectins. The averages f within-line phentypic regressins (table 4) are bth the envirnmental and within-line selectin effects. The difference between the tw regressins estimates the apprximate genetic prgress resulting frm culling f prer lines. The phentypic regressins in the pled analysis wuld be greater than the average within-line regressins due t relatively smaller cntributin (smaller number f recrds) f the prer lines in the pled analysis than in the average f within-line regressin and effects f between-line selectin. The average f withinline regressins shuld give an indicatin f what is expected frm a ppulatin f inbred lines withut culling f pr lines. The results shw that in the inbred ppulatin, except fr BW, WS, FC and SPW, genetic imprvement was made in all ther traits, due t culling f pr lines. Imprvements were as much as.2,.5,.4 and.6 kg/yr fr adjusted WW (adjusted fr inbreeding), FW in bulls, 12W and 18W in females. In the linecrss ppulatin, negative genetic prgress was indicated in all birth and weaning traits, pstweaning traits in males and 12 t 18 m gain in females. Imprvements per year were.1,.9, 2.7,.7,.6 and.8 kg fr gain frm weaning t 12 m, 12W, 18W, SPW, SUW and FAW, respectively, in linecrss females. Thus, as expected, the culling f prer lines yielded greater genetic imprvement in the inbred than linecrss ppulatin because hetersis frm crssing f higherperfrming lines wuld be less than that frm mre extreme lines. Between-line selectin resulted in additinal genetic prgress ver that achieved by restricting selectin t a within-line basis. Expected vs Actual Respnse. Table 6 summaries the expected genetic respnse and estimates f the actual genetic and phentypic respnses per generatin. The phentypic respnse per generatin was btained by muhiplying the regressin n years by the generatin interval. Fr thse traits where the expected genetic respnse was nt calculated, nly the estimated genetic and the average phentypic respnses are listed fr direct cmparisn. The expected genetic respnse in WW and pstweaning traits in linecrss females was btained as the average f the expected respnses in linecrss females and inbred males when WW was adjusted fr inbreeding. In the inbred ppulatin, genetic prgress was expected in WW and all pstweaning traits in males and females studied, except gain frm weaning t 12 rn. Actual genetic imprvement in these traits was generally nt as rapid as expected, and was negative in mst cases. In FE, much mre prgress was realied than expected. In the linecrss grup genetic prgress was expected in WW and all pstweaning traits in males and females studied except gain frm weaning t 12 m in females. Althugh mstly psitive, the estimated genetic respnse

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12 948 NWAKALOR ET AL. was again smaller than expected in all traits except gain frm weaning t 12 t, 12W and 18W. The average phentypic change in the inbred grup was less than the expected genetic respnse in several traits including WW, IW, FE, ADG, 12- t 18-m gain, SPW and FAW and was negative in almst all cases. Hwever, this was greater than the expected genetic prgress in FW in bulls, gain frm weaning t 12 t, 12W and 18W in females. In the linecrss grup, the average phentypic change was greater than the expected genetic change in mst f the traits fr which the latter was cmputed, including FW in bulls and all pstweaning traits in females studied except gain frm weaning t 12 m. Generally, the rati f the average phentypic respnse t the expected genetic respnse in these traits was much greater than unity, indicating that even thugh the phentypic regressins may verestimate the actual genetic respnse, these was apparently a cnsiderable psitive respnse in the traits. Like the estimated genetic respnse, the average phentypic respnse was far belw the predicted genetic prgress fr WW, lw, FE, ADG and gain frm weaning t 12 m. Althugh variable, the results frm this study are nt in harmny with expectatins. Apparently, the actual prgress was belw predictin in mst f the traits studied. The results d nt agree with thse f Brinks et al. (1965), wh reprted that actual prgress was greater than expected in birth and weaning traits and was as great as r greater than expected fr all pstweaning traits studied in a mildly inbred line. The results, hwever, agree clsely with thse f Armstrng (1964) frm the data f inbred lines in the same herd being evaluated and thse f Dickersn et al. (1954) frm swine data, where selectin apparently failed t imprve measurably the genetic merit fr traits studied. Dickersn et al. (1974), als frm swine data, demnstrated that imprvement frm selectin was at least as rapid as expected in all lines and crsses fr pstweaning grwth, but was far belw predictin fr sw prductivity in parental lines selected fr crss perfrmance. Perhaps the departures f bserved respnses t cntinued selectin frm theretical expectatins culd be explained frm the fact that thery predicts a linear (r unifrm rate f) respnse. Hwever, with species like cattle that have a very lw reprductive rate and with verlapping generatins, the rate f genetic imprvement is nt expected t be linear and, in fact, is expected t underestimate the linear respnse. This is because the genes frm a grup f selected individuals may take many years t pass thrugh the ppulatin. The rates f respnse predicted by the classical thery are, therefre, reached nly asympttically, and this can be thught f as an extremely lng (in years) lag-time effect with erratic early respnses (Hill, 1974; McClintck and Cunningham, 1974). As Hill (1971) and I-links (1971) have shwn, the departures frm predictins are larger when animals are retained fr many mre breeding seasns, such as in cattle. Pssibly~ the discrepancy between bserved and predicted respnses can be minimied by imprvements in design and scale f experiments, mre accurate estimates f genetic and phentypic parameters, mre dependable methds f estimating genetic prgress and a reappraisal f the effects f inbreeding n selectin within inbred lines and their crsses. Literature Cited Armstrng, J. B Evaluatin f selectin intensity and genetic changes in experimental herd f Herefrd cattle. Ph.D. Thesis. Clrad State Univ., Frt Cllins. Brinks, J. S., R. T. Clark and N. M. Kieffer Evaluatin f respnse t selectin and inbreeding in a clsed line f Herefrd cattle. USDA Tech. Bull Brinks, J. S., R. T. Clark, N. M. Kieffer and J. J. Urick Estimates f genetic, envirnmental and phentypic parameters in range Herefrd females. J. Anita. Sci. 23:711. Brinks, J. S., R. T. Clark and F. J. Rice Estimatin f genetic trends in beef cattle. J. Anita. Sci. 20:903 (Abstr.). Brinks, J. S. and B. W. Knapp Effects f inbreeding n perfrmance traits f beef cattle in the Western Regin. Clrad State Univ. Exp. Sta. Tech. Bull Brinks, J. S., B. W. Knapp, J. J. Urick and O. F. Pahnish Maternal hetersis in pstweaning traits f cattle. Prc. West. Sec. Amer. Sc. Anita. Sci. 22:181. Brinks, J. S., B. W. Knapp, J. J. Urick and O. F. Pahnish Hetersis in preweaning maternal traits amng lines f Herefrd cattle. J. Anita. Sci. 34:14. Brinks, J. S., J. J. Urick, O. F. Pahnish, B. W. Knapp and T. J. Riley Hetersis in preweaning and weaning traits amng lines f Herefrd cattle. J. A1aim. Sci. 26:278. Buchanan, D. S., M. K. Nielsen, R. M. Kch and L. V. Cundiff Selectin fr grwth and muscling scre in beef cattle.. Genetic parameters and predicted respnse. J. Anim. Sci. 55:526. Dickersn, G. E., C. T. Blunn, A. B. Chapman, R. M.

13 GENETIC PROGRESS IN HEREFORD CATTLE 949 Kttman, J. L. Krider, E. J. Warwick and J. A. Whatley, Jr Evaluatin f selectin in develping inbred lines f swine. Nrth Central Reginal Pub. 38, Missuri Agr. Exp. Sta. Res. Bull, 551. Dickersn, G. E., H. O. Heter, E. V. Krehbiel and A. E. Flwer Effectiveness f reciprcal selectin fr perfrmance f crsses between Mntana N. 1 and Yrkshire swine. I. Expected and actual respnse. J. Anita. Sci. 39:24. Flwer, A. E., J. S. Brinks, J. J. Urick and F. S. Willsn Cmparisn f inbred lines and linecrsses fr perfrmance traits in Herefrd range cattle. J. Anita. Sci. 22:914. Harvey, W. R. and G. D. Bearden Tables f expected genetic prgress in each f tw traits. USDA Agr. Res. Serv., ARS Harwin, G. O The effect f inbreeding and envirnmental factrs n the weaning weight and pstweaning grwth f range Herefrd cattle. Ph.D. Thesis, Clrad State Univ., Frt Cllins. Hill, W. G Investment appraisal fr natinal breeding prgrammes. Anim. Prd. 13:37. Hill, W. G Predictin and evaluatin f respnse t selectin with verlapping generatins. Anita. Prd. 18:117. Hinks, C.J.M The genetic and financial cnsequences f selectin amngst dairy bulls in artificial inseminatin. Anita. Prd. 13:209. Kch, R. M., K. E. Gregry and L. V. Cundiff Selectin in beef cattle.. Selectin respnse, J. Anim. Sci. 39:459. Kch, R. M., L. A. Swiger, Dyle Chambers and K. E. Gregry Efficiency f feed use in beef cattle. J. Anita. Sci. 22:486. Lasley, J. F Genetics f Livestck Imprvement. Prentice-Hall, Inc., Englewd Cliffs, NJ. Lush, J. L Effectiveness f selectin (Summary). J. Anim. Sci. 10:3. McClintck, A. E. and E. P. Cunningham Selectin in dual purpse cattle ppulatins: Defining the breeding bjective. Anita. Prd. 18: 237. Nwakalr, L. N., J. S. Brinks and G. V. Richardsn Estimated genetic change in weaning weight f beef cattle. J. Anita. Sci. 43 : 396. Nwakalr, L. N., J. S. Brinks and G. V. Richardsn Selectin in Herefrd cattle. I. Selectin intensity, generatin interval and indexes in retrspect. J. Anita. Sci. 62:927. Petty, R. R. and T. C. Cartwright A summary f genetic and envirnmental statistics fr grwth and cnfrmatin traits f yung beef cattle. Texas Agr. Exp. Sta. Dept. Anim. Sci. Tech. Rep. 5. Pirchner, F Ppulatin Genetics in Animal Breeding. W. H. Freeman and C., San Francisc. p 154. Prestn, T. R. and M. B. Willis Intensive Beef Prductin. Pergamn Press, Oxfrd, CT. Ray, D. E., C. B. Rubicek, O. F. Pahnish and J. S. Brinks Breeding merit f tpcrss parents fr preweaning traits in Herefrd cattle. J. Anita. Sci. 30:161. Urick, J. J., J. S. Brinks, O. F. Pahnish, B. W. Knapp and T. M. Riley Hetersis in preweaning traits amng lines f Herefrd cattle. J. Anita. Sci. 27:323.