SELECTION IN HEREFORD CATTLE. SELECTION INTENSITY, GENERATION INTERVAL AND INDEXES IN RETROSPECT

I. SELECTION IN HEREFORD CATTLE. SELECTION INTENSITY, GENERATION INTERVAL AND INDEXES IN RETROSPECT L. N. Nwakalor 1, J. S. Brinks 2 and G. V. Richardson 3 Colorado State University, Fort Collins 80523 ABSTRACT Selection intensity and generation interval were evaluated in a Hereford cattle herd made up of 14 inbred lines and 14 linecross groups corresponding to the lines of inbred sires at the Suan Juan Basin Research Center, Hesperus, Colorado. Selection indexes practiced were calculated in retrospect. Analyses of the records collected from 1946 through 1973 involved weaning weight (WW) and postweaning traits in males and females. Analyses by line were performed for the inbreds, while pooled analyses were done on the inbred and linecross populations. From records of 1,239 calves weaned, age of sire averaged 3.75 yr compared with 4.52 yr for age of dam, showing faster generation turnover for sires than for dams. Generation interval determined as actual age of midparent was 4.13 yr. Selection applied for WW, evaluated as annual selection differentials within inbred lines and then pooled over all lines, averaged.55 standard deviations (a)/generation for sires. For females, selection was much less intense. Midparent selection differential amounted to.33o/ generation. For sires, pooled standardized selection differentials per generation over all lines during the postweaning gain period were.49cr,.46tr,.400, -.20e, -.10o and.69o, respectively, for initial weight, final weight, feed consumed, feed efficiency (FE, unadjusted and adjusted) and average daily gain (ADG). Selection of females for postweaning traits was not intense. Selection index actually practiced in retrospect for sires was: I S =.4461 (WW) -.0092 (FE) +.6126 (ADG). The indexes for dams included WW, 12-mo weight (12W), 18-mo weight (18W), mature spring weight (SPW) and mature fall weight (FAW) and were: for inbred dams, I D =.1824 (WW) -.0284 (12W) +.0736 (18W) -.1097 (SPW) -.1097 (FAW); for linecross dams, I D =.2693 (WW) -.2960 (12W) +.0147 (18W) +.1185 (SPW) -.0354 (FAW). The corresponding index selection differentials were.818,.203 and.209. Sire index selection differentials represent about 79% of the total selection differentials. (Key Words: Hereford, Cattle, Selection Intensity, Indexes, Generation Interval.) Introduction Genetic response per year to selection depends on the extent to which parents deviate from average, on the average genetic variation and covariation of all traits directly or indirectly related to traits under selection, and on the generation interval between selected generations. In any population the smaller the proportion of animals selected as parents, the greater will be the selection differential provided that those selected come from the upper segment of 1Present address: Dept. of Anita, Sci., Univ. of Nigeria, Nsukka, Nigeria. 2Professor, Dept. of Anita. Sci., Colorado State Univ. 3Biomerrician, ARS, USDA, Fort Collins, CO 80523. Received March 21, 1985. Accepted October 30, 1985. the merit scale for the trait involved. Longevity in parental stock will increase the generation interval, but at the same time, increase the accuracy of the predicted breeding value from the additional data. This study evaluates the intensity of selection practiced for weaning weight and postweaning traits and the generation interval in males and females at San Juan Basin Research Center Hereford herd. Selection indexes also were calculated in retrospect. Materials and Methods Breeding System. The breeding project was started in 1946 at the San Juan Basin Research Center, Hesperus with the primary purpose of studying the prospects for the utilization of within-breed heterosis in commercial beef production. Over the years 14 established inbred lines of Hereford cattle were used in reciprocal crossing to produce 14 Iinecross groups corre- 927 J. Anim. Sci. 1986.62:927--936

928 NWAKALOR ET AL. sponding to the lines of inbred sires. Linecross cows were mated concurrently to inbred line herd sires to produce linecross progeny. By 1973 the average inbreeding levels in the herd were 25.5 and 36.3% for dam and calf. The mating scheme used to initiate most of the lines was sire x daughter mating. Detailed information on line formation has been given by Anonymous (1975). Cattle Management and Selection of Replacements. Cows were grazed on range throughout the year and fed supplemental hay from late December until early or mid-may. They were placed in individual sire breeding pastures during the breeding season. Until 1967 the breeding season extended 90 d, from early June to early September. Beginning in 1968 the season was reduced to 60 d, from mid-may to mid-july. At the end of the breeding season, sires were removed and cows and calves were run together on native range until weaning in late October to early November at an average age of approximately 200 d. Calves were not creep fed. All bulls were retained uncastrated on pasture; after weaning they were given a 2- to 4-wk adjustment period before they were individually fed during a 140-d performance test. Feeding of the bulls was done in the late afternoon and early morning hours with the bulls chained all night and allowed free exercise during the day. Water was available and daily feed records were kept on each animal. The bulls were weighed on two successive days to establish accurate initial test weights and final weights. All bulls were observed closely for sickness, vaccinated against warts and treated for external parasites. They were retested for tuberculosis and brucellosis during the test period. A semen evaluation and physical examination for breeding soundness was performed on each bull at the completion of test. Until 1971, about one-third of the heifers were retained at weaning for breeding and were fed to gain approximately.45 kg/d through the winter. Replacements were selected primarily on age, weight and need for replacements within specific line or linecross groups. Selected heifers were exposed to bulls at about 15 mo of age. Cows were culled based on age, health, soundness and the results of pregnancy examinations, with most open cows being culled. Beginning in the fall of 1969 some emphasis was placed on most probable producing ability values of cows based on calves' weaning weight. Selection of herd sires was done within lines. Before 1956, selection of replacement inbred bulls was based on an index, giving equal weight to adjusted weaning weight, feed efficiency on test and yearling grade. From 1956 through 1966, selection was on the index: I -- adjusted weaning weight + 50 (postweaning daily gain). Beginning in 1967, bulls were selected on preweaning and postweaning growth performance. Selected bulls were usually first used as yearlings and were used for one to several years depending on whether superior younger bulls of the same line were available. Performance Traits. Weaning weight (WW) was adjusted for sex, age and age of dam. Additional adjustments were made for inbreeding of calf and dam. Adjustment factors used for WW were those derived for the herd from an earlier study of the same herd by Harwin (1963) shown in table 1. Adjustment factors for postweaning traits were calculated from the present study. Postweaning traits of males include initial test weight (IW), final weight (FW), feed consumed (FC), feed efficiency (FE) unadjusted and adjusted to midweight on test and average daily gain (ADG) over the test period. Postweaning traits of females comprise gain from weaning to 12 mo, 12-mo weight (12W, adjusted to 365 d), gain from 12 to 18 mo, 18-mo weight (18W, adjusted to 550 d) and mature weights in spring (SPW), summer (SUW) and fall (FAW). Mature cow weight data were analyzed by least-squares procedures for the effects of year of record, age of cow when weight was taken and lactation status. Age classes ranged from 1 to 10+ and lactation status from 1 to 4 (wet-wet, wet-dry, dry-wet and dry-dry, representing whether or not a cow had a calf in the preceding year and in the year weight record was taken). The weight-by-age class constants for cow weights taken three times each year showed that cows continue to gain weight until 8 yr of age, and then decline slightly thereafter, similar to the pattern shown by the Miles City study of Brinks et al. (1962). Year of record, age of cow and lactation status all had effects (P.01) on female weights and the individual weights from ages of 2 through 10+ were adjusted for the effects using the least-squares constants. Adjustment for age of cow was made to 8-yr-old equivalent while lactation status was adjusted to a wet-wet (lactation in 2 consecutive yr)basis.

SELECTION INTENSITY IN HEREFORD CATTLE 929 TABLE 1. ADJUSTMENT FACTORS (KG) USED FOR WEANING WEIGHT Sex of calf Effect Bull Heifer Sex 0 11.3 Age of dam 2 yr old 28.6 24.9 3 yr old 16.3 13.2 4 yr old 6.8 6.3 5 yr old and over 0 0 Age of calf Inbreeding of calf (Fc) (per 1% Fc) Inbreeding of dam (Fd) (per 1% Fd) Adjustment to a constant age of 205 d. Add gain/d if under 205 d. Stibtract gain/d if over 205 d..186.109.354.358 Average mature weights were obtained for individual cows as the average of weights taken from 2 to 10+ yr of age after adjustments. Evaluation of Selection Applied. Selection applied was measured as the average annual selection differential of parents (Ap), which is the difference in mean performance of selected parents compared with the average of the unselected group from which they came, divided by the average age of parents (P,) when offspring were born. Annual selection differentials were computed within lines separately for sires (AS) and dams (AD). Because each sex of parent contributes equally to the genetic composition of the progeny in the next generation, the net selection differential was the average of sire and dam. Formulas used for calculating annual selection differentials were presented by Dickerson and Hazel (1944), Dickerson et al. (1954), Rendel and Robertson (1950) and Brinks et al. (1965). The formulas that account for sequential culling over years of animals from the herd are: ~nis i AS- NA' AD- ~njdj NA' AS + AD AP---- 2 and ~nia i + Y~njAj 2N or the average age of parents when the offspring were born. Terms ni and nj are the number of progeny by a particular sire and dam, respectively, in a given year; s i and dj are the superiority or inferiority of a particular sire and dam measured as the deviation from the mean of the unselected group in which they were born; and N is the number of progeny in a given year. Only those animals whose sires and dams were born within the respective lines (apart from foundation parents) were used in calculating selection differentials (SD). Foundation animals were assigned a SD of zero and an age at the time offspring were born equal to the number of years after the line was started for calculating their contribution to annual SD. Parents born at the time the line was started or later were assigned their actual ages. Annual SD were computed separately for inbred sires, inbred females and linecross females. Values for respective lines, weighted by the number of offspring contributed by each line, were pooled to obtain the overall annual SD for the herd. Mean annual SD was taken as the unweighted average over all years. Selection differentials were not calculated for the early years of each line when all of either male or female parents were foundation animals and

930 NWAKALOR ET AL. performance records were not available on them. Selection Indexes in Retrospect. The correlation matrix multiplication procedure of Harvey and Bearden (1962) was used to compute the selection indexes actually employed in retrospect. The phenotypic correlation matrix (as the independent variables) was equated to the SD in standard units (s i) matrix (as the dependent variable). Solution of the equations yielded a i (the relative weight expressed as standard partial regression coefficients) for the indexes actually practiced. Indexes so obtained give each characteristic the average emphasis it actually had during selection. Selection differentials in standard measure for the index actually practiced was calculated from the formula (Harvey and Bearden, 1962): I = (als I + azs2 +. 9 9 + ansn) 1/2. The phenotypic correlations used in this study were average estimates obtained for different traits from the studies of Armstrong (1964), Brinks et al. (1964), Petty and Cartwright (1966) and Preston and Willis (1974). Analysis by Line Results Age of Parents. Average ages of parents by line calculated from WW records show that in 8 of the 12 lines used, average age of dams was greater than that of sires, indicating faster replacement rates for sires. Ages of sires and dams in lines 2, 6, 7 and 14 suggest slightly more rapid replacement rates of dams than of sires, resulting from the use of older sires than dams. Generation interval ranged from 2.00 yr in line 7 to 5.83 yr in line 15, which shows substantial differences in parent age between lines for WW. Average parent ages calculated from post-weaning records of males and females were similar to those for WW. In general, generation turnover was faster for sires than for dams. Selection Differentials. Mean SD in standard units per generation for WW (table 2) ranged from -1.70 in line 6 to 1.85 in line 2 for males when the data were unadjusted for inbreeding effects. Lesser pressure was applied in female selection, although SD were positive for all lines. Mid-parent SD were all positive except for line 6, and represent a range of saving from the lower 54% to the upper 36% under truncation selection. Selection pressure patterns for WW adjusted for inbreeding were similar to those for data unadjusted for inbreeding and serve as evidence that no direct attention was given to the level of inbreeding in selecting individual replacements. Selection differential of sires for postweaning traits of males (table 3) was positive for most lines for IW, FW, FC and ADG. Selection for FE was negative for several lines, with negative values representing desirable SD. Standardized SD per generation for inbred females showed little selection of females for postweaning traits. In fact, negative selection was practiced for most lines, especially for mature weights, and in these cases individuals selected to be parents were chosen from the bottom segment of the population. Overall Analysis Weaning Weigbt. From a total of 1,239 WW records for the inbred males and females, age of sire averaged 3.75 yr compared with 4.52 yr for age of dam. Generation intervals, determined as actual age of mid-parent, amounted to 4.13 yr. As would be expected, replacement rates were faster for sires than for dams. Mean pooled annual SD of sires and dams for WW (unadjusted for inbreeding) are shown in table 4. Annual SD showed a more consistent trend after 1955. The value for sires averaged 3.4 kg/yr or.50 standard deviations (o)/generation, corresponding to selecting the upper 69% of male population. Selection pressure for in- TABLE 2. SELECTION DIFFERENTIALS IN STANDARD UNITS PER GENERATION OF INBRED MALES AND FEMALES FOR ADJUSTED WEANING WEIGHT (UNADJUSTED FOR INBREEDING) BY LINE Line &S AD AP 2 Bon 1.85.23 1.04 3 BA.76.06.41 4 Colo.88.00.44 5 Don.55.10.32 6 Ft Lew --1.70.22 --.74 7 LaPI --.11.13.02 9 Mon 1.06.38.72 10 Pros 1.41.08.74 11 Roy.58.37.47 12 SJ.81.25.54 14 Tarr.89.09.49 15 RP.14.29.22

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932 NWAKALOR ET AL. TABLE 4. MEAN POOLED ANNUAL SELECTION DIFFERENTIALS FOR ADJUSTED WEANING WEIGHT Item Inbred males and females Linecross (unadjusted for Fx a) females AS AD AP AD Mean/yr in actual units, kg 3.4 Mean/generation in actual units, kg 14.1 Mean/generation in standard units.50 Truncated value, % 69 SD b 1.0 2.2.6 4.3 9.2 2.7.15.33.10 92 78 95 27.9 26.2 alnbreeding effects. bstandard deviation. bred females was 1.0 kg/yr, or about three and one-half times smaller than for sires. This corresponds to standardized SD per generation of.150, with those selected representing the top 92% of the inbred female population. Midparent annual SD amounted to 2.2 kg/generation and was equivalent to.330/generation and selection of the top 78% of the population. Selection differentials of linecross females averaged.6 kg/yr. Standardized SD per generation amounted to.10o, with the truncated selection value representing the upper 95% of the linecross female population. This indicates slightly less selection for linecross than inbred females for WW. Postweaning Traits. The mean annual SD of sires for postweaning traits of males are presented in table 5. Selection differentials were positive for IW, FW, FC and ADG, but negative for FE. Negative SD would be expected for FE because lower feed intake per unit of gain is advantageous. In inbred females, annual SD (table 6) were negative on the average for gains from weaning to 12 too, 12 to 18 mo and mature cow weights, but positive for 12W and 18W. With the exception of 12W and 18W, for which selection was for the top percentage of the population, the selected individuals for other traits represented the lower segment of the population. In linecross females (table 6), average annual SD were small but positive for all traits except 12W and 18W, which amounted to -.2 and zero, respectively. These values (except for 12W which represent selection of the lower segment) indicate that about the upper 98% of the linecross female population were retained for breeding. Selection Indexes in Retrospect. Indexes actually practiced represent the average weightings actually used in any particular year and line and were determined in retrospect for inbred sires, inbred dams and linecross dams. The index for inbred males included adjusted WW, TABLE 5. MEAN POOLED ANNUAL SELECTION DIFFERENTIALS (AS) OVER ALL LINES FOR POSTWEANING TRAITS OF INBRED MALES IW a FW FC FEU FEA ADG Item (kg) (kg) (kg) (units) (units) (units) Mean/yr in actual units 3.9 4.8 18.5 --.043 -.023.031 Mean/generation in actual units 17.1 21.3 79.4 -.185 --.099.132 Mean/generation in standard units.49.46.40 -.20 -.10.69 Truncated value, % 69 72 77 89 95 57 SD b 34.9 46.0 201.2.917.960.191 alw = initial weight, FW = final weight, FC = feed consumed, FEU = unadjusted feed efficiency, FEA = adjusted feed efficiency, ADG = average daily gain. bstandard deviation. AS

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934 NWAKALOR ET AL. FE (adjusted) and postweaning ADG. Initial weight, FW and FC were not included in the index because these are accounted for, respectively, by WW, ADG and FE. Female indexes consisted of adjusted WW, 12W, 18W, SPW and FAW. Postweaning gains were not included because these are fully described by the various weights. The phenotypic correlations and SD used to calculate the indexes are shown in table 7. The standard partial regression coefficients for the indexes actually practiced are given in table 8. Sire indexes show that ADG received the greatest emphasis and had about one and onehalf times the weight given to WW. Although FE received attention in the expected direction, it did not contribute significantly to the indexes. In both the inbred and linecross dam indexes, WW had the greatest emphasis and was expected because selection of heifers for replacement was done at weaning time. Discussion The generation intervals obtained in this study are within the ranges reported by Armstrong (1964), Flower et al. (1964), Brinks et al. (1965), Koch et al. (1974) and Buchanan et al. (1982). As in the present study, they re- ported faster replacement rates for sires than for dams. The more consistent trend in annual SD observed for WW after 1955 probably reflected the change of the selection index that took place after that year. Also the culling of lines 6, 7 and 8 from the herd in 1955 for lower-thanaverage productivity probably resulted in better stabilization of performance in the herd. The effect of both events was demonstrated in a previous study (Nwakalor et al., 1976) where a strong upward genetic trend was reported for WW. Previous studies (Brinks et al., 1961, 1965; Armstrong, 1964; Flower et al., 1964) reported variable but positive selection intensity for WW. Other researchers (Chapman et al., 1972;Koch et al., 1974; Stanforth and Frahm, 1975; Buchanan et al., 1982) reported positive and relatively intense selection pressure for the same trait. In all cases, much greater pressure was applied in male than in female selection. However, in those studies WW was the only trait selected for; in the present study selection was for preweaning and postweaning growth performance. The SD of sires for postweaning traits of males do not represent very intense selection. However, considering that selection was not for single traits, the overall selection pressure for TABLE 7. PHENOTYPIC CORRELATIONS AND SELECTION DIFFERENTIALS (PER GENERATION IN STANDARD MEASURE) USED TO CALCULATE SELECTION INDEXES IN RETROSPECT Trait a WW FE ADG AS Weaning wt (WW) b 1.00.03.17.55 Feed efficiency (FE) c 1.00 --. 17 --. 10 Avg daily gain (ADG) 1.00.69 Trait d WW 12W 18W SPW FAW Inbred AD Linecross Weaning wt (WW) b 1.00.73.64.45.45.11.10 12-mo wt (12W) 1.00.82.57.57.04 -.04 18-mo wt (18W) 1.00.67.67.02.00 Mature spring wt (SPW) 1.00.87 -.09.05 Mature fall wt (FAW) 1.00 --.09.03 amales. badjusted for inbreeding effects. CAdjusted for differences in body weight. dfemales.

SELECTION INTENSITY IN HEREFORD CATTLE 935 TABLE 8. STANDARD PARTIAL REGRESSION COEFFICIENTS FOR SIRE AND DAM SELECTION INDEXES a Index WW FE ADG AIs f Inbred sires 1 b.4461 -.0092.6126.818 2 c.3945 -.0061.6219.792 Index WW 12W 18W SPW FAW AIDg Inbred dams 1 d.1824 -.0284.0736 -.1097 -.1097 2 e.2690 -.0826.0636 -.1105 -.1105 Linecro~dams 1.2693 -.2960.0147.1185 -.0354.203.241.209 aabbreviations for traits are as defined in table 7. bwith weaning weight adjusted for inbreeding and feed efficiency adjusted to common body weight. CWith weaning weight unadjusted for inbreeding and feed efficiency adjusted to common body weight. dwith weaning weight adjusted for inbreeding. ewith weaning weight unadjusted for inbreeding. fstandardized sire index selection differential. gstandardized female index selection differential. postweaning traits in bulls may be considered reasonable. In females, the SD for postweaning traits indicate that selection was not intense on inbred and linecross dams. Selection differentials for mature weights might be biased since selection of females was done at weaning time. Armstrong (1964) reported greater selection pressure on the sires, amounting to.70,.90 and.78o/generation for IW, FW and ADG, and a positive selection pressure of -.33o/generation for FE for bulls. Brinks et al. (1965) revealed that selection of sires was fairly intense for postweaning traits, averaging 1.10 and 1.46o/generation for 196-d gain and FW. In conformity with the results of the present study, selection was not intense for any of the postweaning traits in females. Buchanan et al. (1982) in their study of Fort Robinson, Nebraska data reported much larger SD for males, representing 79 to 84% of actual mid-parent SD in the three lines studied, similar to the proportions of 79 to 88% reported in an earlier analysis of the same data through 1970 by Koch et al. (1974). A similar study by Stanforth and Frahm (1975) showed that male selection accounted for 80 and 83% of the primary SD for weaning and yearling weights. Nelms and Stratton (1967) reported average annual midparent standardized SD of.142o/generation for ADG and.190o/generation for FW. Other studies (Flower et al., 1964; Chapman et al., 1969, 1972; Bailey et al., 1971) showed that selection for postweaning traits was fairly intense in the males and much less in the females. In some cases, little or no selection was practiced in the females. The inclusion of sire SD of some foundation animals in the annual weighted average could have caused a downward bias in the SD. However, results from this study and those reported in the literature indicate that most of the midparent selection pressure for WW and postweaning traits was applied in male selection as was expected. The standard partial regression coefficients obtained in this study for WW (.446 and.395) and ADG (.613 and.622) are close to the values of.423 and.577 reported, respectively, for the two traits by Armstrong (1964) for the same herd. He also reported much more attention to ADG than to any other trait included in the index. Although Brinks et al. (1965) did not consider ADG directly in the sire index, they reported that FW received much more emphasis than weights or scores recorded

936 NWAKALOR ET AL. before FW. Koch et al. (1974) and Buchanan et al. (1982) noted greatest emphasis for WW in the sire index for WW line, for yearling weight in the sire index for yearling weight line, and for muscling score in the sire index of yearling weight and muscling score line. The sire SD per generation (AIs), in standard measure, of.818 and.792 were similar to the values of.86 and.89 obtained by Armstrong (1964) but were lower than those reported by Brinks et al. (1965), Koch et al. (1974) and Buchanan et al. (1982). There was much greater opportunity for selection in sires than in dams due to a smaller fraction needed for replacement. Sire index SD represented about 79% of the total SD. Literature Cited Anonymous. 1975. Origin and history of cattle lines at the San Juan Basin Research Center, Hesperus, Colorado. Colorado State Univ. Exp. Sta. General Series 945. pp 15-19. Armstrong, J. B. 1964. Evaluation of selection intensity and genetic changes in experimental herd of Hereford cattle. Ph.D. Thesis. Colorado State Univ., Fort Collins. Bailey, C. M., W. R. Harvey, J. E. Hunter and C. R. Torell. 1971. Estimated direct and correlated response to selection for performance traits in closed Hereford lines under different types of environments. J. Anim. Sci. 33:541. Brinks, J. S., R. T. Clark and N. M. Kieffer. 1965. Evaluation of response to selection and inbreeding in a closed line of Hereford cattle. USDA Tech. Bull. 1323. Brinks, J. S., R. T. Clark, N. M. Kieffer and J. R. Quesenberry. 1962. Mature weight in Hereford range cows: Heritability, repeatability and relationship to calf performance. J. Anim. Sci. 21: 501. Brinks, J. S., R. T. Clark, N. M. Kieffer and J. J. Urick. 1964. Estimates of genetic, environmental and phenotypic parameters in range Hereford females. J. Anim. Sci. 23:711. Brinks, J. S., R. T. Clark and F. J. Rice. 1961. Estimation of genetic trends in beef cattle. J. Anim. Sci. 20:903 (Abstr.). Buchanan, D. S., M. K. Nielsen, R. M. Koch and L. V. Cundiff. 1982. Selection for growth and muscling score in beef cattle. I. Selection applied. J. Anim. Sci. 55:516. Chapman, H. D., T. M. Clyburn and W. C. McCormick. 1969. Selection of beef cattle for single traits. J. Anita. Sci. 29:225. Chapman, H. D., T. M. Clyburn and W. C. McCormick. 1972. Comparison of criteria for selecting introduced beef sires. J. Anita. Sci. 35:321. Dickerson, G. E., C. T. Blunn, A. B. Chapman, R. M. Kottman, J. L. Krider, E. J. Warwick and J. A. Whatley, Jr. 1954. Evaluation of selection in developing inbred lines of swine. North Central Regional Pub. 38, Missouri Agr. Exp. Sta. Res. Bull. 551. p 60. Dickerson, G. E. and L. N. Hazel. 1944. Effectiveness of selection on progeny performance as a supplement to earlier culling in livestock. J. Agr. Res. (Camb.) 69:459. Flower, A. E., J. S. Brinks, J. J. Urick and F. S. Willson. 1964. Selection intensities and time trends for performance traits in range Hereford cattle under mass and recurrent selection. J. Anim. Sci. 23:189. Harvey, W. R. and G. D. Bearden. 1962. Tables of expected genetic progress in each of two traits. USDA Agr. Res. Serv., ARS 20-12. Harwin, G. O. 1963. The effect of inbreeding and environmental factors on the weaning weight and post-weaning growth of range Hereford cattle. Ph.D. Thesis. Colorado State Univ., Fort Collins. Koch, R. M., K. E. Gregory and L. V. Cundiff. 1974. Selection in beef cattle. I. Selection applied and generation interval. J. Anim. Sci. 39:449. Nelms, G. E. and P. O. Stratton. 1967. Selection practiced and phenotypic change in a closed line of beef cattle. J. Anita. Sci. 26:274. Nwakalor, L. N., J. S. Brinks and G. V. Richardson. 1976. Estimated genetic change in weaning weight of beef cattle. J. Anim. Sci. 43: 396. Petty, R. R. and T. C. Cartwright. 1966. A summary of genetic and environmental statistics for growth and conformation traits of young beef cattle. Texas Agr. Exp. Sta. Dept. Anim. Sci. Tech. Rep. 5. Preston, J. M. and M. B. Willis. 1974. Intensive Beef Production. Pergamon Press, New York. Rendel, J. M. and Alan Robertson. 1950. Estimation of genetic gain in milk yield by selection in a closed herd of dairy cattle. J. Genet. 50:1. Stanforth, T. A. and R. R. Frahm. 1975. Selection for weaning and yearling weight in Hereford cattle. J. Anim. Sci. 41:259 (Abstr.).