Selection for weaning weight and postweaning gain in Hereford cattle. II. Response to selection

Single trait selection was practiced in three lines of Hereford cattle at two locations. Bulls were selected within sire families for increased weaning weight (WW) in the WW line (WWL), for postweaning gain (PG) in the PG line (PGL) and at random in the control line (CTL). Data include the performance of 2,467 calves produced from 1967 to 1981. Environmental effects were estimated from CTL (method I) and from multiple regression procedures (method II). Phenotypic and environmental time trends were negative for WW and generally were positive for PG. Estimated genetic gains for WW in WWL were 1.07 +/- .51 kg/yr in bulls and .62 +/- .36 kg/yr in heifers using method I and .50 +/- .31 kg/yr in bulls and .10 +/- .17 kg/yr in heifers using method II. Corresponding values for PG in PGL were .85 +/- .40 and 1.03 +/- .24 kg/yr in bulls and .30 +/- .28 and .37 +/- .12 kg in heifers. Correlated genetic gains for WW in PGL were larger than direct WW gains, whereas genetic gains for PG in WWL were smaller than direct PG gains. From method I, estimates of realized heritability (h2R) for WW were .31 +/- .18 in bulls and .22 +/- .13 in heifers. For PG, h2R was .31 +/- .13 in bulls and .06 +/- .12 in heifers. Using method II, h2R for WW was .09 +/- .08 in bulls and .02 +/- .07 in heifers. Corresponding values for PG were .29 +/- .10 and .11 +/- .08. Joint estimates of the realized genetic correlation between WW and PG were .69 +/- .18 and .46 +/- .31 for methods I and II, respectively. Variation in selection response was evaluated using quasi-replicates. Results of this study indicate that selection for PG improved both WW and PG faster than selection for WW.     

Selection for weaning weight and postweaning gain in Hereford cattle. I. Population structure and selection applied

Single trait selection was practiced in three lines of Hereford cattle derived from a common base population. Selection was practiced on males only within sire families for increased weaning weight (WW) in the WW line (WWL), for postweaning gain (PG) in the PG line (PGL) and at random in the control line (CTL). Females were culled on the basis of age or reproductive failure. Progeny of selected bulls were produced in two herds from 1970 through 1981. The data consisted of records on 2,467 progeny of 125 sires and 922 dams. Generations of selection to produce the 1981 calf crop were 1.96, 1.85 and 1.80 for WWL, PGL and CTL, respectively. For calves born in 1981, mean cumulative selection differentials (CSD) were 54.5 kg in WWL and 37.8 kg in PGL. Corresponding values in standard deviation units (SDU) were 2.31 and 1.68, respectively. Secondary selection differentials were 25 to 40% as large as selection differentials for the primary traits. Unintentional selection in the CTL in 1981 was 16.2 kg or .68 SDU for WW and .2 kg or .01 SDU for PG, respectively. Regressions of CSD on year were 4.1 kg or .17 SDU in WWL and 3.2 kg or .14 SDU in PGL. Realized selection differentials were approximately 88% of the potential selection differentials in both lines. Inbreeding coefficients of dam and calves in 1981 were 2.0 and 3.5% in WWL, 2.1 and 3.5% in PGL and 2.9 and 5.8% in CTL.     

Interactions in postweaning production of F1 cattle from Hereford, Limousin, or Piedmontese sires

A 2-yr study was conducted to evaluate the interactions of castration, feeding length, and dietary CP on growth and carcass characteristics of male cattle (bulls and steers) that vary in expression of muscular hypertrophy. Crossbred cows were bred by AI to Hereford, Limousin, or Piedmontese bulls, which represented genotypes with normal, moderate, and hypermuscularity, respectively, but with similar mature weights. Male calves (131 in yr 1 and 120 in yr 2) were placed in pens with individual electronic feeding gates. Calves were fed growing diets until they reached 386 kg BW and then were individually switched to finishing diets for 90 or 132 d. Interactions were observed among sire breed, gender, and feeding length on carcass composition. Bulls were more efficient than steers in producing live weight gain. Length of finishing period accounted for a larger source of variation than gender for weight characteristics, whereas gender was the larger source of variation for carcass composition. Concentration or degradability of dietary CP influenced rate of gain from weaning to 386 kg. Interactions resulting from varying management on carcass characteristics among calves of different sire breeds indicate that unique strategies may be beneficial for the production of meat from these breeds.     

Comparison of two- and three-way rotational crossing and synthetic variety production involving inbred lines of Hereford cattle: postweaning traits

Data for this linecrossing study were the postweaning growth and weight records of calves born from 1967 through 1975. The records consisted of 264 bull (M) and 263 heifer (F) straightline (SL) calves; 108 M and 100 F two-line rotational cross (2W) calves; 130 M and 102 F three-line rotational cross (3W) calves and 58 M and 51 F synthetic variety (SV) calves, all in the Hereford breed. Average inbreeding percentages of calves and dams in Lines 1, 4, 6 and 10 were 24, 28, 34 and 28; respectively. The M calves were fed a growing-fattening diet for 196 d after a 14-d period of adjustment to postweaning environment. The F calves were fed to gain approximately .5 kg daily during winter from weaning to about 12 mo of age and were then grazed on native pasture until about 18 mo of age. For M, average percentage increases over SL by 2W, 3W and SV combined over three generations were respectively, 9.0, 11.0, 11.6 for initial weight; 5.9, 8.1, 8.8 for 140-d weight; 4.8, 7.2, 7.7 for final 196-d weight and 1.5, 4.2, 4.7 for 196-d gain. All above estimates of increased weight or gain over SL were significant except for 196-d gain in 2W. For F, average percentage increases over SL by 2W, 3W and SV, respectively, were 9.0, 10.9, 11.3 for initial weight; 8.2, 9.2, 8.4 for 12-mo weight; 7.5, 7.7, 9.1 for 18-mo weight; 5.2, 5.3, 5.7 for 18-mo score. All these estimated increases over SL were significant.(ABSTRACT TRUNCATED AT 250 WORDS)     

Selection in Hereford cattle. I. Selection intensity, generation interval and indexes in retrospect

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 (sigma)/generation for sires. For females, selection was much less intense. Midparent selection differential amounted to .33 sigma/generation. For sires, pooled standardized selection differentials per generation over all lines during the postweaning gain period were .49 sigma, .46 sigma, .40 sigma, -.20 sigma, -.10 sigma and .69 sigma, 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: IS = .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, ID = .1824 (WW) - .0284 (12W) + .0736 (18W) - .1097 (SPW) - .1097 (FAW); for linecross dams, ID = .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.     

Selection in Hereford cattle. II. Expected and realized response

Data from 14 inbred lines and 14 linecross groups of Hereford cattle at the San Juan Basin Research Center, Hesperus, were used to evaluate expected and realized response in birth and weaning traits and postweaning traits in males and females over a 28-yr period. There were large differences in the means and variances of the performance traits among the inbreds and linecrosses, with the inbreds showing inbreeding depression and greater variability among lines, while the linecrosses manifested within-breed heterosis. Except for gain from weaning to 12 mo, in females, genetic progress was expected in all traits studied, mainly due to sire selection. Regressions of annual trait means on years indicated positive phenotypic trends in the inbreds for heart girth circumference at birth, adjusted weaning weight (adjusted for inbreeding), weaning score, final weight, feed consumption and the yearling weights and gains in females. Changes were negative for other traits. In the linecross group phenotypic trends were positive in all traits except heart girth circumference, weaning age, initial test weight and feed efficiency. Estimated genetic progress per generation due to within-line selection was negative in most of the traits in the inbreds but was considerably positive for the linecrosses for most of the traits. As expected, between-line selection yielded greater genetic improvement in the inbred than in the linecross population. The different patterns of response in the two populations are attributed to high rates and levels of inbreeding. Although variable, the actual progress was below prediction in most of the traits studied.     

Additive and nonadditive differences in postweaning growth and carcass characteristics of Devon, Hereford, and reciprocal-cross steers.

Postweaning growth and carcass characters of 110 steers from a complete two-breed diallel of the Devon and Hereford breeds were examined under two environments. Additive and nonadditive effects were estimated using linear contrasts for several growth and carcass traits. Steers from each of the four breed groups were grown postweaning to slaughter in high- and low-nutrition environments. Weights were recorded every 2 mo. At slaughter, hot carcass weight, longissimus muscle area, kidney and channel fat, and subcutaneous fat at nine sites were measured. Heterosis for postweaning growth rate was 3.9% (P less than .01) and for slaughter weight 5.0% (P less than .01). Within the low-nutrition environment during periods of slow and fast growth, the Devons and Herefords performed differently. The growth rate of the steers differed in the two environments; however, heterosis for slaughter weight was of the same magnitude in both environments. No differences existed between the straightbreds or between the reciprocal crosses for slaughter weight. Crossbred carcasses were 7.4% heavier (P less than .01) than the straightbred carcasses; however, this effect was removed after adjustment for differences in slaughter weight. Heterosis for longissimus muscle area and carcass fatness were not significant after adjusting for carcass weight. Additive differences occurred for carcass traits. Devon carcasses had more kidney and channel fat (P less than .05) at a constant hot carcass weight and differences occurred in the partitioning of fat within the subcutaneous depot. No significant maternal effects were observed for the carcass traits measured. Crossbreeding increased carcass weight without altering composition, and relative performance was not affected by the diverse environments.     

Performance of crosses among Hereford, Angus and Simmental cattle with different levels of Simmental breeding. III. Heifer postweaning growth and early reproductive traits

Postweaning growth and early reproductive traits in heifers whose potential for milk production differed were studied during the years 1976 through 1979. Data were collected on 230 heifers raised by greater than or equal to 3-yr-old Hereford dams. Heifers were comprised of the following breed groups: Hereford (HH), Angus-Hereford (AH), 25% Simmental-75% Hereford (1S3H) and Simmental-Hereford (SH). Postweaning growth rate during a 140-d period was greatest for SH heifers (.75 kg/d), intermediate for AH and 1S3H groups (.70 and .68 kg/d, respectively) and slowest for HH heifers (.64 kg/d). At 1 yr of age, SH heifers were heavier, taller and had larger pelvic areas than all other groups (P less than .01). Least-squares means for yearling weight, hip height and pelvic area for HH, AH, 1S3H and SH heifers were 283, 303, 298 and 317 kg; 111, 112, 114 and 118 cm, and 132, 136, 139 and 148 cm2, respectively. Ninety-one percent of the heifers reached puberty by the end of the breeding period. Crossbred heifers reached puberty at younger ages and had higher pregnancy rates than HH heifers (P less than .01). Least-squares means for pubertal age, weight and height for HH, AH, 1S3H and SH breed groups were, respectively, 407, 371, 382 and 368 d; 300, 302, 305 and 313 kg, and 115, 114, 117 and 119 cm. Percentage of heifers reaching puberty by 14 mo of age was 64, 99, 85 and 94 for HH, AH, 1S3H and SH, respectively. Respective pregnancy rates were 59, 90, 77 and 86%, but were not significantly different among breed groups when only heifers reaching puberty by the end of the breeding period were studied. No differences among breed groups were found for date of pregnancy.     

Selection for increased weaning or yearling weight in Hereford cattle. I. Measurement of selection applied

Selection was applied from 1964 to 1978 for increased weaning weight (WWL) or yearling weight (YWL) in two Hereford lines with an Angus line maintained as an unselected control line (CL). Each line was maintained with 50 cows and four sires (two sires selected each year and each used for 2 yr). Traits analyzed were birth weight (BW), preweaning daily gain (WDG), weaning weight (WW), weaning conformation grade (WG), weaning condition score (WC), weaning to yearling daily gain (YDG), yearling weight (YW), yearling conformation grade (YG) and yearling condition score (YC). After 15 yr of selection, a total of 3.22 generations of selection had occurred in both WWL and YWL. Average selection differentials in standard measure per generation for WWL, YWL and CL, respectively, were: BW, .44, .51, .0; WDG, .95, .81, .09; WW, .97, .85, .09; WG, .66, .57, .09; WC, .60, .38, -.02; YDG, .30, .79, .38; YW, .80, 1.05, .25; YG, .63, .62, .34 and YC, .45, .64, .24. The proportionate contribution of sire selection (delta S) to the average midparent selection differential per generation (delta M) was 70% in WWL and 76% in YWL. Selection indexes in retrospect were also calculated.     

Selection for increased weaning or yearling weight in Hereford cattle. II. Direct and correlated responses

Selection was applied from 1964 to 1978 for increased weaning weight (WWL) or yearling weight (YWL) in two Hereford lines. An Angus line was maintained as an unselected control line (CL). Each line was maintained with 50 cows and four sires each year (two sires selected each year and used for 2 yr). Primary traits measured in the lines were birth weight (BW), preweaning daily gain (WDG), weaning weight (WW), weaning conformation grade (WG), weaning condition score (WC), weaning to yearling daily gain (YDG), yearling weight (YW), yearling conformation grade (YG) and yearling condition score (YC). Averaged over two methods, estimated genetic responses/generation (in standard deviation units) in WWL and YWL were: BW, .29, .26; WDG, .17, .15; WW, .22, .19; WG, .19, .26; WC, .12, .12; YDG, -.02, .04; YW, .08, .14; YG, .19, .16; YC, -.13, -.03. The realized heritability estimates were .23 and .15 for WW and YW, respectively. The realized genetic correlation between WW and YW was .69. Progeny from crosses of selected WWL and YWL sires to Angus cows had similar feedlot and carcass performance. At the end of the study, milk yield and composition were similar for mature cows in WWL and YWL.     

Maternal heterosis and grandmaternal effects in beef cattle: postweaning growth and carcass traits

Crossbred steer and heifer progeny from 5-, 6- and 7-yr-old dams produced in a four-breed diallel crossing experiment involving the Brown Swiss, Red Poll, Hereford and Angus maternal grandsires and maternal granddams were evaluated for postweaning growth and carcass traits to estimate breed mean maternal heterosis, maternal heterosis for specific breed cross females, average maternal heterosis for all crosses, breed grandmaternal effects and net breed effects in crosses. All progeny evaluated were born in 1979 and 1980 and were sired by 7/8 or 15/16 Simmental bulls. Average maternal heterosis was significant for 200-d weight in heifers but not in steers and was not significant for final weight (444-d) in either heifers or steers. The effects of maternal heterosis on postweaning growth were not important. Differences among breeds in mean maternal heterosis values were small for growth-related traits. Breeds did not differ (P greater than .05) in grandmaternal effects for growth-related traits; Brown Swiss tended to be highest, Red Poll lowest, with Hereford and Angus intermediate. Differences in net breed effects in crosses favored Brown Swiss over the three other breeds and were generally significant for growth traits. Average maternal heterosis, though generally positive, was not significant for carcass traits on either an age-constant or weight-constant basis. Differences among breeds were small in grandmaternal effects, specific heterosis and net effects in crosses for carcass traits associated with both weight or composition; generally the Brown Swiss breed was favored on carcass traits associated with weight in the age-constant analysis and generally had a higher lean-to-fat ratio than the three other breeds in both the age-constant and weight-constant analyses.     

Heterosis among closed lines of Hereford cattle. III. Postweaning growth and carcass traits in steers

Feedlot and carcass characteristics of 276 steers from five closed lines of Hereford cattle and reciprocal crosses among these lines were studied. The traits studied were initial weight, final weight, 224-d gain, days on test, hot carcass weight, marbling score, longissimus muscle area, fat thickness, yield grade, dressing percentage and shear force. Year of record was a significant source of variation for most traits. Age of dam was a significant source of variation for growth traits but not carcass traits. Line of sire affected initial weight, final weight, 224-d gain, days on test, marbling score and dressing percentage. Significant heterosis was observed only for hot carcass weight. Heterosis estimates were 1.9% for initial weight, 2.2% for final weight, 2.5% for 224-d gain, -2.1% for days on test, .6% for hot carcass weight, -.6% for marbling score, 0 for carcass grade, .6% for longissimus muscle area, 2.3% for backfat thickness, .9% for yield grade, -.9% for dressing percent and -10.9% for shear force. Initial age on test affected only hot carcass weight. Hot carcass weight, dressing percentage, marbling score, longissimus muscle area and fat thickness were affected by slaughter weight. Slaughter age affected dressing percent and marbling score.     

Tag-SNP selection using Bayesian genomewide association study for growth traits in Hereford and Braford cattle

The aim of this study was to perform a Bayesian genomewide association study (GWAS) to identify genomic regions associated with growth traits in Hereford and Braford cattle, and to select Tag-SNPs to represent these regions in low-density panels useful for genomic predictions. In addition, we propose candidate genes through functional enrichment analysis associated with growth traits using Medical Subject Headings (MeSH). Phenotypic data from 126,290 animals and genotypes for 131 sires and 3,545 animals were used. The Tag-SNPs were selected with BayesB (π = 0.995) method to compose low-density panels. The number of Tag-single nucleotide polymorphism (SNP) ranged between 79 and 103 SNP for the growth traits at weaning and between 78 and 100 SNP for the yearling growth traits. The average proportion of variance explained by Tag-SNP with BayesA was 0.29, 0.23, 0.32 and 0.19 for birthweight (BW), weaning weight (WW205), yearling weight (YW550) and postweaning gain (PWG345), respectively. For Tag-SNP with BayesA method accuracy values ranged from 0.13 to 0.30 for k-means and from 0.30 to 0.65 for random clustering of animals to compose reference and validation groups. Although genomic prediction accuracies were higher with the full marker panel, predictions with low-density panels retained on average 76% of the accuracy obtained with BayesB with full markers for growth traits. The MeSH analysis was able to translate genomic information providing biological meanings of more specific gene products related to the growth traits. The proposed Tag-SNP panels may be useful for future fine mapping studies and for lower-cost commercial genomic prediction applications.     

Selection for postweaning gain in rats: III. Correlated response in maternal performance

Effects from 34 generations of selection, either up (U) or down (D), for 3- to 9-wk weight gain on genetic direct and postnatal maternal effects, dam size and maternal efficiency were evaluated in 25 cross-fostered sets of rats. Direct genetic effects on 12-d pup weight were 14 and 9% above controls (C) for U and D lines, respectively (P less than .01). Postnatal maternal effects on 12-d pup weight were also 11% higher for the U line (P less than .01), but not different from C for the D line. Females of the U line were heavier (P less than .01) by 13% at mating, 12% at parturition and 15% at 12 d of lactation, relative to controls. They also produced 11% greater total litter weight at 12 d and consumed 9% more feed, but were only nonsignificantly lower in feed per unit of pup 12-d litter weight. Females of D line were -8, -7 and -3% relative to controls in the three weights, but did not differ from controls in 12-d litter weight, feed consumed or in feed efficiency during lactation.     

Selection for postweaning gain in rats: II. Correlated response in reproductive performance

Reproductive performance was evaluated on lines of rats selected 34 generations for up (U) and down (D) 3- to 9-wk weight gain and from a control (C). Direct response to 3- to 9-wk weight gain was 16% in U and -8% in D lines. Inbreeding was similar among lines. Both selection lines were poorer in percentage of fertile pairs than the C line, especially so in D (P less than .10). The lines differed (P less than .05) in litter size born with means of 11.4, 10.4 and 9.6 for U, C and D, respectively. The significant positive correlated response in litter size corresponded to that expected from intraline association with female body weight. Selection for increased postweaning gain resulted in 5% earlier age but little change in weight at vaginal opening. The D line was similar to C in age, but 7% lighter in weight at vaginal opening. The U line was significantly higher (14, 12 and 69%) for ovulation rate, number of fetuses and post-implantation losses, but not for pre-implantation losses. The D females were similar to C females in all components of litter size. Males of all lines were similar in age at first mating, but U males were 22% heavier than C males in weight at first mating. A positive correlated response in weight of testes at 91 d of age disappeared when adjusted for intraline association with body weight.     

Growth patterns of Angus, Hereford and shorthorn cattle. II. Relationship of growth patterns of dams with progeny performance

Relationships between estimated growth curve parameters of dams and performance traits of their progeny were studied in Angus, Hereford and Shorthorn herds, each divided into four inbred and two noninbred lines. Growth curve parameters were calculated from the growth function Yt=A(1-Be-Kt), where Yt was weight at age t, A was estimated mature weight, B was an estimate related to early life weight changes and provided for a Y-intercept term and K was estimated general rate of maturing. Least-squares analyses of progeny variables were calculated separately for each breed and sex. Line differences did not influence any of the progeny variables except weaning type score of Shorthorn males (P less than .01). Birth year exerted a curvilinear effect on birth weights of Angus female progeny (P less than .01), a linear influence on 205-d weights of Shorthorn male progeny (P less than .05) and a curvilinear effect on weaning type scores of Angus and Shorthorn male progeny (P less than .01). Regression coefficients on weaning age indicated that calves born earlier in the calving season had lighter birth weights and that older calves at weaning received higher type scores. Inbreeding of the progeny negatively influenced (P less than .05) birth weights of Angus male and Hereford female progeny. Hereford male and Shorthorn female 205-d weights were negatively affected (P less than .05) by inbreeding, while weaning type scores of Shorthorn female progeny were negatively influenced (P less than .001) by inbreeding. Regression coefficients of all progeny variables on inbreeding in all analyses indicated negative trends.(ABSTRACT TRUNCATED AT 250 WORDS)     

Genotype X environment interaction in Hereford cattle: IV. Postweaning traits of bulls

Genotype X environment interactions for postweaning performance traits of bulls produced by different lines of Hereford cattle were investigated in the contrasting environments of Miles City, Montana and Brooksville, Florida. During Phase 1 of the study (1966 to 1973), the performance of bull progeny from two unrelated lines (M1 and F6 previously developed in Montana and Florida, respectively) was compared at each of the two experimental sites. During Phase 2 (1967 to 1974), performance of bulls from two related lines (M1 of Montana origin and F4 derived from an M1 foundation through selection in Florida) was compared at each of the two locations. The line X location interaction effect in Phase 1 was highly significant for weaning weights and average daily gain during the postweaning test, and for end-of-test weight, conformation score, condition score and forecannon circumference. The interaction effect in Phase 2 was highly significant for 205-d and end-of-test weights and significant for all other traits except end-of-test conformation score. These results, consistent with results for traits covered in other papers of the same series (reproductive traits, birth-to-weaning traits of bull and heifer calves combined, and postweaning traits of heifers), indicated the existence of economically important genotype X environment interactions in beef cattle. Results from the series of papers indicated that adaptation to local environment should receive consideration in planning breeding programs, performance tests and interregional transfers of beef cattle.     

Infectious bovine keratoconjunctivitis: evidence for genetic modulation of resistance in purebred Hereford cattle

A vaccination study was conducted in a herd of purebred Hereford cattle representing 4 selection (genetic) lines. For each of 2 years, half of the cattle were vaccinated with a pilus-enriched Moraxella bovis bacterin. Cows were vaccinated before parturition, and calves were vaccinated at 2 to 3 months of age. None of the cattle was vaccinated for 1 year preceding and 1 year after the 2 years in which cattle were vaccinated. There was a significantly (P less than 0.05) lower percentage of infectious bovine keratoconjunctivitis (IBK) in calves during years cattle were vaccinated than during years cattle were not vaccinated. During years cattle were vaccinated, there were lower percentages of IBK in vaccinated calves when compared with the percentages of IBK in nonvaccinated calves. When calves were compared on the basis of selection lines, regardless of the vaccination group, there were consistent differences in the percentages that developed IBK. Although calves with pigmented and nonpigmented eyes (representing all 4 genetic lines) developed IBK, the genetic line of calves with the most pigmented eyes had the lowest (P less than 0.05) percentage of IBK. Also, across all genetic lines, there was less IBK in pigmented eyes than in nonpigmented eyes. Seemingly, vaccination of dams, before parturition, and young calves reduced the occurrence of severe IBK in a herd situation under natural exposure conditions. The resistance or susceptibility in cattle under good management may be influenced by genetic factors.