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Postweaning growth and feed efficiency data were analyzed on 769 F1crossbred steers out of Angus and Hereford dams and sired by Pinzgauer, Tarentaise, Brahman, Sahiwal, Hereford and Angus sires. Breed groups were fed in replicated pens and slaughtered serially at three different dates in each of 2 yr. Quadratic regressions of pen mean weight on days fed and of cumulative metabolizable energy consumption (ME) on days fed were used to estimate gain, ME consumption and efficiency (Mcal ME/kg gain) over time (0 to 213 d on feed) and weight (250 to 470 kg) intervals, and from d 0 to a small degree of marbling (corresponds to USDA low Choice quality grade) and an 18.9% fat trim end point. Brahman-sired crosses were significantly heavier at weaning than all other breed groups. However, during the postweaning period, which included winter months, and while being fed a moderately high concentrate diet, average daily gain of Brahman-sired crosses tended to be less than Angus- or Hereford-, Tarentaise- and Pinzgauer-sired crosses and that of Sahiwal-sired crosses was significantly less than all other breed groups. Differences among breed groups were not significant for feed efficiency in the time constant interval from 0 to 213 d. In a weight interval from 250 to 470 kg, only Sahiwal-sired crosses differed significantly, requiring more Mcal ME/kg gain than other breed groups. Hereford-Angus crosses were more efficient than all other breed groups (P<.05) to the marbling end point and to the 18.9% fat trim end point, primarily because of fewer days on feed and thus less net energy required for maintenance. Pinzgauer-sired crosses were significantly more efficient than Tarentaise-, Brahman- and Sahiwal-sired crosses to the marbling end point (P<.05).

Postweaning growth, puberty and pregnancy data were analyzed on 267 heifers produced by breeding yearling heifers of 12 crossbred groups (Hereford-Angus reciprocal crosses plus crosses produced by breeding Jersey, South Devon, Simmental, Limousin and Charolais sires to Hereford and Angus cows) to Hereford, Angus, Brahman, Devon and Holstein bulls.Effects of sire breed of heifer were significant for all growth traits and for age and weight at puberty. Holstein-sired heifers had the highest (P<.05) feedlot average daily gain (ADG). Brahman-sired heifers, due to their large weaning weights, had the lowest (P<.05) feedlot relative growth rate (RGR). Other differences in feedlot ADG and RGR were relatively small. Brahman- and Holstein-sired heifers had higher pasture ADG (P<.05) and were heavier (P<.05) at 400 and 550 days of age than Hereford-Angus or Devon-sired heifers. Brahman-sired heifers were older and heavier (P<.05) at puberty than heifers from any other sire breed.Effects of sire breed of the heifers' dam (breed of maternal grandsire) were significant for weights at 200, 400 and 550 days, puberty age, puberty weight and percentage pregnant. Heifers from Jersey cross cows had lower (P<.05) feedlot ADG and RGR than heifers from any other crossbred cow group. Heifers from Hereford-Angus cross cows had the lowest (P<.05) pasture ADG and heifers from Charolais cross cows had the highest (P<.05). Heifers from Simmental and Charolais cross cows were heavier than heifers from any other crossbred group at 400 and 550 days of age, although all differences were not significant. Heifers from Hereford-Angus, Limousin and Charolais cross cows were older (P<.05) at puberty than heifers from Jersey, South Devon and Simmental cross cows. Charolais crosses had the highest average puberty weight. Jersey crosses had the lowest. Even though 99% of the heifers from Limousin cross cows reached puberty before the end of the breeding season , only 69% were classified as pregnant which was the lowest (P<.05) percentage pregnant among the cross bred cow groups.

Effects of maternal heterosis were evaluated for postweaning growth and carcass traits of 497 crossbred steers and 356 crossbred heifers produced from 1963 through 1968 in phase 2 of a comprehensive heterosis experiment. Maternal heterosis was estimated by a comparison of two-breed cross progeny from straightbred Hereford, Angus and Shorthorn cows to three-breed cross progeny from F1crossbred cows. Postweaning weights and average daily gains for the three 84-day periods and for the entire 252-day feeding test were evaluated. Carcass traits were evaluated at both age- and weight-constant endpoints.Steers and heifers from crossbred dams were 10.4 kg (5.3%, P<1) and 7.9 kg (4.2%, P<.001) heavier than progeny from straightbred dams at 200 days of age but were only 5.9 kg (1.4%) and 4.9 kg (1.2%) heavier at 452 days of age, because of compensatory effects of maternal heterosis on postweaning average daily gain (−.02 and −.01 kg). Maternal heterosis effects were unfavorable and largest on first-period growth rate of heifers but on last-period growth rate of steers. The unfavorable maternal heterosis for growth rate was associated with slightly fatter carcass composition of progeny from crossbred dams, apparently from greater milk production of crossbred cows. Maternal heterosis effects on carcass traits of steers and heifers at either a constant age or constant.

Postweaning growth data on 282 steers and carcass data on 275 steers produced by artificially inseminating (AI) yearling heifers of 12 crossbred groups (Hereford-Angus reciprocal crosses plus F1crosses produced by mating Jersey, South Devon, Simmental, Limousin and Charolais sires to Hereford and Angus cows) to Hereford, Angus, Brahman, Devon and Holstein bulls were analyzed. For postweaning growth rate, Devon-sired steers were the slowest, Holstein-sired steers were fastest and Hereford-Angus- and Brahman-sired steers were similar and intermediate. Due to their large weaning weights, Brahman-sired steers were heaviest at 452 days, followed closely by Holstein and then Hereford-Angus-AI (Hereford and Angus sires used by artificial insemination) and Devon. Steers from Jersey cross cows had the lowest average daily gain, relative growth rate and 452-day weight. Steers from the other crossbred cows group had similar postweaning growth rates, but due to weaning weight differences, Simmental, South Devon and Charolais crosses had similar 452-day weights that tended to be heavier than those of Hereford-Angus and Limousin crosses.Carcass traits were analyzed once with carcass weight as a covariate and once with slaughter age as a covariate. The analyses produced similar results. Carcasses from Holstein-sired steers had the least fat cover, smallest percentage of kidney, heart and pelvic fat (%KHP), best yield grade, highest estimated percentage retail product and lowest conformation score. Devon-sired steers had the highest %KHP fat. Brahman-sired steers had the poorest yield grade and the lowest marbling score and quality grade. Hereford and Angus-sired steers had the most fat cover, poorest yield grade and lowest estimated percentage retail product but highest conformation, marbling and quality scores. Steers from Hereford-Angus and Jersey cross cows had the most fat cover, smallest longissimusarea, highest %KHP fat, lowest yield grade and lowest percentage retail product. Steers from Jersey crosses also had the lowest conformation scores. Most other differences among cow sire breeds for carcass traits were small.

Preweaning growth and weight at 120 and 200 days were studied in the progeny of 564 2-year-old and 569 3-year-old crossbred cows produced by mating Hereford, Angus, Jersey, South Devon, Simmental, Limousin and Charolais bulls to Hereford and Angus cows. Two-year-olds produced calves by Hereford, Angus, Brahman, Devon and Holstein bulls and 3-year-olds produced calves by Hereford, Angus, Maine-Anjou, Chianina and Gelbvieh bulls. Progeny of Jersey and Simmental cross cows grew fastest (.785 and .790 kg/day, averaged over ages) and were heaviest at 120 (133 and 138 kg) and 200 (189 and 194 kg) days. Progeny of Hereford-Angus cross cows grew least rapidly (.715 kg/day) and were lightest at 120 (125 kg) and 200 (176 kg) days. In 2-year-olds, Brahman-sired calves grew significantly faster than Hereford- and Angus-sired calves (.77 vs.69 kg/day), and both Brahman-and Holstein-sired calves were significantly heavier at 200 days (190 and 174 vs168 kg). In 3-year-olds, Chianina- and Gelbvieh-sired calves grew faster (P<.05) than Hereford- and Angus-sired calves (.81 vs.78 kg/day), and Hereford- and Angus-sired calves were lightest (P<.05) at 200 days.Fifty-nine 2-year-old and 125 3- and 4-year-old crossbred cows (plus purebred Hereford and Angus) were used to estimate milk production. Jersey and Simmental cross cows gave the most milk, whereas Limousin and Charolais cross cows and purebred Hereford and Angus cows gave the least. Heterosis for milk production declined as lactation progressed and averaged 15 percent.

Birth weight, gestation length, dystocia and mortality were studied in 653 2-year-old and 622 subsequent 3-year-old carvings. Dams were Hereford-Angus reciprocal crosses plus crosses of Charolais, Simmental, Limousin, Jersey and South Devon sires on Hereford and Angus cows. Calves from 2-year-olds were sired by Hereford, Angus, Brahman, Holstein and Devon sires. Calves from 3-year-olds were sired by Hereford, Angus, Maine-Anjou, Chianina and Gelbvieh sires. Progeny of Charolais, Simmental and South Devon cross cows were heaviest at birth, (36.5, 35.5 and 36.1 kg; averaged over ages), and had the most dystocia (36.5, 36.5 and 39.5%). Calves from Jersey cross cows were lightest at birth (31 kg), had the shortest gestations (282 days), the least dystocia (17.5%) and had the least mortality (6%). In 2-year-olds, Brahman-sired calves were heaviest at birth (35.3 kg) and had the longest gestations (289 days), the most dystocia (67%), and the highest total mortality (21%). Calves by Hereford, Angus and Devon sires were lightest at birth and had the least dystocia and mortality. In 3-year-olds, calves by Maine-Anjou and Chianina sires were heaviest at birth (38.9 and 39.7 kg) and had the most dystocia (37 and 36%) and the most mortality (11 and 13%). Gelbvieh-sired calves were intermediate and calves by Hereford and Angus sires had the lowest birth weights (34.7 kg), the shortest gestations (283 days) and the least dystocia (12%). Significant curvilinear relationships between birth weight and dystocia and between birth weight and mortality were found at both ages.

Heterosis for carcass characters was estimated from Angus and Hereford crosses. Two data sets were analyzed. Data set I contained records of 1,095 calves placed on feed immediately after weaning. Data set II was made up of records on 210 calves assigned randomly to either immediate feeding or deferred management (wintering, grazing and then feeding).Cattle with Angus sires or dams produced heavier carcasses with higher dressing percentages, conformation scores, marbling scores and final grades, a larger percentage kidney fat and thicker fat cover but lower estimated cutability than cattle with Hereford sires or dams. Steers dressed higher, producing heavier carcasses with higher conformation scores and larger longissimusmuscle areas, whereas heifers were fatter and graded higher. Carcasses from deferred cattle were heavier, with higher marbling scores and grades, less kidney fat, larger longissimusmuscle areas and higher estimated cutability than carcasses of cattle fed immediately after weaning.Heterosis was observed for carcass weight, longissimusmuscle area and measures of fatness in both data sets.

Heterosis for postweaning growth and weight was estimated in crosses of the Angus and Hereford breeds. Two data sets were analyzed. Immediately after weaning, 1,095 calves recorded in data set I were placed on feed, and 210 in data set II were assigned randomly to either immediate feeding or deferred management.Differences between effects of the Angus and Hereford breeds on postweaning performance were not statistically significant. Steers outgained heifers by 10%. Steers exhibited a greater advantage over heifers under deferred management (wintered, grazed and then fed) than when placed immediately on feed.Crossbreds exceeded straightbreds by 5 to 6% for postweaning gain and weight; heterosis effects were similar for steers and heifers. The breed of sire by breed of dam by management interaction in data set II approached significance (P<.10) for ADG from weaning to slaughter. Heterosis estimates for this trait were 90 g (9.6%) in the group placed on feed immediately after weaning and 30 g (3.7%) in the deferred cattle.

Birth and livability data on 995 calves born, weaning data on 915 calves and carcass data on 817 calves produced in two calf crops werestudied. Calves were sired by 51 bulls from 18 Angus herds and 44 bulls from 18 Polled Hereford herds (two to four sires sampled/herd) through AI matings to Hereford and Angus cows at the U.S. MARC.Estimates of genetic variance did not differ significantly between breeds. From pooled analyses, effects of herd origin of sires were significant for birth weight, 200-day weight, postweaning average daily gain, 452-day weight, marbling, final carcass grade and carcass conformation grade. Effects of sire/herd were highly significant for postweaning average daily gain, 452-day weight, carcass conformation, final grade, marbling, fat thickness, longissimusmuscle area and significant for estimated cutability.Genetic variance components for herds and sires/herd were used to compute the intraherd correlation of individual breeding values. Relatively higher correlations for growth traits (averaging .51) than for carcass traits (averaging .16) indicate that selection pressures and responses have been more diverse, among herds, for growth traits than for carcass traits.

Postweaning growth and feed efficiency (megacalories metabolizable energy [ME] /kilogram gain) were determined for 798 steers representing Hereford and Angus straightbred (H and A), Hereford-Angus reciprocal cross (HAx), Red Poll-Hereford and Red Poll-Angus reciprocal cross (Rx), Brown Swiss-Hereford and Brown Swiss-Angus reciprocal cross (Bx), Gelbvieh-Hereford and Gelbvieh-Angus cross (Gx), Maine Anjou-Hereford and Maine Anjou-Angus cross (Mx) and Chianina-Hereford and Chianina-Angus cross (Cix) breed groups. Regression of pen mean weight and cumulative megacalories ME on days fed was used to estimate gain, ME consumption and feed efficiency (megacalories ME/kilogram gain) over (1) time- (0 days to 248 days) and (2) weight-constant (250 kg to 470 kg) intervals and to (3) marbling (0 days to a small degree of marbling, USDA choice quality grade) and (4) fat trim (0 days to 18.9% fat trim) endpoints. Breed groups with the most rapid growth rates and heaviest weights at birth, 200 days and 424 days (Bx, Gx, Mx) required fewer (P<.05) megacalories ME/kilogram gain over time- and weight-constant intervals than slower-gaining, lighter breeds (HAx, Rx). Breed groups reaching the marbling or fat trim endpoint in the fewest days generally required fewer .05) megacalories ME/kilogram gain to marbling (HAx < Cix) and fat trim (HAx < Bx, Mx and Cix) endpoints than did faster-growing, heavier breed groups that reached the endpoints at older ages. Contrary to previous reports, HAx did not gain significantly faster than H and A straightbreds during the postweaning period (the HAx advantage was only 2.0% for postweaning average daily gain). HAx required more (P<.05) megacalories ME/kilogram gain than H and A straightbreds over time- and weight-constant intervals.

Data on growth traits, puberty and pregnancy were analyzed on 490 crossbred females produced by artificial insemination from Hereford and Angus dams by Hereford, Angus, Brahman, Sahiwal, Pinzgauer and Tarentaise sires. Brahman crosses were heaviest (P<.01) at 200 and at 550 days. Brahman and Sahiwal crosses gained at a slower rate than Angus-Hereford, Pinzgauer and Tarentaise crosses from 200 to 400 days, whereas from 400 to 550 days, Brahman crosses gained significantly faster than all other crosses and Sahiwal crosses gained significantly faster than Angus-Hereford, Pinzgauer and Tarentaise crosses. Differences in growth traits among the Angus-Hereford, Pinzgauer and Tarentaise crosses generally were small. Females with Angus dams were significantly heavier at 200 and at 400 days than females with Hereford dams, but females with Hereford dams gained significantly faster than females with Angus dams after 200 days and were significantly heavier at 550 days.Major differences were observed among breed of sire groups for age and weight at puberty. Brahman crosses were significantly older and heavier than all other breed of sire groups at puberty. Sahiwal crosses were older (P<.01) than all remaining breed of sire groups at puberty and were heavier (P<.01) at puberty than Pinzgauer, but not Tarentaise or Angus-Hereford crosses. Pinzgauer crosses reached puberty at a significantly younger age than all other crosses. Females with Angus dams did not differ from females with Hereford dams in weight at puberty, but were 22 days younger (P<.01) at puberty. A significantly higher percentage of Brahman (93%), Sahiwal (98%) and Pinzgauer (94%) crosses, but not Tarentaise crosses (91%), were pregnant at 550 days than Angus-Hereford crosses (82%).

Growth, puberty and pregnancy were studied in 665 crossbred heifers produced by breeding Hereford and Angus cows to Hereford, Angus, Red Poll, Brown Swiss, Gelbvieh, Maine Anjou, and Chianina sires. Estrus was determined from an average of 240 to 476 days of age. Puberty age was defined as age at first observed, standing estrus followed by an estrus within 45 days, except during the last 45 days of estrus detection when it was defined as first standing estrus. Heifers were bred by artificial insemination for 42 days and naturally for 22 days from an average age of 412 to 476 days.Brown Swiss, Gelbvieh, Maine Anjou and Chianina crosses were 5 to 7% heavier (P<.01), while Red Poll crosses were not different (P>.05) from the Hereford-Angus crosses at the 200-day weight. Compared to Hereford-Angus crosses at 400 days of age, Maine Anjou, Chianina and Gelbvieh crosses were 5 to 8% heavier( (P<.01), Red Poll crosses were 4% lighter (P<.01), and Brown Swiss were 3% heavier (P>.05).Sire breed influenced (P<.01) puberty age and percentage of heifers reaching puberty by 270 to 450 days of age. Sire breed groups divided into three groups for puberty age with Gelbvieh (343 ± 5), Brown Swiss (349 ± 4), and Red Poll (354 ± 5) crosses the youngest (P<.01), Chianina (401 ± 5) crosses the oldest (P<.01), and Maine Anjou (374 ± 5) and Hereford-Angus (374 ± 5) crosses intermediate. Chianina crosses were heaviest at puberty (319 ± 4 kg); followed by Maine Anjou crosses (307 ± 4); then Gelbvieh (286 ± 4 kg), Hereford-Angus (284 ± 4), and Brown Swiss (281 ± 3) crosses; with Red Poll crosses (265 ± 3) the lightest.The heritability estimate was .41 ± .17 for puberty age, .40 ± .17 for puberty weight, and the genetic correlation between puberty age and puberty weight was .52 ± .23. Using breed group means, the correlations between puberty age and the following traits were: birth weight (.66), puberty weight (.90), calving as 2-year-olds the first 25 days of the calving period (−.75), percentage pregnant (−.42), milk production as 3- and 4-year-old cows (−.88), and percentage fat trim at a constant age of steer contemporaries (−.70).

Gestation length, calving difficulty, perinatal mortality, calf crop weaned, birth weight, preweaning average daily gain (ADG), relative growth rate (RGR) and 200-day weight were examined on 1,610 calves born and 1,536 calves weaned in 1975 and 1976 from 4- to 11-year-old Hereford and Angus dams and by Hereford, Angus, Brahman, Sahiwal, Pinzgauer and Tarentaise sires.Angus dams were superior (P<.01) to Hereford dams in preweaning ADG, RGR and 200-day weight. Hereford dams produced calves with significantly heavier birth weights and longer gestation length than Angus dams. Effects of breeding group and sex of calf were significant for all traits except perinatal mortality and percentage calf crop weaned. All breed of sire groups except Pinzgauer and Tarentaise differed significantly from each other in gestation length; Sahiwal crosses had the longest (294.2 days) and Hereford-Angus crosses had the shortest (284.2 days) gestation length. Brahman crosses exhibited a significantly higher level of calving difficulty than all other breed of sire groups which did not differ (P>.05) from each other. Differences in perinatal mortality were not significant among sire breed groups. Hereford-Angus reciprocal crosses had a significantly higher percentage calf crop weaned than Brahman and Sahiwal crosses. Differences in percentage calf crop weaned were not significant among all other breed of sire groups. Birth weight was heaviest in Brahman crosses (40.6 kg) and lightest in Hereford-Angus (35.4 kg) crosses. All sire breed groups except Sahiwal and Tarentaise crosses differend significantly from each other in birth weight. Brahman crosses had significandy heavier 200-day weight than all other sire breed groups and had significantly higher preweaning ADG than all other breed of sire groups except Tarentaise. Sahiwal and Hereford-Angus crosses were similar (P>.05) in 200-day weight as were Pinzgauer and Tarentaise crosses. All other sire breed groups except Sahiwal and Pinzgauer differed significantly from each other in 200-day weight. Sahiwal and Pinzgauer crosses did not differ significantly from each other or from Hereford-Angus reciprocal crosses in preweaning ADG.

Gestation length, calving difficulty, perinatal mortality, calf crop weaned, birth weight, preweaning average daily gain (ADG), relative growth rate (RGR) and 200-day weight were studied on 1,591 calves born and 1,494 calves weanedin 1973 and 1974 from 4- to 9-year-old Hereford and Angus dams and by Hereford, Angus, Red Poll, Brown Swiss (domestic and European), Gelbvieh, Maine-Anjou and Chianina sires. Hereford and Angus dams differed (P<.01) for most traits; the Angus dams reflected a generally higher level of maternal performance. Breed of sire effects were important (P<.01) for all traits; the Hereford-Angus, Red Poll and Brown Swiss crosses were superior in perinatal mortality and calf crop weaned whereas the Gelbvieh, Maine-Anjou, Chianina and Brown Swiss crosses were superior in ADG and in 200-day weight. Thus, average weight weaned per cow calving was highest for the Brown Swiss crosses. Level of calving difficulty was greatest (P<.01) in the Maine-Anjou crosses. The Hereford-Angus reciprocal crosses had the lowest level of calving difficulty, but the value was not significantly less than the value for the Red Poll and Gelbvieh crosses. The Brown Swiss, Maine-Anjou and Chianina crosses had a significantly higher level of calving difficulty than that of the Hereford-Angus reciprocal crosses. Age of dam (4-year-olds vs5-year-olds and older) was important (P<.01) for birth weight, ADG, RGR and 200-day weight. The effects of sex of calf were important (P<.01) on all traits studied except perinatal mortality and calf crop weaned.When the data wereanalyzed with birth weight as a covariate on calving difficulty (with abnormal presentations included as difficult births), perinatal mortality and calf crop weaned; breed of sire, breed of dam and sex of calf effects were still significant for these traits. This result reflects the importance of factors, associated with breed of sire, breed of dam and sex of calf, other than birth weight on these traits.

Relationships among body weights, absolute growth rate (AGR), absolute maturing rate (AMR), relative growth rate (RGR) and degree of maturity of body weight (u) were examined for 208 straightbred and reciprocally crossed Hereford, Angus and Shorthorn cows by 16 Hereford, 17 Angus and 16 Shorthorn sires, u is that fraction of mature weight (adjusted for condition) attained at any age (i.e., u = weight/ mature weight). Heritability of weight and u was respectively .68 and .73 at birth, .59 and .64 at 200 days, .87 and .55 at 396 days, .82 and .21 at 550 days, .41 and —.26 at 3 1/3 yr, and .44 and .40 at puberty. The genetic correlations among weights were large. Heritability of age at puberty was .64 ± .31. The genetic correlation between age and weight at puberty was .67 ± .24.Animals growing most rapidly during the preweaning interval, in absolute terms (AGR) and relative to both current (RGR) and mature weight (AMR), tended to grow more slowly at later ages. Animals more mature at one age were generally more mature at all other ages. Variation in mature weight accounted for only 19% of the genetic variation in u through 550 days. Heritabilities were generally larger for RGR than for AMR. On average, 50% of the variation in AGR was independent of mature weight. Correlations between all measures of growth in the same interval were positive but were generally negative between different intervals. Thus, selection for increased growth rate over any age interval, in either absolute or relative terms, would tend to alter the shape of the growth curve over the interval of selection. However, selection for AGR over any interval would increase weight at all ages, while selection for preweaning RGR would tend to decrease birth weight and weight subsequent to 550 days, but increase 200-, 396- and 550-day weights.

Heterosis and reciprocal differences for immature weights, mature weight, degree of maturity of body weight (u), absolute growth rate (AGR), absolute maturing rate (AMR), relative growth rate (RGR) and age at puberty were examined for 208 straightbred and crossbred Hereford, Angus and Shorthorn females. The cows were born from 1960 to 1963 at the Fort Robinson Beef Cattle Research Station, by 16 Hereford, 17 Angus and 16 Shorthorn sires. Mature weight was defined as weight, adjusted by regression for condition score, taken in January, 1969, when the cows were 6 to 9 years of age. Average heterosis for mature weight was 2.5% (P<.05), but neither heterosis for specific two-breed crosses nor reciprocal differences were significant for mature weight.The increased AMR's of crossbred calves through 550 days of age supported the hypothesis that a primary effect of heterosis is to accelerate the maturing process. Heterosis for u was significant at all constant ages except at birth. The largest heterotic effect was for crosses involving Hereford. Reciprocal differences were unimportant for u, AMR and RGR. Heterosis for RGR was significant (P<.05) only during the first wintering period—a time of increased environmental stress.

Postweaning growth and puberty traits were studied in 965 heifers produced by mating Hereford and Angus cows to Hereford, Angus, Jersey, South Devon, Limousin, Charolais and Simmental sires. Estrus was checked twice daily from 250 to either 480 or 510 days of age and age and weight at puberty were evaluated. Puberty was defined as date of first standing estrus.Charolais, Simmental and South Devon crosses were the heaviest at 400 days of age followed closely by Hereford-Angus and Limousin crosses with Jersey crosses the lightest. Jersey crosses were 16% lighter than Charolais and 8.7% lighter than Limousin crosses at 400 days of age.For percentage of heifers reaching puberty at each 30-day period from 300 to 450 days of age, breed crosses separated into three distinct groups. A higher percentage of Jersey crosses and a lower percentage of Charolais and Limousin crosses reached puberty with Hereford-Angus, South Devon and Simmental crosses intermediate throughout this age range. Breed crosses divided into the same three groups on the basis of average age at puberty. Charolais crosses were heaviest at puberty, followed by Limousin and Simmental, then South Devon and Hereford-Angus, with Jersey crosses the lightest.Heifers from Angus dams were 26 days younger and 9 kg lighter at puberty than those from Hereford dams. Heterosis in Hereford-Angus reciprocal crosses for age at puberty was 19.5 days with no effect on weight at puberty.

Postweaning growth and feed efficiency of 1,105 steers from matings of Hereford and Angus dams to Hereford, Angus, Jersey, South Devon, Limousin, Charolais and Simmental sires were analyzed. Differences were large for 405-day weight (range of breed group means = 14%), average daily gain (ADG) (range = 19%) and relative growth rate (RGR) (range = 10%). Charolais and Simmental crosses, followed by South Devon crosses, were the largest, fastest gaining breed groups; Hereford-Angus cross-breds and Limousin crosses were similar in ADG and 405-day weight; and Jersey crosses were the smallest, slowest growing. South Devon and Simmental crosses had the highest RGR, whereas Limousin and Jersey crosses had the lowest. Breed group rankings were similar for ADG and RGR.Feed efficiency was evaluated over age (0 to 217 days on feed) and weight (240 to 470 kg) intervals and to a longissimusfat-constant end point (0 days to 5% longissimusfat) by regression analysis of data for breed groups fed in replicated pens and slaughtered serially at three dates within each year. Breed group differences were large for weight-constant (range = 23%), but reduced for age-constant (range = 9%) and longissimusfat-constant (range = 12%) intervals. ADG from 240 to 470 kg tended to explain relative ranking of breed groups for weight-constant efficiency. When tested against a significant breed-group by year interaction, Jersey crosses were less efficient than Charolais crosses in the age-constant interval. When evaluated to 5% longissimusfat, Limousin crosses were least efficient, but not different (P>.05) from Simmental crosses.

Gestation length, birth weight, dystocia level, calf mortality, preweaning growth and 200-day weight were studied for 2,368 calves out of Hereford and Angus cows and by Hereford, Angus, Jersey, South Devon, Limousin, Charolais and Simmental sires. Charolais and Simmental crosses had faster preweaning average daily gains (ADG) and were heavier at 200 days, but also had larger birth weights and more dystocia. South Devon and Limousin crosses were similar in 200-day weight to Hereford-Angus crosses, but were intermediate in birth weights and more like Charolais and Simmental crosses in level of calving difficulty. Jersey crosses were lightest at birth and 200 days and experienced much less dystocia than other crosses. Gestation length was longest for Limousin (288.1 ± .4 days) and shortest for Jersey (281.8 ± .5 days) and Hereford-Angus (282.9 ± .4 days) crosses. Preweaning relative growth rate was highest for Jersey, intermediate for Hereford-Angus and lowest for South Devon, Limousin, Charolais and Simmental crosses. Hereford by Angus heterosis was a significant effect for birth weight (.9 kg), 200-day weight (8 kg), ADG (.04 kg/day) and calf survival from birth to weaning (5.1%).Dystocia level increased linearly with birth weight both across and within breed groups. Averaged over all dam ages, each 1 kg increase in birth weight within a breed group increased calving difficulty by 1.63 ± .20%. Calf death losses within 24 hr postpartum were 3.7 times higher in calves experiencing difficulty at birth than those born without difficulty (11.5 ± 1.0% vs3.1 ± .7%). Calving difficulty did not influence (>.05) calf survical from 24 hr postpartum untl weaning.

Effect of dystocia, breeding group, cow age and calf sex on peri- and early postnatal (within 24 hr. after birth) mortality was studied. Data obtained from 1967 to 1972 included 5,064 parturitions from 18 breeding groups. Approximately 35% of the parturitions were from 2-year-old cows, 24% from 3-year-olds and 41% from cows 4 years and older. Type of parturition was divided into four classifications: no dystocia, calfpuller, surgical removal (C-section) and posterior presentation. Overall mortality was 8.6% (436 of 5,064).Calf losses at or near the time of birth were four times greater (P < .01) in calves experiencing dystocia (20.4%) than in those not experiencing dystocia (5.0%). Calf losses were not significantly different among the three types of dystocia parturitions. Calf sex, cow age and breeding group significantly (P < .01) influenced the proportion of dystocia parturitions.In dystocia parturitions, calf losses were higher (P < .05) in males (22.4%) than in females (16.3%), but there was no significant difference in mortality between sexes for calves not requiring assistance at birth.Calf losses were higher (P < .01) in 2-year-old cows with no dystocia than in 3-year-olds and those ≥4 years of age, but cow age had no significant influence on calf mortality in parturitions involving dystocia.Mortality ranged from 5.5% for Jersey × Angus to 14.5% for Charolais × Angus. Breeding group affected (P < .01) calf mortality in parturitions involving dystocia, but did not significantly influence mortality in unassisted parturitions.When calving assistance was required, calf mortality was 11.6% higher (P < .01) in straightbred Hereford and Angus calves than the average for Hereford × Angus and Angus × Hereford calves. Death losses in all types of parturitions were 3.8% higher (P < .01) in the straightbred than in the reciprocal crossbred calves of the Hereford and Angus breeds.

Selection applied and generation interval were studied in three 150-cow-6-sire lines selected for (1) weaning weight (WWL), (2) yearling weight (YWL), or (3) an index of yearling weight and muscling score (IXL). Data were birth weights, weaning weights, yearling weights and muscle scores of 2,956 calves raised at the Fort Robinson Beef Cattle Research Station, Crawford, Nebraska during the years 1960 to 1970. Formulas were presented for calculating average cumulative selection differentials and generation coefficients in populations with overlapping generations.Generation interval based on actual age of parents for the 2,956 calves weaned was 4.6 years. Sire age was 4.1 years and dam age was 5.1 years. By 1970 an average of 2.0, 1.8 and 1.9 generations of selection had been practiced in the weaning weight, yearling weight and index lines, respectively.Average sire selection differentials, per generation, in WWL, YWL and IXL were, respectively: weaning weight 1.51, 1.18, 0.86; yearling weight 1.42,1.79, 1.22; muscling score 0.51, 1.11, 2.13 standard deviations.

Selection response was studied in three lines of Hereford cattle selected for (1) weaning weight (WWL), (2) yearling weight (YWL) or (3) index of yearling weight and muscling score (IXL). Selection response was evaluated by several measures of offspring regression on selection in parents. Average estimated response, expressed in standard deviation units per generation, in the three lines, WWL, YWL and IXL, were: birth weight, 0.22, 0.28 and 0.28; weaning daily gain, 0.20, 0.13 and 0.12; weaning weight, 0.23, 0.17 and 0.15; postweaning daily gain 0.28, 0.42 and 0.33; yearling weight, 0.36, 0.43 and 0.33 and muscling score, –.03, 0.01 and 0.24, respectively. Birth weight response per unit of selection was 0.47 and appeared highly correlated genetically with all performance traits. Expected increase in birth weight could be reduced 30% if all emphasis on growth was directed to postnatal growth rate rather than weaning or yearling weight. Response per unit of selection applied was low for weaning gain or weight (0.10 or 0.12) relative to other traits. Selection for postweaning gain or yearling weight may increase weaning gain or weight more than direct selection for these traits. Response in postweaning gain and yearling weight per unit of selection applied was relatively large (0.37 and 0.48). Correlated responses to selection in the three lines suggest that a wide variety of selection patterns will lead to improvement in all traits even though optimum selection indexes may maximize improvement in particular traits.

Records on 1,661 Angus, Hereford and reciprocal cross calves were analyzed to examine effects of breed of sire, breed of dam, sex of calf, year-age of dam and heterosis on percent dystocia, peri- and postnatal survival and preweaning weights and growth rate. Breed of sire, breed of dam, sex of calf and year-age of dam were significant (P<.01) sources of variation for most of the preweaning characters examined.Calves by Hereford sires were 1.4 kg heavier at birth and experienced 9.3% more dystocia than calves by Angus sires. Angus-sired calves were 4.1 kg heavier at 200 days than Hereford-sired calves. There were no significant breed of sire effects for birth date or calf survival.Angus dams gave birth to calves 5.2 days earlier in the calving season with 9.5% fewer incidences of dystocia than Hereford dams. Calves from Angus cows were 1.7 kg lighter at birth and 14.9 kg heavier at 200 days.Male calves were heavier at birth and weaning and exhibited a higher incidence of dystocia and a lower survival rate than females. Three-year-old dams produced heavier calves at birth with less dystocia, with higher survival rates and heavier weaning weight than 2-yearold dams.Heterosis effects were observed for birth weight (3%) and preweaning growth (8%). However, there were no significant differences between straightbred and crossbred calves for percent dystocia or for peri- and postnatal survival. No significant differences between sexes were observed for amount of heterosis.

Effects of heterosis on milk production and maternal heterosis on preweaning growth traits and conformation score were evaluated in reciprocal crossbred and straightbred cows of Hereford (H), Angus (A) and Shorthorn (S) breeds. Maternal heterosis was estimated from the difference between progeny of crossbred and straightbred dams sired by the same bulls of a third breed. A total of 555 and 420 calves were produced by crossbred and straightbred dams, respectively. Milk production and composition were evaluated on a sample of 149 crossbred and 101 straightbred cows ranging from 2 to 5 years of age. Approximately one-half of the cows were developed and managed to calve first as 2-year-olds and one-half as 3-year-olds to evaluate effects of maternal heterosis expressed from 2 through 6 years of age and 3 through 8 years of age in each management regime, respectively.Over all breeds, ages and management regimes, effects of maternal heterosis were 1.7% for birth weight (P<.05), 3.6% for weight at 135 days (P<.01), 4.7% for weight at 200 days (P<.01) and one-sixth of a grade (P< .01) for conformation. These effects of maternal heterosis did reflect greater and more persistent milk production in favor of crossbred cows over straightbred cows by 0.9% at 2 weeks postpartum, 7.5% at 6 weeks postpartum (P<.05), 6.1% at about 14 weeks postpartum and 38% (P<.01) at weaning at about 29 weeks postpartum.

Effects of heterosis on reproduction were evaluated in a comprehensive experiment comparing straightbred Hereford (H), Angus (A) and Shorthorn (S) females to reciprocal cross females among these breeds when mated to produce crossbred calves with equal additive and non-additive genetic composition. This phase of the experiment involved 570 matings (cow-years) of straightbred cows and 687 matings of crossbred cows accumulated over six breeding seasons, 1962 through 1967. Approximately one-half of the cows were developed and managed to calve first as 2-year-olds and one-half as 3-year-olds, to evaluate the effects of heterosis expressed from 2 through 6 years of age and 3 through 8 years of age in each management regime, respectively.Analyses over all breeds, ages and systems of management revealed that the effects of heterosis significantly (P<.05) reduced the interval from parturition to first estrus and the average date of conception.Calf crop weaned was 6.4% greater for crossbred than for straightbred cows (P<.01).

The influence of hormone implants on daily gains and feeder grade of steers grazed under range conditions was studied in two experiments involving 78 and 288 steers. Implants of 24 mg. of stilbestrol and implants containing 200 mg. progesterone and 20 mg. estradiol benzoate increased daily gains by 8 to 11 %. An implant containing 60 mg. 17-alpha-hydroxy-progesterone caproate, 60 mg. testosterone enanthate, and 24 mg. estradiol valerate increased daily gains about 2%, but this difference was not significant. There was no difference in feeder grade between the treatments.After the summer grazing period the group of 288 steers were subdivided and part of the steers were reimplanted. The steers were then placed in feedlots and full-fed to choice slaughter grade. The implants, increased the rate of gain 12 to 22% when compared with gains of steers receiving no implant in either the summer or feedlot periods. One implant in either the grazing or the feedlot phase seemed to be as satisfactory as an implant in each phase so far as average daily gain over both periods was concerned. The steers that were not implanted in either period required 10 days longer to reach choice slaughter grade, but they did have a higher carcass grade than the other groups. There was no difference in days required to reach choice slaughter grade between the various treated groups.There were small but significant differences in carcass grade due primarily to differences in marbling. There was no significant difference in rib-eye area, fat thickness, or dressing percent between the various treatments. The implanted steers required less feed per 100-lb. gain, but the differences were not statistically significant.

The importance of evaluating the breeding potentialities of domestic animals at an early age has long been recognized by livestock breeders. If phenotype at an early age is an expression of genotype, it should be possible to select superior individuals on the basis of their early performance. However, if the phenotype of an individual is dependent largely on factors other than genotype, an animal's performance at an early age is of little value as a criterion of selection. More information on the factors that influence birth and weaning weights of beef calves is needed to determine the effectiveness of selection based on these characters. The purpose of this study was to gain information on the relative importance of some of the factors that influence birth weight, weaning weight, and gain from birth to weaning of beef calves, and to determine the value of each of these characters

The format of this symposium indicates that it is the intent of the program committee that this discussion be concerned primarily with postweaning growth rate with perhaps some treatment of postweaning feed efficiency. To the extent that postweaning growth rate and feed efficiency may be genetically associated with other traits, such associations may be considered. To put things in perspective, it is appropriate to review the primary interests of the different segments of the beef cattle industry. The commercial producer is interested in cows with a long productive life that wean a high percent of heavy, high grading calves; the feeder desires rapid and efficient feed lot gains; and the packer and retailer are interested in the maximum amount of edible portion per unit of live or carcass weight. The consumer expects this edible portion to be tender, flavorful and juicy. Thus, this particular discussion involves the traits that are of direct concern to the feeding segment of the industry.

Calving and subsequent breeding information were obtained on 1889 Hereford and Angus cows bred to bulls representing the Hereford, Angus, Jersey, South Devon, Limousin, Simmental and Charolais breeds. The influence of sire breed, dam breed, dam age, calf sex and calf birth weight on the occurrence of dystocia and the effects of dystocia on subsequent reproductive performance were examined.Sire breed, dam breed, dam age and calf sex were significant (P<.005) sources of variation associated with dystocia when birth weight was not included in the analysis. The regression of birth weight on calving difficulty was a significant (P<.005) source of variation and calving difficulty increased 2.30 ± 0.21% for each kilogram increase in birth weight. When birth weight was held constant in the analysis, dam age was the only main effect significantly (P<.005) associated with percent calving difficulty.Calves sired by Charolais, Simmental, Limousin and South Devon bulls experienced significantly (P<.01) more calving difficulty (30.90±2.41, 32.66±3.09, 30.78±3.19 and 32.34±5.19%) than those sired by Hereford, Angus and Jersey bulls (15.78+-2.27, 9.90±2.55 and 6.46±3.61%). Percent calving difficulty was higher (P<.05) in Hereford-than in Jersey-sired calves. No significant differences in percent calving difficulty were found among the Charolais-, Simmental-, Limousin- and South Devon-sired calves nor between the Angus- and Jersey-sired calves.Dystocia in 2-year-old cows was 36.03±2.96% higher than in 3-year-olds and 44.62± 2.99% higher than in 4 and 5-year-olds. Percent dystocia was 8.59±3.39% higher (P<.05) in 3-year-old than in 4- and 5-year-old cows.Hereford cows had more (P<.005) calving difficulty than Angus cows, 34.78±3.19 compared to 27.02±3.13%. Dystocia was not significantly different between crossbred and straightbred calves, but crossbred calves were 1.55±0.26 kg heavier at birth than straightbreds. Male calves were heavier (P<.005) at birth than female calves, 35.12±1.82 compared to 32.10±1.81 kg, and experienced more (P<.005) dystocia, 28.40±1.71 compared to 16.98±1.70%.There was a higher percentage of calving difficulty in male births than in female births for all ages of cows, but the magnitude of the differences between sexes was greater in 2-year-old cows than in the other age groups.Dystocia had a signicant (P<.005) effect on the percentage of cows detected in estrus during the 45-day AI period, conception rate to AI and total conception rate. Fewer (14.4%) cows experiencing dystocia were detected in estrus during the AI period than those with no dystocia. In all cows, dystocia resulted in a 15.6% lower conception rate to AI and a 15.9% lower overall conception rate.The 2-year-old cows had a lower estrus detection rate, a lower conception rate during the AI period and a longer interval from calving to first breeding and from calving to conception than the 3-, 4- and 5-year-old cows.