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Data were collected from 1953 through 1980 from identical and fraternal twin beef and dairy females born in 1953, 1954, 1959, 1964, and 1969, from crossbred females born as singles in 1974, and their progeny. Numbers of dams that weaned at least one calf and were included in the first analysis were 37, 45, and 56 in the 1964, 1969, and 1974 data sets, respectively. Respective numbers of dams that weaned three calves and were included in a second analysis were 6, 8, 8, 22, 33, and 33 in the 1953, 1954, 1959, 1964, 1969, and 1974 experiments. Individual feed consumption was measured at 28-d intervals from the time females were placed on the experiment at 240 d of age until three calves were weaned or the dams had reached 5 yr of age. Residual feed intake (RFI) and residual BW gain (RG) of heifers that subsequently became dams were determined based on ADG and DMI from 240 d of age to first calving. Various measures of cow efficiency were calculated on either a life cycle or actual lifetime basis using ratios of progeny and dam weight outputs to progeny and dam feed inputs. The correlation between RFI and DMI was large and positive (r= 0.67; P< 0.0001), and RG was highly correlated with ADG (r= 0.75; P< 0.0001). Correlations of RFI with cow efficiency ratios that included harvest weight, carcass weight, or weight of trimmed wholesale cuts as measures of output ranged from –0.05 (P> 0.10) to −0.17 (P< 0.10), indicating that heifers with better (i.e., more negative) RFI values tended to become slightly more efficient cows. Correlations of RG with life cycle and actual lifetime cow efficiency ratios ranged from 0.08 (P> 0.10) to 0.23 (P< 0.05), demonstrating that heifers with better (i.e., more positive) values for RG were somewhat more efficient as cows. The correlations were stronger when cow salvage value was included in the measures of cow efficiency. Correlations of DMI and mid-test metabolic BW (MMW) with life cycle cow efficiency ratios that did not include cow salvage value as output ranged from −0.15 (P< 0.10) to −0.22 (P< 0.01). Correlations of DMI and MMW with actual lifetime cow efficiency ratios varied from −0.20 (P< 0.05) to −0.36 (P< 0.001). Therefore, smaller heifers that consumed less feed had superior cow efficiency ratios. Correlations of RFI with carcass grade, backfat thickness, marbling score, and kidney fat of progeny indicated that heifers with superior RFI would tend to produce leaner offspring.

Data were collected from 1953 through 1980 from identical and fraternal twin beef and dairy females born in 1953, 1954, 1959, 1964, and 1969, and from crossbred females born as singles in 1974, and their progeny. Numbers of dams that weaned at least 1 calf and were included in the first analysis were 37, 45, and 56 in the 1964, 1969, and 1974 data sets, respectively. Respective numbers of dams that weaned 3 calves and were included in a second analysis were 6, 8, 8, 22, 33, and 33 in the 1953, 1954, 1959, 1964, 1969, and 1974 experiments. Individual feed consumption was measured at 28-d intervals from the time females were placed on the experiment until 3 calves were weaned or the dams had reached 5 yr of age. Residual feed intake (RFI) and residual gain (RG) of the heifers that subsequently became the dams in this study were determined based on ADG and DMI from 240 d of age to first calving. Various measures of cow efficiency were calculated on either a life cycle or actual lifetime basis using ratios of progeny and dam weight outputs to progeny and dam feed inputs. Residual feed intake was phenotypically independent of ADG and metabolic midweight (MMW), whereas the correlation between RFI and DMI was positive and highly significant (r= 0.67; P< 0.0001). Residual gain was highly correlated with ADG (r= 0.75; P< 0.0001) and had near 0 correlations with DMI and MMW. Correlations indicated that heifers that ate less and had smaller metabolic midweights from 240 d of age to first calving had superior efficiency ratios as cows. Residual feed intake was not significantly correlated with age at puberty, age at calving, or milk production. Results of this study do not indicate any serious antagonisms of postweaning heifer RFI with subsequent cow and progeny performance traits or with life cycle or actual lifetime cow efficiency. In addition, selection for increased RG would result in earlier ages at calving, but would also tend to result in taller and heavier cows.

The objective of this study was to investigate the association of the fibroblast growth factor 2 (FGF2) gene with embryonic survival and fertilization rate in cattle. This gene was chosen because of its role in regulating trophectoderm expression of interferon-τ, the maternal pregnancy recognition factor in ruminants. To evaluate the effect of FGF2on fertility traits, we produced in vitro-fertilized embryos from 281 Holstein cows and from 7 sires. A total of 4,542 in vitro fertilizations were performed, from which a total of 3,171 embryos were produced. Survival and fertilization rates were assessed at d 7 of embryonic development. Using the pooled DNA sequencing approach, we identified 2 SNP in FGF2, SNP11646 and SNP23. All sires and cows were genotyped for these SNP. For fertilization rate, no significant differences between genotypes were found for either SNP, whereas the effect on survival rate was significant for SNP11646. The survival rate of embryos produced from GG cows for this SNP was 37%, compared with 28 and 29% for embryos produced from AG and AA cows, respectively. Although the molecular mechanisms that cause embryonic mortality have not yet been identified, this study provides the first evidence of association between FGF2and embryonic mortality in cattle. Thus, we propose that FGF2can be used in animal breeding strategies to test for improved reproductive performance.

Hypotheses of a negative association between fraternity size (size of litter in which an individual develops prior to birth or is reared following birth) and ovulation rate or litter size were tested by examining reproduction of females born or reared in varying prenatal and postnatal fraternities. Gilts were randomly assigned to develop prenatally and be reared postnatally in small or large fraternities. Dams of experimental animals were randomly assigned to one of two prenatal fraternity size treatments, either unilateral oviductal ligation (to bear a small prenatal litter) or no ligation (to bear a normal prenatal litter). Whereas this did result in differences (P< .01) in litter size at birth (small = 6.2 ± .4 vs large = 9.6 ± .9), there was considerable overlap in observed litter sizes between ligated and nonligated dams. Consequently, effects of prenatal fraternity size were examined by regression. Distinct differences in postnatal fraternity size were created by randomly assigning piglets to small (5 piglets) or large (10 piglets) postnatal fraternities within 24 h of birth. Differences in postnatal fraternity size were maintained through weaning at 3 wk (small = 4.9 ± .1 vs large = 9.4 ± .2). Weights at birth (regression of birth weight on prenatal fraternity size = −.07 ± .02, P< .01) and weaning (small = 6.09 ± .15 vs large = 5.46 ± .17 kg, P< .01) were heavier for gilts from small prenatal and postnatal fraternities, respectively, compared with gilts from large fraternities. Effects of prenatal and postnatal fraternity size on BW did not persist following weaning (P> .20). Both prenatal fraternity size (regression of fetus number on prenatal fraternity size = −.24 ± .11, P< .05) and postnatal fraternity size (small = 12.03 ± .58 vs large = 10.86 ± .68, P< .10) influenced number of fetuses at 30 d of gestation. Prenatal fraternity size also was negatively associated with ovulation rate (regression of ovulation rate on prenatal fraternity size = −.26 ± .13, P< .05). Differences in ovulation rate between postnatal fraternity classes followed the same pattern, but they were not significant. Neither prenatal nor postnatal fraternity size affected (P> .15) age at first estrus. No interactions of prenatal and postnatal fraternity size were observed for any of the traits examined (P> .15).

Weight and individual feed consumption data collected on 160 beef, dairy and beef × dairy dams and their progeny were used to estimate postweaning feed efficiency of progeny and several measures of lifetime cow efficiency for cows fed either a high or a low energy diet. Cow efficiency was expressed as the ratio of output to input where output included relative values for either progeny slaughter weights, carcass weights or trimmed wholesale cut weights plus cow weight; and input included pre- and postweaning feed consumption of progeny and lifetime feed consumption of the dam. In the first approach, life cycle cow efficiency was estimated by expressing output as a ratio to input when weight and feed consumption were weighted by their expected occurrence in a theoretical herd consisting of 100cows and 20 replacement heifers. Expected occurrence was a function of the age distribution of the cow herd and percentage calf crop. In the second approach, actual lifetime cow efficiency was estimated by expressing output as a ratio to input when all components were weighted equally. Both approaches included efficiency estimates calculated with and without cow calvage value. Diet of dam had little effect on efficiency of postweaning gain when progeny were slaughtered at age-constant end points. Dams fed low energy diets generally were equal or superior to those fed high energy diets in lifetime efficiency. Straightbred Holsteins mated to Hereford sires were less efficient than straightbred Herefords mated to Holstein sires, even though they produced progeny with heavier slaughter weights, carcass weights and trimmed wholesale cut weights, lower preweaning feed consumption and more efficient postweaning gains, in addition to possessing greater salvage value. The low efficiency ratios of Holstein dams were associated with their large metabolizable energy intake. Progeny of Simmental X Holstein and Chianina X Holstein dams tended to be more efficient than progeny of Hereford X Holstein and Angus X Holstein dams in the conversion of feed to postweaning gain. Hereford X Holstein and Simmental X Holstein dams were nearly equal in lifetime efficiency, while Angus X Holstein and Chianina X Holstein dams were slightly less efficient. Efficiency of production improved as number of progeny slaughtered increased.

Postweaning weight gains, feed consumptions and feed efficiencies, and slaughter weights, carcass weights and trimmed wholesale cut weights of progeny of 160 beef, dairy and beef × dairy dams fed high or low energy diets are reported. These traits represent progeny components of input and output needed to estimate lifetime cow efficiency to a slaughter end point. Diet of dam had little effect on postweaning gain, feed consumption and feed efficiency of progeny. Progeny of dams receiving the high and low energy diets also had similar slaughter weights, carcass weights and trimmed wholesale cut weights. In experiments initiated in 1964 and 1969, second and third progeny of Holstein dams had greater postweaning gains, consumed more metabolizable energy (ME) and tended to be more efficient in the conversion of feed into gain than progeny of Hereford, Hereford × dairy or Hereford × beef dams. In the first parity, Holstein progeny were second to Here ford progeny in gain and efficiency of gain. Progeny of Holstein dams had the heaviest slaughter weights, carcass weights and trimmed wholesale cut weights, but tended to have the greatest fat thickness over the longissimus muscle at the 12th rib and the lowest percentage of the carcass in trimmed wholesale cuts. Progeny of the larger breeds (Simmental × Holstein and Chianina X Holstein ) tended to gain more rapidly, consume more feed and convert the feed into gain more efficiently than progeny of the smaller breeds (Hereford X Holstein and Angus X Holstein) in the time constant postweaning interval used in an experiment initiated in 1974. Simmental × Holstein and Chianina X Holstein breed groups ranked highest in progeny output as measured by slaughter weights, carcass weights and weight of trimmed wholesale cuts. Progeny of these two breed groups had low fat thicknesses over the longissimus muscle, low dressing percentages and a high percentage of the carcass in trimmed wholesale cuts relative to Hereford X Holstein and Angus X Holstein progeny.

The precision of estimates of realized genetic parameters obtained from multiple-trait selection experiments is considered. Statistical optimality theory is used in conjunction with Monte-Carlo techniques to investigate the relationship between index weights and the variability of the estimates. Results indicate that arbitrary choice of index weights is ill advised, and that with some prior knowledge (or good guesses) about the magnitudes of the genetic parameters, index weights can be identified that yield estimates with predictable variation.

The influence of breed or breed of sire and dietary regimen on age at puberty in heifers was determined in two experiments. Exp. 1 was a 2 × 2 factorial involving animals of two breeds (Hereford and Holstein), born January through June, 1969 and fed one of two dietary regimens, High (H) and Low (L). Exp. 2 was a 4 × 2 factorial involving crossbred, autumn born heifers out of Holstein dams, sired by Angus (AF), Hereford (HF), Simmental (SF) or Chianina (CF) bulls and fed one of two dietary regimens. In Exp. 1, Herefords were older at puberty than Holsteins. In Exp. 2 CF and SF were older than AF and HF, and CF were older than SF at puberty. In both experiments, H heifers reached puberty sooner than L heifers. There were no breed or breed - of - sire × diet interactions. In Exp. 1, there was a season of birth × diet interaction (P<.01). Heifers on the H regimen that were born in the spring reached puberty earlier than winter-born H heifers while the opposite occurred among L heifers. Regression coefficients of age at puberty on birth date (January 1 = d 1) were −.69(P<.05), −.19, −1.00 (P<.005) and 1.86 (P<.01) for the Holstein H and L and Hereford H and L groups, respectively. Most of the animals in the first three groups reached puberty before June of the year following their birth, while Hereford L-fed heifers reached puberty between June and August, suggesting that date of attainment of puberty may have affected the relationship between age at puberty and birth date. In Exp. 2, the winter environment appeared to delay the onset of puberty. Most of the L heifers reached puberty in the summer, autumn or subsequent spring and few became pubertal during the intervening winter, causing a bimodal distribution of the dates of puberty attainment. Most of the H-fed heifers reached puberty before November, but puberty appeared to be delayed in those that did not. In Exp. 1, H heifers were heavier and had greater weight/height at puberty and consumed more TDN to puberty than L heifers. The opposite occurred in Exp. 2, presumably because seasons of birth differed between experiments and more L than H heifers did not reach puberty before winter in Exp. 2. Because H and L heifers reached puberty at different weights and levels of fatness, as measured by weight/height, dietary regimen did not appear to affect age at puberty by altering the time needed to attain a certain body weight or composition. We concluded that heifers fed diets low in energy reached puberty later than those fed diets high in energy and that breed or breed - of - sire differences in age at puberty were not affected by dietary regimen. Season affected age at puberty and may have altered the effect of nutrition and other factors on pubertal traits.

The interaction between dietary regimen and breed or breed of sire on postpartum reproduction was examined in two experiments. Exp. 1 was a 2 × 2 factorial involving two breeds (Hereford and Holstein) and two dietary regimens, high (H) and low (L). In Exp. 2, females out of Holstein dams and sired by bulls of four breeds, Angus (AF), Hereford (HF), Simmental (SF) and Chianina (CF) were used in a 4 × 2 factorial with two dietary regimens. Postpartum traits were observed for three consecutive postpartum periods (PP1, PP2, PP3), beginning after first calving. In Exp. 1, Holsteins tended to have longer intervals to first estrus (IE) and had longer intervals to conception (IC, P<.025) than Herefords in PP1 but not in PP2 or PP3. Cows fed H diets had significantly shorter IE and IC in PP1 and PP2, but not in PP3. Interval to conception was not recorded in PP3. There were breed × diet interactions for IE in PP1 (P<.06) and PP2 (P<.10) as differences in IE between L and H diets were more pronounced for the Holsteins than for the Herefords. Though not significant, Herefords fed L diets tended to have shorter intervals between estrus and conception than other groups in PP1 and PP2. In Exp. 2, breed of sire did not significantly affect any postpartum reproductive traits. Cows fed the H diet had shorter (P<.10) intervals to first estrus in PP1 and shorter (P<.05) intervals to first ovulation (10) in PP2 than L-fed cows. Interval to first ovulation was also shorter for H-fed cows in PP1, but the differences were not significant. The differences in IE between L and H diets tended to be greater for AF and HF cows than for SF and CF animals for PP1, but the interaction was not significant. The HF cows fed L diets had longer IE in PP2 than HF cows fed H diets, but the other breed - of - sire groups were not influenced by dietary regimen, resulting in a breed of sire × diet interaction (P<.10). Multiple regression analysis indicated that cows fed L diets that produced more fat-corrected milk and consumed less total digestible nutrients (TDN) from 0 to 56 d postpartum had longer IE during PP1. Fat-corrected milk production, TDN intake, weight at calving and change in body weight from 0 to 56 d postpartum were not significantly related to IE for cows receiving H diets during PP1. Data from PP2 and PP3 were pooled across diet groups for multiple regression analysis. None of the factors included in the model were significantly related to IE. We concluded that the influence of diet on postpartum reproductive function interacted with breed and parity and that the influence of level of milk production on IE varied with dietary level and parity. Because diet influenced reproduction more in Holsteins than Herefords, the use of dairy breeds in beef production may necessitate diets higher in energy to achieve maximum reproductive performance.

Log-linear and generalized least-squares approaches to analysis of categorical data are briefly introduced. Two data sets are analyzed to illustrate the methods of analysis.

Effects of cow size on cow productivity and calf performance were evaluated in Hereford cows. Cows were divided into four size groups based on the August weight taken during the summer following breeding for the second calf crop. Wither height was also taken at that time. Differences in the weight to height ratios and condition scores of the four size groups were highly significant. Very large cows were the fattest as well as the largest in body size. Compared to the herd average, small, medium, large and very large cows weaned −1.5, +3.6, +11.2 and −6.7% weight of calf/cow exposed, respectively. Expressed as ratio to cow metabolic weight, calf weaning weight/cow exposed was 1.63, 1.39, 1.51 and 1.15 and weight of calf slaughtered/cow exposed was 4.10, 3.47, 3.77 and 2.90 for small, medium, large and very large cows, respectively. Birth weight, preweaning daily gain and adjusted weaning weight were significantly greater for calves out of medium and large cows than for calves out of small and very large cows. The significant differences in weaning weight attributable to size of dam were maintained throughout the postweaning period, but postweaning growth and efficiency of steers were not significantly affected. At a constant age, all carcass weight and weight-related traits of steers were significantly affected by cow size. At a constant weight, the effects of cow size on most carcass traits of steers were not significant.

Twenty-four postpartum cows were used to determine the effects of suckling and exogenous gonadotropin-releasing hormone (GnRH) on pituitary-ovarian function and reproductive performance. Twelve cows were suckled, and calves were removed from the other 12 within 24 hr after birth. Six cows in the suckled group (S) and six in the nonsuckled group (NS) were injected with 250 µg GnRH on day 5 postpartum (parturition = day 0). The remaining animals in each group were injected with saline. Pituitary response to GnRH was estimated by measuring serum luteinizing hormone (LH) concentrations in blood collected hourly beginning 2 hr before and continuing until 9 hr after injection. Ovarian structures were observed by laparoscopy on days 5 and 8 postpartum. Injection of GnRH caused an increase (P<.05) in circulating LH concentrations in all animals. LH concentrations were highest 2 hr after injection. The area under the LH response curve was greater (P<.05) for NS than for S cows. Average daily LH concentrations during the first 25 days postpartum were higher in NS (2.4 ± .4 ng/ml) than in S cows (1.5 ± .3 ng/ml) because no ovulations occurred in the S group during this period. Ovaries of NS cows had greater (P<.01) follicular volume on day 5 than did ovaries of S cows (1,194 mm3vs195 mm3) because of a greater number of medium and large follicles, but estrogen concentrations were not significantly different on day 5 (NS = 6.7 ± .9 pg/ml; vsS= 7.4 + 1.8 pg/ml). On day 8, follicular volume was increased in GnRH cows and S cows (GnRH = 2,338 mm3vsno GnRH = 588 mm3and NS = 2290 mm3vsS = 637 mm3). Estrogen concentrations on day 8 were higher in S + GnRH cows (9.3 ± 2.1 pg/ml) than in S + saline (4.9 ± .6 pg/ml), NS + saline (5.7 ± .9 pg/ml) and NS + GnRH (6.3 ± .7 pg/ml) cows. Day to day repeatability of estrogen concentrations within animals, within treatment groups was estimated by intraclass (intracow) correlations. Correlations of estrogen values determined from days 6 to 25 were .38, .69, .56 and −.03 for the S + saline, S + GnRH, NS + saline and NS + GnRH groups, respectively. Correlation coefficients between LH and estrogen were .06, .08, .63 and .36 for the NS + GnRH, NS + saline, S + GnRH and S + saline groups, respectively. Intervals from parturition to first estrus and first ovulation were shorter (P<.01) for NS cows (14.9 and 9.6 days) than for S cows (57.6 and 36.8 days). Progesterone concentrations after first ovulations indicated the occurrence of short luteal phases in all NS cows. They had shorter intervals from calving to conception than S cows but required more inseminations per conception. GnRH treatment of NS cows had no significant effect on any intervals. However, five of the six S + GnRH-treated cows had shorter (P<.05) intervals to conception than dis S + saline-treated cows. It was concluded that calf removal hastens reestablishment of postpartum reproductive activity by eliminating a suppressive effect on pituitary gonadotropin release caused by suckling, which in turn causes hastened ovarian follicular development.

A model for swine fecundity that includes the effect of fraternity size is given. Over a range of plausible values for the parameters of the model, the dam-daughter correlation for number born tended to be negative. Thus, selection of replacement gilts born and reared in large litters would not bring about desirable genetic changes in litter size.

Weight and individual feed consumption data collected on 160 beef, dairy and beef × dairy dams and their progeny were used to calculate several measures of life cycle and actual lifetime cow efficiency to weaning and slaughter end points. Accuracy of predictors of cow efficiency that did not require feed intake data for progeny or dams was then examined. Multiple coefficients of determination (R2) for predictors expressed as the ratio of weaning or slaughter weight output to dam weight were similar to those for predictors expressed as the ratio of weight output to metabolic weight of dam. Including progeny metabolic weight along with dam metabolic weight in the denominator of ratio predictors generally reduced R2values. Accuracy of prediction was lower for weaning or slaughter weight alone than for predictors that expressed progeny weight as a ratio to dam weight or dam metabolic weight. The R2values were greater for multiple regression equations that included combinations of progeny weight, dam weight, weight:height ratio of dam, milk production and age of dam at calving than for ratios of progeny and dam weights. A prediction equation that included progeny weaning or slaughter weight, dam weight and age of dam at calving was the most useful from the standpoint of accuracy and simplicity. Life cycle and actual lifetime efficiency were predicted with sufficient accuracy to be useful (R2>.30) at the end of the second parity in all analyses.

Data from 107 beef (Hereford), dairy (Holstein) and beef × dairy cross cows and their calves through three parities were used to develop prediction equations of cow efficiency. Efficiency was defined as the ratio of calf weaning weight output to cow and calf input through three parities, adjusted for age of the dam at completion of three parities. Cows and calves were individually fed in confinement. Prediction equations of weaning efficiency developed from these data were validated using data from South Dakota State University, and prediction equations developed there were validated using the Wisconsin data. Dam weight or weight:height ratio at weaning of the calf in combination with progeny weaning weight accounted for 56 and 59% of the variation in weaning efficiency, respectively. Equations based solely on traits measured early in the life of the cow were of less predictive value; yearling weight and weight: height ratio explained 22 and 25% of the variation in weaning efficiency, respectively. Equations using a measure of cow size or condition and progeny weight were the most parsimonius equations developed from both the Wisconsin and South Dakota data. These equations performed well when validated on independent data. However, yearling weight and weight:height of the dam, the best early traits for prediction of efficiency in the Wisconsin data, accounted for less than 1% of the variation in weaning efficiency in the South Dakota data.

Weights and individual feed consumptions of 160 beef, dairy and beef × dairy dams and their progeny were used to estimate several measures of life cycle cow efficiency. Efficiency was expressed as the ratio of output to input, where output included relative values for slaughter weights, carcass weights and trimmed wholesale cut weights of progeny and weight of the dam, and inputs were pre- and postweaning feed consumption of progeny plus lifetime feed consumption of the dam. In the first approach, inputs and outputs were weighted by their probability of occurrence; probabilities were a function of the age distribution of the cow herd and calf crop percentage in a theoretical herd. The second approach involved dividing the sum of the weight outputs by the sum of the feed inputs when all pieces of information were weighted equally. Weight, height and weight/ height of dam at 240 d of age were not correlated significantly with subsequent efficiency of progeny slaughter weight production, indicating that selection on these traits would be ineffective for increasing lifetime cow efficiency. However, females exhibiting more efficient weight gains from 240 d to first calving tended to produce progeny with more efficient postweaning gains and tended to become more efficient producers on a lifetime basis. Correlations of efficiency estimates with weight of dam at calving were negative and generally highly significant. Height of dam was related positively to postweaning feed efficiency of progeny, but was negatively related to life cycle cow efficiency. Correlations involving weight/ height indicated that fatter dams were less efficient. Level of milk production needed to maximize biological efficiency varied with the breed combinations involved. The importance of fertility and calf survival were demonstrated by the close association of efficiency with slaughter rate and age at calving. Correlations among efficiency ratios calculated at weaning and at slaughter were .88 (P<.01) or above, whereas relationships between pre- and postweaning efficiency were nonsignificant. Correlations between postweaning efficiency and efficiency of slaughter weight, carcass weight and trimmed wholesale cut weight production ranged from .10 (P>.05) to .41 (P<.01).

Several measures of life cycle cow efficiency were calculated using weights and individual feed consumptions of 160 beef, dairy and beef × dairy dams and their progeny. Ratios of output to input were used to estimate efficiency, where outputs included relative values for slaughter weights, carcass weights and trimmed wholesale cut weights of progeny and weight of the dam, and inputs included pre- and postweaning feed consumption of progeny plus feed consumed by the dam over her entire lifetime. In the first approach to estimating efficiency, inputs and outputs were weighted by their probability of occurrence, where probabilities were a function of the age distribution of the cow herd and calf crop percentage in a theoretical herd. The second approach involved dividing the sum of the weight outputs by the sum of the feed inputs when all pieces of information were weighted equally. Both approaches included efficiency estimates calculated with and without cow salvage value. Efficiency estimates that did not include salvage value of the dam were negatively correlated (P<.01) with weight of dam at weaning. Larger, faster-gaining progeny were more efficient in the time constant postweaning phases of experiments initiated in 1964, 1969 and 1974. Correlations of cow efficiency with slaughter weights of progeny summed across parities were positive and highly significant. Metabolizable energy (ME) intake of dams accounted for approximately 69 and 65% of the total ME required to produce slaughter progeny in the first and second approaches, respectively, and showed a large negative association with lifetime efficiency.

Genetic engineering is defined as those technologies that permit directed changes in gene or genotype frequencies or that change life history characteristics of individuals or their gametes such that those individuals can make a disproportionate contribution to the gametic or zygotic pool of a population. It is a collection of technologies, theories and empirically determined facts with contributions from several traditional disciplines. Herein we review existing technologies, make some predictions and advocate a systems approach to evaluate the impact of these technologies on animal production and improvement.

Several measures of life cycle cow efficiency were calculated using weights and individual feed consumptions recorded on 160 dams of beef, dairy and beef × dairy breeding and their progeny. Ratios of output to input were used to estimate efficiency, where outputs included weaning weights of progeny plus salvage value of the dam and inputs included creep feed consumed by progeny plus feed consumed by the dam over her entire lifetime. In one approach to estimating efficiency, inputs and outputs were weighted by probabilities that were a function of the cow herd age distribution and percentage calf crop in a theoretical herd. The second approach to estimating cow efficiency involved dividing the sum of the weights by the sum of the feed consumption values, with all pieces of information being given equal weighting. Relationships among efficiency estimates and various traits of dams and progeny were examined. Weights, heights, and weight:height ratios of dams at 240 d of age were not correlated significantly with subsequent efficiency of calf production, indicating that indirect selection for lifetime cow efficiency at an early age based on these traits would be ineffective. However, females exhibiting more efficient weight gains from 240 d to first calving tended to become more efficient dams. Correlations of efficiency with weight of dam at calving and at weaning were negative and generally highly significant. Height at withers was negatively related to efficiency. Ratio of weight to height indicated that fatter dams generally were less efficient. The effect of milk production on efficiency depended upon the breed combinations involved. Dams calving for the first time at an early age and continuing to calve at short intervals were superior in efficiency. Weaning rate was closely related to life cycle efficiency. Large negative correlations between efficiency and feed consumption of dams were observed, while correlations of efficiency with progeny weights and feed consumptions in individual parities tended to be positive though nonsignificant. However, correlations of efficiency with accumulative progeny weights and feed consumptions generally were significant.

Weights and individual feed consumption collected on 160 beef, dairy and beef × dairy dams and their progeny were used to estimate several measures of lifetime cow efficiency. Dams were fed either a high or a low energy diet. Efficiency was expressed as the ratio of outputs to inputs. Outputs included progeny weaning weights plus cow salvage weight, and inputs were progeny creep feed consumption plus the dam's lifetime feed consumption. In the first approach, life cycle cow efficiency was estimated by expressing weight output as a ratio to feed inputs when weights and feed consumptions were weighted by their probabilities; probabilities were a function of age distribution and percentage calf crop in a theoretical herd consisting of 100 cows and 20 yearling replacement heifers. In the second approach, actual lifetime cow efficiency was estimated by expressing weight outputs as a ratio to feed inputs when all components were weighted equally. Both approaches included efficiency estimates calculated with and without cow salvage value. Dams receiving low energy diets generally had lifetime efficiencies equal or superior to those fed high energy diets in spite of older ages at calving. Dams on the high energy diet had greater salvage value, but did not wean calves of sufficient additional size to offset their own increased metabolizable energy (ME) intake. Dam breeds and breed crosses of smaller size tended to be more efficient than those of large size, demonstrating the effectiveness of mating small dams to large sire breeds for improving cow efficiency. Breeds calving at later ages were less efficient. Efficiency ratios improved as number of progeny weaned increased.

Selection for feed conversion substantially influenced growth and gross feed efficiency of mice. Realized heritabilities and genie correlation for increased gain on fixed feed intake (FF) and decreased feed intake on a constant gain (FG) were estimated to be .56, .73 and -.93, respectively. The genie correlations between FF and 56-day weight and between FG and 56-day weight were estimated to be .67 and -.95, respectively. The relative efficiency in changing FF or FG by selecting directly for 56-day weight was found to be .7 in either case. When efficiency was defined as the incorporation of biomass independent of maintenance, no difference was found between selection treatments. Differences in mature weight and feed intake approached significance with selected lines having higher means than controls. The absence of a correlated response in the ability to use energy for growth indicates that selection for feed to gain ratio changes maintenance requirements but not requirements for growth.

Effects of prenatal and postnatal fraternity size (size of litter in which an animal develops prior to birth or is reared following birth) on long-term reproduction were studied by rearing 178 female ICR mice in standardized prenatal and postnatal fraternities. Three levels of prenatal and postnatal fraternity sizes were used in a 3 × 3 factorial experiment. Prenatal fraternity size was standardized by selectively terminating fetal development in pregnant females carrying at least 14 conceptuses. Prenatal fraternities were standardized to either 6, 10 or 14 fetuses, and postnatal fraternities were standardized by randomly assigning individuals to nurse litters of 5, 10 or 15 pups. Prenatal fraternity size negatively affected average pup weight at birth (P<.05) but had little subsequent effect on growth or reproduction. Postnatal fraternity size negatively affected weight at weaning (P<.01), with mice reared in smaller postnatal fraternities being heavier than those reared in larger fraternities. Following weaning, mice reared in smaller fraternities gained weight less rapidly (P<.01) but still tended to be heavier at maturity (P=.11). Vaginal opening occurred at older ages in females reared in larger postnatal litters (P<.01). An interval mating system was used to examine fraternity size effects on long-term reproduction. Females were exposed to males six times at 8-wk intervals with initial mating at 7 wk of age. Postnatal fraternity size and age at mating jointly affected litter size (P<.05). The effect of postnatal fraternity size on subsequent litter size was negative for the first two matings; mice reared in small fraternities produced larger litters than mice reared in large fraternities. However, the effect was reversed at the sixth and final mating, with mice reared in small fraternities producing smaller litters than mice reared in large fraternities. Such effects, if observed in litter-bearing livestock species, could be beneficially exploited because breeding animals in production systems are seldom retained for their natural lifetime.

The literature on genetic and environmental sources of variation in the incidence of twinning in cattle is reviewed. The possibility of selection for this trait is discussed. Judging from its low heritability, low repeatability and small variance, the trait would not appear to be a likely candidate. These factors, however, were weighed against the potential benefits and the likelihood of achieving a workable system of twinning by other methods. It was concluded that some portion of our experimental cattle resources should be allotted to selection studies of bovine twinning.

The feasibility of systematically crossfostering within control lines was assessed in two lines of rats. This procedure generates full-sib covariances which are free of postnatal maternal effects and the effect of common environment. Also, it will tend to enhance the effective population size since parents will have a higher probability of transmitting genes to subsequent generations. Alterations of husbandry and data recording necessary to implement this procedure were minor. Thus, it was concluded that systematic crossfostering of control lines would be a useful procedure. Postnatal maternal effects on body weight or gain appear to be of less importance in the rat than in the mouse. The nurse component of variance accounted for 51% of the phenotypic variance in 2-week individual body weight in these data. The nurse component of variance dissipated rather rapidly, being almost zero for 6- and 9-week body weights. The dam × nurse component was small for all traits.

THE crossfostering technique was used to investigate the dynamics of genetic and maternal effects on growth in a random bred population of ICR mice. In addition to weights taken every 3 days from birth to 6 weeks and weekly weights thereafter until 84 days, several gains, 6-week tail length and traits derived from the logistic and Richards growth functions were considered. For weights, the variance due to genetic dams was initially large, but from day 6 to day 49 varied from about 10 to 15% of the phenotypic variance. After day 49 this component became relatively more important. The variance due to nurse dams peaked at 12 days accounting for 68% of the phentoypic variance in 12-day weights. After 12 days a progressive diminution of the relative importance of this component was found. A theoretical consideration of the crossfostering design indicated that the direct-maternal genetic covariance was equal to the interaction component of variance between direct genetic and nursing effects of dams, in the absence of fostering effects. This component was generally small and positive for weights and gains. For tail length and gains, the nurse component was more important than the direct genetic component with the exception of 42 to 84-day gain. Genetic variability and postnatal maternal effects on growth function traits were found. The growth function traits considered appear to be low to moderately heritable. It was concluded that of two functions considered, the logistic function was more suitable. Both functions underestimated asymptotic weight.

Environmental factors affecting milk yield were assessed for 279 lactations made by 193 Hereford cows in two herds during 1968 and 1969. Years and sex of calf were significant; whereas, herds and service sires within years were not statistically significant. Dams nursing female calves gave about 56 kg more milk over a 205-day lactation. This result is at variance with some reports in the literature. Of the regression effects, quadratic responses in milk yield due to age of dam and calving date were found to be significant. Peak yield was observed at 8.4 years. Significant linear effects were found for calf birth weight and cow weight, but both of these effects were small in comparison to age of dam and calving date effects.Examination of 205-day calf weight revealed significant effects of years, sires, milk production, calving date, calf birth weight and cow weight; whereas, age of dam effects were not significant. On a within herd-sex-year basis, approximately 60% of the variance in 205-day weight could be attributed to the direct influence of the dam's milk yield. Thus, it was concluded that effects due to age of dam are expressed primarily through differential milk production. Percent protein, percent solids-not-fat and percent fat had small nonsignificant correlations with 205-day weight. Therefore, it appears that milk quantity rather than milk quality was more important in its influence on 205-day weight.

Lactation records (279) from 193 Hereford cows in two North Carolina herds were used to evaluate various sampling schemes for estimating TMY which was defined as the sum of 7 monthly 24-hr, measures. Four prediction equations each based upon three 24-hr, measures were prepared from the North Carolina data. An independent data set provided by workers at the University of Wisconsin was used to test the suitability of these prediction equations. Correlations between observed milk yield in the Wisconsin data (based upon weekly measurements during lactation) and predicted milk yields (based upon three measures and the North Carolina equations) were greater than 0.93. Another independent data set involving three breeds of beef cows was furnished by workers at Texas A&M. Within breed correlations between observed TMY and predicted TMY ranged from 0.94 to 0.99. It appears that the prediction equations are suitable for use over a wide range of environmental and managerial practices.The estimate of repeatability for TMY was 0.38. This value suggests a low to moderate heritability of milk production in beef cows.

Data from three generations of a selection experiment with Yorkshire swine were analyzed. Fraternity size was defined as the number of sibs at 2 weeks of age. In one line, potential replacements were reared in fraternity sizes initially set at six, while in the other two lines fraternity size was not altered. Over the period of study, gilts reared in small fraternities (mean = 5.77) averaged 11.30 pigs born, whereas gilts reared in an unaltered fraternity size (mean = 10.06) averaged 10.19 pigs born. Both of these lines had experienced two generations of selection for increased litter size. A randomly selected control line in which fraternity size (mean = 7.80) was not altered averaged 10.77 pigs born. Gilts reared in small fraternities were heavier at 21 days, but there were no significant body weight differences between lines at breeding or farrowing.

One-hundred and five female rats from 25 litters were either joined with mature males at 3 weeks of age or at 9 weeks of age. Mean age at vaginal opening was about 37 days and was not accelerated by the presence of a mature male. Ninety-eight percent of the younger females pupped. Their average litter size at birth was 8.2. The females joined at 9 weeks had an average of only one more pup born (P < .05) even though they were, on the average, 30 days older (P < .01) and 36 g heavier (P < .01) at parturition than those joined at 3 weeks. This weight advantage was roughly maintained throughout a 3-week lactation. Although pups from the younger females averaged about 2 g less (P < .01) at weaning, it is unlikely that these reduced weaning weights would cause appreciable post-weaning death losses or other husbandry problems. These results indicate that a considerable shortening of the generation interval might be obtainable in selection or inbreeding studies.