Evaluation of the ovine callipyge locus: II. Genotypic effects on growth, slaughter, and carcass traits

A resource flock of 362 F2 lambs provided phenotypic and genotypic data to estimate effects of callipyge (CLPG) genotypes on growth, slaughter, and carcass traits. Lambs were serially slaughtered in six groups at 3-wk intervals starting at 23 wk of age to allow comparisons at different end points. Probabilities of CLPG genotypes were calculated at a position 86 cM from the most centromeric marker of chromosome 18. A contrast of CLPG genotypic effects, based on the paternal polar overdominance model, was used to evaluate callipyge and normal phenotypes. Relationships of traits with slaughter age, carcass weight, or 12th-rib fat depth for callipyge and normal phenotypic groups were estimated by regression. Callipyge and normal lambs did not differ for growth traits measured from birth to slaughter. Callipyge lambs produced 55.9% of live weight as chilled carcass weight compared with 51.7% for normal lambs at the same mean live weight of 48.32 kg. Lighter pelt, kidney-pelvic fat, and liver weights contributed to this advantage of callipyge lambs for dressing percentage (P < .001). Estimated accretion rates of carcass protein at the mean slaughter age were 12.5 and 10.2 g/d for callipyge and normal carcasses, respectively. Corresponding values for carcass fat were 35.2 and 42.1 g/d. Compositional differences in favor of callipyge carcasses were detected at constant values of slaughter age, carcass weight, and 12th-rib fat depth. Callipyge carcasses had 2.56 kg greater fat-free lean and 1.39 kg less fat than normal carcasses at the same mean age of 214.9 d (P < .001). The majority of these differences were established before the initial group was slaughtered and were maintained as age increased. Callipyge carcasses consisted of 24.3% fat and 71.3% fat-free lean, compared with 31.5 and 64.0% for normal carcasses at 25.6 kg of carcass weight. When evaluated at .49 cm of 12th-rib fat depth, callipyge lambs were 15.4 d older and produced 4.1 kg heavier carcasses with 4.3% less fat (P < .001). Effects of CLPG genotypic groups on carcass composition were greater than virtually all reported breed substitution effects. Use of the CLPG mutant allele in structured mating systems can dramatically increase production of lean lamb.

Evaluation of the ovine callipyge locus: I. Relative chromosomal position and gene action

Genotypic and phenotypic data were collected to estimate chromosomal position of the callipyge (CLPG) gene and to test gene action. Nine Dorset rams of extreme muscling phenotype and 114 Romanov ewes composed the grandparent generation of a resource flock of 362 F2 lambs segregating at the CLPG locus. The parent generation consisted of eight F1 sires and 138 F1 dams. The F2 lambs were serially slaughtered in six groups at 3-wk intervals starting at 23 wk of age to allow comparisons at different end points. A linkage group of 25 marker loci (mean of 708 informative meioses per marker) spanning 87.2 cM was developed and improved the previous known coverage and precision of marker order and interval distance from available maps of ovine chromosome 18. Probabilities of each CLPG genotype were calculated at 1-cM intervals (0 to 107 cM). Statistical models included effects of year, sex, sire, regressions on genotypic probabilities, and genotype-specific linear and quadratic regressions on appropriate covariates. Orthogonal contrasts of CLPG genotypic effects evaluated additive, maternal dominance, and paternally derived polar overdominance models of gene action. The most parsimonious model did not include the additive and maternal dominance genetic contrasts. From analyses of four key traits, a consensus for position of CLPG was obtained at 86 cM relative to the most centromeric marker. An F-test with 3 df representing polar overdominance was maximum at position 86 cM (F = 407.4; P < .00001) with leg score as the dependent variable. These results are consistent with assignment of the CLPG locus to the telomeric region of chromosome 18 and support the polar overdominance model of gene action proposed by Cockett et al. (1996). Furthermore, recombinant individuals with definitive phenotypes confined the position of CLPG to a 3.9-cM interval, facilitating positional cloning experiments.

Variability in metabolic rate, feed intake and fatness among selection and inbred lines of mice

Mouse populations differing in metabolic rate have been developed through selection for high (MH) and low (ML) heat loss (HLOSS), along with randomly selected controls (MC). Objectives of this study were to (a) compare MH, ML and MC lines for HLOSS and correlated traits of feed intake, body composition and organ weights; (b) compare three widely used inbred mouse lines with MH, ML and MC for the same traits; and (c) investigate potential genotype by diet interaction resulting from feeding diets differing in fat percentage. Heat loss (kcal/day) of MH and ML mice differed by 37% of the mean and remained significant (33%) when HLOSS was expressed on a fat-free mass basis. MH mice consumed more energy than ML with a greater difference in mice fed high-fat compared with standard diets (27% vs 13.9%). Despite greater energy consumption, MH mice were leaner than ML with a difference in total body fat percentage of 40%. The greatest difference in HLOSS between selection and inbred lines was between MH and C57BL/6J (BL), which differed by 26.3%. MH and BL mice also differed in energy intake (15.5%). Body composition of BL mice was similar to MH when fed a standard diet, but similar to ML when fed a high-fat diet. Crosses between MH and ML or between MH and BL would be useful to investigate the genetic regulation of, and identify quantitative trait loci influencing HLOSS, energy intake and body composition. Feeding of a high-fat diet may allow diet-specific loci influencing body composition to be identified in MH and BL lines.

Divergent selection for heat loss in mice: II. Correlated responses in feed intake, body mass, body composition, and number born through fifteen generations

Divergent selection for heat loss (kcal.kg-.75.d-1), measured in 9- to 11-wk-old male mice, was conducted for 15 generations. Selection for high (MH) and low (ML) heat loss and unselected control (MC) occurred in each of three replicates for a total of nine unique lines. Feed intake in males was measured during Generations 9 through 15. Body mass at commencement of mating in females and at time of measurement of heat loss in males was recorded. Body fat percentage at 12 wk for animals of Generations 6, 10, and 14 was predicted as a function of electrical conductivity and body mass. Litter size was recorded for all generations, and components of litter size were evaluated at Generation 11 in one replicate and Generation 12 in the other two replicates. Feed intake changed in the same direction as heat loss for the MH and ML selections; at Generation 15, the difference between MH and ML (P < .002) was 20.6% of the MC mean. Body mass did not change with selection for heat loss. Differences in body fat percentage were not significant in earlier generations, but at Generation 14, MH and ML were significantly (P < .01) different with MH mice having the lowest fat percentage; MC was intermediate. Selection had a significant (MH vs ML; P < .01) effect on litter size, causing an increase in MH and a decrease in ML. This difference was explained by a difference (P < .01) in ovulation rate. There was no asymmetry of response in feed intake, fatness, litter size, or number of ovulations.

Effects of sire, dam traits, calf traits, and environment on dystocia and subsequent reproduction of two-year-old heifers

A study was conducted over 3 yr to evaluate effects of sire birth weight EPD, calf birth weight and shape, and heifer pelvic area and weight, individually and in combination, on dystocia and subsequent rebreeding of 2-yr-old heifers. Heifers (n = 550), MARC II yearlings, were assigned for breeding to one of four Angus sires with birth weight EPD of -.95, -82, +2.9, and +2.7 kg. At calving, heifers were assisted as needed. A gauge attached to the cal puller recorded applied traction pressure. Analysis of traction pressure detected only slightly larger amounts of variation (2 to 3%) affecting dystocia than the standard five-point scoring system. Dam weight did not affect calving difficulty score (CDS), except dam birth weights were heavier (P < .05) for CDS 5 (Caesarean section) than CDS 1 (unassisted). Dams requiring Caesarean section had smaller pelvic areas (P < .05), with no other differences among CDS. The CDS increased as calf birth weight and cal external measurements increased. Low EPD sires produced calves with smaller (P < .05) birth weights and smaller calf head and food circumferences and caused less dystocia than high EPD sires. The CDS did not affect subsequent pregnancy rates but did affect conception date of the second calf. Calves delivered by Caesarean section were lighter (P < .05) at weaning than other calves but had similar slaughter weights. As mean winter temperature increased (6.1 degrees C) from yr 1 to 3, calf birth weight decreased (4.6 kg) and calving difficulty decreased 23%. Results indicate sire birth weight EPD, calf birth weight and shape, dam pelvic area, and climate affected CDS, and CDS affected subsequent conception date.

Divergent selection for heat loss in mice: I. Selection applied and direct response through fifteen generations

Divergent selection for heat production/loss (kcal.kg-.75.d-1), measured in 9- to 11-wk-old male mice, was conducted for 15 generations. Heat loss was measured for 15 h on individual animals placed overnight in direct, gradient-layer calorimeters. Selection for high (MH) and low (ML) heat loss and unselected control (MC) occurred in each of three replicates for a total of nine unique lines. Repeatability of the heat loss measurement was .45 and the CV was 10.5%. Cumulative realized selection differentials, averaged for the three replicates, were 145.1 and -105.0 (kcal.kg-.75.d-1) and ranged from 136.9 to 149.2 and -17.1 to -101.3 for MH and ML selection, respectively. Cumulative standardized realized selection differentials, averaged for the three replicates, were 10.06 and -9.51 for MH and ML selection, respectively. Direct responses (kcal.kg-.75.d-1) in heat loss after 15 generations were 44.2 for MH and -27.4 for ML as deviations from MC. Asymmetry of response was evident (P = .03) by Generation 10. Realized heritability was .28 +/- .01 based on divergence of MH and ML selection. For selection for higher and lower heat loss, realized heritabilities were .31 +/- .01 and .26 +/- .01, respectively.

A simulation model including ovulation rate, potential embryonic viability, and uterine capacity to explain litter size in mice: I. Model development and implementation

Litter size in mice was studied using a model including ovulation rate, potential embryonic viability, and uterine capacity. Simulated results were compared with experimental results from a selection experiment with mice. The four criteria of selection were selection on number born (LS), selection on an index of ovulation rate and ova success (IX), selection on number born to unilaterally ovariectomized females (UT), and unselected control (LC). Comparisons were made to statistics of the base generation and to responses after 13 generations of selection. Phenotypic and genetic statistics for uterine capacity were generated so that simulations produced the experimental means, standard deviations, and correlations between left and right litter size, as well as responses in number born using the LS, IX, and UT criteria. Statistics for the simulated data generally agreed with observed values. Simulated heritability in the base generation for uterine capacity was .065. Experimental and simulated responses per generation in litter size through 13 generations of selection were .15 and .16, .17 and .18, and .10 and .11 for LS, IX, and UT, respectively. Simulated responses in uterine capacity after 13 generations were 2.19, 1.60, and 3.40 for LS, IX, and UT, respectively. Simulated means for the base generation were 13.22 and 16.30 for ovulation rate and uterine capacity, respectively. Uterine capacity was an important component of the variability in litter size; however, ovulation rate was the more limiting component.

A simulation model including ovulation rate, potential embryonic viability, and uterine capacity to explain litter size in mice: II. Responses to alternative criteria of selection

Direct selection for litter size was compared with selection for ovulation rate, ova success, or uterine capacity and for indexes of ovulation rate with ova success or uterine capacity. Selection was simulated for 10 generations in a mouse population based on a model integrating ovulation rate, potential embryonic viability, and uterine capacity. Two indexes including ovulation rate (OR) and ova success (OS) were I = .291 x OR + 2.19 x OS and I = .165 x OR + .736 x OS. Heritabilities for ovulation rate and ova success, assumed in the simulation and to derive the indexes, were .25 and .06, respectively. Both indexes resulted in the same response in litter size, 12.9% greater than response to direct selection for litter size. Two indexes including OR and uterine capacity (TUC = true total uterine capacity; UC = uterine capacity measured as number born for a female with right ovary excised) were I = .881 x OR + .223 x TUC and I = .876 x OR + .568 x UC. Heritabilities assumed for uterine capacity were .09 (TUC) and .065 (UC). The first index assumed true parameters for uterine capacity (TUC) and resulted in a response in litter size that was 23.9% greater than direct selection. The second index was calculated using parameters estimated under a unilateral-ovariectomy model and resulted in response that was 14.7% greater than direct selection. Selection for OR, TUC, UC, or OS resulted in responses that were 4.5, 48.5, 38.7, or 74.8%, respectively, less than that from direct selection for litter size.

Embryonal survival to 6 days in mice selected on different criteria for litter size

Embryonal survival was compared in mice resulting from four criteria of selection: LS = selection on number born; IX = selection on an index of ovulation rate and ova success; UT = selection on number born to unilaterally ovariectomized females; and LC = unselected control. Selection occurred for 21 generations with three replicates of the four criteria; thereafter, relaxed selection was practiced. The evaluation was performed using mice of two replicates at Generation 35 and one replicate at Generation 36. Data on a total of 289 female mice were recorded. Females, at an average age of 9 wk, were mated to males of the same line. Six days after mating, each female was killed, ovaries were excised, corpora lutea were counted and equated to number of ova shed, and the numbers of implantation sites in each uterine horn were recorded. Least squares means were .84, .91, .85, and .82 for left embryonal survival (left implantations/left ova) and .91, .90, .86, and .87 for right embryonal survival for LS, IX, UT, and LC, respectively. The right side had greater ovulation rate (P < .001) and number of implantations (P < .001). For embryonal survival, the criterion x side interaction was possibly important (P < .09). Selection for litter size by different criteria increased ovulation rate (P < .003) and embryonal survival (P < .05) to 6 d. However, responses in embryonal survival were not greater after UT selection compared with LS or IX selection.

Estimates of heritabilities and genetic and environmental correlations for left- and right-side uterine capacity and ovulation rate in mice

Heritabilities for and genetic and environmental correlations between uterine capacity, ovulation rate, and body mass (BM) were estimated in mice. Uterine capacity was defined as the number of fetuses (LUC or RUC for left or right side) in one uterine horn for unilaterally ovariectomized females. Ovulation rate (corpora lutea, LCL or RCL for left or right ovary) was measured on the remaining single ovary in these same females. Data on 1,931 mice from four selection populations were used. Left ovulation rate and LUC were measured on 958 animals, and RCL and RUC of another 972 animals were recorded. Genetic and environmental variances and covariances were estimated simultaneously using an animal model with a multiple-trait, derivative-free, restricted maximum-likelihood procedure. Averages for heritability and correlation estimates derived from separate analyses of the selection populations are presented below. Heritability of LUC was higher (.33 +/- .06) than that of RUC (.19 +/- .02). Heritability of LCL and RCL ranged from .17 +/- .03 to .27 +/- .06, and heritability for BM was .65 +/- .05. The genetic correlation between LUC or RUC and LCL or RCL ranged between .43 +/- .29 and .68 +/- .05, and between LUC and RUC was .92 +/- .05. Body mass had a higher genetic correlation with LCL and RCL (.70 +/- .12 and .93 +/- .02) than with LUC and RUC (.37 +/- .05 and .47 +/- .12). Environmental correlations between LCL and LUC and RCL and RUC were .32 +/- .09 and .36 +/- .05, respectively.

Uterine mass and uterine blood volume in mice selected 21 generations for alternative criteria to increase litter size

Lines of mice, selected for 21 generations using alternative criteria to increase litter size, were evaluated for uterine mass and uterine blood volume to help explain differences in uterine capacity. For this study, mice were sampled from Generation 27, the sixth generation after relaxation of selection. Mice came from all four criteria of selection (LS = selection on number born to unaltered females; IX = selection on index of ovulation rate and ova success; UT = selection on uterine capacity; and LC = unselected control) in each of three replicates (a total of 12 lines). Measurement was at one of two stages, either 3 d or 6 d of gestation. Matings were at 10 wk of age, and a total of 508 mice (17 to 26 per line-day of pregnancy subclass) were measured. The mean of the three selected groups exceeded the control in uterine mass (P < .001), uterine blood volume (P < .002), uterine mass/body mass (P < .03), and uterine blood volume/body mass (P < .04) but not in uterine blood volume/uterine mass. Greater uterine mass and concomitantly greater uterine blood volume may have been partly responsible for greater uterine capacity resulting from LS, IX, and UT selections.

Differences in pup birth weight, pup variability within litters, and dam weight of mice selected for alternative criteria to increase litter size

Selection for litter size had been practiced for 21 generations and relaxed selection for 13 generations in mice. Three replicates were used with four selection criteria: index of components (ovulation rate and ova success), uterine capacity, litter size, and an unselected control. Especially with selection for litter size and the index relative to the control, number of pups born had increased, and differences also occurred in mating weight. Dams of the three replicates and their litters were used to evaluate the effects of accumulated selection on pup birth weight, variability in weight of littermates, and dam's weight at mating and after littering. Total number born, number born alive, number of males, and number of females were also recorded and studied. Mean pup birth weight did not differ among the criteria; however, variability among littermates in pup weight tended to differ among criteria of selection. Regressions for pup weight and within-litter standard deviation of pup weight on number born were small and negative but significant (P < .001). The distribution of pup weight within litter was normal for 77.2% of the litters, with no differences among the criteria. The difference between weight of male and weight of female pups was significant (P < .001); overall males were 2.5% heavier than females. There was a difference (P < .02) among criteria in mating weight and littering weight; however, the maternal weight gain between mating and littering was not different among criteria. Number born differed (P < .003) among the criteria, but there was no significant difference among criteria in numbers of males and females.(ABSTRACT TRUNCATED AT 250 WORDS)

Serum cholesterol concentration of mice selected for litter size and its relationship to litter size and testis mass

This study assessed the genetic relationship between litter size and serum cholesterol concentration and between litter size and testis mass in mice. Mice were from a long-term experiment in which selection had occurred for 21 generations in three replicated lines per criterion of selection (LS = selection to increase litter size based on number born; LC = unselected control). Thereafter, random mating within lines was practiced. Serum cholesterol concentrations were evaluated in female and male mice from two replicates at Generation 29 and one replicate at Generation 30. Body weights and blood samples were collected from primiparous females 8 d after weaning their pups. Data from males were collected as they came out of breeding cages. In addition, the testes were excised, stripped clean of connective tissue and the epididymides, and weighed. Means for body mass of females and males, serum cholesterol, number born, and testis mass were as follows: 35.2 vs 32.5 g (P < .09), 33.9 vs 30.7 g (P < .08), 117.5 vs 110.5 mg/dL (P < .08), 14.0 vs 10.3 pups (P < .04), and 126 vs 122 mg, respectively, for LS and LC. Serum cholesterol was greater in males than in females (133.3 vs 95.1 mg/dL; P < .001), but there was no interaction between sex and selection criterion. Serum cholesterol concentration was not correlated phenotypically to number born or body mass, but it had a small negative relationship with testis mass. Therefore, we concluded that selection for litter size tended to increase serum cholesterol in addition to the increase in number born but did not change testis mass.

Uterine capacity and ovulation rate in mice selected 21 generations on alternative criteria to increase litter size

After 21 generations of selection for alternative criteria to change litter size in mice, responses in uterine capacity and ovulation rate were evaluated. Females from Generations 22 and 23 were sampled from 12 lines, representing three replicates of four selection criteria: LS = direct selection on litter size; IX = selection on an index of ovulation rate and the proportion of ova shed that resulted in fully formed offspring; UT = selection on uterine capacity measured as litter size from females unilaterally ovariectomized at 4 wk of age; and LC = unselected control. All females in the present evaluation (a total of 1,932) were unilaterally ovariectomized (either left or right ovary excised) at 4 wk, mated at 9 wk, and killed at d 17 of gestation. The number of corpora lutea and number of fetuses were counted to measure ovulation rate and uterine capacity, respectively. Selection in IX, LS, and UT increased (P < .01) ovulation rate from unilaterally ovariectomized females but by a greater amount (P < .01) in IX and LS than in UT. Selection also increased (P < .01) uterine capacity of IX, LS, and UT (average response relative to LC = 1.76 pups); response was at least as great in LS and IX as in UT. Direct selection in UT was successful at improving uterine capacity but was no more effective than IX or LS selection. Cases in which ovulation rate limited expression of uterine capacity in UT may have shifted some selection emphasis to ovulation rate and reduced response in uterine capacity.

Effects of Synovex C implants on growth rate, pelvic area, reproduction, and calving performance of replacement heifers

Two trials were conducted to evaluate effects of Synovex C implants on replacement heifers, given at two different ages. Crossbred heifer calves (n = 370) were allotted to four treatments: 1) nonimplanted controls, 2) implanted at 2 mo, 3) implanted at 6 mo, and 4) implanted at both 2 and 6 mo of age. Heifers implanted at 2 mo gained 7 kg more (P = .01) by 6 mo than those not implanted at 2 mo. No differences were found in 22-mo weights. All implanted heifers had larger (P = .01) yearling pelvic area than controls. All heifers implanted at 6 mo continued to have larger (P = .01) pelvic area at 22 mo. All implanted heifers had higher (P = .05) occurrence of non-ovulatory estrus. No differences were found among treatments in percentage of heifers puberal before breeding, in estrus first 21 d of breeding, or in first-service conception rate. In only one trial, pregnancy first 21 d and total pregnancy in 63-d breeding season were decreased (P = .05) by implanting at 6 mo. At subsequent calving, an interaction existed between the effects of the 2- and the 6-mo implant for calf birth weight and pelvic area:birth weight ratio. A single implant at either 2 or 6 mo decreased (P = .01) calving difficulty score; and implanting at both 2 and 6 mo showed the greatest reduction in calving difficulty. Implants had no significant long-term effects on reproduction or calf production of 2-yr-old cows.(ABSTRACT TRUNCATED AT 250 WORDS)

Alternative methods of selection for litter size in mice: III. Response to 21 generations of selection

Alternative methods of selection to increase litter size in mice have been practiced for 21 generations followed by six generations of relaxed selection. Three replicates were used with four selection criteria: index of components (IX:I = 1.21 x total ovulation rate + 9.05 x ova success), uterine capacity (UT), litter size (LS), and an unselected control (LC). In IX, ovulation rate and ova success were measured by number of corpora lutea and number of pups born/number of corpora lutea, respectively. In UT, uterine capacity was measured and defined as number of pups born to unilaterally ovariectomized (right ovary excised) females. Selection in LS was based on number born to unaltered dams. In all cases, number born was fully formed, live or dead pups. Pups from 16 randomly chosen LC dams and from the top 16 dams in IX, UT, and LS were selected to produce the next generation in each criterion-replicate line. Response in number born, selected criteria deviated from control, was regressed on generation number over the 21 generations of selection. Responses for the IX and LS criteria were quite similar (.14 +/- .01 and .16 +/- .01 pups per generation, respectively), whereas response in UT, with only one functional horn, was slightly lower (.09 +/- .01). The average cumulative selection differentials for IX, LS, and UT at Generation 21 were 32.78 index units, 36.38 pups, and 28.53 pups, respectively. The LC criterion had an unintentional cumulative selection differential of 3.3 pups.(ABSTRACT TRUNCATED AT 250 WORDS)

Environmental effects on neonatal mortality of beef calves

Calving records from 1969 to 1989 from the Roman L. Hruska U.S. Meat Animal Research Center were used to investigate how climatic conditions, in addition to dystocia, age of dam, size of calf, and sex affect calf survival from birth to 1 wk of age. Data were analyzed separately for cows calving with (n = 11,094) or without (n = 72,187) dystocia. Neonatal mortality was described by a logit model and parameters were estimated by maximum-likelihood procedures. Calves born to cows with dystocia were five times as likely to die neonatally than calves born without assistance. Of all calves that died, 43.6% were born with difficulty. Of these calves, survival was lowest for those that were small relative to their genetic group, sex, and age of dam. Large calves had markedly increased mortality only when born to 2-yr-old dams. Average ambient temperature and precipitation on day of calving affected survival nonlinearly and the magnitude of the effect depended on age of dam, sex and size of calf, and dystocia incidence. Calves born to 2-yr-old cows were more susceptible to severe weather conditions than calves born to older cows. The negative effect of precipitation on survival increased with decreasing temperature.

Economical and biological efficiencies of beef cattle differing in level of milk production

Economical and biological efficiencies of beef production to weaning and to slaughter were estimated in three groups, different in milk available (low, medium, and high) to the calves but with the same potential for growth. Data from different breed groups of cows (low [L] = Hereford x Angus, medium [M] = Red Poll x Angus, and high [H] = Milking Shorthorn x Angus) were used. Economical efficiency was the ratio of income to expenses and biological efficiency was the ratio of calf weight to total feed energy required. Income was derived from cull cows and calves at weaning or carcasses of calves fed to slaughter. Feed and non-feed expenses for the cowherd and for calves to weaning or to slaughter were included in economical efficiency. Efficiencies were estimated assuming observed reproductive rates and energy requirements for maintenance, as well as for equal reproductive rates and equal energy requirements for maintenance in the M and H groups. With the observed reproductive rates and maintenance requirements, biological efficiencies to weaning and to slaughter were 28.1, 27.2, and 27.5 g of weaning weight and 22.0, 20.4, and 20.3 g of carcass weight per megacalorie of ME for L, M, and H, respectively; the corresponding values using equal reproduction and equal maintenance in M and H were 28.3, 27.2, and 27.4 g of weaning weight and 22.1, 20.5, and 20.5 g of carcass weight per megacalorie of ME.(ABSTRACT TRUNCATED AT 250 WORDS)

Genetic variation in liver mass, body mass, and liver:body mass in mice

Genetic variation for liver mass (LM), body mass (BM), and liver:body mass (LM/BM) was examined for outbred populations of laboratory mice. Liver mass and body mass data were collected on 170 pureline sires at 12 wk of age, representing four outbred stocks of laboratory mice; 523 of their male and female two-way-cross progeny at 9 or 12 wk; and 214 four-way-cross offspring at 12, 14, or 16 wk. Genetic differences for LM, BM, and LM/BM were found among the base sire lines and between two-way crosses. Heritabilities and genetic correlations for LM, BM, LM/BM, and LM/MBM (MBM = BM.75) were estimated using offspring-sire regression within and across characteristics. Estimates of heritabilities and genetic correlations were also derived from full-sib covariances in the two-way-cross generation. Heritability estimates pooled over all analyses were .53, .54, .36, and .40 for LM, BM, LM/BM, and LM/MBM, respectively. Body mass was highly genetically correlated (.87) with LM and lowly correlated with LM/BM. Previous research has indicated possible positive relationships between LM/BM and maintenance energy requirements in mature, nonlactating, nonpregnant animals. A selection index was developed for increasing BM but restricting genetic change in LM to zero. Selection using this index would be 40% as efficient in increasing BM as selection on BM alone but may hold maintenance energy requirements at a stable level.

Effects of three methods of selection for litter size in mice on pre-implantation embryonic development

Characteristics of preimplantation embryonic development to Day 3.5 of gestation were evaluated in lines of mice after 21 generations of selection for litter size or components of litter size. Selection criteria were direct selection for number born (LS), selection on an index of ovulation rate and the proportion of ova shed that resulted in fully formed pups (IX), selection for number born in unilaterally ovariectomized females as an indication of uterine capacity (UT), and an unselected control (LC). Comparison of the average distributions of embryonic stage of development on the left side of the uterus showed that selection (average effect of LS, IX, and UT vs. LC) tended to advance (p = 0.07) the average stage of embryonic development at Day 3.5 and shift the distribution (p = 0.10) by increasing the frequency of expanded blastocysts and decreasing the frequency of pre-morula embryos. A similar shift in the distribution on the right side of the uterus was not statistically significant. Selection decreased (p = 0.06) variability in developmental stage among embryos within the right uterine horn. These selection criteria evaluated in the mouse appear to have changed the frequencies of genes that affect some determinants of average stage of embryonic development and uniformity of development within a uterine horn at Day 3.5 of gestation.

Use of a simulation model to evaluate the influence of reproductive performance and management decisions on net income in beef production

A stochastic dynamic model of reproduction and a deterministic cow-herd economic simulation model were used to evaluate how management decisions and reproductive performance interact to influence net income in a cow-calf operation (1,000 cows) for 1 yr of production. The stochastic model was used to determine herd performance when length of breeding season (45, 70, or 120 d) interacted with three postpartum intervals of anestrus (48, 65, or 90 d) and three conception rates at first service (60, 70, or 80%). Short, moderate, and long postpartum intervals were used to reflect differences in reproductive performance. In addition, replacement heifers were bred beginning either 3 wk ahead of the cow herd or at the same time as the cow herd. Fifty-four simulations were generated. Inputs into the economic model were herd performance, livestock and feed prices, nonfeed costs, and feed requirements for 1 yr of production. Feed requirements were calculated separately for each postpartum interval to reflect three different body condition scores, thin, moderate, and good, to correspond with long, moderate, and short postpartum intervals. Net income was greatest with 70-d breeding seasons when the postpartum interval was short or moderate. When the postpartum interval was long, net income was greatest with 120-d breeding seasons because pregnancy rates, as a result of the long breeding season, were highest and feed costs were lowest for thin cows. Overall, net income was greatest when cows were managed to have postpartum intervals of moderate length. Breeding heifers 3 wk before the cows provided the most economic benefit with long postpartum intervals.

Effects of breeding season length and calving season on range beef cow productivity

A 5-yr study was conducted beginning in 1983 with 460 cows to evaluate the effects of three breeding seasons (30, 45, and 70 d in length) and two times of spring calving, March (early) and April (late), on cattle production under Nebraska Sandhills range conditions. Criteria evaluated included pregnancy and weaning percentages, calving date and distribution, cow weights and body condition at four intervals, calf birth and weaning weights, and cow productivity. The 30-d breeding season included a 10-d estrus synchronization and AI period; in the other breeding seasons only natural breeding was used. The same sires were used over the entire study period. Percentage of cows pregnant and percentage of calves weaned were lower (P less than .01) for cows bred for 30 d than for cows bred for 45 or 70 d. Average calving dates were similar among the breeding groups within the early and late calving herds. Pregnancy rates from AI were higher (P less than .01) for the cows calving in April (64%) than for the cows calving in March (41%). Cows calving in April lost less weight between precalving and prebreeding and were heavier (P less than .05) at prebreeding time than the cows calving in March. Calf weaning weights were not different (P greater than .10) among any of the breeding season groups or between the two calving herds when calves were weaned at a similar age. Cow productivity (calf weaning weight per breeding female) was highest (P less than .05) for the cows bred for 70 d (186 kg), intermediate for the cows bred for 45 d (172 kg), and lowest for cows bred for 30 d (162 kg).(ABSTRACT TRUNCATED AT 250 WORDS)

Distribution of time to first postpartum estrus in beef cattle

The function of a distribution that describes postpartum interval (PPI) under any experimental treatment is useful for simulation modeling, understanding the effects of stimuli on the endocrine system, and estimating the average PPI in experiments terminated before all animals have expressed estrus. This study was undertaken to compare the fit of three statistical distributions, the Weibull, the log-normal, and the linear hazard rate (LHR), to the empirical distribution of PPI for five treatment regimens: no bull exposure postpartum, bull exposure from 53 d postpartum, bull exposure from 3 d postpartum, and bull exposure from an average of 63 d postpartum for 2-yr-old cows and for mature cows. The Weibull and the log-normal distributions deviated considerably from the empirical distribution. The LHR distribution with parameters changing over three different regions gave an excellent fit. The resulting hazard rate (instantaneous probability of a cow expressing her first estrus at time t postpartum) revealed a low probability of expressing estrus within 27 d postpartum (43 d for 2-yr-olds). For cows not exposed to bulls, the hazard rate increased slowly with time. For cows exposed to bulls after 3 d postpartum, the hazard rate increased rapidly between d 27 and d 50. For cows exposed to bulls after 53 d postpartum, the hazard rate increased instantaneously approximately 12 d after initial exposure to bulls. This increase was also seen when cows were exposed to bulls beginning at a constant date (at an average of 63 d postpartum). Because of lack of fit, the Weibull and the log-normal distributions should not be used in survival analysis of PPI.(ABSTRACT TRUNCATED AT 250 WORDS)