Reproductive rate and genetic variation in composite and parental populations: experimental results in sheep.

Mean and genetic variability for ewe performance of two composite populations were compared with those of their contemporary parent lines to assess relative potential for continued response from selection. Data were from the U.S. Meat Animal Research Center, 1973 to 1983. The 4,415 ewes by 508 sires of Finnsheep (F), Rambouillet (R), Dorset (D), and Composite 1 (C1 = F/2 + R/4 + D/4) lines were mated in April, August, and December of each year. The 1,763 ewes by 320 sires of Finnsheep (F), Suffolk (S), Targhee (T), and Composite 2 (C2 = F/2 + S/4 + T/4) were mated in October only. Data were adjusted for effects of year, season, and age of ewe at breeding, plus interval between lambings, for the accelerated lambing lines and for year and age at breeding for the annual lambing lines. Superiority of composite over parent lines in lambs weaned per ewe exposed was 36% for accelerated and 44% for annual lambing lines; that superiority arose largely from dominant heterotic effects for fertility and viability. Weighted mean paternal half-sib estimates of heritability for mean lifetime ewe performance of composite vs parental lines were 32 vs 23% for litter birth weight, 25 vs 28% for lamb birth weight, 26 vs 11% for number born, 11 vs 23% for conception rate, 5 vs 10% for number weaned, -1 vs 19% for lamb weight at 42-d weaning, but 6 vs 23% for number and 8 vs 28% for litter weight weaned per ewe exposed. Heritability was higher in composites than in parental lines for numbers born, but it was lower for the more heterotic traits (fertility, lamb viability) and, hence, for total weaned lamb output, without change in phenotypic variation. Thus, even an equal response in composites from continuing selection for weaned lamb output may depend mainly on more intense selection permitted by a higher reproductive rate.     

Heterosis for survival and dentition in Hereford, Angus, Shorthorn, and crossbred cows

Records of 328 Angus, Hereford, Shorthorn, and first-cross cows produced from 1960 to 1963 were used to estimate heterosis (h) and individual (gI) and maternal (gM) average breed effects on cumulative survival (CS) by ages to 12 yr, longevity (L), and size (SI) and condition of incisors at 10 to 15 yr of age. Reasons for cow disposal were also studied. The 155 cows born in 1960 and 1961 first calved at 3 yr of age, and the 173 born in 1962 and 1963 first calved at 2 yr of age. Analyses for CS and L were done for both actual culling of cows open in two consecutive years (AC) and imposed culling of any open cow (IC). The model for CS and L included cow birth year-breed of cow's sire (Y-S), sires within Y-S, breed of dam (D), and D x Y-S. For SI, the model included breed of cow, year of recording, and quadratic effect of age. Linear contrasts were used to estimate h, gI, and gM for quantitative variables, and chi-square was used for discrete variables. Under both AC (P less than .05) and IC, crossbreds exceeded straightbreds in CS to 12 yr (16.7 and 8.5%) and L (1.36 and .99 yr). Estimates of gI and gM for CS and L were generally small, except for the low (P less than .05) gM of Shorthorn cows under IC. Mortality and culling for emaciation, cancer eye, or prolapse was less for crossbreds than for straightbreds and increased with age. Crossbreds had longer (P less than .01) and better (P less than .001) teeth than straightbreds. The longer productive life of crossbred cows reduces rearing costs for replacements and increases sales of calves and cull cows.     

Breed effects on crossbred cow-calf performance

Effects of seven breeds of cow's sire and 12 breeds of cow's maternal grandsire on preweaning performance of crossbred cows and their calves were examined in data from two experiments conducted at the Roman L. Hruska U.S. Mean Animal Research Center. Data included 1,836 records over three to five parities for 516 cows by 143 sires and by 307 maternal grandsires. The statistical model fitted effects of calf sex, parity, cow birth-breeding year or cow-calf birth year, the breed effects and their interactions. Deviations of breed of sire or equivalent grandsire effects on each trait from the mean for Hereford x Angus cows ranged from -1.6 to 5.5 kg (P less than .001) for calf birth weights, -15 to 1% (P less than .001) for calving difficulty, nonsignificant for preweaning calf mortality and -2 to 27 kg (P less than .001) for calf weaning weight. Deviations were nonsignificant for conception rate and calves weaned per cow exposed to breeding, but -2 to 40 kg (P less than .001) for calf weight weaned per cow exposed for breeding, -7 to 78 kg (P less than .001) for cow weight and -20 to 2% (P less than .001) for body condition score. The advantages of Holstein and Brahman cross over Hereford x Angus cows of 23 and 13% in weight of calf weaned/cow-breeding exposure must be compared with the expected greater feed requirements from 7 or 8% heavier cows and at least 50% higher milk production, which emphasizes the need to include input measures and costs in breed evaluation schemes.     

Genetic variation in composite and parental populations: expectations for levels of dominance and gene frequency.

Usefulness of multibreed composites depends on: 1) adaptation of the average breed effects combined, 2) amount of initial heterosis retained, and 3) the rate of further improvement from selection. Potential improvement could be increased, relative to parental stocks, by retained heterosis in reproductive rate and by any increase in additive genetic variation in the composite. A single-locus, two-allele model with additive plus dominant gene effects was used to clarify expected changes from varying: 1) degree of dominance (d) from none to over-dominance, 2) variance among parental lines in frequency of a dominant allele (Vq), and 3) mean gene frequency in the composite (qc). Simulation for a three-breed composite (1/2, 1/4, 1/4) showed that expected heterosis retained in the composite is proportional to d and Vq and is highest for any d and Vq when qc is intermediate. Percentage of change of the composite from parental mean in additive genetic variance (delta Vac,%) increases most with parental diversity (Vq) when d = 0 but changes with higher d from positive when q is less than .5 to negative when q is greater than .5. Therefore, the expected association of delta Vac with level of heterosis retained (delta H,%) is always smaller for higher d and changes from positive when qc less than .5 to negative for the more likely higher equilibrium values of qc. Thus, greater selection intensity (from a higher reproductive rate) in composites than in parent stocks may not be accompanied by increased genetic variability and change in response per unit of selection applied may be limited.     

Twinning in cattle: I. Foundation animals and genetic and environmental effects on twinning rate

Foundation cows were selected using prior records from one of two sources, private herds or other projects at the Research Center. Comparing twinning rates before and after selection, the repeatability was lower for those from the first (.08) than for those from the second (.16) source with a combined value of .12. Realized heritability of single-parity twinning rate estimated from selection of parents and response in daughters of foundation females was .06. Paternal half-sib estimate of heritability of twinning rate was .02 +/- .07. Estimates of repeatability computed from calving records of females born in the project indicate that permanent environmental effects on twinning rate in cattle are small. Mean calving rate of females born in the project was 1.11 in the data set that included all data and 1.09 in the data set that excluded females from highly selected parents. Twinning rate was greater (P less than .05) in fall than in spring calving (1.13 vs 1.06). Data on twinning rate of a sample of the Swedish Friesian breed were summarized. Mean twinning rate of the Swedish Friesian breed is 2.57%. Age adjusted mean twinning rate of daughters of 32 half-sib sons of one particular Swedish Frieisan sire averaged 5.4% and ranged from .9% to 13.6%. There was no indication of a bimodal distribution, which would be expected if a single gene with a major effect on twinning rate were segregating. The estimated genetic standard deviation (sigma-xn) for mean twinning rates of the 32 sire progeny groups was 1.8%. Observed range among son progenies was .127 or 7.2 sigma-xn, in reasonable agreement with the hypothesis that twinning rate in this population is inherited as a quantitative trait.     

Twinning in cattle: III. Effects of twinning on dystocia, reproductive traits, calf survival, calf growth and cow productivity

An evaluation of natural twinning in beef cattle revealed that cows birthing twins had shorter (P less than .01) gestation lengths, more (P less than .01) retained placentas, more (P less than .01) dystocia, more (P less than .01) days to estrus, lower (P less than .01) conception rates and more (P less than .01) days to pregnancy than cows birthing singles. Days to estrus, conception rate and days to pregnancy were not affected by number of calves reared (1 vs 2) in cows birthing twins. Survival at birth was greater (P less than .01) for single- than for twin-born calves, but twins and singles did not differ (P greater than .05) in postnatal survival. When dystocia was experienced, calf survival at birth was 95% vs 73% for singles vs twins compared with 99% vs 92% when no dystocia was experienced. Calves born twins were lighter (P less than .01) at birth, 100 d and 200 d, but twins and singles did not differ in postweaning gains. Total calf weights at 100 d per cow calving were 12% greater (P less than .01) in cows birthing twins vs singles when twin calves reared by foster dams were excluded. The potential increase in cow productivity for total calf weight at 100 d is 40% if calf survival rates of twins with dystocia relative to survival rates of twins without dystocia were comparable to survival rates of singles with and without dystocia, and if cows birthing twins were fed and managed to obtain conception rates equal to those of cows birthing singles. Identification of cows gestating twins to provide for their higher prepartum nutritive requirements and calving assistance at parturition is necessary to make twinning in cattle an economically viable technology.     

Effect of EDTA-tris on an Escherichia coli isolate containing R plasmids

Solutions of ethylenediaminetetraacetate (EDTA)-tris combined with antibiotics have been shown to be effective in treating selected cases of persistent bacterial infections. Basic techniques in microbial genetics, including mating frequencies, chemical elimination of R plasmids, isolation of plasmid DNA and agarose gel electrophoresis, were used to determine if EDTA-tris has a curing effect on an R plasmid as part of its clinical action. Results of this study indicated that EDTA-tris by itself eliminated an antibiotic resistance marker from a clinical isolate of Escherichia coli and when combined with another chemical curing agent altered the isolate's mating frequency.     

Lamb production and its components in pure breeds and composite lines. II. Breed effects and heterosis

Ewe performance was compared for five pure breeds of sheep (Finnsheep, F; Rambouillet, R; Dorset, D; Targhee, T; Suffolk, S) and the crosses in development of two maternal composite lines (C1 = 1/2F1/4R1/4D and C2 = 1/2F1/4T1/4S). The data involved 10,959 breeding season records of 4,219 ewes by 412 sires. Ewe production and the components (fertility, litter size, neonatal and preweaning lamb survival and mean lamb weaning weight) were adjusted for age and standardized across season of lambing and years. The D and F ewes produced more weight of lamb/ewe exposed than R, S and T ewes because of higher D and F fertility, higher D lamb survival and larger F litters. First cross and inter se generations of C1 and C2 ewes averaged 17 to 27% higher fertility than the parental mean. Litter size averaged about one lamb higher for F than for other pure breeds, but only slightly higher for C1 and C2 than for the mean of F and other breeds. Lower neonatal survival for F than for other breeds and crosses was associated with the larger F litters and with 2 to 8% positive heterosis in the crosses. Preweaning survival of suckled and nursery lambs was low for F and S and positive heterosis ranged from 9 to 19% in crosses. Mean lamb weaning weights were highest for S, lowest for F, with little heterosis in crosses. Crossbred ewes reared .3 to .4 more lambs than mean for parental pure breeds. Heterosis in C1 and C2 ranged from 11 to 28% for lambs born, 27 to 43% for lambs weaned and 29 to 44% for weight of lamb weaned/ewe exposed. Decline in heterosis with inter se mating of crosses was no greater than expected from the reduction in predicted heterozygosity.     

Retrospective analysis of axial skeleton osteosarcoma in 22 large-breed dogs

Medical-records of 22 large-breed dogs (>15 kg) with osteosarcoma (OSA) of the axial skeleton were reviewed to determine prevalence of metastasis and survival associated with this neoplasm. All dogs were treated with more than 1 mode of therapy including palliative radiation (n = 12), definitive radiation (n = 8), surgery (n = 7), chemotherapy (n = 12), or some combination of these therapies. Metastasis was documented in 10 of 22 dogs (46%), and the median survival for all dogs was 137 days. Primary cause of death was local tumor recurrence (54%). Breed (retriever versus purebred versus mixed-breed survival was 100, 182, and 264 days, respectively) and radiation therapy protocol (survival in dogs treated with palliative radiation therapy versus those treated with definitive radiation therapy was 79 and 265 days, respectively) were significantly related to survival (P < .05). Prevalence of metastasis and median survival for large-breed dogs with axial skeleton OSA seems to be similar to that reported for large-breed dogs with appendicular skeleton OSA. Definitive radiation therapy may have a role in the treatment of axial skeleton osteosarcoma.