Applications of sexed semen in cattle production.

Sexed semen will contribute to increased profitability of dairy and beef cattle production in a variety of ways. It could be used to produce offspring of the desired sex from a particular mating to take advantage of differences in value of males and females for specific marketing purposes. Commercial dairy farmers, those who produce and market milk, could use sexed semen to produce replacement daughters from genetically superior cows and beef crossbred sons from the remainder of their cow population. To increase the rate of response to selection, seedstock dairy cattle breeders could produce bulls for progeny testing from a smaller number of elite dams by using sexed semen to ensure that all of them produced a son. Using sexed semen could then reduce the cost of progeny testing those bulls, because fewer matings would be necessary to produce any required number of daughters. Commercial beef cattle farmers, producing animals for eventual slaughter, could use sexed semen to capitalize on the higher value of male than female offspring for meat production. They could also use sexed semen to produce specialized, genetically superior replacement heifers from as small a proportion of the herd as possible. This would allow the remainder of the herd to produce male calves from bulls or breeds with superior genetic merit for growth, feed conversion efficiency, and carcass merit. Single-sex, bred-heifer systems, in which each female is sold for slaughter soon after weaning her replacement daughter, would be possible with the use of X-chromosome-sorted semen. Use of sexed semen would make terminal crossbreeding systems more efficient and sustainable in beef cattle. Fewer females would be required to produce specialized maternal crossbred daughters, and more could be devoted to producing highly efficient, terminal crossbred sons.

Inbreeding and reproduction in mice divergently selected for response to a dietary toxin.

This study was conducted to determine whether inbreeding coefficients of selected parents or of progeny differed between lines of mice selected for increased or decreased responsiveness to a nutritional toxicosis. A second objective was to determine whether the influence of inbreeding of parents and/or progeny on reproductive traits differed between those lines. Mice were selected divergently for 8 generations for the effect on post-weaning growth of endophyte-infected fescue seed in their diet. Forty pairs (or in Generation 7, 20 pairs) were selected and mated per generation in each line. Inbreeding increased 0.5 to 0.6% per generation in both lines, a rate close to that predicted from genetic theory. Inbreeding coefficients of selected parents were not higher in the susceptible than in the resistant line. A difference would have been expected if the inbreeding coefficient had been correlated with susceptibility to toxicosis. The magnitudes of inbreeding depression for reproductive traits did not differ significantly between lines. The average inbreeding coefficient of the potential litter tended to be higher in nonfertile than fertile matings (P = 0.10), but inbreeding coefficients of sires and dams did not differ between successful and unsuccessful matings. Inbred litters tended to be born earlier than noninbred litters (P = 0.10). Inbred dams produced smaller litters than noninbred dams (main effect P < 0.05) but only when the litter also was inbred (interaction P < 0.01). Sex ratio was not influenced by inbreeding of sire, dam or litter, but there was a higher proportion of male progeny in the susceptible than in the resistant line (P = 0.01). To avoid reduced reproductive fitness, laboratory animal populations should be managed to minimize inbreeding of progeny and dam.

Possibilities for genetic manipulation of sex ratio in livestock.

Reports are reviewed in which genetic differences for sex ratio of offspring were found among breeds, breed crosses, strains, lines, line crosses, families or individuals. Although reports in which genetic variation could not be identified outnumber those in which it could, important genetic variation does exist in at least some populations. The direct or correlated response of sex ratio to selection is reviewed. Again, results are equivocal, but there is ample evidence that sex ratio will respond to selection in at least some populations of laboratory organisms. Segregation distortion in the mouse and in Drosophila is examined. Segregation distortion occurs when a heterozygous male mated to a homozygous female does not produce the two offspring types in equal proportions and the disparity cannot be accounted for by sampling or differential mortality. Although the most well-known systems have no effect (in the mouse) or only a moderate effect (in Drosophila) on sex ratio, systems in which sex ratio is subject to segregation distortion are known to exist in insects and could exist in mammals. Finally, the possibility is discussed that XX/XY chromosome chimaeric bulls, born co-twin to a heifer, might produce a preponderance of heifer progeny. A few reports suggest this might be the case, but other research is not in agreement.

Beef cattle breeding à la Jefferson.

?Even more than most disciplines in the Animal Sciences, quantitative genetics is dependent upon models. Models, by definition, are abstractions of reality. Invariably they require simplifying assumptions, which should be but sometimes are not clearly specified. One thesis of this article, illustrated by examples, is that many of the assumptions upon which animal breeding theory and practice are based are not valid. Some proportion of research resources should be devoted to challenging or verifying those assumptions and following up those areas of enquiry suggested by the outcome of such research. A further thesis is that the selection of topics and priorities for animal breeding research should be a matter of choice by individual scientists and should not be determined by steering committees or directed by administrative fiat. Hopefully, the resultant mutation, cross-fertilization, assortment, recombination and selection of ideas that would result would bestow upon our discipline higher fitness from multiple-peak epistasis, and minimal danger of extinction (or petrification) from over-specialization. A final thesis is that true creativity by research scientists should be nurtured and rewarded and that work in traditional areas of breeding and quantitative genetics should be continued-but done better.

The effect of United States versus New Zealand Angus germplasm on characteristics of crossbred calves.

Blood selenium concentration at birth and growth from birth through finishing were analyzed on progeny from two birth years of Angus bulls of United States (US) or New Zealand (NZ) origin. Dams of the calves were Polled Hereford x Angus crossbreds whose sires had been selected divergently from the national sire summary to be superior (+) or inferior (-) for yearling weight EPD (G) and for total maternal effects (M) on weaning weight. There was no evidence of differences attributable to calf paternity or dam genetic group in concentration of blood selenium in calves at birth. For birth year 1988, progeny of US sires grew nonsignificantly more rapidly than progeny of NZ sires in most life-cycle phases. In 1989 calves, NZ-sired offspring grew nonsignificantly more rapidly than US-sired offspring through a year of age. Averaged across years, preweaning and postweaning growth rates were similar for calves of US and NZ paternity. Divergent selection of the calves' maternal grandsires for yearling weight EPD was successful; calves in +G groups had more rapid gains through 1 yr of age than did calves in -G groups. Also, growth rate through 4.5 mo of age was influenced by divergent selection of maternal grandsires on total maternal weaning weight EPD; calves from +M groups gained more rapidly than calves from -M groups.

Prediction of fertility from calfhood traits of Angus and Simmental heifers.

The objectives of this study were to quantify the relationships between traits observed before the first breeding season and fertility of 946 Angus and 351 Simmental heifers and to use those traits to develop prediction equations for heifer fertility. Logistic regression methodology was used. Traits investigated were Julian birth date, age of the heifer's dam, birth weight, actual weaning and yearling weights, weaning and yearling weight ratios, 205-d weight, 365-d weight, and birth-weaning, weaning-yearling, and birth-yearling ADG and relative growth rate (RGR). In both breeds heifers that were younger at the start of the breeding season were less likely to conceive, but this effect was more important for Angus (logistic regression coefficient, b = -.032; P less than .01) than for Simmentals (b = -.015; P = .06). Weaning weight ratio was positively associated with heifer fertility (b = .025; P = .01 and b = .028, P = .04, respectively, for Angus and Simmental), whereas actual weaning weight was related curvilinearly to fertility of Angus heifers. The likelihood of conception was highest for Angus heifers weighing greater than or equal to 240 kg at weaning. The only postweaning trait associated with heifer fertility was weaning-yearling RGR. The likelihood of conception was highest for Angus heifers growing between .15 and .30% per day (P = .01), whereas fertility increased continuously (P = .04) for Simmental heifers as weaning-yearling RGR decreased. The maximum variations in fertility explained by models including all possible explanatory variables were 11.5 and 9.2% for Angus and Simmental, respectively. Results suggested that growth-related traits were relatively more important as a predictor of fertility for Simmental heifers and that age at the start of the breeding season was more important for Angus heifers. The combination of Julian birth date and weaning-yearling RGR produced the best models to predict heifer fertility for both breeds.

Estimation of repeatability, heritability and breed differences for lamb production.

Production records of crossbred ewes, including weight at mating (WTM), fertility (FRT), number of lambs born per ewe lambing (BRN), number of lambs weaned (WEN), litter weight in April (WTA) and litter weight at weaning (WTW), were analyzed for the estimation of breed differences, genotype x environment interactions and repeatability and heritability of ewe production. All ewes were from Suffolk- or Columbia-type dams. Breed of sire of ewe was Finnsheep, Dorset, North Country Cheviot or Romney, Suffolk-cross (SX) ewes were heavier at mating than Columbia-cross (CX) ewes. Cheviot-cross (NX) and Finnsheep-cross (FX) ewes were the most fertile on hill pastures, while Dorset-cross (DX) ewes were the most fertile on irrigated pastures. The FX ewes had larger litters at birth and at weaning. Columbia-cross ewes had larger litters than SX ewes on hill pastures, but the SX ewes had larger litters than CX ewes on irrigated pastures. Finnsheep-cross ewes on both types of pastures and DX ewes on irrigated pastures had heavier litter weights than NX or Romney-cross ewes at weaning. Repeatability estimates for WTM, FRT, BRN, WEN, WTA and WTW were .61, .15, .19, .08, .07 and .09, respectively. Heritabilities, from the sire-of-ewe component, were .40, .02, .12, .0, -.02 and -.05 for WTM, FRT, BRN, WEN, WTA and WTW, respectively.

Production efficiency of crossbred ewes and that of their daughters and granddaughters.

Ewes of eight crossbred groups, born in 1973 and 1974, were maintained under two pasture management systems for five and four production years, respectively. After that experiment terminated, those crossbred ewes produced Polypay-sired lambs in 1979 and were then sold. Ewe lambs from those three-bred crosses were backcrossed to Polypay rams to produce lambs from 1980 to 1983. Five ancestor-descendant data sets (three involving dams and daughters and two involving granddams and granddaughters) were examined to study relationships between production efficiency of ancestor ewes and similar traits measured on their descendants. Ancestor cumulative prolificacy was positively but not significantly associated with cumulative prolificacy in the descendants. Regressions of descendant net revenue on ancestor net revenue were predominantly negative but generally were not significant. There was a tendency for prolific ancestors to produce costly but less profitable descendants. These results may be due to economic conditions (high feed costs and low lamb values) that persisted during the course of the experiment.

Crossbreeding among British and continental European dual-purpose breeds in the coastal Pacific Northwest.

From 1979 through 1987, British breed and crossbred cows were mated to Simmental, Pinzgauer or Tarentaise bulls or to Hereford-Angus crossbred bulls. Beginning in 1982, continental European crossbred females also entered the herd, to be mated for first calving to Hereford-Angus or Angus bulls and as cows to continental European or Hereford-Angus bulls. In progeny of British breed and crossbred cows, dam breed effects on birth and weaning weight were not important, but continental European crossbred calves were heavier than British crossbred contemporaries at birth and weaning. Pinzgauer- and Simmental-sired calves were heavier at birth than Tarentaise crosses, but calves sired by each of the continental breeds had similar weaning weights. Maternal heterosis was greater than direct heterosis effects on weaning weight (8% vs 5%), but only direct heterosis (13%) influenced birth weight. Within progeny of continental European crossbred cows, calves with a Tarentaise maternal grandsire were lighter at birth, with no significant difference among other maternal grandsire breeds. Maternal breed effects on weaning weight were not significant. Nevertheless, in contemporary years, continental European crossbred cows reared calves that were 10% heavier than calves reared by British crossbred cows. Pacific Northwestern cattle producers could achieve substantial increases in weaning weight from introducing inheritance from continental European dual-purpose breeds into cow herds and calf crops of British ancestry.

Growth and physiological responses to toxicosis in lines of mice selected for resistance or susceptibility to endophyte-infected tall fescue in the diet.

In three experiments, mice from lines selected for resistance (R) or susceptibility (S) to growth depression from endophyte-infected fescue seed in the diet were fed diets containing infected (E+) or non-infected (E-) seed. Activities of liver enzymes known to participate in oxidation, reduction, or hydrolysis or in conjugation of xenobiotics were measured in these mice. In all experiments, E+ caused greater reduction in initial ADG of S than of R mice. In Exp. 1, liver cytochromes P450 and b5 activities were not affected by line, diet, or their interaction. These enzymes were not evaluated in subsequent experiments. In all experiments, glutathione-S-transferase (GST) and uridine diphosphate glucuronosyltransferase (GRT) activities differed between lines. Resistant mice had significantly higher GST activity on both diets in Exp. 1, on E- in Exp. 2, and on E+ in Exp. 3. Resistant mice had higher GRT activities on E+ in Exp. 1, on E- in Exp. 2, but after 4 wk on either diet in Exp. 3. Before test diets were imposed in Exp. 3, GST and GRT activities were higher in R-line mice. Divergent selection created lines that differed in response to tall fescue in the diet. Postweaning growth of resistant mice was less severely depressed by E+, although susceptible mice later expressed compensatory gain. Activities of two detoxification enzymes generally were higher in livers from R-line mice, suggesting a biochemical mechanism for the difference. Using such traits, it may be possible to select ruminants for resistance to fescue toxicosis.

Differences between progeny of beef sires in susceptibility to fescue toxicosis.

Fifteen progeny of two Polled Hereford sires were fed endophyte-infected tall fescue seed (E+) to investigate variability in susceptibility to fescue toxicosis. One sire, bred in Missouri, was reputed to produce calves that were resistant to fescue toxicosis. The Control sire, from Virginia, had unknown merit for susceptibility. There were two phases in which E+ was included in the diet (2 and 4) and three in which it was not (1, 3, and 5). Voluntary feed intake, serum prolactin, cholesterol, and alkaline phosphatase concentrations; ability to maintain homeothermy (afternoon minus morning rectal temperature); and heat-transfer inefficiency (afternoon rectal minus afternoon surface temperatures) were monitored. Intake and prolactin concentration were decreased by E+, but sire groups did not differ in responses across phases. Cholesterol levels were lower during E+ phases, and the rate of increase in cholesterol concentration after withdrawal of E+ differed between groups. Alkaline phosphatase concentrations were lower during E+ phases; calves from the Control sire showed greater reduction than calves sired by the Missouri sire. Ability to maintain homeothermy was reduced by E+, but the reduction did not differ between sire groups. Heat-transfer inefficiences were not indicative of fescue toxicosis in this study. Evidence was not compelling to support the existence of differences between the sire groups for susceptibility to toxicosis. However, progeny of the Missouri bull had higher feed intake and lower rectal and body surface temperatures through nearly all phases of the trial. This may account for the reputation of the Missouri bull to sire calves with relatively high resistance to fescue toxicosis.

Genetic, environmental and interaction effects on lifetime production efficiency of crossbred ewes.

A total of 399 crossbred ewes born in 2 yr were maintained either on irrigated or dryland hill pastures. The ewes were sired by North Country Cheviot, Dorset, Finnsheep or Romney rams mated to Suffolk or Columbia-type ewes. The crossbred ewes were mated to Hampshire rams throughout the study and had the opportunity for either 4 or 5 yr of production. Annual feed cost was calculated based upon estimated annual metabolizable energy (ME) requirements for maintenance, ewe growth, pregnancy and lactation. Income from weaned feeder lambs and from orphan lambs sold shortly after birth, feed cost and net revenue were analyzed during each ewe's first production year, as well as for the entire period of the experiment. Crossbred group significantly affected income from lambs produced, feed cost and net revenue for the ewes' first production year. Crossbred group and crossbred group X management system interactions generally influenced lifetime income from lambs produced and from orphans sold, gross income, total cost and net revenue per ewe. Finnsheep X Suffolk ewes performed best on irrigated pastures but were below average for lifetime net revenue on hill pastures. Finnsheep X Columbia ewes performed best on hill pastures and, with Dorset X Suffolk and Dorset X Columbia ewes, were well above average for lifetime net revenue on irrigated pastures as well. Suffolk crossbreds consistently performed better on irrigated than on hill pastures. While most ewes with Columbia-type inheritance were more efficient on hill than on irrigated pastures, this result was not always consistent.

Phenotypic relationships among early life traits and lifetime ewe production efficiency.

In least-squares analyses of variance, 10 ewe lamb traits were used, singly and in various combinations, to predict lifetime production efficiency. Ewes belonged to eight crossbred groups and were raised on two management systems for either 4 or 5 yr of production. Lifetime production efficiency components were total feed and ewe ownership cost, gross income and net revenue. A ewe lamb's type of birth, actual and adjusted weaning weight (WWt and AWWt), postweaning weight (PWWt) and first year gross income (FGROS) were significantly phenotypically related to one or more components of lifetime production efficiency. While triplets were better than either of the other two birth types, single-born ewes were better than twin-born ewes for lifetime production efficiency. Any of the three ewe lamb weights (AWWt, WWt or PWWt) appeared to be an accurate estimator of lifetime production efficiency. The effect of first year productivity was largely a part-whole relationship with lifetime production because the regression coefficients of subsequent lifetime production efficiency (not including first year production) on first year productivity were near zero. Postweaning average daily gain, ewe age at first estrus and date of first lambing (at approximately 12 mo of age) did not have significant relationships with lifetime production efficiency. When type of birth, date of birth, weaning weight and first year gross income were analyzed simultaneously, weaning weight and FGROS had the greatest effect on lifetime production efficiency. When AWWt and type of birth were considered simultaneously, both had significant relationships with lifetime production efficiency.

The effect of selection in retrospect on lifetime production efficiency in sheep.

Within 32 crossbred group-management system-birth year subgroups, "selection in retrospect" was practiced based upon several ewe lamb traits, singly or in combination, to assess what the effect of such selection would have been on ewe lifetime production efficiency. Ewes were from two birth years, eight crossbred groups and two management systems. Ewe's type of birth, date of birth, actual and adjusted weaning weight (WWt and AWWt), postweaning weight (PWWt) and first yr gross income (FGROS) were the ewe lamb traits involved in the single trait paper selection schemes. Only the ewe lamb weights (WWt, AWWt and PWWt) caused a significant increase in the production efficiency of selected populations over those of the corresponding unselected populations. When postweaning weight was used jointly with the ewe's type of birth in an independent culling level selection scheme, both the group composed of heavier multiple-born ewes and the group composed primarily of single-born ewes were more productive than the group composed of lighter weight multiple-born ewes. Backward selection of the elite ewes having the best production efficiency also showed that the heavier the ewe lamb, the more efficient she was expected to be. Examination of ewes surviving the entire duration of the experiment failed to identify any early life trait that predicted longevity. Furthermore, surviving ewes were equally variable in their early life traits to the populations at large. Longevity, however, was positively related to lifetime efficiency.

Genetic aspects of longevity in Angus and Hereford cows.

Thirty years and 23 yr of life history data from a Hereford herd in Arizona and an Angus herd in Wyoming, respectively, were analyzed. Longevity averaged 4.21 +/- .06 for years from first calving to disposal (FST), 7.40 +/- .06 for years from birth to disposal (AGE) and 3.46 +/- .06 for lifetime number of calves weaned (NUM) in Herefords and 4.49 +/- .13 (FST), 6.68 +/- .12 (AGE) and 3.66 +/- .11 (NUM) in Angus. In the Hereford herd, heritability estimates for traits measuring longevity, estimated from daughter-dam regression and paternal half-sib analyses, ranged from .16 to .26. In the Angus herd, heritability estimates from daughter-dam regression ranged from .03 to .05. In the Hereford herd, genetic correlations of birth weight and weaning weight with longevity, from daughter-dam regression, were negative and generally of low magnitude, whereas genetic correlations between weaning condition score and longevity were positive and moderate. Analogous estimates from paternal half-sib analyses all were positive and moderate to high. Phenotypic correlations between early life traits and longevity traits in Herefords all were near zero. In the Angus herd, curves for age-specific survivorship and age-specific survival rate varied markedly among sires. This study suggested the existence of moderate genetic variation for longevity traits in beef cattle. None of the traits expressed early in life that were examined would, however, be reliable predictors of genetic or phenotypic merit for longevity.

Genetic differences in concentration of immunoglobulins G1 and M in serum and colostrum of cows and in serum of neonatal calves.

Blood samples obtained 24 and 36 hr postparturition from 187 calves in 2 years were analyzed for serum concentration of immunoglobulins G1 (IgG1) and M (IgM). Serum and colostrum samples also were obtained from their dams, and corresponding immunoglobulin levels were determined. Calves were evaluated for their ability to acquire and absorb immunoglobulins and cows for their ability to produce immunoglobulins. Mathematical models included sources of variation for breed of sire, sire within breed, breed of dam, age of dam and sex of calf. Of these factors, breed of sire, breed of dam and age of dam were the most important. Simmental- and Pinzgauer-sired calves tended (p = .07) to have lower IgG1 and IgM levels than calves sired by Hereford, Hereford X Angus and Tarentaise bulls. Calves of Hereford X Angus dams had consistently higher immunoglobulin concentrations than calves of Hereford dams. Hereford X Angus cows tended to have higher colostrum concentrations and lower serum concentrations of the two immunoglobulins than Hereford cows. A fetal sire effect was demonstrated for serum IgM concentrations in that cows mated to Simmental bulls had lower (P = .05) concentrations than cows mated to bulls of other breeds. Advancing age of dam was associated with higher concentrations of immunoglobulins, except for serum IgM in the calf and in the cow. Correlations between serum and colostrum concentrations of both immunoglobulins in the dam were low, as were correlations between cow colostrum concentration and calf serum concentration of IgG1. IgM in the colostrum was positively associated with IgM in calf serum, however. Heritability estimates for calf serum IgG1 concentration were high (.52 +/- .28 and .69 +/- 30 at 24 and 36 hr, respectively), while estimates for heritability of IgM concentrations were intermediate (.30 +/- .26 and .35 +/- .26 at 24 and 36 hr, respectively). There was no evidence that sires within breeds influenced, through the fetus, immunoglobulin concentrations in serum or colostrum of their mates. Relatively high repeatabilities for all traits imply that observed differences among cows were due in part to genetic and (or) permanent environmental differences.

Sleep time following anesthesia in mouse lines selected for resistance or susceptibility to fescue toxicosis.

In previous work, a mouse line selected for resistance (R) to fescue toxicosis had higher activities of two hepatic Phase II detoxification enzymes than a mouse line selected for fescue toxicosis susceptibility (S). The primary objective of the present study was to determine whether those same lines also differed in hepatic Phase I enzyme activity, estimated from sleep time (ST) following sodium pentobarbital anesthesia. Additional objectives were to determine whether ST differences between lines were modulated by endophyte-infected fescue in the diet (with or without an enzyme inducer) and whether ST of individual mice was correlated with the effect of a toxin-containing diet on the postweaning growth of those mice. In Exp. I, 24 males from each line were randomly assigned to each of five diets: control (commercial rodent food meal); E+ (50% endophyte-infected fescue seed, 50% control); E+P (the E+ diet supplemented with 1,000 ppm phenobarbital); E- (50% endophyte-free fescue seed, 50% control); and E-P (the E- diet supplemented with 1,000 ppm phenobarbital). After 4 wk on these diets, ST was measured on all the mice. A second ST was recorded on each mouse by randomly sampling one-fourth of the population after 1, 2, 3, or 4 wk on a pelleted rodent food diet. Regardless of diet, R mice had shorter first and second ST than S mice (P < 0.01), suggesting higher hepatic Phase I microsomal enzyme activity. Mice on both phenobarbital-supplemented diets had shorter first ST than mice whose diets did not include that microsomal enzyme inducer (P < 0.01). In Exp. II, ST was measured on male and female R and S mice (n = 280) after they had been fed the E- diet for 2 wk, then the E+ diet for 2 wk, and then a pelleted rodent food diet for 2 wk. Growth response to the E+ diet was the percentage of reduction in gain on the E+ diet compared to gain on the E- diet the previous 2 wk. As in Exp. I, S mice slept longer than R mice (P < 0.01). The residual correlation between ST and gain reduction associated with the E+ diet equaled 0.04. Thus, an animal's apparent Phase I enzyme activity did not predict its growth rate depression on the toxin-containing diet. Based on these and previous studies, divergent selection for toxicosis response in mice was successful partially by causing divergence in activities of hepatic Phase I and II detoxification enzymes.

Doe reproduction and preweaning litter performance of straightbred and crossbred rabbits.

Reproductive characters (N = 430 matings) of four doe genetic groups and preweaning performance of litters (N = 280) were evaluated in a 15-mo study. Doe genetic groups were New Zealand White (NN) and Californian (CC) straightbreds and Californian X New Zealand White (CN) and New Zealand White X Californian (NC) crossbreds. Does and litters were provided either a commercial control or a 74% alfalfa pelleted diet. Sires of litters included NN, CC and Flemish Giant (FG) straightbred bucks. The linear and quadratic regression on age of dam affected (P less than .01) litter birth weight. The NN does were heavier than CC does at first breeding at 154 d and produced heavier litters at birth and weaning (all P less than .01). Percentage survival of kits from birth to weaning and doe and preweaning litter feed intake and efficiency were greater from NN than CC does (all P less than .01). Numbers born and reared/litter to weaning at 28 d were likewise greater for NN vs CC straightbred does. Heterosis estimated from comparing litters of reciprocal crossbred vs straightbred does was significant for number born, litter birth and weaning weights and doe and preweaning litter feed intake. Reciprocal crossbred differences were observed (P less than .05) for doe breeding weight, number born and weaned per litter, litter birth and weaning weights and doe and preweaning litter feed intake, favoring CN does reared by NN straightbred dams over NC does. Diet, in consistent favor of the 74% alfalfa diet, significantly affected all traits studied with the exception of doe longevity, doe fertility percentage, and doe and litter feed efficiency.(ABSTRACT TRUNCATED AT 250 WORDS)

Breed, heterotic and diet effects on postweaning litter growth and mortality in rabbits.

The effects of sire breed, dam genetic group and diet on postweaning litter growth and mortality were evaluated in 208 litters of rabbits. Sire breeds were New Zealand White (NN), California (CC) and Flemish Giant (FG). Dam genetic groups were NN and CC straightbred and California X New Zealand White (CN) and New Zealand White X Californian (NC) reciprocal crossbreds. Litters were fed either a commercial diet or a 74% alfalfa diet. Litter size and weight at 56 d and litter gain (the difference between 56- and 28-d litter weights) tended to be lower in NN- compared with CC-sired litters; while feed intake was significantly lower and mortality was higher in progeny of NN sires. Litter 56-d weight, litter gain and litter feed efficiency (gain/feed intake) were improved (P less than .05) in litters sired by FG vs NN or CC bucks. Direct heterosis (CN and NC compared with NN and CC litters) for litter size, growth and feed-related traits was generally low (less than 11% in absolute value), while moderate heterosis levels (-21 and -22%) were observed for diarrhea-related and total mortality. Differences between NN and CC straightbred dams for litter size and weight at 56 d and litter gain were small, but in favor of NN dams. Diarrhea-related and total mortality were higher (P less than .05) in litters reared by NN vs CC straightbred dams.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of straightbred and crossbred rabbits for milk production and associative traits.

Two hundred twenty-five lactation and litter performance records from 82 does representing four genetic groups and two diets were analyzed to quantify breed, heterosis, reciprocal F1 cross and diet effects for milk production and associative traits. Doe genetic groups were New Zealand White (NN) and Californian (CC) straightbred and Californian X New Zealand White (CN) and New Zealand White X Californian (NC) reciprocal crossbreds. Pelleted diets fed to does and litters were either a commercial control or a 74% alfalfa diet. Three sire breeds of litters included NN, CC and Flemish Giant (FG) straightbreds. Doe genetic group and diet were important sources of variation (P less than .05) for all traits examined except for litter milk efficiency (litter gain/milk intake) and doe feed efficiency (milk yield/feed intake). The sire breed of litter effect did not influence (P greater than .05) lactational performance of does nor associative preweaning traits. Straightbred NN does were heavier at kindling, yielded more milk, reared a heavier litter by 21 d and were more efficient in converting feed into milk than were straightbred CC does (P less than .01). Significant heterosis was detected for milk production and for litter size and weight at 21 d. Reciprocal differences between crossbred doe groups were observed (P less than .05) for litter milk efficiency and doe feed intake. The 74% alfalfa diet was superior to the commercial control diet for effects on milk production and litter size and weight at 21 d, although doe feed intake was increased.(ABSTRACT TRUNCATED AT 250 WORDS)