Maternal care of rabbits in the lab and on the farm: endocrine regulation of behavior and productivity.

Maternal behavior in rabbits has been well described in the wild, the laboratory, and the farm. Salient characteristics include: (a) the construction of a nest (inside an underground burrow or a box), composed of straw/grass and body hair and (b) the display of a single, brief (ca. 3 min) nursing bout per day. The onset and decline of nest-building in mid and late pregnancy are controlled by specific combinations of estradiol, progesterone, testosterone, and prolactin. Following parturition (kindling) does can mate and become pregnant again. Management strategies have been used on the farm to enhance productivity, impinging on specific reproductive processes (e.g., use of hormones to synchronize estrus, artificial insemination at kindling, doe-litter separation on specific days of lactation to increase sexual receptivity and fertility). Knowledge about the rabbit's reproductive physiology and behavior will be enriched by integrating research coming from the laboratory (where estrus, pregnant-only, or lactating-only animals are the main categories investigated) with studies performed on the farm or research station (where pregnant-lactating rabbits are prevalent). Similarly, the high productivity demands of modern rabbit farms will benefit from the information obtained in the laboratory where specific issues in reproductive neuroendocrinology are explored with methodologies that are not amenable for farm use.

Fiber production and properties in genetically furred and furless rabbits.

The objectives of this study were to describe and compare means and measures of variability of fiber characteristics and fiber production between genetically furred and furless rabbits and among classes of furless rabbits. An F1 generation of rabbits was produced by mating New Zealand White does to a rare, furless Mini Lop buck. All F1 offspring had normal coats of fur. Inter se random matings of the F1 stock (barring full-sibling matings) were made to produce the F2 generation that consisted of approximately 75% furred and 25% furless progeny. Furless animals were further subjectively classified into 3 distinct classes (1 to 3) having increasingly more fur. In yr 1, 17 furred and 20 furless rabbits (age 28 to 49 d) were randomly assigned to growing pens, and in yr 2, 17 additional furless and 9 additional furred rabbits were included to increase the size. After 6 wk, the rabbits were weighed, and a measured area of fur (approximately 20 cm2) was shorn from the left flank of each rabbit. This fiber was weighed and measured for staple length, fiber diameter, prickle factor (% of fibers > 30 microm in diameter), and fiber curvature. Fiber production per unit area of skin was calculated and fiber production per animal was estimated. In yr 2, all of the furless and 2 of the furred rabbits were shorn over their entire bodies to obtain direct measurements of total fur weight. Furless rabbits were 9% heavier (1,941 vs. 1,783 g of BW, P < 0.01) and produced approximately 90% less fiber per unit area of skin than furred rabbits (1.74 vs. 15.83 mg/cm2, P < 0.01). The fibers from furless rabbits were shorter (1.54 vs. 2.56 cm, P < 0.01) and coarser (15.8 vs. 14.5 microm diameter, P < 0.01) than those from furred rabbits and exhibited greater prickle factor (11.3 vs. 3.5%, P < 0.01) and curvature values (47.5 vs. 38.5 deg/mm, P < 0.01). Class 3 furless rabbits were heavier than rabbits of classes 1 and 2 (2,075 vs. 1,817 and 1,981 g of BW, respectively, P < 0.05). Means for actual total fiber production per animal for classes 1 to 3 were 0.64, 2.07, and 8.68 g, respectively, compared with 23.0 g for furred rabbits (P < 0.01). Although some similarities were present, several of the correlations involving fiber properties and BW were substantially different (e.g., BW vs. staple length and fiber diameter vs. weight of fiber per unit area) for furred and furless groups. These results, and those reported elsewhere from a series of experiments, support the potential for production of furless rabbits in arid and tropical environments.

Breed and heterotic effects on postweaning traits in Altex and New Zealand White straightbred and crossbred rabbits.

Postweaning data from 1,111 straightbred and reciprocally crossbred rabbits were analyzed to evaluate Altex and New Zealand White (NZW) breeds for individual growth and litter traits. The Altex is a recently developed sire breed, whereas the NZW is a popular commercial dam breed. Individual fryer growth traits were weaning (28 d; WW) and market (70 d; MW) weights and ADG. Litter traits included litter size (LSW) and total weight of litter at weaning (LWW), 28 to 70 d total feed intake (LFI), feed efficiency (LFE = total litter gain/LFI), survival rate, and within-litter MW uniformity. Least squares models consisted of fixed effects of sire breed, dam breed, season of weaning, doe parity, two- and three-way interactions, and random effects of sire within sire breed, litter within sire x dam breed, and(or) residual error (depending on whether an individual or a litter trait was analyzed). Crossbreeding parameters (direct breed additive, maternal breed, and individual heterosis) were estimated. Altex sires increased WW, ADG, and MW by 40 g (P < 0.10), 2.5 g/d, and 152 g (P < 0.001), respectively. Individual growth traits were not significantly influenced by the maternal breed effect. Litter size at weaning and LWW means were numerically similar for Altex and NZW dams. Direct heterosis increased ADG (1.7 g/d; P < 0.01) and MW (66 g; P < 0.10). In straightbred Altex compared to NZW fryers, ADG and MW were increased by 3.6 g/d and 216 g, respectively (P < 0.001). In Altex (sire) x NZW (dam) crossbred compared to NZW straightbred fryers, WW and MW were heavier (55 and 218 g; P < 0.10 and < 0.001) and ADG was more rapid (4.2 g/d; P < 0.001). For litter traits, Altex compared to NZW sires increased LFI by 1.28 kg (P < 0.10). Individual crossbreeding parameters did not affect (P > 0.05) other litter traits. No relationship existed between breed type of fryer and survival status (chi2 = 2.81; P > 0.25). For litter traits, straightbred Altex had significantly greater LFI by 2.45 kg and increased LFE by 0.015 units relative to NZW. Combined direct breed additive and heterosis effects increased LFI by 1.84 kg (P < 0.05) in Altex (sire) x NZW (dam) crossbreds compared to NZW straightbreds. Also, 25% more Altex (sire) x NZW (dam) crossbred fryers were marketable (body weight > or = 1.8 kg) by 63 d of age than NZW straightbred fryers. These data suggest that crossing Altex bucks to NZW enhanced breeding efficiency of fryer growth performance.

Teaching international animal agriculture.

Students who major in animal science at U.S. institutions are generally exposed to a curriculum that emphasizes commercial, large-scale production of the few traditional food animals: cattle, poultry, sheep, and swine. Globally, most farmers live in lesser-developed countries under limited-resource conditions of land, feed supplies, equipment, and capital. The promotion of commercial animal production enterprises may not be appropriate for such farms because it can subject farmers to considerable economic risk. Rather, use of limited numbers of large livestock, locally adapted breeds, or smaller livestock (e.g., ducks, goats, guinea pigs, and rabbits) may be more appropriate under subsistence, integrated farming systems. In this global context, a course in international animal agriculture has been taught for 15 yr to undergraduate and graduate students. The course consists of a review of traditional and potential livestock species well suited for impoverished families on small farms and methods to implement sustainable livestock projects, including feasibility, design, implementation, monitoring, and evaluation stages. To enhance student understanding, global food issues and challenges are illustrated with case studies. A term paper is also assigned for which students choose three suitable livestock species or local breeds that would be complementary on a small crop farm (< 5 ha). Daily dietary requirements of protein and energy per family member are calculated. Itemized enterprise budgets and production tables are prepared. Early in the course, the general consensus of students was that people who are malnourished and live in poverty have low personal ambition and motivation, and that their problems should be amenable to solution by application of American technology and expertise. The course modifies such attitudes and enhances a student's critical thinking and problem-solving abilities and communication skills. Course evaluations indicated that students believed that it is important to acquire some international knowledge and understanding when seeking a job, and that certain animal science courses should contain some international content. Students gain an understanding of global animal agriculture and an appreciation of the complexity of food production and hunger issues.

Maternal nest quality and its influence on litter survival and weaning performance in commercial rabbits.

Two hundred fifty parturition records on measures of maternal nest quality were collected over four seasons (S) from does (d; n = 108) of Californian (CAL), New Zealand White (NZW), and CAL x NZW breed types (DB). Service sires (s; n = 85) were from four sire lines (SL): CAL, NZW, and control and selected synthetics. Sires were nested within SL and 6-mo breeding cycle (C). Nest quality traits included fur weight (FW), nest structure (NS) and fur placement (FP; scores 1 to 5), and kit placement (KP; scores 1 to 4). Additional doe and litter characters studied over eight parities (P) were gestation length, doe body weight at parturition, litter size and weight at birth and weaning, estimated milk yield, doe feed intake, and neonatal and preweaning survival rates (NSR and PSR). A mixed model was used that included SL, C, SL x C, s/(SL x C), DB, SL x DB, d/(SL x DB), S/C, SL x (S/C), DB x (S/C), P, SL x P, DB x P, (S/C) x P, and residual error. Neither SL, s/(SL x C), nor SL x DB influenced (P > .05) nest traits. Nest quality traits did not have normal distributions. Although residual correlations were moderate to high among nest traits studied (.21 < r < .67), correlations relating nest quality to doe and litter traits were low (r < .21). The CAL had lower (P < .05) nest trait scores than NZW purebred does. The NZW does had higher (P < .05) FW (1.9 g) and FP (.36 units) scores than CAL x NZW does. Season, but not parity, influenced (P < .05) NS and KP. From path analysis, relative to other doe and litter characters, nest traits accounted for 21.2 to 35.3% of total variation (across doe breed types) in NSR. However, nest traits had low determination (< 5%) for PSR and litter weaning weight.

Preweaning variables' influence on market traits in rabbits.

Influence of several preweaning variables on market trait determination was investigated using 338 fryer rabbits from 52 litters. Variables measured at 1, 8, and 15 d included litter size, individual kit body weight (BW), within-litter rank score for BW, and litter uniformity (coefficient of variation for BW among littermates; CV). Daily milk consumption in wk 1, 2, and 3 was used to determine average weekly dominance rank score and total weekly milk consumption. Market traits were individual average daily gain (ADG) and litter feed intake (FI) from 28 to 70 d, and 70-d BW (BW70), litter uniformity (CV70), litter size (LS70), and within-litter rank score for BW (RANK70). Market traits were analyzed separately by week involving preweaning variables and season of weaning as covariates. The week models provided the most accurate predictions of ADG, BW70, FI, and LS70 (R2 = .542, .712, .906, and .970, respectively). Prediction of CV70 and RANK70 was poor (R2 = .202 to .369). Season influenced ADG, BW70, and FI (partial R2 values of .285 to .435). Path analysis indicated that milk consumption and BW were more important in BW70 determination (28.4, 26.4, and 36.9% of total variation in wk 1, 2, and 3, respectively) than litter size (partial R2 < or = 2.0%). In wk 1, litter size plus dominance rank score primarily determined LS70 (direct and indirect contributions of 62.3 and 21.5%). Because of major seasonal influences, management should take measures to optimize feed intake and to improve doe milk yield and fryer gain performance.

Mass selection for 70-day body weight in rabbits.

Five generations of phenotypic mass selection for increased 70-d body weight (market age, MW) were conducted in an experimental population of rabbits. Market-age weight was recorded for 1,616 rabbits from 336 litters (sired by 121 bucks and reared by 321 does). Additional growth and carcass traits measured in control (C) and select (S) lines were 28-d weaning weight (WW), 28- to 70-d average daily gain (ADG), carcass yield percentage (CY), loin primal cut percentage (LP), and lean-to-bone ratio in the loin primal cut (LBR). Average inbreeding coefficients in litters by the fifth generation were 10.3 and 11.2% for C and S lines, respectively. After five generations the difference in weighted cumulative selection differential between lines for MW was 1,015 g. Using an animal model with REML and regression procedures, observed and realized estimated heritabilities for MW were .12 and .11, respectively. Estimates of heritability by REML were .04, .17, .37, .25, and .35 for WW, ADG, CY, LP, and LBR, respectively. Bivariate REML estimates of direct genetic, common litter, and residual correlations between MW and a second growth trait (WW or ADG) were all positive and moderate to high in magnitude (range of .56 to .98). At generation five, mean direct breeding values of S and C line rabbits for MW were 133.2 and 12.0 g. Genetic trends were 29.1 and 29.4 g per generation for MW, estimated according to regressions of estimated mean breeding value (obtained from mixed-model analyses) or of observed S minus C line mean differences on generation number, respectively. Correlated response for WW, ADG and LBR was consistently favorable, whereas results for other carcass traits studied were inconclusive. Selection was effective for increasing MW performance.

Breed and environmental effects on postweaning growth of rabbits.

Growth records of 4,270 weanling rabbits born between March 1985 and December 1989 were studied to evaluate the effects of breed and month of birth on postweaning growth performance of four medium-sized breeds. Californian (CAL), New Zealand White (NZW), Palomino (PAL), and White Satin (WS). Traits examined were 28-d weaning weight (WW), postweaning gain (GAIN), attainment of 1,600-g market weight by 76 d of age (MKT), and approximate age at 1,600 g (AGE). Least squares models included breed, month of birth, sex, and year of birth as fixed effects and litter within breed by month and by year and the residual as random variables. The NZW had significantly higher GAIN and MKT and lower AGE than the other three breeds. White Satin had the highest WW, followed by CAL, NZW, and PAL. White Satin had higher GAIN and lower AGE than PAL or CAL but did not differ for MKT. Poorer performance was seen during the summer, but the NZW tended to be less affected by the environmental extremes than the other breeds. The effects on GAIN of mean monthly temperature and daylength and the interrelationships of these with estimated milk production and litter size at weaning were evaluated by regression methods for the 2,100 NZW fryers. Temperature and daylength had significant effects on GAIN, with lowest GAIN in the summer, but the individual contributions to the variance were small because of some redundancy when month, temperature, and (or) light were included in the same model. Curvilinear trends were observed that favored GAIN as estimated milk production increased but decreased GAIN as litter size at weaning increased. In the hot, humid climate of southern Louisiana important breed differences were noted. There were also indications that daylength may be an important factor in postweaning fryer performance.

Comparison of rabbit breed types for postweaning litter growth, feed efficiency, and survival performance traits.

Postweaning data on 643 rabbits from 122 litters representing four breed types, New Zealand White (NZW) and Californian (CAL) purebreds, CAL X NZW (CXN), and Flemish Giant (FG) crossbreds (the latter group was a collection of FG X CAL, FG X Champagne D'Argent [CHA], and 1/2 FG X 1/4 CAL X 1/4 CHA), were gathered over five seasons and compared for growth, feed efficiency, and survival-related performance traits. Evaluation criteria included litter size and weight at weaning (28 d); 28- to 70-d litter feed intake, weight gain, feed efficiency, and mortality rate; and litter and average market weight (70 d). The least squares model included main effects of breed type, season of birth of the litter and parity of dam, litter size at weaning as a linear covariate, and the random error. Breed-type differences were not detected for litter size and weight at weaning and feed efficiency. Purebred NZW and CAL litter trait performances were comparable (P greater than .05). Purebred NZW litters consumed less feed than CXN and FG crosses, gained weight less rapidly than FG crosses, and weighed less per fryer at 70 d than CXN and FG crosses (P less than .05). Feed intake was lower and average market weight was lighter for CAL purebred litters than for CXN and FG crossbred litters (P less than .05). The CXN and FG crossbreds only differed (P less than .05) for average market weight (2,078 vs 2,192 g). Mortality rate was lower (P less than .05) in CXN crossbred litters than in CAL purebred litters.(ABSTRACT TRUNCATED AT 250 WORDS)

Evaluation of purebred and crossbred rabbits for carcass merit.

Two hundred twenty-six fryers representing four breed types, New Zealand White (NZW) and Californian (CAL) purebreds, CAL x NZW (CxN), and Flemish Giant (FG) terminal crossbreds (the latter group including FG x CAL, FG x CHA [CHA = Champagne D'Argent], and 1/2FG x 1/4CAL x 1/4CHA), were appraised for carcass merit. Evaluation criteria included the following: preslaughter and carcass weights; pelt, visceral, giblet, abdominal fat, and dressing percentages; percentages of carcass in loin, forequarter, and hindquarter primal cuts; and weight of lean in loin and ratio of lean to bone weight in loin cut. The least squares model consisted of breed type (B), season of birth (S), and gender (G) as fixed effects, B x G and S x G interactions, litter within B and S as a random variable, and random residual. Breed type influenced (P less than .10) all traits except giblet and forequarter cut percentages. Season of birth significantly influenced all all traits except percentage of loin cut. Gender of rabbit affected (P less than .05) only the percentage of forequarter cut. A B x G interaction was observed (P less than .05) for pelt and visceral percentages. The NZW control purebreds were generally inferior (P less than .05) to the three other breed types for carcass yield and cutability traits. Purebreds were lighter for preslaughter, carcass, and loin lean cut weights and lower for loin lean-to-bone ratio than were CxN crossbreds (P less than .05). The FG crossbreds had heavier preslaughter and carcass weights and a lower abdominal fat percentage (P less than .01) than NZW, CAL and CxN. Results of this study provide corroborative evidence in support of terminal crossbreeding using CAL sires with NZW dams and FG purebred or crossbred sires with CAL and CHA purebred or crossbred dams to improve carcass merit.

Prediction and causation of litter market traits from preweaning and weaning characteristics in commercial meat rabbits.

Market data on 1,315 rabbits from 201 litters from Californian (CAL), New Zealand White (NZW), CAL X NZW and NZW X CAL dams bred to CAL, NZW and Flemish Giant sires were subjected to multiple regression and path analyses. Market traits observed in litters at 56 d included average kit weight (A56W), litter size (LS56), total litter weight (L56W) and within-litter uniformity in individual weights (LCV). Preweaning variables as covariates included in the model were dam metabolic body weight (DMW), litter born (LSB), litter birth weight (LBW), milk yield from 1 to 21 d (MY) and feed intake from 1 to 28 d of the dam and litter (FI). Results from multiple regression analyses indicated linear and quadratic effects (P less than .20) due to LSB and MY for all four market characters. The LBW influenced (P less than .05) LS56 and L56W, and FI affected (P less than .05) LS56, L56W and LCV. Separate analyses were conducted involving 28-d weaning and feed intake variables as covariates: litter size weaned (LSW), litter weaning weight (LWW) and litter feed intake from 28 to 56 d (LFI). The three weaning covariates were important (P less than .05) for all market traits except LS56 (LWW was not significant). The most accurate regression equations were obtained from the weaning model for prediction of L56W and LS56 (R2 = .68 and .78). Path analyses revealed that preweaning covariates generally had direct rather than indirect effects on market traits. Both direct and indirect effects of weaning covariates were important for market traits. Results suggest that litter market traits of size and weight can be predicted with a reasonable degree of accuracy.

Effects of breed, parity, day of lactation and number of kits on milk production of rabbits.

Lactation records (n = 86) from 60 does of four breeds (Californian, New Zealand White, Palomino and White Satin) were analyzed to assess the effects of breed, parity, day of lactation and number of kits on milk production. Breed of doe tended (P less than .07) to be important for mean milk yield according to ANOVA results. Californian does had numerically higher production than did does of the other breeds. Doe body weight, litter size born alive and weaned and litter weaning weight, likewise, were not influenced (P greater than .05) by breed of doe. Significant linear and quadratic relationships were found between milk production vs day of lactation, and milk production vs number of kits. However, breed x days and breed x number of kits interactions (P less than .05) indicated that the individual breeds responded differently to two of these effects. Peak lactation occurred at approximately 20 d after kindling. As kit number increased, milk yield also increased to a predicted maximum when 12 kits were suckling. Parity tended (P less than .10) to influence lactation yield in a curvilinear manner, increasing steadily through the seventh parity and declining thereafter. A nonsignificant residual correlation (.34) between milk production and doe body weight was observed. Corresponding correlations between milk production were high for litter size born alive and weaned (r = .62 and .87, respectively) and litter weaning weight (r = .86). Although lactation curves are unique to each particular breed, milk yield is influenced by several factors.

Cholesterol level of rabbit meat and trait relationships among growth, carcass and lean yield performances.

Data from New Zealand White rabbits (n = 89) were collected on growth, carcass and lean yield traits and on total lipid and cholesterol contents of total carcass lean tissue. Postweaning growth traits included individual 28-d weaning weight, 56-d market weight and average daily gain (28 to 56 d). Carcass yield traits included preslaughter, abdominal fat, giblet, pelt, visceral and carcass weights and dressing percentage; lean yield traits consisted of uncooked lean percentages from forequarter, hindquarter and loin primal cuts, adjusted total lean weight and overall meat to bone ratio. Experimental rabbits were reared under similar environmental conditions of diet, housing and management, were similar in age (range of 4 d) and were all slaughtered on the same day. Analysis of variance results revealed detectable (P less than .05) effects of litter source for all traits examined, except for loin lean percentage, meat to bone ratio and cholesterol content. In a separate analysis, litter size class influenced (P less than .05) the same traits (as the above litter source effect) with the exception of visceral weight and hindquarter lean percentage. Sex of rabbit did not (P greater than .05) have an effect on any carcass measurements. Cholesterol content of uncooked rabbit meat (total ground lean tissue) was 163.6 +/- 3.1 mg/100 g DM. Correlation coefficients were low (r less than .25) in absolute value between total cholesterol content and all other traits investigated. Individual 56-d market weight was related to a number of carcass parameters: dressing percent (r = .60), carcass weight (r = .93) and adjusted total lean weight (r = .89).(ABSTRACT TRUNCATED AT 250 WORDS)