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.

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.

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.

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.