Livestock Research for Rural Development 15 (2) 2003

Citation of this paper

Rabbit growth performance in a subtropical and semi-arid environment: effects of fur clipping, ear length, and body temperature

S D Lukefahr and C A Ruiz-Feria*

Department of Animal and Wildlife Sciences,
MSC 228, Texas and M University, Kingsville, TX 78363, USA
s-lukefahr@tamuk.edu
*Department of Physiology, University of Tennessee Health Center
894 Union Avenue, Memphis, TN 38163, USA

Abstract

 

In this 42-day growth experiment, the effect of ear length and body temperature in rabbits whose fur was electrical clipped compared to controls (non-clipped) on growth performance was studied. Two breeds (Altex and New Zealand White) and six diets (combinations of 0, 25, and 50% lablab forage plus commercial pellets (100, 75, or 50%) with or without cactus pads) were involved in the study, but reported previously.  Growth traits included initial body weight (age ranging from 28 to 33 days), ADG, and final body weight, as well as ear length (taken at the end of the study), which were measure on 152 weanling rabbits in the main experiment. A subset of 48 rabbits were randomly chosen and had daily body temperatures taken at 1400 hour using rectal thermometers. Correlations between ear length and growth traits were moderate to high (range of 0.50 to 0.70), but were low between body temperature and growth traits (range of 0.09 to 0.17), while a zero linear correlation was found between ear length and body temperature. Altex had longer ears than New Zealand Whites (10.92 versus 10.08 cm), but no difference in mean body temperatures existed between breeds (P>0.05). Fur clipping generally improved growth traits (P<0.05), and somewhat reduced body temperatures (means of 40.02 and 39.75 °C in clipped and non-clipped rabbits, respectively; P<0.001). A stronger association existed between ear length and ADG in fur clipped versus non-clipped rabbits (linear regression coefficients of 2.721 compared to 1.326 g/d per cm increase in ear length; P<0.05). In conclusion, further studies are needed to elucidate thermoregulatory mechanisms unique to the rabbit.

Key words: growth, heat stress, prediction, rabbits, thermoregulation

Introduction 

It is generally accepted that the greatest potential for small-scale rabbit meat production is in the lesser developed countries. However, a limiting factor affecting growth of rabbits in tropical and arid climates, which is common to the lesser developed countries, is caloric stress as associated with high ambient temperature (McNitt et al 2000). The wild European rabbit, Oryctolagus cuniculus (the progenitor of domesticated rabbits), displays the instinctive behaviour of burrowing underground to create tunnels to escape from predators but also to avoid exposure to high ambient temperatures.  Even in desert regions, such as in Egypt and Tunisia, rabbits are observed to thrive when allowed to burrow underground (Finzi et al 1988). Of course, domesticated rabbits reared in hutches are denied this behavioural expression, and, as a consequence, become highly vulnerable to heat prostration as related to dysfunctional thermoregulation.
 

Photo 1: A traditional scene in Egypt involving domesticated rabbits
being allowed to burrow underground

 

Genetic selection for heat resistance in rabbits is a subject of great importance, but one that has received little research inquiry, except for studies conducted mostly in Egypt. Obeidah (1975) first reported estimates of heritability for heat tolerance characters (0.12 and 0.47 for body temperature and respiration rate, respectively) in young Giza White rabbits. Genetic correlations between body weight and body temperature were generally positive, and between body weight and respiration rate were negative. Toson (1983) reported somewhat higher heritabilities for body temperature (0.28 to 0.45) and lower heritabilities for respiratory rate (0.20 to 0.45) in local breeds.  Egyptian studies (Afifi et al 1984; Hanafi et al 1984) have also reported significant breed differences in body temperature and respiration rate. Shafie et al (1970) documented that Baladi White rabbits were more thermo-tolerant than Baladi Black rabbits, while Baladi Red rabbits were intermediate. Ibrahim (1988) observed that the Baladi breed had longer and thicker down and guard hairs, but had less dense fur coats than exotic breeds (Bouscat and Flemish Giant) from temperate climates, and that these hair traits were moderately to highly heritable (range of 0.28 to 0.45). Finzi et al (1994) also reported that Baladi rabbits had better resistance to heat stress than an exotic breed (New Zealand White) in terms of control of body temperatures via their circadian cycles.

 

Photo 2: Rabbit possessing incredibly large ears (left) reared in
a tunnel-housing system in a hot climate (Courtesy of A. Finzi)

 

The objective of this investigation was to determine the relative importance of ear length and body temperature on growth performance for two breeds of rabbit (Altex and New Zealand White) that were subjected to electrical fur clipping and reared in a subtropical, semi-arid environment in south Texas.

 

Materials and Methods

Background, breeds, diets, and fur clipping treatments

The experiment was conducted at Texas A and M University-Kingsville (TAMUK) from May 23 to July 4, 1996. During the experiment, mean ambient temperature, relative humidity, and wind speed velocity at 1400 hour were 32.3 ± 3.7 °C, 48.6% ± 1.8, and 18.9 ± 7.5 km/hr, respectively. Region, climate, breed populations, and management background aspects, as well as fryer growth and feed performance results (according to breed, diet, and fur clipping treatments), were previously described by Ruiz-Feria and Lukefahr (1998). Briefly, commercial-bred Altex (a large sire breed) and New Zealand White (NZW) rabbits were used in the 42-day growth experiment. Diets consisted of 100, 75 or 50% commercial pellets (Nutrena®, Cargill-Nutrena Feeds Division, Minneapolis, MN) replaced by 0, 25 or 50% fresh lablab (Dolichos lablab) leaves (18.9% DM) on a DM basis. The level of pellets fed to pens of rabbits receiving 25 or 50% lablab was determined based on average feed consumption of pens of control rabbits fed ad libitum on the previous day.  In addition, each pen was offered or not offered fresh prickly pear cactus (Opuntia stricta) pads (5.7% DM) fed free choice.

 

The main experiment involved 152 rabbits in the 42-day experiment.  Two fryers per breed were randomly assigned to each of thirty-eight pens.  Initially, half of the pens were randomly sampled (involving 19 of 38 pens) where fryers had their entire body coats clipped of fur (~1 mm above skin level) using electric clippers, requiring approximately 5 minutes per rabbit.  Three weeks later, it was necessary to reclip these same animals due to fur regrowth.  Each pen represented one of six diets (lablab x cactus diet combinations as described above), and one of two clipping groups. Hence, there were three pens as replicates for each treatment class combination (3 lablab x 2 cactus x 2 clipping classes) for a total of thirty-six pens.  Two additional pens were accommodated with control animals (commercial feed pellets only) to increase the sample size.

 

Traits measured

 

Initial and final body weights and ear length were recorded and average daily gain was calculated during the 42-day growth experiment.  At the end of the experiment, ear length measurements were also taken on all rabbits using 0.1 cm precision. Asubset of 48 rabbits from 12 pens had body temperature measurements recorded daily at 1400 hour using rectal thermometers, and averaged over the 42-day period. Only one of the forty-eight rabbits died during the study. 

Statistical procedures

Average daily gain (ADG) and final weight were analyzed using multiple regression procedures (Harvey 1990). Model main effects included diet (lablab forage level of 0, 25 or 50%, cross-classified with the exclusion or inclusion of cactus for a total of six diet treatment groups), breed (New Zealand White or Altex), and non-clipped (controls) or fur clipped groups, and first-order interactions. Although there were pen replicates by treatment class existed in the experiment, in preliminary analyses, variances due to pens were close to zero for growth traits, and so this source was eliminated from all models. In addition, independent continuous variables included initial body weight, ear length, and body temperature, which were added to regression models to identify variables that significantly impact ADG and final weight. However, for final weight, the covariate of initial body weight was appropriately excluded from the model. Best-fit prediction equations were obtained using stepwise regression procedures in which partial linear and quadratic effects for covariates (and their interactions with main effects) were tested (P<0.25) for inclusion in final models.

 

Results and Discussion

 

Descriptive statistics

 

Descriptive statistics for all traits measured in the growth experiment (unadjusted for model effects) are shown in Table 1. Whereas, age at the beginning of the study was somewhat earlier for New Zealand White than for Altex fryers, in preliminary analyses the age effect was not important for all traits examined (P>0.05). As expected, Altex rabbits were superior for growth traits than New Zealand White rabbits. In a more recent and larger experiment (n=1,111 fryers) as reported by Medellin and Lukefahr (2001), this same trend was substantiated in terms of statistical significance. For ear length and body temperature, however, statistical values were more numerically similar between breeds.

 

Correlations among traits
 

Product-moment correlations among all traits measured in the study are provided in Table 2. Although correlations were high between final weight and its component traits (initial weight and ADG), as expected, a correlation of only 0.24 was computed between initial weight and ADG. The low correlation is possibly attributed to the several factors imposed upon the animals, such as lablab and cactus forages versus only commercial feed pellets, and clipping versus no clipping of fur. Residual correlations (adjusted for these factors in our models) among the same traits would be expected to be higher. Moderate to high correlations (r of 0.50 to 0.70) were observed between growth traits and ear length. However, this finding does not imply a cause-and-effect relationship insofar as larger ears enhancing growth performance. Rather, the relationship could simply reflect heavier rabbits having larger ears. Results from multiple regression analyses (reported below) will shed more light on this subject. More interesting were the weak correlations detected between body temperature and growth traits, and especially the zero correlation between body temperature and ear length.  A residual correlation of only -0.14 was observed between body temperature and ear length from an analysis whereby these two variables were adjusted for model main effects. Italian researchers (Valentini et al 1985; Finzi et al 1986; Moera et al 1991) have also examined cues of physiological stress in rabbits exposed to high temperatures in artificially controlled climatic chambers. In agreement, Finzi et al (1986) reported a weak association between ear surface area and body temperature in response to heat stress tests. However, the climatic chambers had minimal air flow which may have limited the thermoregulatory effectiveness of the ear.

 

 

ANOVA results for ear length and body temperature

 

The main effects of lablab level (P<0.05), breed (P<0.001), and cactus x breed interaction (P<0.05) were important for ear length. Least-squares means for 0, 25, and 50% lablab were 10.67, 10.50, and 10.33 cm, respectively. Altex had longer ears than New Zealand Whites (10.92 versus 10.08 cm) by the end of the experiment. Changes in rank for breed means across cactus groups did not occur. The covariate of initial body weight was positively associated (P<0.001) with ear length; for every 100 g increase in initial weight, ear length was 0.17 cm longer by the end of the 42-d experiment. The final model for ear length explained 49.4% of total variation for this character.

 

For body temperature, both lablab forage and clipping treatments were important (P<0.05 and P<0.001, respectively). Least-squares means for 0, 25, and 50% lablab were 39.92, 39.92, and 39.81 °C, respectively. More dramatic was the mean difference in body temperature between non-clipped and fur clipped rabbits (40.02, and 39.75 °C; P<0.001). Initial body weight was positively associated (P<0.05) with body temperature; however, for every 100 g increase in initial weight, body temperature was only 0.039 °C higher. The final model accounted for 66.7% of total variation for body temperature.

 

In addition, a supplemental analysis was performed to determine the effect of ear length as a covariate on body temperature (R² = 72.6%). The addition of ear length to the same base model resulted in highly significant differences (P<0.01) in body temperature between non-clipped and fur clipped groups (39.95 compared to 39.77 °C). Also, significant differences (P<0.05) between lablab level and cactus treatments were found, while significance was approached (P=0.10) for the mean difference between Altex and New Zealand White breeds (39.89 and 39.82 °C). In addition, an interaction between lablab level and clipping treatment was detected (P<0.05) for body temperature. However, upon close examination of the subclass means, no rank change occurred and only small differences existed between clipping group and lablab level. For ear length as a linear covariate, an interaction was also observed between clipping treatment and ear length (P=0.10). In other words, the slope of the linear plus quadratic regression lines varied between clipping groups (non-clipped: -0.0499 linear and +0.3506 quadratic coefficients, and clipped: -0.0992 linear and -0.0561 quadratic coefficients) as clearly shown in Figure 1.

Figure 1: Relationship between ear length and body temperature in rabbits fur clipped  and controls (non-clipped)

 

The curvilinear nature of the relationships in the above figure may explain, in part, why an overall linear correlation of zero was obtained (Table 2). In control (non-clipped rabbits), body temperature tended to decline slightly from 40.17 to 39.94 °C for rabbits having ear length values between 9.5 to 10.5 cm ear length, but became increasingly elevated as ear length increased from 10.5 to 12 cm.  A plausible explanation for this relationship is that rabbits with longer ears were generally heavier and may have possessed more fur, partly due to increased feed appetite (which could increase body temperature). However, individual feed intake and fur weight measurements were not taken to substantiate this possible explanation. Of course, feed intake and growth rate are highly correlated, and final weight had a correlation of 0.70 with ear length (Table 2). The increased digestive and metabolic activities could, in part, explain the higher body temperatures.  In contrast, also from Figure 1, rabbits that were fur clipped tended to show only a slight linear decline in body temperature from 9.5 to 10.5 cm ear length, but more rapidly decreased in rabbits with ear length values beyond 10. 5 cm. For rabbits with longer ears, the effect of fur clipping may have effectively resulted in more efficient thermoregulatory function.

 

In contrast, for fur clipped rabbits, body temperature was fairly constant at 39.78 °C for rabbits having ear length values between 9.5 to 10.5 cm. But, beyond 10.5 cm, body temperatures steadily declined. Clipping of fur may have allowed the ear to become more functional in terms of thermoregulation. It is likely, too, that larger ears (as associated with greater profusion and(or) supply of blood capillaries) improved the effectiveness in cooling the blood, especially if exposed to hot, less humid conditions when good air flow or ventilation was prevalent. However, more research is warranted to further explore these factors and the nature of their inter-relationships. 

Multiple regression results for growth traits including ear length and body temperature as covariates 

As stated previously, statistical results for growth traits due to diet, breed, and clipping treatments have already been published (Ruiz-Feria and Lukefahr 1998). The effect of initial body weight as a linear covariate was not important (P=0.33), and so was excluded from the final model. However, the linear covariate of ear length was very highly significant (P<0.001), while a significant clipping treatment x ear length interaction (P<0.05) was detected for ADG. Rabbits that were not fur clipped had a slower rate of growth than rabbits that were fur clipped, although this trend was evident only beyond the threshold or critical ear length value of 10 cm (Figure 2). Linear regression coefficients of 1.326 and 2.721 for non-clipped and clipped groups were obtained. However, the inclusion of ear length in the model for analyzing ADG only accounted for 6.9% of total variation, while a total R² value of 61.8% was observed.

Figure 2: Relationship between ear length and average daily gain in rabbits fur clipped and controls (non-clipped)) 

For the data subset involving body temperature measurements, this character was added as a covariate to the main effects model (also including initial body weight as a covariate) for analyzing ADG.  However, body temperature had no effect on ADG performance; neither linear (P=0.75) nor linear and quadratic regressions were significant (P=0.78). In an additional analysis involving the main effects model with initial body weight, body temperature, and ear length as linear covariates, it was again observed that body temperature had no influence on ADG (P=0.91), whereas ear length had a strong relationship (P<0.01) as noted previously.

 

The analysis of final weight revealed no interactions between ear length and main effect treatments (P>0.05), although a strong, overall linear relationship existed (P<0.001). For every 1 cm increase in ear length, final weight was heavier by 149 g. A total R² value of 51.1% was noted. In a separate analysis, body temperature as a linear or as a linear and quadratic covariate source did not affect final weight (P>0.25). However, when both ear length and body temperature were included in the model, ear length was very highly significant (linear regression coefficient of 199 ± 42; P<0.001), but body temperature was not significant (P=0.17). A total R² value of 69.6% was observed. In agreement, in the report by Hanafi et al (1984), no significant relationships were found between body temperature and body weights in growing rabbits. The reason for the discrepancy between regression coefficients of 149 and 199 in prediction of final weight based on ear length was the former value was from analyses involving the larger data set (n=152) and the latter from the data subset (n=47). 

 

Conclusions

 

One dilemma of this study was the seemingly automatic correlation between body weight and ear length. It would also be useful to conduct genetic experiments in which rabbits are directly selected for ear length in one line and for body weight in another line, and, after several generations of selection, determine if indirect responses occur for either trait in each line. Such an experiment would reveal the extent that the two traits are genetically correlated. An animal model approach could also be employed to make the same determination.

This experiment offered insight into the means and measures by which rabbits are able to express thermoregulation. In hot climates, the effect of fur clipping promoted more rapid growth, especially in rabbits with longer ears.  In contrast, ear length was less influential in rabbits whose fur was not clipped. Although lower body temperatures resulted from fur clipping, body temperature had no linear association with growth performance, except for initial body weight. Further studies involving larger numbers of animals, fewer imposed experimental factors (e.g., breed and diet), and more detailed measurements (fur density, respiration rate, and physiological stress indicators) are needed to elucidate thermoregulatory mechanisms unique to the rabbit. New knowledge could be applied to improve rabbit production through enhanced management that provides optimal animal comfort (achieved through genetic or environmental means), ultimately bestowing greater benefits to farmers.

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