Effects of Dietary Crude Protein on Growth Performance, Nutrient Utilization, Immunity Index and Protease Activity in Weaner to 2 Month-old New Zealand Rabbits *

: An experiment was conducted to determine the effects of different dietary crude protein (CP) levels on growth performance, nutrient utilization, small intestine protease activity and immunity index of weaner to 2 month-old New Zealand rabbits. Eighty weaner rabbits were allocated in individual cages to five treatments in which they were fed diets with CP at 14%, 16%, 18%, 20% and 22%, respectively. The growth performance and nutrient digestibility of rabbits increased firstly when dietary CP increased, then decreased. The average daily gain was the highest and feed conversion rate was the lowest when dietary CP reached 20%, namely 34.9 g/d and 2.74:1, respectively. Maximum CP digestibility was 72.1% in the 18% CP group, maximum crude fiber digestibility of 28.4% occurred in the 16% CP group and was significantly different from other treatments (p<0.01), apparent digestibility of Lys and Val followed the same trend as CP digestibility, and reached their maximum when dietary CP was 18%. Apparent digestibility of Cys, Tyr, Leu and Thr also had a similar trend to CP digestibility. Nitrogen retention (RN) increased with CP level (p>0.05), and was highest for 20% CP treatment (1.5 g/d). The effect of CP level on the rate of digestible nitrogen (DN) converted RN was small. The spleen index, thymus index, chymotrypsin and trypsin activities in small intestine were highest when dietary CP was 16%, which were 1.0, 2.8, 15.7 U/g and 125.7 U/g, respectively. There was no significant difference among treatments (p>0.05). According to the above results, the appropriate dietary CP level from weaner to 2 month-old meat rabbits was 18-20%. (Asian-Aust. J. Anim. Sci. 2004. Vol 17, No. 10 : 1447-1451)


INTRODUCTION
Protein is an important component for life processes, and is a material for renewing and repairing tissue.Studies on dietary protein were popular in the in the 1970s and 1980s, but seldom dealt with meat rabbits.It is believed that the dietary crude protein requirement of growing rabbits is 16%, which comes from NRC (1977).Omole (1982) reported the appropriate crude protein level for meat rabbits was 18%-22%.Abdella (1988) indicated that body gain, protein digestibility, slaughter performance and economy benefits were the greatest when the dietary crude protein level was 16%.Wang (1991) pointed out that the average daily gain, feed conversion rate and slaughter rate were the best when the dietary crude protein level was 15.3%-17.9%.Wang (1999) reported that the body gain, feed consumption and slaughter performance were ideal when the dietary crude protein level was only 16.5%.
The data obtained by Moughan et al. (1988) allow estimation of the amino acid requirements of growing rabbits according to their respective proportions of body protein.Several authors have studied the total amino acid requirements for rabbits on a dose-response basis (de Blas, 1998).Recent studies (Taboada et al., 1994(Taboada et al., ,1996;; de Blas et al., 1996) have determined the lysine, sulphur and threonine requirements.Results for growth were consistent with those obtained by Moughan et al. (1988) based on amino acid composition of the whole body.But, information regarding the requirement of amino acids is scarce.
The objectives of the research were to examine the effects of different dietary CP levels on growth performance, nutrient utilization, small intestine protease activity and immunity index of weaner rabbits, and to determine the appropriate dietary CP level in weaner to 2 month-old meat rabbits.

Anim이s and diets
Eighty weaned New Zealand rabbits (mean body weight 1.13±0.47kg, equal numbers of male and female) were randomly allocated to five groups according to average body weight, with sixteen rabbits per group.Rabbits were individually housed in metabolism cages which can separate urine from faeces.Each cage contained a feeder to provide free access to feed and a nipple waterer to provide free access to water.
Table 1 gives the composition of diets.The diets were designed according to NRC (1977) and were pelleted.Eighty weaned rabbits were fed each of five experimental diets.The CP levels of the five experimental diets were 14%, 16%, 18%, 20% and 22%, respectively.

Experimental procedures
All rabbits were divided into five groups according to average body weight.Feed was offered in equal portions at 8:30 and 17:30 daily.Total experiment consisted of a 7-day adjustment period followed by an 18 day experimental period (including a 7 day collection of faeces, urine and residual feed daily).Faeces, urine and residual feed were collected from rabbits in the metabolism cages.After the feeding trial, 10 ml blood samples were collected from each rabbit and 6 rabbits from each experimental group were slaughtered.The serum was separated by centifugation for chemical analysis.

Chemical analysis and statistics
All experimental rabbits were weighed from weaning to 2 months old, and the average daily gain was calculated.The average daily feed consumption was recorded and feed/gain ratio was calculated.Feed and faeces samples were analyzed for dry matter (DM), crude fiber (CF), nitrogen (N), crude fat and energy according to the procedure of Yang (1993).Urine samples were analyzed for nitrogen (N) according to the procedure of Yang (1993).
Spleen and thymus weight was measured after slaughter and spleen index and thymus index were calculated according to the following formulae: spleen index=spleen weight/body weight, thumus index=thymus weight/body weight.
The small intestine chyme was collected after slaughter, chymotrypsin and trypsin activities were analyzed according to the method of Xu (2002).
Data collected from the experiments were subjected to analysis of variance using the General Linear Model Procedure of SAS (1985).Treatment means comparison was done using Duncan's Multiple Range Test.

Effects of different dietary CP on growth performance
Effect of different dietary CP on growth performance of experimental rabbits is shown in Many researchers have studied the effect of dietary protein levels on production performance of growing rabbits.Although protein levels were different, all researchers thought that low dietary protein levels can reduce production performance and high dietary protein levels can increase economic profit, gain, slaughter performance and nutrient digestibility.In this study, ADG rose from 14% CP group to 20% CP group, then fell for the 22% CP group F/G ratio descended then ascended when CP increased.Tang (1987) reported that ADG and F/G ratio were 19.4 g/d and 3.8 when dietary CP was 19.5%, which were higher and lower than those of 16.7% and 12% CP groups.Wang (1999) reported that ADG was 29 g/d when dietary CP was 16.5%, which was higher than those of 13.5%, 15%, 18%, 19.5% and 21% CP groups.F/G ratio was 3.5 when dietary CP was 18%, which was lower than those of 13.5%, 15%, 16.5%, 19.5% and 21%CP groups.Xu (1982) reported that many traits were the best for 6 to 14 week-old rabbits when dietary protein was 17%, ADG and microorganism concentration in intestine decreased in 22% protein group.Similar finding were reported by Wang (1991).

Effects of different dietary CP on nitrogen balance (Table 5)
Effects of dietary CP on N balance of experimental rabbits are shown in Table 6.N intake, faces N, urine N, digestible N (DN) and retained N (RN) of different treatments increased with increasing dietary CP levels.RN/N intake and the efficiency of DN converted into RN were the highest when dietary CP was 16%, and were 44.8% and 66.8%, and had significantly different among different treatments.The regression formula of RN (y, g/d) and dietary CP levels (x, %) was y=-0.76+0.19x-0.004x2,R2=0.8312.In this study, Urinary N losses increased from 14% CP group to 20% CP group, and fell for the 22% CP group.Urinary N losses of 18% CP group, 20% CP group and 22% CP group were higher than those of 14% and 16% CP group (p<0.05).There were no significant differences among 18%, 20% and 22% CP group and between 14% and 16% CP group (p>0.05).The regression formula of urinary N losses (y, g/d) and dietary CP levels (x, %) was y=-0.01 x2+0.45x-3.59,R2=0.9027.
This research illustrated that RN increased when dietary CP increased, that is to say that the synthesis of body protein and product protein may increase with increasing dietary CP, consistent with results of Singh et al. (1988).

Effects of different dietary CP on immunity indexes
Effects of dietary CP on immunity indexes of experimental rabbits are shown in Table 6.Spleen weight, thymus weight, spleen index and thymus index were not significantly different among different groups, but spleen index and thymus index were the highest when dietary CP was 16%.
Protein and amino acids are the basic structural units of the body's immune system, and have a close relationship with the growth and development organs of the immune system.Immunity index reflects immunity function of animals in certain context.For a healthy animal, immunity function is better when immunity index is bigger.This research illustrated that spleen and thymus index of 16% CP group were the greatest and higher than that of 14% group, which indicated immunity function of weaner to 2 monthold rabbits will drop if dietary CP level is lower than 16% and may affect rabbits' immune responses.

Effects of different dietary CP on small intestine protease
Effects of different dietary CP on the activity of small intestine protease are shown in Table 7.The activity of small intestine chymotrypsin and trypsin were the highest when dietary CP was 16%, but there were no significant differences among different treatments.
Chymotrypsin and trypsin are two important enzymes in digesting and breaking down protein.This research indicated that their activities do not vary with the change of dietary protein and CP digestibility.

CONCLUSIONS
Growth performance of weaner to 2 month-old meat rabbits rose then fell when dietary CP increased from 14% to 22%.ADG was the highest and feed/gain ratio was the lowest when dietary CP was 20%, so growth performance of weaner to 2-old month meat rabbits was the best when dietary CP was 18-20%.
The digestion of CF was highest with 16% CP group.The digestibility of CP, Lys and Val were highest with 18% CP group.Apparent digestibility of Cys, Tyr, Leu, Thr and RN was greatest in the 20% CP group.
Spleen index, thymus index, chymotrypsin activity and trypsin activity of weaner to 2 month meat rabbits were highest when dietary CP was 16%.

Table 1 .
The composition of experimental diets

Table 2 .
Effects of dietary CP levels on growth performance of experimental rabbits (mean土SD)

Table 3 .
Effects of CP levels on nutrient apparent digestion of experimental rabbits (%)(mean±SD)

Table 4 .
Effects of different CP levels on amino acid apparent digestibility (%) (mean±SD) were higher than that of the 14% CP group.Among them, the ADG of the 20% CP group was the highest but there was no significant difference in ADG (p>0.05).The feed/gain (F/G) ratio of 14% CP group, 16% CP group, 18% CP group and 22% CP group were higher than that of 20% CP group and there was a significant difference between 14% CP group and other groups (p<0.05).The regression formula of ADG (y, g/d) and dietary CP levels (x, %) was y=-13.24+5.37x-0.15x2,R2=0.8997. group

Table 5 .
Effect of different protein levels on nitrogen balance (mean±SD)

Table 6 .
Effect of different protein levels on immunity index of experimental rabbits (mean±SD)

Table 7 .
Effect of different protein level on enzyme activities in small intestine of rabbits (U/g) (mean±SD)