ANIMAL FEED SCIENCE AND TECHNOLOGY

ELSEVIER Animal Feed Science Technology 70 (1998) 15 I- 160

Performance response of lactating and growing rabbits to dietary threonine content

J.C. de Blas a, * , E. Taboada b, N. Nicodemus a, R. Campos ‘, J. Piquer a, J. Mkndez b

” Departumento de Pmduccicin Animal, ETSI Agrdnomos, Uniwrsidad Polite’cnim de Madrid.

28040 Madrid, Spain

’ COREN SCL, C/ Juan XXIII 33, 32003 Orense. Spain

Accepted 17 February 1997

Abstract

This study investigated the threonine requirements of rabbits. Five diets, containing from 5.4 to 7.2 g/kg crude threonine, were formulated by supplementing a basal diet with L-threonine. Feeding trials were carried out using 370 rabbit does and 1150 weanling rabbits, grown in individual (n = 190) or in commercial (n = 960) cages, and slaughtered at 2.035 * 0.025 (SE) kg of body weight. Milk production was measured in 80 lactations. Carcass traits were determined in 115 rabbits. The apparent threonine digestibility (%) was 63.8 + 1.3 in the basal diet and 93.0 + 6.0 for L-threonine, as estimated by the difference method. A minimal dietary concentra- tion of 6.4 crude and 4.4 g/kg digestible threonine should be included to maximize the performance of lactating does. For growing rabbits, these values were 6.0 crude and 4.0 g/kg digestible. An excess of dietary threonine tended to impair intake and performance and to reduce the weight of perirenal fat. The results of this study also demonstrated the need for using digestible rather than total values to express the threonine requirements of rabbits. 0 1998 Elsevier Science B.V.

Keywords: Rabbits: Threonine requirements: Threonine digestibility: Reproduction; Growth

1. Introduction

After lysine and methionine, threonine is the next most limiting amino acid in non-ruminant animals. At present, crystalline threonine is available at a relatively low cost. As a consequence, there is an increasing interest in the feed industry for a better

* Corresponding author. Tel.: + 34-1-5492357; fax: + 34-1-5499763; e-mail: cdeblas@pan.etsia.upm.es

0377.8401/98/$19.00 0 1998 Elsevier Science B.V. All rights reserved. PII SO377-8401(97)00063-l

152 J.C. de Bias et al./Animul Feed Science Technology 70 (1998) 151-160

knowledge of threonine requirements of rabbits. Practical recommendations have been established at 5.5 and 7.0 g/kg in diets for growing rabbits and lactating does, containing, respectively, 10.5 and 10.9 MJ DE/kg (Lebas, 1987).

Previous studies conducted with pigs (Wang and Fuller, 1989; Fuller et al., 1989) have shown a relatively high threonine requirement for maintenance. Other work in pigs (Lewis and Speer, 1975; Cole et al., 1983; Edmond and Baker, 1987; Saldana et al., 1994) indicated that an excess of dietary threonine resulted in a decrease in feed intake and performance. Furthermore, threonine digestibility can depend on the source of dietary protein, as in other non-ruminant species. For this reason, the recommendations should be expressed on the basis of digestible, instead of crude, threonine in the diet.

The aim of this work was (i) to determine the apparent digestibility of crystalline threonine as compared with that of threonine in a basal diet, and (ii) to measure the performance response of highly productive does and growing rabbits to an increase in dietary threonine, by using five isoenergetic diets (10.1 MJ DE/kg) with crude threonine levels ranging from 5.4 to 7.2 g/kg.

2. Materials and methods

2.1. Diets

A basal diet (diet A) was formulated to meet or exceed all the essential nutrient requirements for lactating does, except for threonine, whose content was limited to 5.4 g/kg (as fed basis). Four additional diets (B, C, D and E) were formulated by supplementating the basal diet with L-threonine, so that the threonine content was 5.8, 6.3, 6.8 and 7.2 g/kg (as fed basis), respectively. The ingredient and chemical composition of the basal diet are shown in Table 1.

2.2. Digestibility trial

A group of 18 New Zealand X Californian growing rabbits from 45 to 60 days of age was allotted at random to diets A and E (nine rabbits per diet) to determine the apparent digestibility of energy and threonine in the basal diet, and the digestibility of threonine in diet E. Following a lo-day period of adaptation to each diet, total collection of faeces was made on four consecutive days.

2.3. Rabbit does feeding trial

Three hundred and seventy Hy Plus doe rabbits (74 per diet) were randomized in blocks by previous litter parity. A 60&y adaptation period was allowed before recording rabbit performance. Data from 130 rabbit does were discarded because of lack of fertility, disease or mortality. Therefore data from only 240 rabbit does were analyzed. Remating interval after parturition was fixed at 7 days and weaning age at 30 days. Does that failed to conceive or lost their litters were immediately given the opportunity to remate. The experimental diets were offered ad libitum in late pregnancy

J.C. de Bias et al./Animal Feed Science Technology 70 (1998) 151-160 153

Table 1 Ingredient and chemical composition of the basal diet

Item

Ingredient, g/kg Wheat bran Lucerne meal Wheat grain Sunflower meal Wheat straw Molasses Pork lard Calcium carbonate Dicalcium phosphate Sodium chloride Choline chloride L-Lysine oL-Methionine Robenidine premix” Vitamin/mineral mixh

Chemical analysis, g/kg DM Ash Crude protein Threonine Lysine Methionine Cystine Starch Ether extract Crude fibre Acid detergent fibre Neutral detergent fibre Acid detergent lignin Calcium Phosphorus Gross energy, MJ/kg DM

330 197 149 130 100

39 20 13.5 8.2 5.0 0.3 2.6 1.4 1.0 1.7

94 164

5.9 9.0 4.4 2.9

209 62

159 183 313

38 11 7.0

18.4

a6.6% of active ingredient. Supplied by Cyanamid IMrica. ‘Provided by Colbom Dawes. Mineral and vitamin composition (g/kg): Mn, 13.4; Zn, 40; I, 0.7; Fe, 24; Cu. 4; Co, 0.35; riboflavin, 2.1; calcium pantothenate, 7.3; nicotinic acid, 18.7; menadione, 0.65; wtocopherol, 17; thiamin, 0.67; pyridoxine, 0.46; biotin, 0.04; folic acid, 0.1; cyanocobalamin, 7 mg/kg; vitamin A, 6700.000 IU/kg; vitamin D,, 940,000 IU/kg.

(from day 28 on> and throughout lactation; all the other animals received a restricted amount (140 to 150 g/d) of feed. Productive traits were recorded and accumulated over a S-month production cycle. Feed consumption and weight of does were determined at the beginning and at the end of the experimental period.

Animals were housed in flat-deck cages measuring 600 X 500 X 330 mm high. A cycle of 16 h light and 8 h dark was used throughout the experiment. Building heating systems and forced ventilation allowed the temperature to be maintained between 18 and 23 “C.

154 J.C. de Bias et al. /Animal Feed Science Technology 70 (1998) 151-160

2.4. Lactation trial

Eighty rabbit does from the previous group (16 per diet) were used to measure milk production during one lactation. Does were separated from their litters after parturition. Milk production was estimated daily from weight loss of does during suckling. Feed intake of does and litters was determined separately every 10 days. Young rabbits growth from 21 days of age until weaning was measured.

2.5. Growth trial

One hundred and forty one litters (1150 rabbits) of both sexes from the lactation trial were assigned to the treatments. After weaning, rabbits had ad libitum access to the same solid feed they received during the lactation period, until they reached approxi- mately 2 kg of body weight. Feed intake and length of the experimental period were recorded per cage. The rabbits were divided in two groups of 190 and 960 animals caged individually or in groups of eight rabbits each, respectively.

One hundred and five rabbits (21 per diet) were chosen at random from the group grown in commercial cages. Once they reached the pre-established weight, they were fasted during two hours and transported to a commercial slaughterhouse. Carcass dressing percentage was measured for all the animals. The total weight of primal joints (fore and hind legs, loin and kidneys) was also determined. The perirenal fat was dissected in 40 carcasses (eight per diet).

2.4. Analytical methods

Chemical analysis of diets was made by the method of Robertson and Van Soest (1981) for acid detergent fibre and acid detergent lignin, Van Soest et al. (1991) for neutral detergent fibre, Longstaff and McNab (1986) for starch, and the Association of Official Analytical Chemists (1984) for dry matter, ash, crude protein, ether extract, and crude fibre. Gross energy was determined by adiabatic bomb calorimetry. Amino acid contents were determined by using high pressure liquid chromatography (Cohen et al., 1989).

2.7. Statistical analysis

Data were analyzed by using the GLM procedure of SAS (1985). Treatment sums of squares were partitioned into linear, quadratic and cubic effects. Mean comparisons were made using orthogonal contrasts. Lactation and rabbit does feeding trials were analyzed as a randomized complete block design with parity number of does as a block effect, and dietary threonine content as the main source of variation. Initial weight of does was used as a linear covariate for all the traits studied. Day of effective mating was used as a linear covariate in the lactation trial. Interactions between dietary threonine content and day or week of lactation were studied using a repeated measures analysis (SAS, 1985). Data from growth trial were analyzed as a completely randomized design with dietary threonine content as the main source of variation. Weaning and slaughter weight of

J.C. de Bias et al./Animal Feed Science Technology 70 (19981 151-160 155

rabbits were used as linear covariates in the growth and carcass trials, respectively. Data are presented as least square means.

3. Results

3. I. Digestibility trial

Digestible energy content (MJ/kg feed) of the basal diet was (mean * SE) 10.12 k 0.20. Apparent threonine digestibility (%) in the basal diet and in diet E was 63.8 k 1.3 and 71.4 rt 1.3, respectively. From these values, digestibility of the supplementary t_-threonine was calculated by difference, obtaining an estimate of 93.0 _t 6.0.

3.2. Lactation trial

Dietary threonine content did not affect feed intake of does, total or maximal milk production (Table 2). However, a significant interaction was found between dietary threonine content and period of lactation on feed intake and milk production (P = 0.06 and 0.01, respectively), which were respectively 6.3 and 9.5% lower in diet A than for the average of the other four diets in the period of maximal milk production (1 l-20 days). Maximal milk production was also reached 1.8 days earlier (P = 0.006) in animals receiving diet A than for the other diets. Feed intake and rate of growth of young rabbits in the period 21-30 days were cubically related (P = 0.03) to dietary threonine content, having a maximum for diet B (containing 5.8 g/kg of crude threonine).

Table 2 Effect of dietary threonine level on milk production and feed intake of rabbit does and growth and feed intake of young rabbits

Dietary crude threonine content (g/kg) Diets

5.4

SE“

5.8 6.3 6.8 7.2

Milk production lactation (kg) per 5.40 5.65 5.68 5.83 5.78 0.18 Milk production I- 10 days (kg) 1.72 1 .I2 I .65 1.78 1.69 0.07 Milk production 1 l-20 days (kg) 2.38 2.59 2.55 2.63 2.65 0.08 Milk production 21-30 days (kg) 1.30 1.33 1.41 1.41 1.43 0.06 Maximal daily milk production (g) 211 290 287 300 300 9 Day of lactation of maximal milk production 15.9 11.1 17.6 17.4 18.2 0.5

Daily.feed intake, g Rabbit does, whole lactation 340 356 346 343 360 8 Rabbit does, l-10 days 318 336 309 309 330 12 Rabbit does, 1 l-20 days 363 387 381 312 402 9 Rabbit does, 21-30 days 339 345 348 345 348 9 Young rabbits, 21-30 daysb 167 206 167 194 170 12 Growth rate of rabbits (21-30 days), g/db young 21.4 23.9 22.1 19.7 21.7 1.0

“SE = Standard error of means (n = 16). hCubic response to dietary threonine content (P = 0.03).

156 J.C. de Bias et al./Animal Feed Science Technology 70 (1998) 151-160

3.3. Rabbit does feeding trial

The effect of diet on productive traits is shown in Table 3. Dietary threonine content did not affect average weight, live weight gain or replacement rate of does. Total feed intake by does and pups @I, g/d) showed a cubic trend (P = 0.05) with dietary threonine content (thr, g/kg), reaching a maximum for a level of 6.3 g/kg of crude threonine. The regression equation obtained was (n = 240):

PI = 23.0( f621) - 17.7( f229)thr + 32.0( k29)thr2- 3.24( f 1.47)thr3

Parturition interval was quadratically affected (P = 0.05) by dietary tbreonine con- tent, showing a minimal value for a level of 6.3 g/kg of crude threonine. Treatments had no effect on number of rabbits born. However, dietary threonine content affected quadratically mortality of young rabbits throughout the lactation period and litter size at weaning. These traits reached again optimal values for 6.3 g/kg of dietary crude threonine content. The combined effects of dietary threonine content on parturition interval and litter size at weaning implied a highly (P < 0.001) quadratic effect on numerical productivity per doe and year, which was 16 and 21% higher at the optimal dietary crude threonine content (6.4 g/kg) than at the extreme diets A and E, respectively.

Dietary threonine content affected quadratically (P = 0.01) litter weight at 21 days of age (LW21, kg), litter weight at weaning (LWW, kg) and overall feed efficiency (OPE,

Table 3 Effect of dietary threonine content on reproductive traits of rabbit does

Dietary crude tbreonine content (o/o) Diets

5.4 5.8 6.3

SE”

6.8 1.2

n

Average weight of does, kg Feed intake, g/db Parturition interval, d’ No. born total per litter No. born alive per litter No. weaned per litterd No. weaned per cage and yea? No. dead 1-2 1 days of age per litter f No. dead 22-30 days of age per litterC Litter weight at 21 days of age, kgg Litter weight at weaning, kgg Feed efficiency, No. weaned/kg feede Feed efficiency, kg weanedjkgfeedg 0.299 0.305 0.315 0.326 0.292 0.010

50 48 49 46 41 4.17 4.14 4.15 4.19 4.14 0.03

342 356 381 369 356 9 49.0 49.1 48.3 46.6 50.8 1.6 11.4 10.7 10.7 10.8 11.3 0.3 10.8 10.0 10.2 10.2 10.0 0.3 7.62 8.07 8.69 8.55 7.19 0.26

60.3 62.1 69.5 69.9 57.6 2.1 2.83 1.54 1.31 1.41 1.87 0.29 0.34 0.39 0.18 0.21 0.38 0.07 2.68 2.80 2.93 2.89 2.66 0.08 4.83 5.17 5.49 5.41 5.03 0.16 0.471 0.479 0.499 0.513 0.443 0.019

aSE = Standard error of means. bCubic response to dietary tbreonine content (P = 0.05). ‘Quadratic response to dietary threonine content (P = 0.05). dQuadratic response to dietary threonine content (P = 0.004). ‘Quadratic response to dietary tbreonine content (P < 0.001). ‘Cubic response to dietary tbreonine content (P = 0.009). gQuadratic response to dietary threonine content (P = 0.01).

J.C. de Blas et al./Animal Feed Science Technology 70 (19981 151-160 1s:

kg weaning weight/kg of total feed intake by does and young rabbits, which reached optimal values for a dietary crude threonine content of 6.4, 6.5 and 6.3 g/kg. respectively. The regression equations obtained were (n = 240):

LW21 = -8.O( f4.6) + 3.4( f 1 S)thr - 0.27( +O.l2)thr*

LWW = - 17( k9.1) + 6.9( t2.9)thr - 0.53( *0.23)thr’

OFE = - 1 .l( kO.6) + 0.46( f0.19)thr - 0.036( +0.015)thr2

3.4. Growth trial

The effect of diet on average daily gain and carcass traits is presented in Table 4. Rate of growth was significantly higher (P < 0.001) when rabbits were grown in individual vs. commercial cages (42.2 vs. 36.2 g/d, SE 0.41), but no interaction was found between dietary threonine content and cage density.

Average daily gain was cubically affected (P = 0.05) by dietary threonine content in animals individually caged, both in the two weeks after weaning and in the whole

Table 4 Effect of dietary threonine content on growth and carcass traits of rabbits

Dietary crude threonine content (So) Diets SE”

5.4 5.X 6.3 6.8 7.2

Individual fattening trial

Two weeks after weaning period Feed intake, g/d Average daily gain, gh

X6.5 41.9

Whole fattening period Feed intake, g Average daily gain, gb Mortality. D/c

117 41.3

0

CollectiLle fattening period Feed intake, g/dC Average daily gain, g Mortality, %

Carcass traits Perirenal fat, %’ Carcass dressing percentage, VI Proportion of primal joints in dressed carcass, %

109 36. I

6.2

16.9 57.1 x1.2

90.9 X6.4 X6.4 X5.2 1.5 44. I 43.7 43.5 42.5 0.7

120 117 117 116 I .3 42.1 43.2 41.X 41.9 0.6

5.3 0 0 0 2.3

113 113 107 108 2.0 31.4 36.2 35.X 35.5 0.5

x.3 7.3 7.x 6.X 0.02

16.X 19.7 12.1 14.6 1.6 57.1 57.4 57.9 51.4 0.4 X1.5 Xl.6 80.9 Xl.2 0.3

“SE = Standard error of means (n = 38 individual growth traits, n = 24 for collective growth traits, except n = 192 for mortality; n = 21 for carcass traits, except n = 8 for perirenal fat. bCubic response to dietary threonine content (P = 0.05). ‘Cubic response to dietary threonine content (P = 0.03). ‘Cubic response to dietary threonine content (P = 0.006).

158 J.C. de Bias et al./Animul Feed Science Technology 70 (1998) 151-160

fattening periods. The highest rate of growth was obtained for around 6.0 g/kg of dietary crude threonine content. Both lower and higher levels impaired this trait by about 3-4%. The same trend was observed in the collective fattening trial, although in this case the differences did not reach statistical significance.

The dietary threonine content had little influence on intake, mortality and feed efficiency, although average feed intake in the collective fattening period was cubically affected (P = 0.03) by diet, having a maximum for a 6.0 g/kg of dietary crude threonine content.

Perirenal fat was cubically affected (P = 0.006) by dietary threonine content, show- ing a maximum for diet C (6.3 g/kg of crude threonine content), and decreasing sharply at higher threonine levels. Neither the carcass dressing percentage, nor the proportion of primal joints in the dressed carcass, were affected by dietary threonine content.

4. Discussion

This experiment indicates that a minimal dietary concentration of 5.8 or 3.8 g/kg of crude or digestible threonine, respectively, is required to achieve a maximal feed intake and milk production in the period from 11 to 20 days of lactation. Furthermore, dietary threonine content affected quadratically several productive traits of rabbit does, as parturition interval, young rabbit mortality before weaning, numerical productivity and feed efficiency. Optimal values for these traits were obtained for a dietary content of 6.4 or 4.4 g/kg of crude and digestible threonine, respectively. Both lower and higher levels elicited an impairment of rabbit does performance, around 20 and 10% for numerical productivity and feed efficiency, respectively, for the extreme diets studied. A similar quadratic effect of the dietary threonine content on piglet growth has been reported by Lewis and Speer (1975), when determining the threonine requirements of the lactating sow.

Results from the growth trial showed that a 6.0 or 4.0 g/kg of crude and digestible threonine is optimal to obtain a maximal daily gain during the fattening period. A growth depression with threonine concentrations greater than the optimum has also been observed in pigs (Cole et al., 1983; Edmond and Baker, 1987; Saldana et al., 1994). Moreover, the 4.1 g/kg digestible threonine requirement for finisher pigs reported by Saldana et al. (1994) is close to the 4.0 g/kg obtained in this study with fattening rabbits.

Dietary threonine content did not influence feed efficiency in the fattening period. This lack of effect might be explained by a parallel effect on body fat retention, estimated in this study by the weight of the perirenal fat. A similar effect has been observed by Henry (1995) in growing pigs.

The optimal dietary crude threonine content obtained in this study were higher for growing rabbits, but lower for rabbit does, than feeding standards proposed by Lebas (1987). In the case of rabbit does, these recommendations (7.0 g/kg) are close to the content of diet E, which led to an impairment of performance in this work. A part of the discrepancies might be related to the differences in dietary energy concentration (10.9 in feeding standards vs. 10.1 MJ DE/kg in this study). Another part might be explained by

J.C. de Bias et al./Animal Feed Science Technology 70 (19981 151-160 159

differences in dietary threonine digestibility. Diet E contained 25% of its threonine as highly digestible L-threonine. If most of the dietary threonine is supplied by less digestible sources, crude threonine requirements would be higher than those obtained in this study. The differences observed between threonine digestibility for the whole basal diet and the supplemented L-threonine (71.4 vs. 93.0) emphasizes the need of using digestible rather than crude units to express threonine requirements for rabbits, which agrees with previous, similar observations for lysine (Taboada et al., 1994) and sulphur amino acids (Taboada et al., 1996).

Optimal digestible threonine in diets for growing rabbits obtained in this study (4.0 g/kg) is 66.7% with respect to the digestible lysine requirements (6.0 g/kg) obtained previously (Taboada et al., 1994). This value is similar to the 64% threonine: lysine ratio observed by Moughan et al. (1988) in the whole body of 53 days old New Zealand rabbits. This result might indicate that maintenance requirements for threonine are not so high in rabbits as in pigs, as in this case estimations based on ideal protein calculations should be lower than optimal requirements.

Acknowledgements

This research was supported by CDTI-EUREKA Program EU-619.

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