H. ENTING1
J. POS1
R.E. WEURDING2
A. VELDMAN1
1Schothorst Feed Research,
Lelystad, The Netherlands
2Animal Science Group, Wageningen Institute of Animal Sciences,
Wageningen Institute of Animal Science
The Netherlands
In a series of experiments, it was shown that slowly digestible starch (SDS) has a positive effect on broiler chicken performance compared to rapidly digestible starch (RDS). Based on these results, it was hypothesized that a gradual starch digestion may have an amino acid sparing effect and therefore enhance growth efficiency of broiler chickens. Two growth experiments were performed with broiler chickens in order to investigate the effect of SDS on broiler performance and the interaction between starch digestion rate and amino acid level. In the first experiment, differences in starch digestion rate were reached by processing of the starch sources used, while differences in amino acid level were obtained by the addition of casein and glutamine. In the second experiment the birds were fed either a pea/maize based diet (SDS) or a tapioca/maize based diet (RDS). Both types of diet were formulated with five levels of digestible lysine, varying from 8.5 to 11.0 g/kg. The minimal levels of other amino acids varied accordingly. In both experiments the feed conversion ratio (FCR) was lower for broilers on diets containing a high amount of SDS compared to broilers on diets with RDS. Adding extra amino acids decreased FCR for birds on diets with RDS, but not for birds on diets with SDS. In both experiments, this resulted in a significant interaction between starch digestibility rate and digestible amino acid content. It was concluded that SDS seems to have an amino acid sparing effect.
Introduction
Starch is an important energy source for broiler chickens. It supplies more than 50 % of the metabolizable energy in practical broiler diets. Starch digestibility is often considered to be 100 %. However, several authors have reported incomplete starch digestion in broiler chickens for cereal and legume grains (Guillame, 1978; Hesselman and Åman, 1986; Rogel et al., 1987; Yutste et al., 1991; Weurding et al., 2001). Moreover, results from digestibility studies by Yutste et al. (1991) and Weurding et al. (2001) showed considerable differences in site and rate of starch digestion between feedstuffs. Therefore, it might be important to take starch digestion characteristics into account when optimizing broiler chicken feeds.
In a preliminary experiment, it was shown that performance of broiler chickens was better on a diet with slowly digestible starch (SDS) than on a diet with rapidly digestible starch (RDS; Weurding et al., 2003). Weurding (2002) demonstrated that the improved performance with SDS was independent of starch source and technological treatment of these sources.
In order to explain the positive effect of SDS on bird performance, it was suggested that starch digestion rate might affect metabolic responses of insulin or synchronisation of energy and protein availability (Weurding et al., 2003). SDS could lead to a more continuous supply of glucose, which can change insulin response (Björck et al., 2000). Insulin plays a key role in protein deposition during growth (Fox, 1996). Moreover, a more continuous supply of glucose to the posterior part of the small intestine with SDS could prevent the use of amino acids as an energy source for the gut wall. In order to test a possible effect of starch digestion rate on amino acid requirements, two experiments were carried out, which are described in this paper.
Materials and methods
a) Animals and Housing
Two experiments were performed to investigate the interaction between starch digestion rate and amino acid content in relation to broiler performance.
In experiment 1, 4,080 sexed, newborn Cobb 500 male and female broiler chicks were obtained from Cobroed, Lievelde, The Netherlands, and were divided over 24 floor pens. Each pen contained 85 male and 85 female chicks. The pens were divided in six blocks of four pens. Four dietary treatments were randomly assigned to a pen in each block.
In experiment 2, 6,800 sexed one-day-old male and female broiler chickens of the Cobb 500 strain were used. The chicks were housed in forty floor pens with 85 male and 85 female chicks per pen. The pens were divided in four blocks of ten pens. From d 0-9, chicks were fed a starter diet containing peas, tapioca and maize. From d 9-18, chicks were fed one of ten experimental diets, which were randomly assigned to a pen in each block. Average body weight at the start of the experimental period (d 9) was 230 g.
In both experiments, 23 h light and 1 h dark intervals were used and chicks had unrestricted access to feed and water. In the first experiment, the diets were supplied as mash to maximise the contrasts in starch digestion rate (the processed and unprocessed starch rich feedstuffs were both finely milled) while in the second experiment the diets were provided as pellets. The experimental protocols of both experiments were in agreement with the standards for animal experiments and were approved by the Ethical Committee of Schothorst Feed Research.
b) Diets
In both experiments the diets were formulated according the nutrient standards used in Dutch practice.
Experiment 1: For the grower phase (day 14-30) a diet was formulated with peas and maize as starch sources (PM-0, SDS sources). In order to increase starch digestion rate, expanded and pelleted peas and maize (PM-EP) were used. In order to test the interaction between starch digestion rate and amino acid content, in each diet (PM-0 and PM-EP) protein levels were raised by adding 1% casein and 0.5% synthetic glutamine. These protein additions resulted in an increase of 0.5 g digestible lysine compared to the other diets.
Experiment 2: Two diets containing peas and maize (PM, SDS sources) and two diets containing tapioca and maize (TM, RDS sources) were formulated with digestible lysine contents of 8.50 and 11.00 g/kg respectively. In order to get diets with intermediate digestible lysine contents, diets with the same starch sources were mixed to give digestible lysine contents of 9.13, 9.75 and 10.38 g/kg. During formulation of the diets, the minimum ratio of other digestible amino acids to digestible lysine was equalised in all diets.
c) Statistical Analysis
For experiment 1, data for feed intake, weight gain and FCR were analysed by ANOVA. The effect of processing (PROC, influencing starch digestion rate), the addition of extra amino acids (+AA) and the interaction between these factors on performance were tested using a model including overall mean, block, processing, extra amino acids, interaction terms and a residual error term.
For experiment 2, data for feed intake, weight gain and FCR were analysed by ANOVA. The effect of starch source (SS), digestible lysine content (LYS) and the interaction between these factors on performance were tested using a model including overall mean, block, starch source, digestible lysine content, interaction terms and a residual error term.
Results and discussion
a) Experiment 1
Broiler performance from day 14-30 of the first experiment is presented in Table 1. The results show that processing of PM diets resulted in a (not significant) increase of the FCR. These results confirm earlier findings of Weurding et al. (2003), in which diets with a high content of SDS resulted in a lower FCR compared to diets with a low content of SDS.
Furthermore, the results show a significant (P = 0.02) interaction between processing and amino acids for FCR during this period. The addition of extra amino acids did not affect FCR of the birds receiving the PM-0 diet but reduced the FCR of the birds receiving the PM-EP diet from 1.72 to 1.64.
b) Experiment 2
The results from day 9-18 of the second experiment are presented in Table 2. The results show that the weight gain was higher and the FCR was lower for birds on PM diets than for those on TM diets (P < 0.01). These results confirm the effect of SDS on broiler performance, which was also found in the first experiment. Adding digestible lysine to the diet reduced feed intake linearly (P < 0.01) and quadratically (P < 0.10) from 662 to 649 g and increased weight gain linearly (P < 0.01) from 458 to 480 g. An interaction between starch source and digestible lysine content on FCR was observed (P < 0.05). FCR was lower for birds on PM diets than those on TM diets and the difference was most pronounced with lower digestible lysine contents in the diets.
The positive effect of SDS on broiler performance was confirmed in both experiments. From the results in Tables 1 and 2 it is clear that birds given diets containing a high content of SDS grew faster and more efficiently than those on diets with a lower content of SDS. Both experiments also showed a similar interaction between starch digestion rate and digestible amino acid content on FCR of broiler chickens. The second experiment showed that the effect of SDS is more pronounced for birds on diets with low amino acid levels, indicating that amino acid supply and glucose supply are unbalanced in diets with RDS. This could mean that a diet with synchronized starch and protein digestion (e.g. a more gradual starch digestion) results in better performance.
The explanation may be found in hormonal responses to glucose absorption which affect protein deposition. It may be hypothesized that gradual starch digestion results in a lower but longer lasting insulin peak than rapid starch digestion. This hypothesis is supported by data from Björck et al. (2000) who reported a correlation between the glycemic index and the insulinemic index. Elevated insulin levels are required for amino acid transport and uptake by body cells (Fox, 1996). On the other hand, asynchrony of starch and protein digestion may increase the oxidation of amino acids to meet the energy demand of gut tissues (Vaugelade et al., 1994; Flemming et al., 1997). When glucose is metabolized in the gut tissues of the posterior part of the small intestine, as may be the case when diets with SDS are fed, amino acids may be spared and can thus be used for muscle growth.
Based on the results of these experiments it can be concluded that SDS results in a lower amino acid requirement compared to RDS. Therefore, SDS seems to have a protein sparing effect. This effect cannot fully explain the total improvement in performance when feeding SDS. It is likely that energy utilization is also improved because of the prolonged elevated plasma glucose levels.
References
Björck, I., Liljeberg, H. and Östman, E. (2000). British Journal of Nutrition, 83, Suppl. 1, S149-S155.
Classen, H.L. (1996). Animal Feed Science and Technology, 62: 21-27.
Fox, S. I.. (1996). Human Physiology, pp 588 (Wm. C. Brown Publishers).
Guillaume, P.J. (1978). Archiv für Geflügelkunde, 42: 179-182.
Hesselman, K. and Åman, P. (1986). Animal Feed Science and Technology, 15: 83-93.
Oates, C.G. (1997). Trends in Food Science and Technology, 8: 375-382.
Fleming, S. E., Zambell, K. L. and Fitch, M. D. (1997). American Journal of Physiology, 273: G968-978.
Rogel, A.M., Annison, E.F., Bryden, W.L. and Balnave, D. (1987). Australian Journal of Agricultural Research, 38: 639-649.
Vaugelade, P., Posho, L., Darcy-Vrillon, B., Bernard, F., Morel, M. T. and Duee, P. H. (1994). Proceedings of the Society for Experimental Biology and Medicine, 207: 309-316.
Weurding, R.E., Veldman, A., Veen, W.A.G., Van der Aar, P.J. and Verstegen, M.W.A. (2001). Journal of Nutrition, 131: 2329-2335.
Weurding, R.E. (2002). PhD Thesis, Wageningen Institute of Animal Sciences, Wageningen University, The Netherlands.
Weurding, R.E., Enting, H., and Verstegen, M.W.A. (2003). Animal Feed Science and Technology, 110: 175-184.
Yutste, P., Longstaff, M.A., McNab, J.M. and McCorquodale, C. (1991). Animal Feed Science and Technology, 35: 289-300.
From Proceedings of the “17th Australian Poultry Science Symposium”, New South Wales, Australia.
