Rapidly digestible protein influences starch and protein digestive dynamics, digestibility and concentrations of amino acids in portal circulation in broiler chickens

The digestive dynamics of starch and protein are considered pivotal to broiler performance (Liu and Selle, 2015). Moreover, it was proposed that the rates of starch digestion and glucose absorption exceed the rates of protein digestion and amino acid absorption and this asynchrony in digestive dynamics compromises the performance of broiler chickens. If so, it follows that the provision of rapidly-digestible protein should enhance broiler performance in a reciprocal manner to the slowly digestible starch concept. A rapid protein summit diet was formulated principally by partially replacing soybean meal with casein in the slow protein foundation diet, a blend of both diets constituted an intermediate diet. Thus the objective of this study was to examine the hypothesis that the dietary provision of rapidly digestible protein enhances the performance of broiler chickens. The absorption of nutrients is considered to be a more important rate limiting factor on the growth performance of broiler chickens than their digestion (Croom et al., 1999). However, following their absorption, amino acids may be subject to catabolism in small intestinal enterocytes for energy provision to the gut (Wu, 1998) and this is a partial determinant of their entry into the portal circulation and their post-enteral availability for protein accretion. Therefore, plasma concentrations of free amino acids in the portal (anterior mesenteric vein) circulation was determined in birds offered the foundation and summit diets.

A ‘slow-protein’ foundation and a ‘rapid-protein’ summit diet were formulated to be isoenergetic (12.97 MJ/kg). Protein (220 g/kg) in the foundation diet was derived from soybean meal (252 g/kg), canola meal, maize and standard quantities of supplemental lysine, methionine and threonine. Protein (209 g/kg) in the foundation diet was derived from casein (50 g/kg), at the expense of soybean meal (82 g/kg), supplemental arginine, isoleucine and tryptophan in addition to the sources already outlined. Digestible amino acid levels in the dietary treatments were comparable. Each dietary treatment was offered to eight replicate cages (6 bids per cage) from 7 to 28 days post-hatch. Birds had unlimited access to feed and water during the experimental feeding period under a ‘16-hours-on’ lighting regime and room temperature was gradually reduced from 32°C at day 1 to 22°C at day 28. Growth performance and parameters of nutrient utilisation (AME, ME:GE ratios, N retention, AMEn) were determined by standard procedures. At day 28, all birds were weighed, euthanised and digesta samples collected from four small intestinal segments. Protein (N) and starch digestibility coefficients and disappearance rates in proximal jejunum (PJ), distal jejunum (DJ), proximal ileum (PI) and distal ileum (DI) were determined using acid insoluble ash (Celite) as the dietary marker. Disappearance rates (g/bird/day) were calculated from the following equation:

Disappearance rate = feed intake_(g/bird/day) x dietary nutrient_(g/kg) x digestibility coefficient.

Blood samples were taken from the anterior mesenteric vein of birds following euthanasia (intravenous injection of sodium pentobarbitone). Blood samples were centrifuged and the decanted plasma samples were then kept at -80o C prior to analysis. Concentrations of eighteen proteinogenic amino acids in plasma from the portal circulation were determined as fully described in Selle et al. (2016). The feeding study was conducted so as to comply with specific guidelines approved by the Animal Ethics Committee of the University of Sydney.

The overall performance of male broiler chicks from 7 to 28 days post-hatch (weight gain of 1576 g/bird, FCR of 1.463) compared very favourably with 2014 Ross 308 performance objectives (weight gain of 1387 g/bird, FCR of 1.479). The effect of rapid versus slow protein diets on protein (N) and starch apparent digestibility coefficients and disappearance rates in four small intestinal segments are shown in Table 1. The transition from foundation to summit diet significantly (P < 0.001) increased protein (N) digestibility coefficients in DJ by 10.4% (0.740 versus 0.670), in PI by 8.57% (0.798 versus 0.735), in DI by 6.11% (0.816 versus 0.769) and significantly (P < 0.005) accelerated protein (N) disappearance rates in DJ by 9.27% (16.5 versus 15.1 g/bird/day), in PI by 9.09% (18.0 versus 16.5 g/bird/day) and in DI by 6.36% (18.4 versus 17.3 g/bird/day). The transition from foundation to summit diet significantly (P < 0.05 - < 0.001) increased starch digestibility coefficients in DJ by 4.46% (0.844 versus 0.808), in PI by 3.96% (0.946 versus 0.910), in DI by 3.11% (0.962 versus 0.933) and significantly (P < 0.005) accelerated protein (N) disappearance rates in PJ by 16.5% (26.8 versus 23.0 g/bird/day) DJ by 14.5% (32.4 versus 28.3 g/bird/day), in PI by 13.8% (36.1 versus 31.7 g/bird/day) and in DI by 13.3% (36.7 versus 32.4 g/bird/day).

The treatment effects on distal ileal amino acid digestibility coefficients and concentrations of free essential amino acids in plasma taken from the anterior mesenteric vein are shown in Table 2. The transition from foundation to summit diets increased (P < 0.001) the digestibility of all amino acids; the average apparent digestibility coefficients of nine amino acids were increased by 5.82% (0.854 versus 0.807) in the distal ileum. The dietary transition increased plasma concentrations of methionine by 42.2% (16.5 versus 11.6 μg/ml; P < 0.005) and threonine by 39.1% (81.8 versus 58.8 μg/ml; P < 0.005) in the portal circulation. Curiously, the balance of amino acids was not influenced by treatment.

As anticipated, the transition from foundation to summit diet significantly accelerated protein (N) disappearance rates in the three posterior small intestinal segments. Therefore, substitution of soybean meal in the foundation diet with casein and supplemental amino acids in the summit diet had the intended impact of generating a ‘rapid protein’ diet. The importance of biophysical and biochemical starch-protein interactions on energy utilisation in poultry is accepted (Rooney and Pflugfelder, 1986), although these interactions have yet to be described precisely. It may be that more readily and rapidly digestible protein ameliorates these interactions along the digestive tract, thereby increasing starch digestibility and disappearance rates. This positive influence of rapid protein on starch/energy utilisation appeared to be driving significant improvements in AME of 0.35 MJ, ME:GE ratios by 6.81%, N retention by 7.11% and in AMEn of 0.32 MJ (data not shown). Starch and protein disappearance rate ratios were calculated to exclude the confounding influence of feed intakes. On this basis starch:protein disappearance rate ratios in the distal ileum were positively correlated to parameters of energy utilisation including AME (r = 0.509, P < 0.05) and ME:GE ratios (r = 0.573, P < 0.025). These outcomes demonstrate the relevance of starch and protein digestive dynamics to growth performance of broiler chickens. However, there is a precedent for this outcome as unequivocal impacts of starch and protein digestive dynamics were reported by Sydenham et al. (2017). In this study there were significant (P < 0.001) quadratic relationships between starch:protein disappearance rate ratios in the proximal jejunum with 15-28 day weight gain (r² = 0.722) and FCR (r² = 0.702).

The transition from slow to rapid protein diets significantly increased distal ileal digestibility coefficients of protein (N) by 6.11% (0.816 versus 0.769) and the average of essential amino acids by 5.82% (0.854 versus 0.807). The likelihood is that the magnitude of these improvements is somewhat more pronounced than would be expected to stem from the differences in diet composition. Nevertheless, the dietary transition increased ileal digestibility of methionine by 3.66% and threonine by 8.09% but increased their concentrations in the portal blood-stream by 42.2% and 39.1%, respectively. Supplemental methionine comprised 35% of the dietary total and the corresponding figure for threonine was 14%. The combined portal plasma concentrations of the six amino acids present in the summit diet in both ‘free’ and protein-bound forms were increased by 18.6% from 156 to 185 g/ml by the transition to the summit diet. The corresponding increase for the four proteinbound amino acids was a more modest at 6.1% from 147 to 156 g/ml. While not conclusive, this outcome appears to be consistent with the possibility that supplemental amino acids are less likely to undergo catabolism in the gut mucosa than their protein-bound counterparts. Both amino acids and glucose are catabolised in avian enterocytes (Watford et al., 1979) to meet the substantial energy requirements of the digestive tract. However ‘free’ or supplemental amino acids do not require digestion and are rapidly absorbed in the upper small intestine where more starch/glucose is available as an alternative energy substrate. So it is possible supplemental amino acids are more likely to be spared from catabolism in the gut mucosa than their protein-bound counterparts because of their more proximal sites of absorption. Considerable research remains to be completed if the mechanisms by which ‘rapid protein’ influences starch digestive dynamics and energy utilisation and influences the transition of amino acids across the gut mucosa are to be identified. However, it does appear that such research would be advantageous for chicken-meat production.