I. INTRODUCTION
Both glucose and amino acids are essential for muscle protein deposition and feed conversion efficiency and total tract nitrogen retention was reported to be influenced by protein and starch digestion in broiler chickens (Liu et al., 2013). Liu and Selle (2015) found that 76% of the variation in the feed-conversion ratio (FCR) could be attributed to starch and protein digestion rates in sorghum-based diets. Quadratic relationships between proximal jejunal starch to protein disappearance rate ratios with weight gain (R = 0.849; P < 0.001) and FCR (R = 0.838; P < 0.001) in broiler chickens from 15 to 28 days post-hatch were reported by Sydenham et al. (2017). Embracing the concept of digestive dynamics and applying it in practical diet formulations requires an understanding of variations in protein and starch digestion rates in common feedstuffs. Five studies have been completed to quantify digestion rates of starch and protein in common feed ingredients used in Australia (Liu et al., 2019) and the objective of the present study was to formulate practical broiler diets based on these pre-determined values to establish the relevance of starch and protein digestion rates on feed conversion efficiency of broiler chickens.
II. MATERIALS AND METHODS
The feeding study complied with specific guidelines approved by the Animal Ethics Committee of The University of Sydney. Five studies were previously completed to quantify starch and protein digestion rates in common poultry feedstuffs. Digestion rates were quantified by fitting exponential models between apparent digestibility coefficients of starch and protein (N) in proximal jejunum, distal jejunum, proximal ileum and distal ileum with their corresponding mean retention times by using acid insoluble ash as the marker (Liu et al., 2013).
Table 1 - Diet composition and calculated nutrient specification in six completed diets (g/kg).
In the present study, six iso-energetic and iso-nitrogenous diets (12.60 MJ/kg; 11.5 g/kg dig Lys) based on wheat, red sorghum, soybean meal, canola meal and canola seed were formulated to contain six ratios of starch and protein digestion rate ranging from 1.43 to 2.45 as shown in Table 1.
Table 2 - The influence of starch and protein digestive dynamics on growth performance from 7-35 days post-hatch.
Figure 1 - The correlations between FCR and starch, protein digestion rate and starch:protein digestion rate ratios in broiler chickens from 7-35 days post-hatch.
All diets contained similar ideal protein ratios and were steam-pelleted at 80˚C and then crumbled for birds from 7 to 14 days post-hatch. The same diets were offered as pellets from 15 to 35 days post-hatch. Each of the six dietary treatments was offered to 6 replicate cages (6 birds per cage) or a total of 216 off-sex male Ross 308 chicks (parent line). Chickens had ad libitum access to feed and water. Initial and final body weights were determined, and feed intakes were recorded from which feed conversion ratios (FCR) were calculated. The incidence of dead or culled birds was recorded daily and their body-weights used to adjust FCR calculations. ANOVA and linear and quadratic correlation were performed using JMP® 13.0.0 and significance was determined at P < 0.05.
III. RESULTS AND DISCUSSION
Growth performance results are shown in Table 2 where the 4.6% mortality rate was not influenced by dietary treatments (P > 0.25). There were no significant treatment differences in feed intake, weight gain and FCR; however, there were quadratic relationships between starch and protein digestion rates, and their ratios, with FCR (Figure 1). Weight gains and feed intakes were not correlated with starch and protein digestion dynamics. The quadratic relationship (R2 = 0.40, P = 0.003) between FCR and starch digestion rates can be described as:
𝑦 = 0.366𝑥2 − 2.423𝑥 + 5.456.
This indicates the optimal FCR of 1.446 was predicted when starch digestion rate equals 3.31 min-1. Increasing starch digestion rate tended to compromise feed conversion efficiency or increase FCR. The quadratic relationship (R2 = 0.43, P < 0.001), between protein digestion rates and FCR can be described as:
𝑦 = 0.251𝑥2 − 1.014𝑥 + 2.474.
This suggests the optimal FCR of 1.450 was predicted when protein digestion rate equals 2.02 min-1. Increasing protein digestion rate tended to improve feed conversion efficiency or decrease FCR. The quadratic relationship (R2 = 0.42, P < 0.001) between starch:protein digestion rate ratios and FCR can be described as:
𝑦 = 0.095𝑥2 − 0.316𝑥 + 1.712.
This shows that the optimal FCR of 1.450 was predicted when starch:protein digestion rate ratios were equal to 1.663. Narrowing the rate ratios tended to improve feed conversion efficiency or decrease FCR.
In general, starch is more rapidly digested than protein and feed conversion efficiency may be influenced by both rate and extent of starch and protein digestion (Liu and Selle, 2015). Liu et al. (2016) compared the influence of corn starch (rapidly digestible starch) and fish meal (rapidly digestible protein) inclusions on broiler performance and found that protein digestion rates were more influential than starch digestion rates. However, in the present study, starch and protein digestion rates were equally important in respect of FCR. Across the experimental range, retarding starch digestion rates, or accelerating protein digestion rates, or condensing starch:protein digestion rate ratios, will improve feed conversion efficiency or reduce FCR. Therefore, the present study confirmed the importance of starch and protein digestive dynamics in practical broiler diets and demonstrated the possibility of including starch and protein digestion rate data in least-cost feed formulations.
ACKNOWLEDGMENTS: The authors would like to thank the AgriFutures Chicken-meat for funding the ‘Formulating broiler diets based on protein and starch digestive dynamics’ project (PRJ010216).
Presented at the 30th Annual Australian Poultry Science Symposium 2020. For information on the next edition, click here. 
