I. INTRODUCTION
Globally, maize and wheat are the major cereal grains used in broiler feed. Maize is predominant in Asia and the Americas, whereas wheat is in Australia, New Zealand and Europe. Extensive research has been undertaken in refining dietary strategies to optimise broiler performance when offered reduced crude protein (CP) diets. In feed, these grains have been optimized by the inclusion of feed enzymes and unbound (crystalline or synthetic) amino acids. Reduced CP diets have demonstrated beneficial outcomes in enhancing litter quality, minimising foot pad dermatitis (Dunlop et al., 2016) and reducing nitrogen emissions (Lemme et al., 2019).
Despite improvements in the formulation of reduced CP diets, poor bird performance is still observed in CP levels < 180 g/kg (Belloir et al., 2017). Underlying factors are often attributed to potential deficiencies in amino acids, including valine, isoleucine and arginine (Dozier and Kriseld, 2019). To date, comparative studies evaluating the influence of wheat and maize within the context of reduced-CP diets have not been completed.
A series of reduced-CP diet feeding studies have been completed by the Poultry Research Foundation (unpublished data). Step-wise reductions in CP from about 210 to 165g/kg have been evaluated, without the inclusion of feed enzymes. The growth performance of birds offered the lowest CP maize-based diets has been compromised but only to a limited extent. This was also the case with the first wheat-based study; however, in two subsequent studies bird performance was notably compromised. Thus, the impression was formed that maize- based diets are more conducive to reductions in CP than wheat-based diets. Therefore, the aim of this paper is to review the compositional differences between wheat and maize, and how these factors might influence broiler performance in the context of reduced-CP diets.
II.WHEAT VS MAIZE: INFLUENCING FACTORS ON BROILER PERFORMANCE
In poultry feed, environmental and economic factors have typically dictated the use of maize and wheat on a geographical basis. However, maize has been preferred due to the perception that maize has a greater nutritional value compared to other grains (Cowieson, 2005). Cereal grain composition is variable in terms of starch, protein, fibre, oil and amino acid content, much of which is influenced by intrinsic and extrinsic factors such as growing conditions, variety type, starch structure, drying temperature and anti-nutritive factors (Cowieson, 2005). The relevant properties of maize and wheat are shown in Table 1.
Table 1 - A comparison of the properties of maize and wheat in vitro and in vivo.
Interestingly, despite compositional differences, bird performance is relatively equivalent when birds are fed standard CP levels of > 220 g/kg between the two grains (unpublished data). However, in the context of reduced CP diets, two recent studies (unpublished data) suggest that the compositional differences between wheat and maize have a direct influence on bird performance, namely in feed conversion ratio, bird weight and feed intake. These studies have shown that birds offered wheat- compared with maize-based diets performed more poorly in their feed conversion ratios, weight gain and feed intake (P < 0.001). In the initial study, birds were offered 180 and 162.5 g/kg CP wheat or maize-based diets, with starch capped or uncapped. Birds offered wheat-based diets had decidedly inferior performance compared with birds offered maize-based diets. In a subsequent study, reduced CP levels (165 g/kg) wheat- or maize-based diets were offered to male broilers from 7 to 35 days post-hatch. Inferior (P < 0.001) performance was again observed in birds offered wheat-based diets. In both studies, such observations were not anticipated, but highlight a need to understand the underlying causes.
The literature on reduced CP diets often attributes poor bird performance to amino acid deficiencies (Dozier and Kriseld, 2019). Whilst this may be the case in certain circumstances, other factors should be considered since the chemical and nutrient composition between maize and wheat differ. Of these factors, possibly three are of greatest importance. The first and most important is the levels of CP with wheat containing 51% more CP than maize (Table 1). Within the context of reduced- CP diets, wheat-based diets contain large amounts of unbound amino acids that may lead to imbalances of amino acids at sites of protein synthesis (Selle et al 2019). Amino acids in excess have been shown to impede broiler performance in reduced CP wheatbased diets, by inducing ammonia toxicity (Noda, 1975; Namroud et al., 2008). According to Noda and Chikamori (1976), blood ammonia levels regulate feed intake via the central nervous system. It is theorised that surplus amino acids undergo deamination, mostly in the liver, producing ammonia that needs to be detoxified via a condensation reaction in which ammonia and glutamic acid are converted to glutamine and excess nitrogen subsequently excreted as uric acid (Watford and Wu 2005). Higher concentrations of ammonia in the systemic plasma have been observed to depress feed intake, resulting in reduced liveweight gain and inferior feed conversion ratios (Ospina-Rojas et al, 2014; Namroud et al., 2008).
The second factor is the competition of nutrient uptake in the intestine between glucose and unbound amino acids (Moss et al., 2018). This stems from the rate of starch digestion with wheat being more rapid than maize (0.118 versus 0.08 min-1; P = 0.048) as reported by Liu et al (2019). Furthermore, feed grain content increases in reduced-CP diets diminish the capacity of the bird to efficiently absorb nutrients (Carré, 2004) since glucose and amino acids compete for co-absorption via common Na+-dependent transport systems (Vinardell, 1990). It is theorised that slowly digestible starch in maize-based diets may spare amino acids from catabolism in the gut mucosa and therefore enhance the post enteral availability of amino acids (Moss et al., 2018). In addition, according to Herwig et al. (2019), slowly digestible starch activates nutrient sensing mechanisms related to unabsorbed nutrients, such as the ileal brake, thereby enhancing the extent of digestion by increasing transit time.
The third factor is the issue of gut viscosity as a result of increased wheat inclusion. The aim of reduced CP diets is to reduce CP through minimizing the use of soybean meal, whilst increasing feed grain. Increases in wheat content have a greater negative influence compared to maize. Unlike wheat, maize has very low levels of soluble NSP (Choct,1997). Soluble NSP is an anti-nutritive factor, and is understood to increase digesta viscosity, which has been shown to hinder bird performance as it negatively impacts digestion and nutrient absorption (Choct and Annison, 1990).
III. CONCLUSION
In the context of reduced-CP diets, the higher levels of feed grain inclusion of wheat and maize have not been sufficiently considered. Composition differences between wheat and maize may influence broiler performance, by virtue of differences in crude protein and starch digestive dynamics. These factors highlight a need for further research to better understand the influence of feed grain in the context of reduced- CP diets and how that impacts on broiler performance. This will hopefully allow for implementation of enhanced dietary strategies.
ACKNOWLEDGEMENTS: The authors would like to acknowledge the financial support and guidance of AgriFutures Chicken-meat and Evonik Nutrition & care GmbH, and the technicians that have contributed to the data used to produce this paper.
Presented at the 30th Annual Australian Poultry Science Symposium 2020. For information on the next edition, click here. 
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