Growth Performance of Broiler Chickens Offered Maize- Versus Wheat-Based, Reduced Crude Protein Diets

Global chicken-meat production has been predicted to increase considerably between 2005 and 2050 (Alexandratos and Bruisma, 2012) and this demands sustainable practices. Reducing dietary crude protein (CP) substantially without compromising broiler performance has the potential to provide tangible reductions in nitrogen excretion and environmental pollution, with improved bird welfare outcomes and economic benefits (Kidd and Choct, 2017). Substantial reductions in dietary CP for broilers will reduce requirements for soybean meal particularly. However, substantial CP reductions usually compromise growth performance in association with increased fat deposition. Several reasons have been suggested for the poor performance of broilers offered substantially reduced CP diets that include a variety of both amino acid (AA) and non-AA acid limitations (Waldroup, 2017; Siegert et al., 2015). Nevertheless, the impact of the feed grain used in reduced CP diets on broiler performance has received little attention. Thus, the purpose of this study was to compare maize- and wheat-based diets on broiler performance in the context of substantial reductions in dietary CP.

A total of 216 off-sex male Ross 308 broilers were offered either maize- or wheat-based, iso-energetic (12.85 MJ/kg) diets, steam-pelleted at a conditioning temperature of 80°C from 7 to 35 days post-hatch. The diets were formulated to contain 222, 193 and 165 g/kg CP as shown in Table 1. All diets were formulated to standardised ileal digestible lysine level of 11.50 g/kg, glycine equivalents of 14.51 g/kg and the dietary electrolyte balance was maintained at 250 mEq/kg. Each diet was offered to 6 replicate cages (6 birds per cage) as a 2 x 3 factorial array of dietary treatments. Weight gains, feed intakes and feed conversion ratios (FCR) were determined from 7 to 35 days post-hatch as were relative abdominal fat-pad weights. Experimental data were analysed via the SPSS Statistics 24 program (IBM Corporation. Somers, NY). The feeding study fully complied with specific guidelines (2016/973) approved by the Animal Ethics Committee of the University of Sydney.

Significant treatment interactions (P < 0.001) were observed for growth performance parameters as shown in Table 2. These interactions were driven by the extremely poor performance of birds offered the 165 g/kg CP wheat-based diets. Weight gain was inferior by 34.6% (1549 versus 2370 g/bird), feed intake by 18.3% (2843 versus 3481 g/bird) and FCR by 24.9% (1.840 versus 1.473) in comparison to their 165 g/kg CP maize-based diet counterparts. Birds offered the 165 g/kg CP maize-based diet had higher weight gains by 7.05% (2370 versus 2214 g/bird; P = 0.049) than the corresponding 222 g/kg CP diet. Similarly, the 193 g/kg CP maize-based diet supported a weight gain advantage of 8.22% (2396 versus 2214 g/bird; P = 0.023) where the significant P values are based on pair-wise comparisons. Instructively, with the transition from 222 to 165 g/kg CP diets, maize-based diets generated a feed intake increase of 8.51% (3481 versus 3208 g/bird; P = 0.004); in contrast, wheat-based diets triggered a decline in feed intake of 18.5% (2843 versus 3487 g/bird; P < 0.001). However, maize-based diets increased relative fat-pad weights by a two-fold factor (12.77 versus 6.42 g/bird) but wheat-based diets did not influence fat deposition.

The growth performance of the majority of broiler chickens comfortably exceeded 2019 Ross 308 objectives in the present study but with the obvious exception of birds offered the 165 g/kg CP wheat-based diets as their performance was remarkably inferior. However, there is a precedent for this as equally poor performance of birds offered reduced CP wheat-based diets were observed in another of our studies (as yet unpublished data). The underlying reasons for these unexpected outcomes are obscure but it does appear that maize-based diets are more conducive to reductions in CP than wheat-based diets. However, it is imperative that the underlying causative factors are identified.

Substantial differences in broiler growth performance were observed between maize and wheat-based diets when dietary CP was reduced to 165 g/kg where feed grain inclusions were 721 and 751 g/kg, respectively. The relevance of starch and protein digestive dynamics in reduced-CP diets have been considered by Liu and Selle (2017) and in this context it is noteworthy that wheat starch digestion rates are more rapid than maize as demonstrated under both in vivo (Liu et al., 2019) and in vitro (Giuberti et al., 2012) conditions. In reduced CP diets, more slowly digestible maize starch may spare AA from catabolism in the gut mucosa, thereby enhancing their post-enteral availability. In contrast, more rapidly digestible wheat starch may increase AA catabolism in enterocytes along the posterior small intestine. Also, rapidly digestible wheat starch may flood the anterior small intestine with glucose to the extent that AA and glucose are competing for intestinal uptakes via co-absorption with sodium through their respective Na+-dependent transport systems. This is supported by Moss et al. (2018) who reported significant negative correlations between proximal ileal glucose and AA digestibility coefficients in 12 of the 16 AA assessed.

The protein content of wheat is greater than maize, consequently there are more unbound AA in a reduced CP, wheat-based diets because soybean meal inclusions are less in wheat-based diets. The 165 g/kg CP diets contained either 113 g/kg soybean meal with maize but only 48 g/kg soybean meal with wheat-based diets. Consequently, wheat contained 49.4 g/kg unbound AA as opposed to 38.5 g/kg for maize when the 165 g/kg CP diets are compared in the present study as shown in Table 1.

This 28.3% increase in unbound AA may disadvantage wheat-based diets due to greater imbalances of AA at the sites of protein synthesis (Selle et al., 2019). Surplus AA undergo deamination which generates ammonia that requires detoxification as excessive plasma ammonia concentrations have been associated with depressed feed intakes in rats (Noda and Chikamori, 1976). Moreover, increasing systemic ammonia levels have been associated with depressions in weight gains, feed intakes and feed efficiencies (Namroud et al., 2008) and inferior feed conversion ratios (Ospina-Rojas et al., 2014) in broiler chickens offered reduced CP diets. Thus, it is tempting to speculate that the grossly inferior performance of birds offered 165 g/kg CP wheat-based diets may have been at least partially due to excessive plasma ammonia levels and further studies would be required to confirm this. Furthermore, the wheat-based 165 g/kg CP diet contained 3.82 times more unbound supplemental L-leucine compared with the equivalent maize-based diet and excessive free leucine has been reported to reduce both valine and isoleucine availability producing negative feedback on feed intake, resulting in inferior broiler growth performance, through branched-chain AA antagonisms (Smith and Austic, 1978; Burnham et al., 1992).

In conclusion, this study would suggest that substantial CP reductions in wheat-based broiler diets may be better realised by more modest dietary feed grain increases in the formulation which would be facilitated by the partial substitution of soybean meal with feedstuffs containing lesser protein contents. Limiting wheat inclusions, dietary starch levels and quantities of unbound AA may permit lower CP thresholds to be achieved without compromised growth performance and increased fat deposition.