Comparative performance of broiler chickens offered nutritionally equivalent diets based on six diverse, ‘tannin-free’ sorghum varieties with quantified concentrations of phenolic compounds, kafirin, and phytate

Additional keywords: condensed tannin, conjugated and bound phenolic acids, ferulic acid, free, polyphenols.

Most energy in broiler diets is derived from starch; however, the utilisation of starch/energy in sorghum-based broiler diets may be substandard. For example, the amount of energy required to generate 1 kg of liveweight gain in broilers offered a sorghum-based diet was shown to exceed that for a wheat-based broiler diet (20.9 vs 19.8 MJ/AME.kg gain; P < 0.05) as reported by Black et al. (2005). On the basis of both in vitro and in vivo data the digestibility of sorghum starch is inferior in comparison to the starch component of maize. The in vitro potential starch digestibility of 14 maize samples was 95.0 in comparison to only 70.4 g/100 g dry starch in 11 sorghum samples (Giuberti et al. 2012). In a review of 11 studies, the mean ileal starch digestibility coefficient of 0.950 (range: 0.873–0.993) in broilers offered maize-based broiler diets exceeded the mean value of 0.883 (range: 0.846–0.921) in seven sorghum assays (Truong et al. 2016). Condensed tannin would be a contributing factor to this disparity if it were present in grain sorghum (Nyachoti et al. 1997); however, sorghum crops now grown in Australia almost certainly do not contain condensed tannin (Khoddami et al. 2015). The present study follows two similar studies by Khoddami et al. (2015) and Truong et al. (2015a). In the first study sorghum-casein diets containing 809 g/kg of six red sorghum varieties were offered to broilers from 7 to 23 days post-hatch. Among other findings, concentrations of conjugated phenolic acids were negatively correlated with ME : GE ratios (r = –0.832; P < 0.05) or the efficiency of energy utilisation. In the second study, conventional diets based on two of the six sorghum varieties were compared in broiler chickens from 7 to 28 days post-hatch. One sorghum was clearly inferior in terms of weight gain, feed conversion efficiency and parameters of nutrient utilisation and this appeared to be associated with a higher kafirin concentration (61.5 vs 50.7 g/kg). One contention is that three factors in grain sorghum, or colloquially the ‘Bermuda Triangle’, are negatively influencing starch/energy utilisation in sorghum-based broiler diets (Liu et al. 2015). The three apices of this triangle are ‘non-tannin’ phenolic compounds, kafirin, the dominant protein fraction in sorghum and phytate, a ubiquitous constituent in plant-sourced feedstuffs. Based on the Clorox bleach test (Waniska et al. 1992), the six sorghum varieties used in this study possessed non-pigmented testas and thus did not contain condensed tannin. However, the established anti-nutritive properties of condensed tannin may extend to the balance of phenolic compounds, which are abundant in sorghum, and the anti-nutritive properties of phenolic compounds are not solely the province of condensed tannin as proposed by Khoddami et al. (2015). It is widely held that the close proximity of kafirin protein bodies to starch granules in sorghum endosperm interferes with starch utilisation (Taylor 2005). The presence of phytate in broiler diets has anti-nutritive effects and although the negative impacts of phytate on the utilisation of protein and amino acids are established, these impacts may extend to starch and glucose utilisation (Selle and Ravindran 2007; Selle et al. 2012). To investigate this proposition, concentrations of polyphenolic compounds, free, conjugated and bound phenolic acids, kafirin, and phytate in six diverse grain sorghum varieties were quantified. Six nutritionally equivalent diets containing 620 g/kg sorghum were formulated and offered to broiler chicks in a 7 to 28 days post-hatch bioassay in order to examine relationships between the nominated components of sorghum and broiler performance parameters.

Grains of six diverse sorghum varieties (Block I, HP, Liberty, Tiger, MP, JM) grown in New South Wales and Queensland and harvested in 2012, 2013 or 2014 (Table 1) were analysed for phenolic compounds, kafirin and phytate. Kafirin was quantified by procedures adapted from those of Wallace et al. (1990) and Hamaker et al. (1995), which has been described in detail (Truong et al. 2015a). Total phosphorus (P) levels were determined by inductively coupled plasma mass spectrometry and phytate-P or phytate concentrations by high performance liquid chromatography procedures. Kafirin and phytate concentrations and other relevant characteristics including AusScan NIR profiles of the six sorghum varieties are shown in Table 2. Concentrations of polyphenols, free, conjugated, bound and total phenolic acids are recorded in Tables 3–7 inclusive. The complex methodologies used in their quantification have been documented by Khoddami et al. (2015).

Table 1.  Background information for the six sorghum varieties

Table 2.  Characteristics of grain sorghums including crude protein, amino acid profile kafirin, phytate concentrations and NIR AusScan values

Table 3.  Concentrations of polyphenolic compounds in six grain sorghum varieties (analyses completed in duplicate)
Means within columns not sharing a common suffix are significantly different (P < 0.05); nd, item not detected

Table 4.  Concentrations of free phenolic acids in six grain sorghum varieties (analyses completed in duplicate)
Means within columns not sharing a common suffix are significantly different (P < 0.05); nd, item not detected

Table 5.  Concentrations of conjugated phenolic acids in six grain sorghum varieties (analyses completed in duplicate)
Means within columns not sharing a common suffix are significantly different (P < 0.05)

Table 6.  Concentrations of bound phenolic acids in six grain sorghum varieties (analyses completed in duplicate)
Means within columns not sharing a common suffix are significantly different (P < 0.05); nd, item not detected

Table 7.  Concentrations of free, conjugated, bound and total phenolic acids in six grain sorghum varieties (analyses completed in duplicate)
Means within columns not sharing a common suffix are significantly different (P < 0.05)

On the basis of the above data, six broiler diets containing 620 g/kg sorghum were formulated to be nutritionally equivalent with an energy density of 12.95 MJ/kg and similar profiles for key amino acids (Table 8). The diets were steam-pelleted at a conditioning temperature of 84°C and crumbled after sorghum grain was ground through a 3.2-mm hammer-mill screen. Feather-sexed, male broiler chicks (Ross 308) were housed in an environmentally controlled facility, initially fed a proprietary starter ration, weighed at Day 7 and distributed among 48 cages so that mean bodyweights in each cage and their variations were almost identical. The six dietary treatments were offered to eight replicates (six birds per cage) from 7 to 28 days post-hatch. Bodyweights were determined on Days 7 and 28 and feed intakes recorded to calculate feed conversion ratio (FCR) with adjustments made from the weight of dead or culled birds, which were monitored on a daily basis. Total excreta were collected from 23 to 26 days post-hatch from each cage to determine apparent metabolisable energy (AME), ME : GE ratios, nitrogen (N) retention and N-corrected apparent metabolisable energy (AMEn). AME values (MJ/kg) on a dry matter basis and were calculated using the following formula:

Table 8.  Composition and nutrient specifications of broiler diets based on six grain sorghum varieties

ME : GE ratios were calculated by dividing AME values by the gross energy (GE) of the relevant diets. N retention was calculated using the following formula:

N-corrected AME values were calculated by correcting to zero N retention by applying the factor of 36.54 kJ/g N retained in the body (Hill and Anderson 1958). Acid insoluble ash was used as the inert dietary marker and acid insoluble ash concentrations were determined by the method of Siriwan et al. (1993). N content of the diets and excreta were obtained using an FP-428 determinator (Leco Corporation, St Joseph, MI, USA) and starch content were determined by a procedure based on dimethyl sulfoxide, α-amylase and amyloglucosidase, as described by Mahasukhonthachat et al. (2010). On Day 28, digesta was collected in its entirety from the proximal and distal halves of the jejunum and ileum to determine apparent digestibility coefficients in four small intestinal segments: proximal jejunum (PJ), distal jejunum (DJ), proximal ileum (PI) and distal ileum (DI). The four small intestinal segments were defined by the end of the duodenal loop, Meckel’s diverticulum, the ileal-caecal junction and their midpoints. Apparent digestibility coefficients of starch and protein (N) were calculated from the following equation:

Feed intakes over the final 2 days of the 7 to 28-day feeding period were recorded. Starch and protein (N) disappearance rates (g/bird.day) were deduced from feed intakes over the final 2 days from the following equation:

Ratios of starch to protein disappearance rates in the four small intestinal segments were calculated. Experimental data were analysed using the IBM SPSS Statistics 20 program (IBM Corporation, Somers, NY, USA). Statistical procedures included one-way univariate ANOVA using general linear models procedures, Pearson correlations, linear and multiple regressions. A probability level of less than 5% was considered to be statistically significant. There were numerous significant relationships between sorghum characteristics and bird performance parameters but many of these were complicated by significant correlations between sorghum characteristics. Therefore, focus was placed on the most significant linear relationship with the highest correlation coefficient (r =) between a given factor in sorghum and selected bird performance parameters. Where this sorghum factor was not correlated with others, valid multiple linear regressions were detected where possible. In a valid model, the two or more unrelated sorghum factors have significant impacts on a combined and individual basis. The feeding study complied with specific guidelines approved by the Animal Ethics Committee of Sydney University.

Kafirin concentrations of the six sorghums averaged 51.9 g/kg with a range from 41.4 to 67.1 g/kg. Phytate concentrations averaged 8.02 g/kg with a range from 4.93 to 9.79 g/kg (Table 2). Concentrations of phenolic compounds are shown in Tables 3–7. These include eight categories of polyphenols (Table 3), six free (Table 4), six conjugated (Table 5) and six bound phenolic acids (Table 6). Total free, conjugated and bound phenolic acid and the overall sum of phenolic acids are shown in Table 7.

There were no significant differences between the dietary treatments in weight gain (P > 0.80), feed intake (P > 0.10) and FCR (P > 0.80) from 7 to 28 days post-hatch as shown in Table 9. The overall weight gain of 1490 g/bird and feed intake of 2302 g/bird exceeded Ross 308 objectives by 6.7% and 12.2%, respectively, but FCR were inferior by 4.5% (1.545 vs 1.479). The overall mortality rate of 1.4% was not related (P > 0.65) to dietary treatments.

Table 9.  Effects of grain variety in six sorghum-based diets on growth performance of broiler chickens from 7 to 28 days post-hatch

The effects of grain variety in six sorghum-based diets on parameters of nutrient utilisation are shown in Table 10 where there significant treatment differences for ME : GE ratios, AMEn (P < 0.001), AME and N retention (P < 0.02). Among the sorghum-based diets, Liberty (white) was superior and Block I (red) inferior across all parameters. Advantages held by Liberty relative to Block I were 0.48 MJ (12.26 vs 11.78 MJ/kg; P = 0.002) in AME, 7.11% (0.753 vs 0.703; P < 0.001) in ME : GE ratios, 10.5% (61.19 vs 55.40%; P = 0.002) in N retention and 0.58 MJ (11.68 vs 11.10 MJ/kg; P < 0.001) in AMEn. The probability values in parentheses are on the basis of pair-wise comparisons between the two dietary treatments.

Table 10.  Effects of grain variety in six sorghum-based diets on parameters of nutrient utilisation of broiler chickens from 7 to 28 days post-hatch
Means within columns not sharing a common suffix are significantly different at the 5% level of probability

The effects of sorghum variety on starch digestibility coefficients and accumulative starch disappearance rates in four small intestinal segments of broiler chickens at 28 days post-hatch are shown in Table 11. There were no significant differences between dietary treatments for starch digestibility coefficients where average digestibilities progressively increased along the small intestine from 0.684 in PJ, to 0.753 in DJ, 0.838 in PI and to 0.871 in DI. Significant differences in starch disappearance rates were observed in PI (P < 0.005) and DI (P < 0.001). Starch disappearance rates ranged from 34.60 to 40.91 g/bird.day (P < 0.001) in PI and from 36.52 to 42.59 g/bird.day (P = 0.004) in DI where Liberty was the superior and JM the inferior sorghum variety.

Table 11.  Effects of grain variety in six sorghum-based diets on apparent starch digestibility coefficients and accumulative starch disappearance rates (g/bird.day) in four small intestinal segments of broiler chickens at 28 days post-hatch
Means within columns not sharing a common suffix are significantly different at the 5% level of probability

The effects of sorghum variety on protein (N) digestibility coefficients and disappearance rates included significant treatment effects (P = 0.033–0.003) for protein digestibility in all four small intestinal segments (Table 12). Average protein digestibility coefficients were 0.344 in PJ, 0.445 in DJ, 0.627 in PI and 0.692 in DI. The lowest digestibility coefficients were recorded for sorghum variety JM in the three caudal small intestinal segments. There were also significant treatment effects (P = 0.026 – <0.001) in four segments for accumulative protein disappearance rates. Average protein disappearance rates were 7.11 in PJ, 9.24 in DJ, 13.05 in PI and 14.38 g/bird.day in DI.

Table 12.  Effects of grain variety in six sorghum-based diets on apparent protein (N) digestibility coefficients and accumulative protein (N) disappearance rates (g/bird.day) in four small intestinal segments of broiler chickens at 28 days post-hatch
Means within columns not sharing a common suffix are significantly different at the 5% level of probability

The effects of sorghum variety on starch : protein (N) disappearance rate ratios in four small intestinal segments of broilers at 28 days post-hatch included significant effects in DJ (P < 0.02), PI and DI (P < 0.001) as shown in Table 13. In the final two segments Liberty had the highest starch : protein ratios of 3.50 and 3.36, respectively, and Block I the lowest ratios of 2.54 and 2.41, respectively.

Table 13.  Effects of grain variety in six sorghum-based diets on starch : protein (N) disappearance rate ratios in four small intestinal segments of broiler chickens at 28 days post-hatch
Means within columns not sharing a common suffix are significantly different at the 5% level of probability

Linear regressions between concentrations of phenolic compounds, kafirin and phytate in six grain sorghums with parameters of nutrient utilisation and distal ileal digestive dynamics of broilers offered sorghum-based diets appear in Table 14. Conjugated vanillic acid was negatively correlated with AME (r = –0.879; P < 0.025) but it was not significantly correlated with bound ferulic acid. In combination, these two phenolic acids were negatively correlated (r² = 0.980; P > 0.005) and the multiple linear regression equation is tabulated. The polyphenolic flavan-4-ols were the most significantly related (r = –0.919; P < 0.015) with ME : GE ratios but were not correlated with conjugated vanillic acid. Collectively, both phenolic compounds were negatively related (r² = 0.986; P > 0.0025) to ME : GE ratios and the multiple linear regression equation is tabulated. Kafirin was dominant in respect of its negative relationship (r = –0.887; P < 0.025) with N retention in this respect. There were no significant correlations with AMEn, although the negative relationship with flavan-4-ols (r = –0.795; P < 0.06) approached significance. In respect of distal ileal starch : protein disappearance rate ratios, flavan-4-ols (r = –0.960; P < 0.005) held the most significant negative correlation.

Table 14.  Linear regressions between concentrations of phenolic compounds, kafirin and phytate in six grain sorghums with parameters of nutritive utilisation and distal ileal digestive dynamics of broilers offered sorghum-based diets

Among our results, the lack of significant differences in growth performance across the six sorghum-based diets was somewhat surprising. This observation may stem from the absence of any significant differences in starch digestibility coefficients in the four small intestinal segments. However, the overall distal ileal starch digestibility coefficient of 0.871 for sorghum-based diets in the present study is both substandard and consistent with previously published data (Truong et al. 2016). After correcting FCR for weight gain (25 g ≡ 0.01), the overall efficiency of feed conversion of 1.508 for gain-corrected FCR remained inferior to the 1.479 Ross 308 performance objective. It is also noteworthy that the diets were formulated to an energy density of 12.95 MJ/kg; however, the average AME recorded in broilers was 12.04 MJ/kg. Arguably, this shortfall vividly illustrates the substandard energy utilisation of broiler chickens offered sorghum-based diets.

It is appreciated that correlations do not establish causation; nevertheless, there were numerous correlations between concentrations of phenolic compounds, kafirin and phytate in six grain sorghums with bird performance parameters that either approached (P < 0.10) or were (P < 0.05) significant. An interpretation is complicated by the fact that several of these sorghum characteristics were themselves significantly correlated. However, the significant negative regression between conjugated vanillic and bound ferulic acids in tandem with AME (P < 0.025) suggests that phenolic acids in grain sorghum negatively impact on AME as an indicator of energy utilisation. ME : GE ratios may be a better indicator of energy utilisation; taken individually, flavan-4-ols, conjugated ferulic and benzoic acids, kafirin and apigeninidin were negatively correlated with ME : GE ratios to significant extents. Flavan-4-ols was the most significantly correlated factor (r = –0.919; P < 0.015) with ME : GE ratios as shown in Fig. 1. In addition, the combined negative impact of flavan-4-ols and conjugated vanillic acid on ME : GE ratios was highly significant (P < 0.0025). This outcome suggests that (non-tannin) polyphenols and phenolic acids in sorghum negatively impact on energy utilisation. On an individual basis, several sorghum characteristics including kafirin, total phenolics, apigeninidin, 7-methoxy-apigeninidin, conjugated benzoic and vanillic acids were negatively correlated with N retention to significant extents. However, kafirin was dominant (r = –0.887; P < 0.025) in this respect. It was not possible to find any valid multiple linear regressions for this parameter; however, this outcome suggests that kafirin, in addition to phenolic compounds, can compromise nutrient utilisation in broilers offered sorghum-based diets.

Fig. 1.  Linear relationship (r = –0.919; P < 0.01) between flavan-4-ol concentrations in six grain sorghum varieties and ME : GE ratios of broiler chickens offered corresponding, nutritionally equivalent diets containing 620 g/kg sorghum.

There were significant, negative correlations between flavan-4-ols, conjugated ferulic acid, phytate, total phenolic compounds, bound ferulic acid and kafirin, with distal ileal starch : protein disappearance rate ratios on an individual basis. Valid multiple linear regressions could not be detected. These ratios are indicators of starch : protein digestive dynamics of sorghum-based diets and it appears that total phenolics, kafirin and phytate may all influence the bilateral bioavailability of starch and protein.

Several significant negative relationships between polyphenols, notably flavan-4-ols, and phenolic acids, where ferulic acid is dominant, with broiler performance parameters were observed. Thus, it appears that phenolic compounds, other than condensed tannin (absent in these Type I sorghums), can negatively influence starch/energy utilisation in broilers offered sorghum-based diets. The complex interactions between starch and phenolic compounds were recently reviewed by Zhu (2015) and in their extensive review, Tomasik and Schilling (1998) stated that phenolics readily form starch complexes and probably have a greater propensity to bind with amylose than amylopectin. However, the researchers did allow that the structure and stability of phenol-starch complexes are not clearly understood. It appears that phenolic compounds may interact with starch through hydrogen bonds, covalent bonds or chelation via their carboxyl and hydroxyl groups (Yu et al. 2001). Interactions between polyphenols and starch molecules were reported by Barros et al. (2012) where phenolic extracts from both tannin and non-tannin sorghums interacted with starch, including hydrophobic and hydrogen bonding with amylose. Phenolic extracts from sorghums were shown to increase the generation of resistant starch under in vitro conditions in the Barros et al. (2012) study. Interestingly, negative relationships between phenolic intakes and blood glucose responses have been observed in humans (Thompson et al. 1983) and phenolics have been shown to inhibit Na⁺-dependent intestinal glucose uptakes in rats (Welsch et al. 1989).

In the present study, ferulic acid was the dominant phenolic acid in soluble, conjugated and insoluble, bound forms. Conjugated ferulic acid was negatively correlated with ME : GE ratios (r = –0.919; P < 0.015) and bound ferulic acid in combination with conjugated vanillic acid was negatively correlated with AME (r = –0.990; P < 0.005). Khoddami et al. (2015) reported negative correlations between conjugated ferulic acid with AME (r = –0.808; P = 0.052), ME : GE ratios (r = –0.831; P = 0.042) and AMEn (r = –0.769; P = 0.074) in a recent study involving six red sorghum cultivars. Thus, the implication is that phenolic acids have a deleterious impact on energy utilisation in broilers offered sorghum-based diets. Instructively, Kandil et al. (2012) found that phenolic acids in feed grains play an important role in the resistance of starch to hydrolysis under in vitro conditions. Phenolic acids are capable of cross-linking with cell wall macromolecules via ester and ether linkages through reactions involving their carboxyl and phenolic groups (Yu et al. 2001). Ferulic acid is the dominant phenolic acid in sorghum but it is also found in barley, maize, triticale and wheat (Kandil et al. 2012). Interestingly, ferulic acid has been shown to influence starch pasting profiles as determined by rapid visco-analysis of maize and sorghum, which suggests that phenolic acids have the capacity to interact with starch (Beta and Corke 2004). Also, Hung et al. (2013) reported that ferulic acid has the capacity to complex with debranched starch and that these starch-ferulic acid complexes were associated with increases in resistance of starch to digestion.

Liberty, the white sorghum variety, contained lower concentrations of polyphenols than five red varieties (Table 3). This was to be expected as the colouration of red sorghums is due to polyphenolic pigments including anthocyanins and anthocyanidins (Taylor 2005). However, Liberty also contained lower concentrations of bound phenolic acids by 35.1% (229 vs mean of 353 μg/g) and total phenolic acids by 34.1% (303 vs mean of 460 μg/g) in comparison to the five red sorghum varieties (Table 7). In a comparison by Liu et al. (2015), another sample of white sorghum (Liberty) contained lower concentrations of bound phenolic acids by 48.3% (282 vs 545 μg/g) and total phenolic acids by 41.3% (374 vs 637 μg/g) in comparison to a red sorghum (Buster). Thus, it appears that white sorghums may routinely contain lesser concentrations of both polyphenols and phenolic acids than red sorghums and, anecdotally, white sorghums as a feed grain for pigs and poultry are considered to be superior to red varieties under Australian conditions.

The significant negative relationships between absolute kafirin concentrations on an individual basis with N retention and ME : GE ratios (Fig. 2) imply that kafirin compromises the efficiency of nutrient utilisation in sorghum-based diets. Salinas et al. (2006) claimed that kafirin, as a percentage of protein in 12 sorghum hybrids, was negatively correlated with AME and TMEn to significant extents in caecetomised roosters. Moreover, Truong et al. (2015a) found, in a comparison of broiler diets based on two sorghums with kafirin concentrations of 61.5 and 50.7 g/kg, that the sorghum with the lower kafirin concentration was associated with substantial and significant advantages in AME (13.61 vs 12.55 MJ/kg), ME : GE ratios (0.806 vs 0.769) and AMEn (12.38 vs 11.35 MJ/kg).

Fig. 2.  Linear relationship (r = –0.891; P < 0.02) between kafirin concentrations in six grain sorghum varieties and ME : GE ratios of broiler chickens offered corresponding, nutritionally equivalent diets containing 620 g/kg sorghum.

It seems reasonable to conclude that kafirin compromises starch/energy utilisation in sorghum-based diets, which is consistent with opinions expressed in Taylor (2005), Wong et al. (2010), de Mesa-Stonestreet et al. (2010) and other research groups. Kafirin protein bodies and starch granules are both embedded in the glutelin protein matrix of sorghum endosperm (Selle et al. 2010). This close proximity facilitates any physical or chemical starch-protein interactions. It has been argued that kafirin physically impedes the swelling of starch granules and their gelatinisation (Chandrashekar and Kirleis 1998). Chemical interactions between starch and protein in sorghum and other feed grains are considered to be important (Rooney and Pflugfelder 1986); however, as discussed by Truong et al. (2016), these starch-protein interactions have not been precisely defined. One possible interaction could involve disulfide cross-linking between the β- and γ-kafirin fractions located in the periphery of protein bodies and starch granule-associated proteins, which may be amplified by steam-pelleting of broiler diets (Selle et al. 2013). Although based on indirect evidence, it has been argued that the kafirin proportion of sorghum protein is escalating in sorghum crops grown in Australia as an inadvertent consequence of breeding programs (Selle 2011). Kafirin is not a readily digestible protein (Selle et al. 2010) but arguably nutritionists can accommodate for this because kafirin only comprises some 15% of total protein in sorghum-based diets. Thus, in the context of protein, the prospect of ‘high-kafirin’ sorghums may not be overly adverse. However, if kafirin additionally compromises energy utilisation then the prospect of ‘high-kafirin’ sorghums constitutes a potentially tangible problem for chicken-meat production.

The six sorghums contained an average level of 2.26 g/kg phytate-P or 8.02 g/kg phytate, which are very similar to values recorded in an earlier local survey (Selle et al. 2003). Phytate was negatively correlated with starch disappearance rates in the distal jejunum (r = –0.845; P < 0.04) and proximal ileum (r = –0.890; P < 0.02) on an individual basis (data not shown). This is consistent with a recent report (Truong et al. 2015b) in which phytase supplementation of maize-based broiler diets significantly increased starch disappearance rates in the proximal jejunum (58.0 vs 43.4 g/bird.day) and proximal ileum (80.8 vs 71.4 g/bird.day). This positive ‘starch/energy effect’ of phytase may stem from enhanced glucose absorption to a greater extent than improved starch digestion. Interestingly both phytate and phenolic compounds have been shown to depress starch digestibility in vitro (Thompson and Yoon 1984) and both components may share analogous anti-nutritive properties (Selle et al. 2010). Thus, it appears that phytate may depress energy utilisation in sorghum but to a lesser extent than phenolics and kafirin. Responses to exogenous phytases in sorghum-based diets are not always robust (Selle et al. 2013); this may be because exogenous phytases does not counteract the deleterious impacts of kafirin and phenolic compounds in this context.

This study provides evidence that ‘non-tannin’ phenolic compounds, kafirin and, to a lesser extent, phytate collectively exert a negative influence on starch/energy utilisation in broiler chickens offered sorghum-based diets. More detailed considerations of the likely responsible underlying mechanisms may be found in two reviews (Selle et al. 2013; Liu et al. 2015). One recommendation arising out of this study is that white sorghum varieties with low-protein contents would be advantageous. White sorghums inherently contain lower concentrations of polyphenols and, quite possibly, phenolic acids. Low-protein cultivars axiomatically contain less kafirin; moreover, data generated by Taylor et al. (1984) indicates that the proportion of kafirin increases at the expense of glutelin as protein contents in sorghum increase. Exogenous phytases are routinely included in broiler diets but in the context of lower phenolic and kafirin concentrations, phytases may generate more robust responses. Over the longer term, this study also suggests that sorghum breeding programs should be re-directed in order to reduce kafirin proportions of protein in grain sorghum, which should benefit both starch and protein utilisation in broiler chickens offered sorghum-based diets in the future.