Effects of dietary inorganic chromium supplementation on broiler growth performance: a meta-analysis

Introduction

Chromium (Cr), as an essential trace element, plays a vital role in insulin action and promotes the efficiency of glucose, protein, and fat metabolism, which was also recognized as a dietary supplement by human beings (Haq et al., 2016). Chromium usually exists in organic and inorganic forms with different biological availability and absorption rates (Haq et al., 2016). Among inorganic sources, the most common forms of chromium are the metallic form (Cr⁰), trivalent form (Cr³⁺), and hexavalent form (Cr⁶⁺) (Haq et al., 2016). Trivalent chromium is the most stable oxidation state and is considered a highly safe form, whereas hexavalent chromium is a known toxin, mutagen, and carcinogen (Chowdhury et al., 2003).

Chromium is usually not considered an essential trace mineral in the broiler industry (Amata, 2013). However, studies showed that chromium could increase weight gain, feed conversion ratio, high density lipoprotein, and improve lean muscle development and nutrient digestion (Sahin, Sahin & Kucuk, 2003; Al-Bandr, Ibrahim & Al-Mashhadani, 2010). The beneficial effects of chromium can be observed more efficiently under environmental, dietary, and hormonal stress (Amata, 2013). Though currently there are no National Research Council (NRC, 1994) recommendations for chromium in broiler diets (National Research Council, 1994), lots of studies suggested that supplementation of chromium at different levels and combinations improved growth performance of broilers (Toghyani et al., 2012; Mohammed et al., 2014; Huang et al., 2016). Samanta, Haldar & Ghosh (2008) revealed that supplementation of CrCl₃ at a dose of 0.5 mg/kg significantly improved the weight gain and food conversion ratio compared to the control group of birds, whereas Zheng et al. (2016) suggested that 0.4 mg/kg of CrCl₃ had remarkable effects on carcass characteristics but not on growth performance. Some researchers also reported that average daily gain (ADG), average daily feed intake (ADFI) and, feed conversion ratio (FCR) were unaffected by CrCl₃, Cr-yeast, or Cr picolinate (CrPic) treatments (Król et al., 2017; Lu et al., 2019).

Additionally, many studies indicated that chromium supplementation, regardless of its source in the broiler diet, positively affected growth performance and carcass traits under heat stress conditions  (Toghyani et al., 2012; Huang et al., 2016). According to Zha et al. (2009), inorganic chromium did not affect growth performance, but organic chromium did during heat stress conditions. Therefore, the effect of chromium addition, especially inorganic chromium addition, on broiler growth performance has not been concluded. However, chromium is still widely used as a common feed additive so far. In fact, there are two concerns regarding the effect of chromium supplementation. First, is adding chromium beneficial to growth performance? Second, what is the appropriate additive amount if it is beneficial?

Meta-analysis is a kind of statistical method that integrates research results of all types in the same field. The differences between studies are removed by meta-analysis, making the corrected data comparable, creating more objective and convincing conclusions (Sauvant et al., 2008). A meta-analysis entails consideration of the following aspects. (1) the results of included studies, (2) the comparisons of these results, (3) the effect of each comparison, (4) stability and reliability (Higgins & Cochrane Collaboration, 2020). Generally, meta-analysis is conducted to assess the strength of evidence present on treatment (Haidich, 2010), determine whether the effect is positive or negative, and further obtain the quantified estimate of the effect. The Cochrane Handbook provides an essential reference for meta-analysis (Higgins & Cochrane Collaboration, 2020). According to this handbook, the general steps are as follows: setting inclusion or exclusion criteria, retrieval process, data extraction, using models to estimate effect size, constructing forest plot, publication bias, sensitivity analyses, and subgroup analyses.

The main goal of this study is to explore the effect of inorganic chromium supplementation on broiler growth performance and the responses of different broiler sex and strains to chromium addition through meta-analysis. Moreover, a reasonable quantitative model needs to be built to explain the observed value and further provide a theoretical reference for the management practice of broiler.

Materials & Methods

Results

Selection process

The search strategy yielded 405 citations (Fig. 1). No additional citations were found in references. After duplicates were removed, 352 full texts were retrieved. After carefully reading the title/abstract of these publications, irrelevant studies and studies that failed to report ADG, ADFI, FCR, or addition level were excluded. Finally, 9 studies were included in quantitative synthesis (meta-analysis).

Relationship between performance and inorganic chromium addition

Chromium addition was used as the independent variable, and the performance indices (ADG, ADFI, and FCR) were used as the dependent variables. The result indicated a quadratic relationship between the adjusted ADFI and chromium addition (Y_ADFI = 84.022 + 1.839E−03X − 5.850E−07X², n = 33, P = 0.026) (Table 2). The maximum value of the ADFI (88.358 g/d) was reached when chromium addition was 1.572 mg/kg (Fig. 5). Similarly, the result showed a quadratic relationship between the adjusted FCR and chromium addition (Y_FCR = 1.802 − 7.000E−05X + 1.508E−08X², n = 33, P = 0.033) (Table 2). The minimum value of FCR (1.559 g/g) was reached when chromium addition as 2.321 mg/kg (Fig. 6). For ADG, our model may not be appropriate (maybe too complicated), and the results showed a lack of convergence (Table 2). In order to ensure the consistency of the model, we did not change the model.

Table 1:

Summary of weighted mean differences from meta-analyses and stratified analyses by different factors.

	Stratification	No. of studies	Random effects model			Heterogeneity
			WMD	95% CI	P	I2	P
ADG
Main analysis		9	1.872	−0.376–4.121	0.103	99.5%	<0.001
Study area	China	4	0.410	−0.078–0.898	0.099	0%	0.469
	Iran	3	2.484	0.154–4.813	0.037	88.7%	<0.001
	Egypt	2	3.311	−1.321–7.942	0.161	99.8%	<0.001
Time	2000–2010	3	2.129	−2.199–6.457	0.335	99.5%	<0.001
	2010–2020	6	1.602	0.663–2.541	0.001	78.4%	<0.001
Sex	Both	3	3.578	−0.173–7.328	0.062	99.8%	<0.001
	Female	2	0.456	−0.590–1.501	0.393	0%	0.442
	Male	4	0.890	0.012–1.768	0.047	69.1%	0.021
Broiler strains	Hubbard	1	5.671	5.640–5.703	<0.001	–	–
	Cobb 500	3	0.910	0.629–1.191	<0.001	0%	0.503
	Arbor Acres	2	0.450	−0.339–1.239	0.263	48.3%	0.164
	Ross 308	3	2.484	0.154–4.813	0.037	88.7%	<0.001
ADFI
Main analysis		9	1.023	0.217–1.828	0.013	58.7%	0.013
Study area	China	4	0.157	−0.646–0.960	0.701	0%	0.847
	Iran	3	2.461	−0.279–5.111	0.079	71.7%	0.029
	Egypt	2	1.363	1.329–1.397	<0.001	0%	0.333
Time	2000-2010	3	0.891	−0.164–1.946	0.098	42.1%	0.178
	2010-2020	6	1.308	−0.148–2.763	0.078	65.5%	0.013
Sex	Both	3	1.363	1.329–1.397	<0.001	0%	0.568
	Female	2	0.784	−0.927–2.494	0.369	0%	0.013
	Male	4	1.154	−0.955–3.262	0.284	77.8%	0.004
Broiler strains	Hubbard	1	1.363	1.329–1.397	<0.001	–	–
	Cobb 500	3	0.564	−0.909–2.036	0.453	0%	0.826
	Arbor Acres	2	−0.002	−0.929–0.889	0.966	0%	0.912
	Ross 308	3	2.416	−0.279–5.111	0.079	71.7%	0.029
FCR
Main analysis		9	−0.060	−0.118—0.001	0.045	100%	<0.001
Study area	China	4	−0.040	−0.063—0.018	0.001	99.6%	<0.001
	Iran	3	−0.063	−0.175–0.049	0.267	100%	<0.001
	Egypt	2	−0.094	−0.219–0.031	0.141	100%	<0.001
Time	2000-2010	3	−0.063	−0.173–0.048	0.265	100%	<0.001
	2010-2020	6	−0.059	−0.120–0.003	0.063	100%	<0.001
Sex	Both	3	−0.116	−0.193 to −0.039	0.003	100%	<0.001
	Female	2	−0.055	−0.104 to −0.006	0.028	99.8%	<0.001
	Male	4	−0.021	−0.035 to −0.007	0.003	98.8%	<0.001
Broiler strains	Hubbard	1	−0.158	−0.158—0.157	<0.001	–	–
	Cobb 500	3	−0.047	−0.006—0.027	<0.001	99.7%	<0.001
	Arbor Acres	2	−0.025	−0.055–0.004	0.095	98.4%	<0.001
	Ross 308	3	−0.063	−0.175–0.049	0.267	100%	<0.001

DOI: 10.7717/peerj.11097/table-1

Table 2:

Regression relationships inferred using mixed effected models.

Dependent variable	Independent variable	Intercept	SE	Pvalue	Quadratic	SE	Pvalue	R2
ADFI	Cr addition	84.022	3.467	<0.001	−5.850E−07	<0.001	0.026	0.344
FCR			1.802	0.047	<0.001	1.508E−08	<0.001	0.033	0.368
ADG		46.478	0.994	<0.001	7.881E−07	<0.001	0.054	0.385

DOI: 10.7717/peerj.11097/table-2

Discussion

Chromium was considered as an essential trace mineral for the boiler in manipulating growth performance and improving carcass composition. However, studies have different conclusions on the effect of chromium supplementation. Also, NRC did not specify the amount of chromium in poultry diets (NRC, 1997). Some researchers suggested that supplement of chromium at different levels in poultry improved feed intake and efficiency because of potentiation of insulin action, which shifts the overall metabolic responses more towards the anabolic side (Samanta, Haldar & Ghosh, 2008; Ahmed et al., 2005). Studies have also shown an adverse effect or no effect of chromium on feed intake, FCR, or body weight gain of broilers (Lu et al., 2019; Zha et al., 2009; Naghieh et al., 2010). This may be related to the lower bioavailability of inorganic chromium than organic chromium. On the other hand, Lu et al. suggested that the broiler’s chromium requirement might be higher under environmentally stressed conditions (Lu et al., 2019). Therefore, the growth performance of broilers was not significantly enhanced under normal environmental conditions.

The purpose of this study was to undertake a meta-analysis of trials assessing the effects of inorganic chromium supplementation on broiler growth performance and provide a more accurate dietary addition of inorganic chromium. To our knowledge, the present study is the first meta-analysis of the association between inorganic chromium dosage and ADG, ADFI, and FCR of broilers. A total of 9 studies that met our eligibility criteria were included in the meta-analysis  (Toghyani et al., 2012; Mohammed et al., 2014; Huang et al., 2016; Zheng et al., 2016; Lu et al., 2019; Zha et al., 2009; Kaoud, 2010; Kheiri & Toghyani, 2007; Moeini et al., 2011). By analyzing the data from 9 studies, we found that adding inorganic chromium had no significant effect on the ADG of broilers (Fig. 2) but significantly increased the ADFI and reduced the FCR of broilers (Figs. 3 and 4). The higher ADFI and lower FCR in this study agreed with early reports (Toghyani et al., 2012; Samanta, Haldar & Ghosh, 2008; Amatya, Haldar & Ghosh, 2004), and this may be attributed to the ability of chromium to regulate the level of glucose by activating insulin secretion and enable proper metabolic transformations of carbohydrates, proteins, and lipids (Haq et al., 2016; Chowdhury et al., 2003; Vincent, 2009). The result of ADG was in accord with previous studies reporting that dietary CrCl₃ supplementation had little effect on broiler’s growth performance (Lu et al., 2019; Moeini et al., 2011; Ghazi et al., 2012). Ghazi et al. believed that dietary supplements of 0.6 mg/kg CrCl₃ had no statistically significant effect on body mass and weight gain at 42 days of age (Ghazi et al., 2012). Even with the increase of chromium addition (0.8 or 1.2 mg/kg CrCl₃), Moeini et al. (2011) reached the same conclusion as Ghazi et al.. Therefore, the effects of chromium supplementation on broiler weight gain, food intake, and feed conversion ratio are not consistent. We speculated that the reason might be related to heat stress. Stressors such as high environmental temperature increase the production and release of corticosteroids, affecting glucose and mineral metabolism, hypercholesterolemia, gastrointestinal lesions, alterations in immune system functions, and decreased tissue vitamin and mineral concentrations in poultry as well as humans (Siegel, 1995). Besides, the released corticosteroids increase chromium mobilization from tissues and its excretion by urinary output (Sahin & Sahin, 2002), which may exacerbate a marginal chromium deficiency or an increased chromium requirement. Previous studies conducted under heat stress conditions indicated that dietary chromium supplementation positively affected broiler growth performance  (Toghyani et al., 2012; Huang et al., 2016; Zha et al., 2009; Moeini et al., 2011), which may be directly related to chromium deficiency and increased chromium demand caused by corticosteroid release. Due to the lack of information from the included studies, no subgroup analysis of environmental stress conditions was conducted in this study. In other words, this meta-analysis could be influenced by stresses, such as temperature or cage area, which might considerably increase the heterogeneity of the data and lead to the fact that broiler’s weight gain had no significant change to the addition of inorganic chromium. Besides, the chromium source, administration level, background of the broiler strains, broiler age, and farm hygiene may be the reasons for the non-significance. However, under the premise that strict literature screening criteria had been established in this study, we tend to believe that environmental stress conditions might be the main factors.

Compared with the overall analyses of ADFI and FCR, our subgroup analyses produced more conservative effect estimates at country and published year group. Inorganic chromium addition was not associated with ADFI and FCR in the subgroup analyses, except for Egypt and China (Table 1). Although the overall analyses showed no significant association between ADG and inorganic chromium, subgroup analyses of country and published time indicated an inverse association in Iran and 2010s (Table 1). We believed that the broiler management practices and strategies were diverse in different areas and periods, which may lead to the inconsistent use of chromium and affect the broiler’s growth performance in the broiler industry. The subgroup analyses of ADG, ADFI, and FCR were consistent with the overall analyses regardless of broiler sex, indicating the high stability of meta-analysis results and then confirmed by sensitivity analyses (Figs. 7, 8 and 9). Besides, we found that male broiler chickens had better growth performance than females (higher ADG), which suggested that males might be more sensitive to chromium addition. Studies on different animals showed that chromium increased lean body mass (Page et al., 1993; Lindemann et al., 1995; Boleman et al., 1995), and the carcass lean rate of male broiler is always higher than that of female, which may be the reason for the higher ADG of male broiler chickens. The subgroup analyses stratified by broiler strains indicated no significant association between chromium and growth performance in Arbor Acres. Meanwhile, the result was difficult to estimate if only very few studies are included in Hubbard (Table 1). Compared with Ross 308, Cobb 500 had better growth performance (lower FCR) with inorganic chromium supplementation. Bjedov et al. reported that Cobb 500 showed lower FCR, higher performance index, and lower mortality than other broiler strain groups (Bjedov et al., 2011). Rosário et al. (2008) evaluated broiler performance under a canonical discriminant analysis and found a clear distinction between strains, where Cobb 500 presented the highest multivariate performance means. However, studies also found no statistically significant difference in body weight and feed conversion between Cobb 500 and Ross 308 (Kassab & Neemy, 2013; Shaheen et al., 2019), suggesting that researchers have not reached a consensus on different broiler strains’ growth performance. Thus, differences among broiler strains still cannot fully explain the reason why broilers showed different growth performance to inorganic chromium supplementation. We speculate that inorganic chromium has complicated effects on the physiological and metabolic process, whereas these are still poorly understood.

With the increase in chromium addition, the ADFI and FCR showed a significant negative and positive quadratic relationship, reaching an extreme value when chromium addition was 1.572 mg/kg and 2.321 mg/kg, respectively (Figs. 5 and 6). A lack of convergence indicated that the model did not fit the data of ADG, the reason of which might be that the model was too complicated. However, we did not modify the model in order to ensure consistency. The amount of chromium addition has always been controversial in the poultry industry. Kaoud reported that 0.2 mg/kg CrCl₃ supplementation improved aspects of growth performance, carcass traits, and immune response (Kaoud, 2010), while Lu et al. believed that 3.2 mg/kg dietary Cr supplementation did not affect on average daily gain, average daily feed intake and, gain: feed of broilers during the starter and grower periods (Lu et al., 2019). This meta-analysis found that the growth performance might be improved with the inorganic chromium addition ranging from 1.6 mg/kg to 2.3 mg/kg. This result is similar to an experimental study reporting that 2.0 mg/kg dietary inorganic chromium supplementation significantly improved broiler’s growth performance (Norain et al., 2013). We tend to consider 2.3 mg/kg as the appropriate additional amount because the FCR reached a minimum at this point. The feed constitutes 70%–80% of the cost in raising broiler chickens. Therefore, changes in FCR can have a significant impact on the profitability of an operation (Aggrey et al., 2010). According to the positive quadratic relationship between FCR and chromium addition, the FCR gradually increased after higher than 2.3 mg/kg. It is worth noting that the broiler’s FCR may not change significantly even with high inorganic chromium supplementation (3.2 mg/kg) as mentioned above (Lu et al., 2019), which may indicate that there was not a simple quadratic relationship between inorganic chromium and broiler performance. Hence, more experimental data are needed to support and explain this speculation.

The literature sources greatly influence meta-analysis, however, all the studies we selected were from English databases. Paper published in other languages, conference proceedings, and some unpublished results may have an impact on this meta-analysis. In addition, the number of studies included was limited. Although there was no publication bias in these studies, only one study was included in some subgroups, which does not lead to robust results. Finally, the dose range of chromium was narrow, and the model may change when pharmacologically relevant doses rather than nutritionally relevant doses are used. Thus, the results of this paper may have some limitations.

Conclusions

A meta-analysis is a statistical analysis that combines the results of multiple scientific studies. The effect of inorganic chromium supplementation on broiler growth performance has always been controversial. Through meta-analysis, our study showed that the males and Cobb 500 broilers might be more sensitive to chromium supplementation and provided more accurate inorganic chromium supplementation for the management practice of broiler. However, this study still has some limitations. The fewer included studies may lead to higher heterogeneity and thus affect the results of the meta-analysis. Additionally, no subgroup analysis of environmental stress conditions was conducted due to the lack of related information. We expect more experimental studies and systematic reviews to provide support and explanation for the response mechanism of broiler performance to feed additives.

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