Introduction
According to the FAO, livestock is one of the fastest growing sectors of global agricultural production. It is evolving in response to the rapid increase in demand for products of animal origin, especially in developing countries. In developed countries, through intensification practices that have helped increase yields and efficiency while bringing down costs, the demand for livestock products is stagnating and have to face many challenges, among them, animal wellbeing must be monitored with increased vigilance and the fair and sustainable use of antibiotics must be found. Also, we can expect that animal production will increasingly be affected by the food-feed competition and natural resources as well as evolving in a carbon-constrained condition. Finally, farmers must use excreta in ways that do not pose a threat to human health or cause blooms of blue-green algae and threaten ecosystem biodiversity.
Besides, modern farms and agricultural operations work far differently than those a few decades ago, primarily because of advancements in technology. It can be expected that these advanced devices and the precision they achieve, will improve sustainability by increasing efficiency, safety, with lower environmental footprint (Flachowsky and Kamphues, 2012). To be available and used accurately in the field, technology should go beyond mere tool making, and address how we process valuable data. With all these constraints and changes, multicriteria approach become necessary to produce sustainable animal protein. In that context, meta-analysis and mechanistic modeling that allow identifying trends and outcomes in multiple studies to draw conclusions based on results of a wide range of experiments rather than single experiments, thereby increasing reliability, are tools all indicated.
The current review will show some insights of animal performance and health status obtained from synergy between meta-analysis and animal trials to develop multicriteria approach to manage growth performance and animal health by (1) founding the between balance dietary calcium (Ca) and phosphorus (P), (2) assess the possibility of lowering dietary crude protein (LCP), and (3) to find alternatives to antibiotics growth promoters.
1. Meta-analysis of experimental data, a knowledge generator tool
Meta-analysis constitutes a scientific approach which carries out critical reviews and statistical studies based on previous research results in order to improve and empirically quantify knowledge on a subject. In animal science, meta-analysis has proven to be an efficient way to renew already published data to return to the user by creating new empirical models allowing to progress in both understanding and prediction aspects (St-Pierre, 2001; Sauvant et al., 2005, 2008). The progress is allowed by (1) the reduction of all biases and imprecision and, (2) by enlarging a priori the domain of validity of the model. This methodology has improved over time with several published meta-analyses in animal science increases with a rate of about 15% stressing an actual success (Sauvant et al., 2019). Meta-analysis is a time-consuming exercise that should ideally be done by specialist in the subject under study.
The main steps are the definition of the research question that allow the identification of keywords for the exhaustive publication search, the selection of publications, the coding and construction of the database, the graphical examination of the meta-design, the choice of the statistical model and the post-analyses (Figure 1). Note that this is an iterative process and it is not uncommon to return to the previous steps during the analysis.
Figure 1. Meta-analysis steps (Sauvant et al., 2005)
A meta-analysis can be performed for different purposes, namely to (1) built mechanistic models (e.g. Létourneau-Montminy et al., 2015), (2) empirical laws of response ΔY = f (ΔX), for example of a causal feeding practice quantified by ΔX, which has been studied in a set of experiments, (3) increase the statistical power of an effect, (4) raise a doubt on the discordant results, (5) test and increase the possibilities of generalization, (6) explain the variability of the results, and (7) answer a question not asked in the tests. In animal science, meta-analysis are essentially interested in relations between variables and are mainly aimed at predicting the average quantitative response (ΔY), within the experiment, of a character Y with one or more X quantitative explanatory/predicting characters treated as independent covariables [i.e. ΔY = f (ΔX)]. Under these conditions, the experiment effect corresponds to the variations between studies not considered by the covariables (Sauvant et al., 2019).
2. Meta-analysis as a tool to generate empirical relationships
Finding the right amount and balance between dietary calcium and phosphorus
In the new millennium, many countries including Canada have been under tremendous pressure to address environmental concerns with legislation aimed at reducing nitrogen (N) and phosphorus (P) excretion, including moratoriums in some provinces such as Quebec. Animal manure nevertheless remains valuable as fertilizer when manage correctly and can replace chemical fertilizers successfully see more advantageously regarding soils structure. A better understanding of the fate of dietary P use by animals is necessary to optimize its use and enhance sustainable practices. The optimization of P utilization is complicated by the multiple criteria, such as growth performance, bone mineralization, and manure P used for assessment of requirements. This is further complicated by Ca concentration which is an important factor in the variability of P utilization by animals. In fact, first as constituents of skeletal hydroxyapatite and second through common hormonal regulation pathways, P and Ca are closely related. Besides, excessive intake of Ca can reduce P absorption and exacerbate symptoms of P deficiency (Létourneau-Montminy et al., 2012; Létourneau-Montminy et Narcy, 2013) by forming insoluble complexes in the gastrointestinal tract (Narcy et al., 2013; Heaney and Nordin, 2002) and thereby reducing efficiency (Suttle, 2010). Therefore, meta-analysis has been performed in pigs (Létourneau-Montminy et al., 2012, 2013) and broilers (Létourneau-Montminy et al., 2010; Couture et al., 2019a; Hedli et al., 2019) with many objectives among them, (1) to evaluate the impact of nonphytate P (NPP), phytate-P (PP), Ca and phytases on digestible P, retained P, bone ash, and growth performance by generating empirical equations and (2) to study the impact of Ca on phytase efficacy where their was confounding results in the literature.
Using apparent total tract digestibility data in pigs (Nexp = 86; Ntrt = 377) and ileal digestibility data in broilers (Nexp = 95; Ntrt = 480), a linear relationship between NPP and digestible P was established in both pig and broiler, so it is likely that P transport across the digestive wall did not represent a limiting step for P absorption, at least in the conditions of the experiments introduced in the database. Both NPP from mineral, animal and plant origin are highly digestible (from 56 to 70% in broilers and from 73 to 78% in pigs).
Regarding PP, large difference was observed between species with 21% of PP available without phytase in pigs as compared with more than 60% in broilers in which the digestibility of PP is lower in high Ca diets (Ca × PP, P<0.001) which was not the case in pigs (No interaction). The previous findings are consistent with results showing that broilers presented intestinal endogenous phytase activity depending of Ca (Applegate et al., 2003) and P supply (Valable et al., 2017). The digestibility of NPP is also reduced in both species by high Ca diets (Ca ×NPP). Likewise, in high-Ca diets, the unabsorbed dietary Ca would interact with P in the chyme to form insoluble salts, resulting in reduced absorption of dietary P (Heaney and Nordin, 2002).
Meta-analysis has showed that this reduction in digestibility in high Ca and low NPP diets induces reduction of feed intake, gain and feed conversion ratio in both pigs and broilers. The impact on feed conversion ratio needs more research to understand why the decrease of gain is higher than that of feed intake. Besides, given these two minerals are deposited together into bone, in a ratio of 2.2:1 to form a hydroxyapatite-like (Ca10(PO4)6(OH)2) compound (Crenshaw, 2001), unless the decrease in dietary Ca concentration could ameliorate P digestibility, it may cause an imbalance between Ca and P that leads to extra urinary losses of P and impairs P retention (De Rauglaudre et al., 2020). In fact, increasing dietary Ca negatively affects both retained P and growth performance in low-NPP diets but increases retained P and does not influence growth performance in high-NPP diets (Figure 2; Létourneau-Montminy et al., 2012).
Figure 2. Responses of retained phosphorus (g/kg) and average daily gain (g/day) to dietary concentrations of non-phytate P (NPP, g/kg) and calcium (Ca, g/kg) with phytate P (PP) 2.2 g/kg, microbial phytase (PhytM) 0 FTU/kg and plant phytase (PhytP) 0 FTU/kg. The black and gray lines represent retained phosphorus and average daily gain, respectively. The solid and dotted lines represent 1.1 and 2.1 g NPP/kg, respectively.
Regarding microbial phytase, at the time we did the meta-analysis, it was stated that feeding poultry or swine with wide Ca:P ratios decreases the efficacy of phytase, and therefore lower ratio were recommended when using phytase (e.g. Qian et al., 1996; Kornegay et al, 1996; Sebastian et al., 1996). However, examination of the data indicates that a significant portion of the literature published over the 90s concerning the efficacy of phytase at different dietary Ca and P ratios is misleading because these references do not make the distinction between the concentrations of Ca and P in which phytase results in maximum animal performance and the concentrations of Ca and P in which phytase is most efficient in liberating P. The doubt has been raised by 2 meta-analysis works in broilers showing that the effect of phytase is more marked in high Ca and low P diet in terms of precaecal P digestibility, growth performance and bone ash in broilers (Létourneau-Montminy et al., 2010, 2012; Couture et al., 2019a) while in pigs, there was no effect of Ca on phytase efficacy in liberating P. Nevertheless, in pig an increase in P urinary losses will occur if phytase is added in a low Ca diet (Narcy et al., 2012).
In sum, with the meta-analysis tool it has been showed that Ca did not reduce the efficacy of phytase in liberating P in both species. The equations predicting apparent total tract digestibility in pigs and precaecal digestibility in broilers were integrated into mechanistic models (Létourneau-Montminy et al., 2015; Couture et al., 2019b) to predict absorption.
Although powerful, meta-analysis is limited by the data available. Health status in broilers is closely associated with litter quality (e.g. litter moisture). In this connection, Ca and P homeostasis is achieved through physiological mechanisms involving osmoregulation, suggesting the indirect role of these minerals with litter quality. More precisely, the antidiuretic hormone of birds, arginine vasotocin will regulate the allostatic response by adjusting the free water clearance, as well as glomerular filtration rate. In this manner, all this process is orchestrated by kidneys and the lower gastrointestinal tract of birds (cloaca, colon, and digestive ceca), plays an important role in avian osmoregulation (Braun, 2015). On the other hand, wet litter is an important welfare problem in broiler that has not to be neglected in the overall strategies to reduce the use of antibiotics. It is expected that an altered osmoregulation will impact gut permeability, as happens when there is heat stress (Ruff et a., 2020). Under these conditions, it is hypothesized that immune system will be more active because of inflammatory responses induced by endotoxins, mycotoxin and pathogens. The more common nutritionally induced causes of wet litter is polyuria, an increase urine output (Collett et al., 2012).
Regarding Ca and P, decreasing Ca is associated lower litter moisture (Enting et al., 2009; Pos et al., 2003) that led to lower incidence of footpad dermatitis (FPD; Rousseau et al., 2016). However, this effect is more marked when reducing Ca in a low P diets (Rousseau et al., 2012, 2016). Moreover, when comparing 5.5, 7.0 and 8.5 g Ca/kg diet, litter moisture followed a quadratic function with lower value obtained at 7.0 g Ca/kg, and a further reduction when adding 1000 FTU/kg microbial phytase (Herisson et al., 2019). It is worth noting that the diet supplying 7.0 gCa/kg and 1000 FTU/kg phytase resulted in the best growth performance and bone mineralization. Parathormone has been showed to enhance urine volume in birds because it causes an osmotic diuresis due to Ca mobilization from bone and the high P excretion from kidney (Clark and Mok, 1986). Thus, it appears that the underlying mechanism of wet litter is link to an imbalance Ca and P or large excess in both that lead to high plasma concentration, then to osmotic diuresis that then to polyuria, wet litter and even FPD.
3. Meta-analysis as a state-of-the-science tool
Reducing crude protein and balancing amino acids profile: environmental and animal performances lessons
Nitrogen excretion is a major concern in the swine and poultry industry given the implications on environment (e.g. soils acidification, water eutrophication) that has led to worldwide actions aiming to reduce the level of N emissions through different strategies, within them: livestock feeding strategies and low-emission animal housing systems (e.g. Directive EU 2016/2284). To face these challenges, low crude protein (LCP) strategies that are characterized basically by a reduction of soybean meal and a supplementation with limiting AA have reemerged. This strategy works quite well in pigs with a reduction of 2 to 4% CP between NRC (1998) and NRC (2012) (Wang et al., 2018). With the development of industrial synthetic AA technology, supplementary feed grade AA, such as L-valine and L-isoleucine have become available for use in livestock diets, resulting in the potential for further reduction in dietary CP. Nevertheless, a challenge remains in broilers where published researches evaluating the effects of LCP strategies on growth performance of broilers has been conducted through gradient reduction of CP content performed in different ways (ingredients and nutrients) without consensus strategy and with inconsistent results in maintaining growth performance.
A meta-analysis has been performed in broilers (Alfonso-Avila et al., 2019; Minussi et al., 2020) to (1) characterize the nutritional strategies followed in literature in LCP studies and (2) to quantify the impact of LCP strategies on growth performance, N balance, uric acid, daily water consumption, litter moisture and footpad dermatitis as well as their main modulating factors. The response variable data were taken from a database of 64 publications including 152 trials in which diets were recalculated based on INRA-AFZ tables (Sauvant et al., 2004). For N balance, only the experiments with constant dietary energy and digestible Lysine content were selected. Since this meta-analysis was performed based on the modulation of the dietary CP, the experiments were independently encoded, thus including a minimum of two dietary treatments (e.g. LCP and control). The studies with more than one growth phases were categorized into two code, 0 to 21 days and 22 to 42 days. Nitrogen balance parameters (intake, excreted, retained and efficiency) were calculated according to Belloir et al. (2017) given only few studies have performed classic N balance. Recalculations of parameters to generate new Y variables to assess a research question is another utility of meta-analysis.
Study of the meta-design showed that only 12 experiments have provided essential amino acid at or exceeding 95% of the recommendation (Ajinomoto-Eurolysine S.A.S, 2015; Rostagno, 2011). This confirms that it's not possible to study the impact of LCP itself on growth performance by meta-analysis tool, meeting nevertheless one of the objectives of the meta-analysis which is to take stock of what is available in the literature. Then, most manuscripts of the database used corn-soybean meal diets. As a result, a decrease in CP was associated with a lower soybean meal, whereas the starch that coming from corn was naturally increased (-1 point % CP = -1.83%; Figure 2a). Also, given the high supply of potassium (K) by soybean meal, its concentration was decreased when decreasing CP (-1 point % CP = 0.05%; Figure 2b) which impacts dietary electrolyte balance (DEB; Figure 2c). Finally, oligosaccharides are also linearly reduced (-1 point % CP = 0.38%; Figure 2d). Therefore, other nutrient than just CP varied in LCP diets.
Figure 3. Meta-design: within-experiment relation between (a) starch, (b) Potassium (c) Dietary Electrolyte Balance (DEB) = [Na + K] − [Cl], and (d) Oligosaccharides.
Performed models showed that N efficiency (% intake) was linearly decreases with increasing CP (P<0.001); a decrease 1 % point of CP increases N efficiency by about 2.3%, whereas N excretion was decreased by about 10% (P<0.001). Although not possible to quantify through meta-analysis given the few studies, it is known that this excretion is mainly composed of uric acid (70-80%), that can be converted into ammonia by litter microbes (Ferguson et al., 1998), and of other compounds such as urea, creatinine, and AA (Nahm, 2003; Braun, 2015).
The model generated for litter moisture showed that reducing dietary CP per 1 % point of CP would reduce by 1.2% the litter moisture (P<0.001). The effect on litter moisture seems to be mainly due to the reduction of daily water consumption. The model generated in the current meta-analysis showed a reduction of 7.3 mL/d of daily water consumption per % point CP. Excessive dietary protein supply in birds must be catabolized and excreted via the kidneys in the form of uric acid which implies higher water consumption (McNabb et al.,1973; Chrystal et al., 2019, 2020). Thus, a large urinary flow is required for broilers to excrete an excess of N metabolic products (Nishimura, 2008). In addition, since dietary mineral profile and CP have an additive effect on water consumption (McNabb et al., 1973), reducing the content of soybean meal that supply a significant amount of K (21 g/kg; Figure 2b) and the concomitant change in the DEB (Figure 2c) reduces water consumption, thereby impacting the excreta moisture (Ahmad and Sarwar, 2006).
Litter quality, namely litter moisture and N content, is increasingly important in broilers production, especially because it causes footpad dermatitis (FPD), a tangible welfare issue that can lead to carcass downgrades. Given the known and now quantified impact of CP on litter moisture, a database including LCP diet and measuring FPD has been built to quantify the impacts of CP on the incidence and severity of this condition (Minussi et al., 2020). The different FPD scoring systems were standardized to footpad score (FPS), varying from 0 to 200. The FPD incidence and FPS linearly decrease (P<0.001) by 15.8% points and 20.5 points (23.2% and 29.1% relatively to the highest CP treatment), respectively, for each % point of dietary CP reduction. In addition, when the FPS of the control treatment is included as a factor in the model, a significant interaction is found between CP and control FPS (P<0.001), indicating that benefit of low protein diets increases with higher initial severity of the condition.
It is worth noting that wet litter being the primary cause of ammonia emissions with N excretion and taken together wet litter and ammonia are one of the most serious performance and environmental factors affecting broiler production (Aziz and Barnes, 2010; Francesch and Brufau, 2003) indicating the LCP strategies in broiler diets is also advantageous in improving health status and animal performance and appears to be a sustainable way of raising broilers in a context of antibiotics growth promoter’s removal.
Find alternatives to antibiotic growth promoters
Sustainably improving agricultural productivity to meet increasing demand for animal protein in an ambition to feed 9 billion of people by 2050 is one of the challenges of the 21st century. Sustainability implies affordable, safe, and low footprint production. Regarding safety and especially the increase in antibiotic resistance, the reduction of antibiotics use in livestock production is an urgent public health problem for both human and veterinary medicine and contributes to the dynamic “one Health”. In this way, adapted therapeutic strategies are envisaged involving less and better antibiotic use (specific treatment with good dose). To do this, animal and more specifically intestinal health must be monitored with relevant biomarkers using a holistic approach including prevention and early detection of diseases.
The ban of antibiotics use have led to many studies, either to understand the role of antibiotic growth promoters on animals in order to efficiently replace them, or to found alternatives to antibiotic therapeutic but especially to preventive antibiotics to maintain or improve animal health and performance (Gadde et al., 2017). Usually, the sought alternatives are aimed at improving gut health with feed additives such as prebiotics, probiotics, proteobiotics, organic acids, phytobiotics (Vidanarachchi et al., 2005., Gadde et al., 2017). However, there is no clear definition of gut health. It refers to positive aspects of the gastrointestinal (GI) tract, such as effective digestion of feed and absorption of nutrient, the absence of GI illness, normal and stable intestinal microbiota, and effective immune status. Even more complicated is how gut health can be measured? All these considerations have fueled the search for biomarkers of the digestive inflammation in swine and poultry, but there is still no gold standard and the solution will probably involve a panel (Ducatelle et al., 2018).
Several reviews have tried to highlight the effects of alternatives to antibiotic growth promoters (AGP), but with little success given the wide responses variation. To provide an overview of the alternatives that have been evaluated in vivo and quantify their impacts in terms of growth performance in comparison to negative controls was performed in broilers (Rouissi et al., 2020), as previously done in pig (Schweer et al., 2017). The objective of this study was (1) to identify the most selected alternatives to AGP for broilers and to quantify their effects on growth performance relative to an antibiotic-free control diet and (2) to shed light on the effective modulating factors.
The information was extracted from peer-reviewed papers published from 2000 to 2017 and retrieved from various public databases. This selection of databases contains articles of enough diversity to make sampling bias unlikely. The keywords used in the search were: ‘chicks’ or ‘poultry’ or ‘broiler’ + ‘prebiotic’, ‘essential oil’, ‘probiotic’, ‘organic acid’ and ‘performance’, ‘alternatives’, ‘necrotic enteritis’. This results in 130 publications. Then the articles were retained for the meta-analysis if they met the following criteria: (1) broiler trial fed with non-antibiotic growth promoter, (2) inclusion of an antibiotic-free control diet (C-), (3) reporting of feed conversion ratio, (4) diet composition, and (5) broiler genetic line. Only 79 publications totaling 150 experiments were kept. They were grouped into four sub-database categories, namely organic acids (Nexp = 30), prebiotics (Nexp = 50), probiotics (Nexp = 30), and essential oils (Nexp = 40). In each of the databases a product largely dominated, i.e. butyrate (70%) for organic acids, mannan-oligosaccharides (MOS; 84%) for prebiotics, Bacillus subtilis (77%) for probiotics, and origan based essential oils (65%) and they were thus isolated in 4 other sub-database and studied. Given large difference in doses and broilers age it was not possible to compare statistically these four products, they were thus studied independently. It should be noted that there is no doubt that this meta-analysis is subject to publication bias, negative or inconclusive trials being often nonpublished.
Quantifying the effects of alternative to antibiotics growth promoters on growth performances.
Average daily weight gain (ADG), average daily feed intake and feed conversion ratio (FCR) were modeled as Y variables. No effect on feed intake was found, whereas daily gain increased linearly and quadratically (P<0.05) with dose of butyric acid, mannan-oligosaccharides and essential oil and linearly with Bacillus subtilis by up to respectively 7%, 8%, 7% and 9%. Metabolizable energy content had a linear effect (P < 0.05) on feed conversion ratio in butyric acid and MOS databases. Improvement in FCR reached 2.4%, 8%, 7% and 3% respectively for butyric acid, MOS, essential oil and Bacillus subtilis. By quantifying the effect of non-antibiotic growth promoters, the model proposes a rational basis for their use in broiler diets. These alternatives have then been tested in the field (Rouissi et al., 2019) but some of then did not resulted in the same effect on growth performance showing that studies around their mode of action to finetune their use are still needed.
Highlighting modulating factors of alternative to antibiotics growth promoter’s response.
Given inconsistencies in the results found in the literature on alternatives to AGP, with equivalent or even better responses than antibiotics while other reported no effect, a specific focus on founding modulating factor of their response have been also done. This has been tested in two works, the one of Rouissi et al. (2018, 2020) just described and the one of Létourneau-Montminy et al. (2019) and Gabarrou et al. (2020) that have evaluated the effects of a specific blend of oleoresins of spices and essential oils (Oleo) based on an exhaustive database including 25 trials with Oleo allowing to quantify its impact on ADG and FCR of broilers.
In both meta-analysis, feed composition was used to recalculate the dietary nutrient profile, especially apparent metabolizable energy (AME), crude protein and digestible amino acids, from INRA tables of feedstuffs (Sauvant et al., 2004). The AME requirement, standardized ileal digestibility of lysine (SID Lys) and CP values were then compiled based on bird genetic line (ROSS 308, 2014; COBB 500, 2015) and the dietary concentration in each experiment was expressed in % of the requirement. In addition to the continuous dose of non-antibiotic alternative, the dietary provision relative to requirement, the genetic line, the presence of challenge (pathogenic bacteria administered orally) and the bird age (starter, grower, finisher) were tested, as well as their interactions. Also, in the works of Létourneau-Montminy et al. (2019) and Gabarrou et al. (2020) another strategy was implemented with the objective of testing the hypothesis that alternative to AGP response was influenced by the challenge the bird experienced. Therefore, when a positive control (C+) with antibiotic growth promoter and a negative control (C-) without antibiotic were both present in the same trial (n=9), the relative difference between them (C+/C-) was also calculated as Challenge Acuity Index and used as an X variable.
An interaction between AME in % of requirement and prebiotic effect have been found by Rouissi et al. (2018) showing that the more the bird is energy deficient the more its response to prebiotics was important. Besides, Létourneau-Montminy et al. (2019) and Gabarrou et al. (2020) showed that the effect of Oleo of FCR expressed in percentage of C- was negatively correlated (Linear, P=0.03; Quadratic, P=0.02; R2=85%) with the Challenge Acuity Index (C+/C-) indicating that in challenge condition (when C+ performed better than C-), the reduction of FCR with Oleo addition was higher (Figure 4). This new index is interesting to better understand the variations observed between studies for a similar alternative to AGP and will be for sure reuse in the future.
Figure 4. Relationship between the effect of feed conversion ratio (FCR) of bird receiving Oleo in comparison to negative control (C-) without antibiotic and the difference in terms of FCR between control positive with antibiotic (C+) and C-.
Conclusion
The matching of the constant demand for animal proteins and the genetic progress we have witnessing in the last decades is a challenge, especially from nutrition point of view. In parallel, there is an unprecedented increase in data available on a subject. For these advances to translate into affordable and safe animal protein produce with a minimal environmental footprint, adequate and precise nutritional management is necessary. This undoubtedly involves multi-criteria methods adaptable to different production contexts.
By incorporating the results of numerous publications into meta-analysis approaches, insights to improve broiler and pig nutrition and health are proposed. Indeed, there is an equilibrium to find between Ca and P to fulfil tissues growth without too much excesses to maintain a good litter quality and animal health. This strategy, that deserves more attention in pig, can even be a lever to reduce the use of antibiotics. Although meta-analysis and modeling approaches are ongoing and will help to make progress, animal trials on microbial phytase of second generation are still needed to fine-tune the equilibrium given few data available in our database and the fact that they are a source of P. Besides, in broilers, meta-analysis clearly showed that reducing CP in broilers is a way of improving health by improving litter quality and reducing ammonia emissions this with a lower environmental footprint, health effects expected to be similar in pigs. In broilers, the maintenance of growth performance is however challenging but is the subject of many recent studies and it can therefore be expected that effective strategies for protein decreases will be developed. Finally, some alternatives to AGP has been found and their effects quantify in terms of growth performance. However, other biomarkers must be studied and proposed to better understand the variation in the effects found in the field.
Meta-analysis is now widely accepted and applied in animal sciences, especially in nutrition. Its implications in systemic approaches and mechanistic modeling is promising. Precision farming and large datasets facing heterogeneity and lack of data to explain it are in the crosshairs of metaanalysis. There is also a growing interest in using meta-analysis to interpret individual laboratory databases by grouping the results of various experiments.
![Meta-design: within-experiment relation between (a) starch, (b) Potassium (c) Dietary Electrolyte Balance (DEB) = [Na + K] − [Cl], and (d) Oligosaccharides.](/_next/image/?url=https%3A%2F%2Fimages.engormix.com%2FE_articles%2F0_96.gif&w=1200&q=75)
