The use of more circular diets in pig production – consequences and challenges

Introduction

As the world population continues to grow, along with the consumption of animal-sourced foods, animal production in the world is expected to grow in the decades ahead. The OECD and FAO estimated that, due to population growth and the ongoing transition towards a higher intake of animal products, global livestock production will grow by 14% from 2020 to 2029 (OECD/FAO, 2022; Ritchie and Roser, 2019). However, the statistic differs between continents and regions of the world. This development will result in increased demands for ingredients for animal feeds and will increase feed-food competition. At the same time, food production, including animal source food from pigs has a major impact on environmental emissions, climate change and land-use (Poore and Nemecek, 2018; Sporchia et al., 2021; Parlasca and Qaim, 2022). To reduce its impact and to use scarce resources for food production, such as land and phosphate, efficiently, a food systems approach has been proposed to reduce the competition between use of land and scarce resources for food and feed production (Van Zanten et al., 2019). In a circular food system, arable land is primarily used for the production of plant biomass for human food while farm animals consume low-opportunity cost feed materials (LCF; “circular ingredients”) including crop residues, co-products from the food industry, food waste and grassland from marginal lands unsuited as arable land. These feed materials are sometimes called LCF since no major alternative opportunities are lost because of their use as ingredients in animal feed, thus reducing the competition for resources between feed and food production (Van Zanten et al., 2018). Depending on the context and scale considered, there are many definitions/descriptions of circularity and their quantification often involves aspects related to 1. reducing input of (scarce) resources, 2. reducing emission levels (pollutants and GHG emissions), 3. reducing material losses/waste, 4. increasing input of renewable and recycled resources and 5. maximizing the utility and durability of products (Puente-Rodríguez et al., 2022). The first four also apply to animal production and feeding of animals.

The increase in the world population and increasing concerns about the contribution of animal production to climate change will force the animal production sector to consider the choice of raw materials for animal feeds and include e.g. the carbon footprint of feed ingredients in diet optimization and provide information on the carbon footprint of end products of animal production (meat, milk and eggs). Livestock production makes an estimated contribution of 55-60% to the emissions caused by food production (Gerber et al., 2013; Poore and Nemecek, 2018). The enhanced concern for the environmental footprint and potential limitations in the availability of more circular ingredients will influence the composition of animal feeds of the future. Bikker and Jansman (2023) addressed possible consequences of the implementation of more circular food systems for the type, volume and nutritional characteristics of feed materials and complete feeds, and considered consequences for nutrition, physiology and metabolism of monogastric farm animals. Different scenarios for improving the sustainability of future pig production systems have been explored (Rauw et al., 2020). To be prepared for future in this context, we need to consider the consequences of feeding more circular diets to pigs. A variety of potential effects should be considered as feeds of the future will likely contain less starch and more fiber. Moreover, the use of specific (new) ingredients might urge us to consider effects of specific constituents in these ingredients, which could be antinutritional or bioactive (“pro-nutritional”) of nature. We should consider potential effects on feed intake, enzymatic and fermentative digestion, gut health, growth responses, nutrient efficiency as well as potential effects on the quality of final products. In addition, we need to consider whether the current genotypes of animals used in pig production are suited to consume more circular diets, provide an adequate growth response with a high nutrient efficiency, and contribute to a long term sustainable pig production.

More circular feed ingredients in pig diets

Table 1 provides an overview of major co-products from the present industrial use of crops for the production of human food and biofuels that are used as ingredients for diets of pigs and other animal species. The production of refined carbohydrates (e.g. flour, starch, and sugar), industrial fermentation and oil crushing contribute to a large volume of co-products from cereal grains, sugar beets and oil seeds as the major groups included in animal diets in Europe. The relative contribution to the total volume of ingredients for compound feed in the EU was estimated as 19% for oilseed co-products, 12% for cereal grain and sugar beet co-products, 2% for former foodstuffs and 1% for meat and dairy co-products (EU, 2023). At present, the majority of compound feed (including on-farm mixed) is provided by intact cereal grains (64%), with a limited contribution of intact oil seeds and legume seeds (2%) on a total volume of 276 million tons of compound feed in the EU.

In the context of circularity, attention has also been given to the production and use of so-called novel feed ingredients, which can be grown in or obtained from other production systems using alternative environments or resources. Examples are aquatic proteins (e.g. macroalgae, microalgae, duckweed), insect protein, grass and other green leaf proteins. The production of such ingredients might use waste streams which are not fit for human consumption as main resource (e.g. insects), or use marginal land on which only grass can be grown. This would fit well in the concept of circularity. Although some of them principally show promise as future pig feed ingredients, their production volumes are still small, related to production technologies that are still under further development and related to production costs that are relatively high so far.

Figure 1 provides an overview of potential effects of feeding more circular diets on feed and nutrient intake, nutrient digestibility, responses related to gut functionality and to growth performance and nutrient excretion. As outlined above, more circular diets have a different ingredient composition, and likely contain less starch, more fiber, plant phytate and could have a lower energy density if the lower starch concentration is not compensated by additional inclusion of oil or fat sources. Moreover, particular ingredients in such diets may contain specific constituents considered as anti-nutritional or beneficial of nature e.g. with a prebiotic or antimicrobial nature. When formulating more circular diets both traditional ingredient and nutrient constraints and safety margins applied may need to be revisited related to the inclusion and nutrient composition of circular ingredients. In addition, in such context also more attention should be paid to a proper characterization of the nutritional value of novel feed ingredients or deviations in nutritional value once ingredients are used at a higher inclusion level than currently applied. More circular diets could affect voluntary feed intake, in part related to a lower nutrient density. Beaulieu et al. (2008) and Quiniou et al. (2012) showed that reducing dietary energy content by reduction of fat content or dilution with wheat bran resulted in an increase in feed intake until the maximum intake capacity was reached. Older pigs (> 50-60 kg) were capable to compensate for a reduction in energy concentration of approximately 15%, whereas a lower capacity was observed in younger pigs. Circular diets could have a lower enzymatic nutrient digestibility e.g. in case diets have a higher fiber content and a larger part of the energy in such diets has to be obtained from fermentation activity by the microbiome in the hindgut of pigs. The influence of more circular diets on the intestinal microbiome in both the small and large intestine and consequences for gut health and function need to be considered. Aluthge et al. (2019) recently described the complex interactions between diet composition and the microbiome in the gut in relation to gut health and function and also emphasize the need for further exploration of the microbiome-phenotype relationships. In line, over the past years the faecal microbiota composition was shown to explain part of the phenotypic variation in digestive efficiency. Such relationships should be further explored and understood using a wider range of diets, including more circular diets (Déru et al., 2022). Finally, faeces and manure composition could be affected as N- and P-digestibility and utilization might be reduced, due to the lower intrinsic digestibility of some circular ingredients or to a less optimal dietary amino acid profile of such diets which could reduce N-efficiency.

Figure 1. Schematic representation of characteristics of diets based on circular feed ingredients (mainly co-product and former foods) in a more circular food production system, interactions in the digestive tract and influence on animal performance and health. Abbreviations: ANFs = antinutritional factors; NE = net energy; NSPs = non-starch polysaccharides; AAs = amino acids, GIT = gastro-intestinal tract. (Reproduced from: Bikker and Jansman, 2023).

As an example of investigating the effects of more circular diets, the consequences of gradual replacement of soybean meal, palm kernel expeller and cereal grains by bakery products and legume seeds in the diet of growing-finishing pigs was studied (Table 2; van Helvoort and Bikker, 2023). A control diet (I) high in cereal grains, with soybean meal and palm kernel meal, a diet (II) high in bakery products with approximately 50% bread meal, and a diet (III) high in legume seeds with 22.5% peas and 25% field beans were evaluated from 25 kg live weight to slaughter at 125 kg. Efficiency of N and P utilization was calculated using reference values for N and P concentrations in the body of pigs. Although rather extreme diets were used, the dietary treatments had limited effects on growth performance and feed efficiency of the pigs. The efficiency of N and P utilization was reduced by approximately 4-5% and 8-9%-units, respectively (P< 0.001) due to the higher N and P content of the diets with bakery products and legume seeds. The carbon footprint of the diets expressed in g CO2 eq. per kg of carcass gain was reduced by inclusion of bakery products with 64% including and 38% excluding land use change (LUC) (P< 0.001), respectively. In contrast, inclusion of legume seeds reduced the carbon footprint including LUC with 23% but increased the carbon footprint excluding LUC with 23% (P< 0.001). This study showed that cereal grains and soybean meal can be replaced by bakery products and legume seeds with minor effects on growth performance of the pigs, but efficiency of N and P utilization as relevant sustainability parameters may be substantially reduced.

Table 2. Effects of gradual increasing the level of bakery products (BP) and legume seeds (LS) at the expense of cereal grains on the growth performance, carbon footprint of carcass gain and N- and P- efficiency of growing-finishing pigs (van Helvoort and Bikker, 2023).

In diets with more by-products of plant origin with a high fiber content, a larger part of the dietary energy originates from fermentation of the organic matter by the intestinal microbiome, in particular in the hindgut. The fermentation capacity of pigs influences the potential inclusion of fiber rich ingredients, which is larger in older pigs and sows compared to young pigs. Dietary fiber content and physical chemical characteristics of fiber can also affect feed intake, passage rate of digesta in the various compartments of the GIT and extent and rate of fermentation of fiber fractions by the intestinal microbiome. As the intestinal microbiome is part of the barrier function of the gut in animals (Fassarella et al., 2021; Barko et al., 2018), the effects of diet composition on the microbiome composition and activity also needs to be considered from the perspective of gut health and gut functionality. Fiber from plant derived ingredients is a heterogeneous fraction of the diet, often referred to as non-starch polysaccharides (NSP). The generic term ‘fiber’ is used for a diverse group of complex carbohydrates consisting of NSP, oligosaccharides, resistant starch, and lignin. The fraction is further characterized by chemical and physical characteristics, e.g. related to their solubility in the conditions prevailing in the digestive tract. More recently, their water-holding capacity, viscosity, binding ability, absorptive capacity, faecal bulking capacity as well as their fermentability have been considered in relation to their effects on the digestive process and gut function (Williams et al., 2019). The fiber fraction not only interferes with the process of enzymatic digestion, largely in the stomach and small intestine, but also with the microbiome and its functionality in different segments of the gut. A proper amount and balance between soluble and structure fiber supports adequate functionality of the gut and supports the establishment of the “healthy” microbiome in different parts of the GIT (Bach Knudsen et al., 2012; Pandey et al., 2023).

Further processing of circular ingredients and diets

Many circular ingredients of plant origin, including rapeseed meal and sunflower seed meal, are high in fiber limiting their enzymatic and fermentative digestibility in the GIT of pigs and negatively interfering with the digestibility of protein and fat in the diet. Therefore, much attention has been given to the application of processing technologies to increase their nutritional value and limit their negative effects on the nutritional value of the diet. Processing techniques such as mechanical and hydrothermal processing can be used to alter the physical properties of the diet in order to improve the fermentability of organic matter to a variable extent in the hindgut of pigs and limit effects on e.g. endogenous protein losses in the GIT (de Vries et al., 2012, Agyekum and Nyachoti, 2017; Li et al., 2021). The application of exogenous enzymes in high fiber diets has been studied extensively in pigs. However, inconsistent results have been obtained regarding their effects on nutrient utilization and growth performance. Factors such as variation in dietary ingredient composition, and enzyme characteristics (i.e., source and dose) have been indicated as major factors explaining the variation in observed effects (Adeola and Cowieson, 2011; ArandaAguirre et al., 2021; Lannuzel et al., 2022). To increase the potential of using fiber rich circular ingredients in pig diets, further refined processing methodologies in combination with more precisely targeted exogenous enzyme cocktails or novel strategies to implement fermentation treatments on circular ingredients prior to feeding, should be considered.

Conclusions

Pigs can play an important role in further developing circular food production systems because of their ability and digestive (enzymatic and fermentative) capacity to utilize a wide range of byproducts (circular ingredients) as part of their diet. Whereas animal nutritionists tend to focus on the animal, diet and/or farm perspective, the concept of circularity of food production should be considered at systems level in the context of local, regional or global food production. Use of more circular diets reduce the feed-food competition and in most cases reduce the carbon footprint of meat produced. At animal level, however, other sustainability parameters such as N- and P efficiency might be compromised when feeding more circular diets related to e.g. a lower N and P digestibility of by-products of plant origin. Application of novel processing methods or targeted supplementation of novel enzymes might help to limit these trade-offs. Further research should explore the applicability and potential limitations of using more circular diets, to further enhance the circularity of meat production.

Acknowledgements

Part of the research presented in this paper was conducted within the framework of the publicprivate partnership ‘‘Feed4Foodure-III; Sustainable animal nutrition in circular agri-food systems” (TKI Agrifood nr. LWV20203) and funded by the Ministry of Agriculture, Nature and Food Quality (project number BO-55- 001-015), and “Vereniging Diervoederonderzoek Nederland”.

Presented at the 2024 Animal Nutrition Conference of Canada. For information on the next edition, click here.