LEVERAGING LIVESTOCK TO PROMOTE A CIRCULAR FOOD SYSTEM

Livestock provide multifaceted services to human societies worldwide. In developing countries, they are crucial assets and safety net for rural poor, and they provide nutrients-dense food to nourish people. In developed economies, growth in demand for animal-derived food is slowing while attention is growing over the role of livestock farming in an enhanced circular food system for sustainability. This analysis, focusing on the modern food systems in developed countries, aims to highlight the unique function of livestock that helps people re-harvest and upcycle crop and food residues generated along the food chain that are otherwise unfit for human consumption. First, human-unusable crop and food residue materials are described in three broad categories based on their characteristics and potential feeding attributes; the magnitude of biomass materials that are already used in routine animal feeding as well as residues that remain as underutilized resources are illustrated using the USA as an example. Then, the research and technology development critically needed for the future is discussed. As the world strives to produce more food with smaller environmental and climate footprints, upcycling the residual biomass via livestock for food production presents a viable pathway toward improved resource use, reduced pollution and enhanced food system efficiency. The primary function of agriculture is to produce food, fiber and fuel to serve human needs. Livestock farming is an important component of the modern agriculture and food systems with a double role to play. One is obvious—to produce nutrient-dense foods such as meat, milk, and eggs for people. Such foods are particularly critical to the world’s poor as the main source of essential proteins and micronutrients for reducing stunting and wasting[1]. The other role is not as obvious to the general public but equally, if not more, important—livestock animals can feed on crop and food residues that are unfit for humans to produce meat, milk, and eggs, thereby helping to maximize the beneficial use of biomass already produced and also to lower resource, environmental and climate burdens. The latter function is the essence of a circular food system that aims to extract maximal value from existing biomass to serve human needs. As the world strives to produce more food to feed the growing population, particularly the surging demand for animal-derived food in developing countries, leveraging livestock to enhance food system efficiency and promote a circular food system is imperative. The modern food and beverage systems generate large amounts of residual biomass from farm to fork. For example, 50%–70% of orange fruit is left in the pulp when making juice. In the USA, 4–7 Mt of oranges are used for juice-making each year[2], leaving 2–4 Mt in the residual pulp[3]. Another example is grain milling; the process leaves behind up to 25% residues. Annual mill residues amount to about 11 Mt in the USA[4]. Further down the food supply chain, 13%–14% of fruit and vegetables delivered to supermarkets remain unsold[5], which amounts to 6 Mt per year in the USA. Moreover, about 40 Mt edible food is estimated to be wasted by consumers (e.g., in restaurants and homes)[6]. Additionally, certain non-food systems also generate large volumes of plant-based biomass materials. For example, the USA ethanol industry generates DDGS (dried distillers grains with solubles) as byproduct, amounting to 44 Mt per year[7]. The various food-beverage-fiber-biofuel residues are generally human-indigestible, unpalatable or undesirable biomass (IUUB), which are unfit for direct human consumption under normal circumstances. However, these materials are still rich in nutrients, e.g., calories, proteins and minerals (Table 1), therefore they have considerable biological value. Capturing the nutrients contained in this biomass in ways that enable the regeneration of food for people would be highly desirable. Table 1 Nutrient profile of select food-beverage-fiber-biofuel processing byproducts commonly used in livestock feeding Dry Matter (% as fed) Crude Protein (% DM) Crude Fiber (% DM) Phosphorus (g/kg DM) Gross Energy* (MJ/kg DM) Notes Flour milling byproducts (wheat grain) 87.9 (87.0) 17.7 (12.6) 7.5 (2.6) 8.9 (3.6) 19.2 (18.2) Wheat milling byproducts, including the parts of wheat kernel that are richest in proteins, vitamins, lipids and minerals. Useful in ruminant, swine, poultry or fish diets. Soybean meal (whole soybean) 87.7 (88.7) 49.5 (39.6) 7.2 (6.2) 7.1 (6.1) 19.5 (23.6) Byproduct after oil extraction from soybeans; the most important protein source used to feed livestock animals. DDGS (maize grain) 89.0 (86.3) 29.5 (9.4) 7.9 (2.5) 7.9 (3.0) 21.4 (18.7) Byproduct of maize-based ethanol facilities in most cases, containing primarily unfermented grain residues (protein, fiber, fat and minerals). As a commodity, DDGS is fed to all classes of livestock animals. Citrus pulp, dried (citrus fruit, fresh) 90.3 (15.8) 7 (6.5) 14 (2.9) 1.0 (2.0) 17.6 (18.1) The solid residue after fresh fruits are squeezed for juice, consisting of peel (60% to 65%), internal tissues (30% to 35%) and seeds (up to 10%); used as a cereal substitute in ruminant feeds, due to its high energy content and good digestibility for ruminants. Cotton seeds, whole 92.3 21.8 28.1 5.9 23.8 The remains after cotton is ginned; can be crushed and the oil extracted, then the meal fed to adult ruminants. Data source: Feedipedia[8]. In many cases, nutrients became concentrated in the byproducts, as compared to the raw unprocessed substrates (data in parenthesis)[8]. *The amount of energy in the feed. Livestock are the ideal means for assisting societies to achieve this goal. As natural bio-processors, livestock have an innate ability to digest a wide range of biomass types and extract the contained nutrients for growth, maintenance and production. Around the world and historically, livestock have had a critical role in maximizing the beneficial use of biomass already produced, such as crop residues or food scraps, to serve human needs[9]. Modern food systems are exceedingly complex and versatile, resulting from the intensification of primary production on farms, specialization of food processing/manufacturing postharvest, plus globalization of food sourcing and distribution via international trade. Decoupled livestock and crop farming has not only changed the long practice of on-site recycling of nutrients in manure to cropland but also disrupted efficient reuse of various IUUB materials in animal feeding. Fortunately, this spatial divide does not mean a total severing of the services animals provide to society. In fact, large amounts of residues from crop and food processing industries (as byproducts) have been developed into commercial feeds, which are routinely used for livestock production. Additional to the industrial scale byproducts, there has been an upcycling of various crop residues or food scraps, such as unsold fruit and vegetables, in feeding dairy cows (Fig. 1). From the food system perspective, livestockcrop integration has maintained its functionality to a large extent. Nevertheless, massive amounts of IUUB materials, especially at the consumption stage of the food supply chain, remain underutilized or wasted. There is an opportunity for transformative changes to treat and manage these materials as feed resources instead of landfill wastes. Fig. 1 Plant-based residual biomass fed to cows on a Chinese dairy farm (residues from tofu-making, tea-making, garlic packaging, cottonseeds and pelleted dried beets pulp), and two dairy farms in Pennsylvania, USA (apple cores, and unsold fruit, vegetables, and baked goods). Photo credits: Zhengxia Dou, Joe Bender. IUUB materials from the food system are diverse and versatile. To facilitate discussion, these are grouped here into three broad categories based on relevant characteristics and potential feeding attributes. Type 1: Food-beverage-fiber-biofuel processing residues. Such residues are integral parts of the raw materials but are not meant for human consumption because of food culture/tradition as well as current processing technology. Examples include orange pulp, wheat screenings, cottonseeds, and DDGS from maize-based ethanol industry. Large-scale processing facilities typically manage their IUUB materials as byproducts (also now called coproducts) for animal feeding. Such feedstuffs are easy to handle, conform to feeding standards and safety regulations, with extended shelf-life. As commercial feedstuffs, their nutritional attributes, e.g. protein, minerals and digestibility are well-established. Such high-quality feeds have been widely incorporated into animal feeding programs, contributing to the enhanced productivity of modern dairy, beef, swine and poultry operations. Additionally, a wide range of IUUB materials are generated by numerous small-scale food and beverage processing facilities that are scattered across a country or region. Some of these facilities may operate steadily year-round, others periodically dependent on seasonal stocking supply and demand dynamics. Examples include residues from chocolate factories (steady year-round), apple pomace from cider or juice production (postharvest only) in apple producing areas, or wet brewers grains from local or specialty breweries. Different from the mass-produced coproducts from large processing centers described above, these IUUB materials often leave the production sites as raw residues without further treatment. They can be used for animal feeding on nearby farms through private arrangement with or without formal marketing-distribution channels. The nutritional attributes of these residues are generally steady and thus their incorporation into animal feeding programs is viable. A practical challenge is their relatively short shelf-life as untreated wet residues can be prone to spoilage. Type 2: Food retail-distribution