Utilization of Agro-industrial By-products in Animal Feed

The growing agro-industry generates tons of organic residues (straws, bagasse, pulps, husks, pomace, whey, etc.) that are traditionally discarded, causing environmental problems. However, these materials contain valuable nutrients that can be reintroduced into the production cycle as animal feed. In fact, agro-industrial by-products are often rich in fiber, proteins, and bioactive compounds. Their use addresses both the need to reduce feed costs (by substituting imported grains) and environmental goals (decreasing waste and the carbon footprint) (1). Research organizations report on national programs where tables of nutritional value and maximum inclusion levels of these residues have been developed for each animal species.

Classification of Useful Agro-industrial By-products

Agro-industrial by-products can be grouped by origin. Examples include derivatives of cereals (bran, husks from grains) and their processing; of roots and tubers (beet pulp, potato peels); of oilseeds (soybean, sunflower, canola seed meals); of fruits and vegetables (citrus pulp, olive pomace, tomato pulp, vegetable waste) (2). Also included are by-products from sugarcane (bagasse, molasses) and the brewing industry (spent malt grains, spent yeast) or the dairy industry (whey). In practice, the local supply of residues varies by region: for example, in Mediterranean areas, residues from olives, grapes, and tomatoes are abundant (3). The diversity of materials requires evaluating each one: some are fibrous, others are rich in starch or protein, and all require knowing their composition before formulating a diet (4).

Advantages of Using Agro-industrial By-products

• Economic: They reduce diet costs by partially substituting conventional grains or forages. For example, in Chile, the increase in input prices has motivated producers to seek cheap alternatives with "good nutritional value." The use of residues near the farm avoids transportation costs for foreign raw materials.

• Environmental: They decrease pollution from agro-industrial residues (preventing their decomposition in the field) and reduce the system's carbon footprint. Spanish studies highlight that recycling these sources into animal feed promotes the circular economy, reduces import dependency, and lowers enteric methane emissions (1).

• Nutritional: Many by-products provide useful nutrients. For example, wheat or sorghum bran is rich in dietary fiber and offers good-quality proteins; citrus pulp contains fermentable carbohydrates that degrade quickly in the rumen. Residues like tomato pulp or wine lees provide protein and beneficial secondary metabolites, while olive pomace includes fats and phenolic compounds. Whey, although liquid, is abundant in lactose, fat, and proteins of high biological value. Together, these resources can improve the energy and protein density of the ration and add functional components (fermentable fibers, antioxidants, probiotics) that promote the digestive health of the animals.

Limitations and Risks

• Physical and Chemical Contaminants: By-products may contain foreign material (metal, glass, stones) from the production chain, which can cause physical damage to the animal. They can also harbor residues of agrochemicals or veterinary drugs (pesticides on fruits, antibiotics in poultry bedding). The presence of mold or other pathogens (fungi on husks or forages) can introduce dangerous mycotoxins. Although these risks do not usually completely prevent their use, they require adopting control measures and maintaining conservative inclusion levels.

• Nutritional Variability: The composition of residues can fluctuate depending on the season and the industrial process. While some standardized by-products offer a consistent composition, others (like fresh pulps or harvest by-products) show great variation between batches (4). Therefore, it is crucial to analyze their nutrient content (protein, fiber, energy) before using them.

• Palatability: Some residues may be less palatable. For example, poultry feathers require special treatment (autoclave or hydrolysis) to be digestible, and products with strong flavors or odors can decrease animal consumption. Molasses or flavoring additives are often added to improve acceptance.

• Conservation: Many by-products have a high water content (e.g., citrus pulp 65-70% moisture), which makes transport and storage difficult without losing quality. It is common to ensile them to preserve them anaerobically. However, ensiling changes the original composition and can produce alcohols or other metabolites that, in excess, can be toxic. In general, it is recommended to dry or ensile wet residues to eliminate pathogenic microorganisms, and in some cases, treat them with alkalis (e.g., calcium hydroxide) to improve their digestibility. The enzymatic degradation of lipids (in the case of seeds or oilseed cakes with fat) causes rancidity, feed refusal, and the risk of mycotoxins if not dried properly.

Processing and Conditioning

To maximize their nutritional value, residues are subjected to various treatments:

• Ensiling: Fermenting under anaerobic conditions (e.g., covering pulp or bagasse silos with plastic sheets) allows for the conservation of high-moisture by-products. According to Spanish studies, ensiling is simple and cheap, although it produces fermentations with alcohols that must be controlled to prevent poisoning.

• Drying and Grinding: Solar or mechanical drying of pulps and bagasse reduces weight and prevents unwanted fermentations. Once dry, fibrous materials (hemp stalk, sugarcane bagasse, husks) are ground or pelletized to homogenize the ration. Thermal processing (e.g., cooking potato pulp) or mechanical pressing (water extraction) are common to facilitate preservation.

• Chemical/Physical Treatments: Heat or chemical agents are often applied to protect nutrients. In oilseed cakes (e.g., soybean, sunflower), coatings with fats and applied starches or formaldehyde have been used to reduce the ruminal degradability of protein. Treatments with formaldehyde (0.5–3% on nitrogen) or with heat (120 °C) decrease the ruminal degradability of proteins and starches, increasing the bypass fraction and improving animal efficiency. In other cases, alkalizing agents (sodium hydroxide or lime) are used to increase fiber digestibility, as recommended by Spanish guides.

• Controlled Fermentation: Cultures of bacteria or fungi (solid-state fermentation) can be used to enrich the substrate with protein or degrade anti-nutritional compounds. This biotechnological approach, along with enzymatic techniques, opens the way for the production of functional ingredients from residues (e.g., products fermented with yeasts or fungi).

Recommendations by Animal Species

The utilization of a specific by-product must be adapted to the species and the production system:

• Ruminants (cattle, sheep, goats): Their digestive system allows them to utilize difficult fibers. Many residues can be incorporated at moderate levels (e.g., 20-40% of the total ration) without affecting performance. In Chile, it was reported that apple pulp (6-7% CP) was used at up to 50% of the diet for steers, with gains of ~1.2 kg/d and 22 L/d in dairy cows. Tomato pulp (18-21% CP) was incorporated at up to 40%, achieving ~1.1 kg/d in steers and no production reduction in cows (22-24 L/d). In contrast, by-products with anti-nutrients (tannins) must be limited—for example, grape pomace did not exceed ~15% of the diet because higher levels inhibited ruminal fermentation. It is recommended to introduce them gradually, analyzing the productive response and adjusting to the animal's physiological state. There are technical tables from Chile and Spain that suggest maximum inclusion levels for each by-product and species.

• Monogastrics (pigs, poultry): These animals digest crude fiber poorly. Therefore, residues with high digestibility are chosen or they undergo intensive processing. A successful case is brewer's spent grain (BSG): studies have shown it can be included at up to 20% in the rations of fattening pigs without affecting their growth. However, poultry and pigs do not digest cell wall polysaccharides well (arabinoxylans, β-glucans); to use BSG in poultry, specific enzymes (xylanase, β-glucanase) are often added. In general, residues rich in structural fiber (untreated straw, bagasse) are not recommended for poultry and pigs, as highlighted in specialized literature. Therefore, for these species, residues with very high protein quality (flours from animal or microbial by-products) or those that have been previously fermented are preferred.

• Other animals (equines, goats, fish): In general, similar criteria to ruminants are applied if they can degrade fiber, or to monogastrics if they cannot. Each case requires local evaluation. In all cases, it is essential to balance the ration, monitor consumption, and animal health when introducing a new by-product.

Practical Examples

Several experiences illustrate the potential benefits:

• Fruit Pulp in Ruminants: In Chile, a research program successfully integrated apple and tomato pulp into the diets of cattle and sheep. In beef cattle, 50% apple pulp maintained gains of ~1.2 kg/d, and 40% tomato pulp yielded ~1.1 kg/d (without reducing consumption). In fattening sheep, the inclusion of 30–40% of these residues did not affect weight gain (~240–250 g/d).

• Brewery Grains in Pigs: Various studies recommend up to 20% of spent malt grain (BSG) in the diets of fattening pigs, with no loss of performance. In Latin America, some breweries provide this by-product at a low cost to local farms, highlighting its "valuable composition" (fiber, residual protein) and its annual availability.

• Cheese Residues (Whey): In Mexico, whey is being researched for its high nutritional and functional content. Although much of it is discarded, processes have been developed to convert it into protein concentrates and fermented foods, obtaining high-value products and reducing the environmental impact.

• Citrus and Sugarcane: In citrus regions, orange pulp is commonly used after ensiling. Molasses from sugarcane, a residue of the sugar industry, is used to improve the palatability of ruminant rations. In several American countries, ensiling plants for fruit and vegetable residues have been implemented for livestock feed, showing success in the effective conservation and use of biomass.

These experiences demonstrate that the technical utilization of agro-industrial residues can become a strategic ally: it adds value to what was once waste, improves profitability, and promotes more sustainable livestock farming.

References: Recent academic and technical sources that document practical cases and the scientific basis for the utilization of residues in animal feed were consulted (1). Each statement is accompanied by its specific source for further reading, if needed.

(1, 3) Investigan la utilización de residuos vegetales en alimentación animal | RETEMA

https://www.retema.es/actualidad/investigan-la-utilizacion-de-residuos-vegetales-en-alimentacion-animal

(2, 4) Subproductos Agropecuarios en la Alimentación de Rumiantes | Intagri S.C.

https://www.intagri.com/articulos/ganaderia/subproductos-agropecuarios-en-la-alimentacion-de-rumiante