Álvaro García
This paradox of abundance coexisting with scarcity highlights a critical imbalance in the global agri-food system. Turning this challenge into opportunity requires innovative strategies that recover value from waste streams, reduce environmental impact, and enhance resource efficiency. One promising approach lies in rethinking how we use food and crop residues that are unfit for human consumption but still rich in nutrients. Each year, vast amounts of fruits and vegetables are discarded for being bruised, overripe, or irregular, even though they contain valuable sugars, water, fiber, and micronutrients. The question is whether this biomass can be safely preserved and reused within the food system rather than lost to landfills.
Ruminants offer a natural solution. Cows and other livestock can transform fibrous plant materials that people cannot eat into nutrient-dense foods like milk, dairy products, and meat. However, fruit and vegetable scraps are highly perishable and too wet to store for extended periods. To overcome this challenge, researchers have explored ensiling, a well-established preservation method based on natural microbial fermentation. In this process, chopped plant materials are tightly packed to exclude air, allowing beneficial microbes to convert sugars into organic acids. The resulting drop in pH halts spoilage, stabilizes nutrients, and creates a feed that can be stored for months.
A recent proof-of-concept study applied this approach to supermarket-style fruit and vegetable discards by blending them with drier crop residues to balance moisture and improve fermentation. The goal was to determine whether these co-ensiled products could serve as safe, nutritionally viable feed ingredients for sustainable dairy and beef production.
Turning leftovers into feed
A research team from the University of Pennsylvania conducted a series of small, controlled ensiling trials to assess feasibility. In one set, they ensiled mixed fruits and vegetables (ten types reflecting common U.S. supermarket discards) on their own. In others, they co-ensiled the same mix with crop residues and by-products such as corn cobs (CC), corn stalks (CS), spent mushroom compost (SMC), mushroom stems, and wet brewers’ grains (WBG). These combinations mattered because produce is extremely moist, around 85 percent water, while the residues are much drier. Balancing the moisture helps fermentation proceed cleanly and reduces the liquid seepage that can wash away soluble nutrients.
Across six-week runs, the researchers sampled the silage at days 3, 7, 14, 28, and 42 to track how preservation evolved. In the fruit-and-vegetable-alone treatment, pH fell rapidly to about 4.2 by day 3 and then stabilized between 3.7 and 4.1, the ideal range for well-preserved silage. Fermentation acids rose into the desirable 5–13 percent of dry matter (DM) range, with lactic acid dominating, a hallmark of stable fermentation. Co-ensiled batches with corn cobs or spent mushroom compost behaved similarly: pH dropped from 6.0–6.6 to 4.2–4.5 within three days, then settled just below 4.0, while acids climbed into the same target range. One co-ensiled variant later showed higher acetic acid (not lactic) as the main acid, indicating less optimal fermentation, but acetic acid still enhances aerobic stability and can be metabolized by cows.
These rapid pH drops demonstrate that even very wet mixes can be stabilized quickly, which is essential for discouraging undesirable microbes. The team found no butyric acid, typical of Clostridia activity a group of bacteria that does well in unstable high-moisture silages, causing off-odors, nutrient losses, and poor feed intake. Despite starting with materials below the typical “safe” dry-matter threshold of 30–32 percent, the rapid acidification prevented Clostridia from developing.
Some practical challenges were that when fruit and vegetables were ensiled alone, liquid effluent pooled in the container, about 160–210 mL/kg of substrate by day 42. That effluent contained soluble carbohydrates and minerals, signaling nutrient losses and a handling issue if scaled to farm conditions. Co-ensiling with dry residues sharply reduced seepage while preserving soluble nutrients, a two-for-one benefit that makes practical sense for farmers.
The team went beyond pH and acids by sending finished samples for nutritional analysis. Crude protein (CP) ranged from about 6.9 to 18.1 percent DM across treatments. Co-ensiling with WBG or mushroom stems increased protein levels, reaching values like grass or alfalfa hay, while mixes with corn residues resembled corn silage (around 8–9 percent CP). Fiber values were also within normal forage ranges: ADF between 20 and 40 percent DM and NDF between 32 and 58 percent. One caution was that mushroom refuse pushed lignin higher (around 18.7 percent DM), which can limit digestible energy, suggesting that they should be used sparingly in total mixed rations.
Ash contents stayed below 10 percent DM, typical for good forages, except in mushroom-heavy mixes, where ash rose and would need to be diluted. Total digestible nutrients (TDN) averaged 50–60 percent DM, comparable to medium- to high-quality forages. From a feed hygiene standpoint, mold and yeast counts remained within safe limits if anaerobic conditions were maintained. When a lid seal cracked, counts increased, mirroring the importance of airtight storage that farmers already practice in silage management.
To estimate how these feeds might behave in the rumen, the researchers conducted in-vitro incubation trials, using rumen fluid to digest feed samples at body temperature for 24 hours. They formulated a standard dairy ration and replaced 5 or 10 percent of it with this novel feed. Another set included a ground corn and protein mix to balance nutrients in the 10 percent diets. Gas production averaged 86–106 mL per container, and fermentation indicators such as pH, ammonia-N, and volatile fatty acids remained like the control diet. In practical terms, the rumen microbes accepted the new ingredient at these inclusion levels. Microbial community analysis showed slight change with 5–10 percent inclusion, confirming that fermentation remained normal.
What the results mean
Several conclusions emerge from this study. First, the preservation process worked effectively. Within days, fruit and vegetable discards, alone or paired with crop residues, reached the acidic, lactic-dominated state typical of stable silage without producing butyric acid. That is the foundation of any viable system: if the material cannot be stored safely, it cannot be fed safely.
Second, the finished feeds closely resembled conventional forages in composition. Protein, fiber, ash, and energy values all fell within normal nutritional ranges. While mushroom-rich mixtures carried higher lignin and ash, co-ensiled combinations with corn residues and WBG offered a balance more suitable for practical feeding. The added benefit of reduced effluent loss makes co-ensiling especially appealing at the farm level.
Third, the in-vitro results provided encouraging early evidence. At 5–10 percent of the ration, this novel feeds-maintained fermentation stability without disrupting rumen function. Although live-animal trials are still needed to assess performance, milk yield, and methane emissions, these findings confirm technical feasibility. Future work should quantify methane and carbon dioxide outputs to understand the net climate effect of recycling food waste through ruminants.
Scaling this approach will require addressing real-world challenges. Food waste composition changes with seasons and sources, so clear guidelines for moisture targets, mixing ratios, and quality checks are essential. Food safety protocols should specify acceptable inputs (pre-consumer, plant-based materials only), as well as screening for contaminants and maintaining anaerobic conditions during storage. Planning will also determine success: collection routes, transport distances, and farm-scale handling must balance costs and sustainability. Collaborative pilot projects among retailers, haulers, and farmers will help refine the system.
For farmers, these feeds can strengthen feed security and lower costs when conventional ingredients are expensive or scarce. For retailers and municipalities, nutrient recovery reduces disposal costs and environmental impact. For the planet, every kilogram of produce that becomes milk or meat instead of methane in a landfill represents a tangible gain.
Ensiling, a centuries-old technique, can turn discarded produce and crop residues into safe, nutritious cattle feed. It is not a silver bullet, but it is a practical, scalable step toward a circular food system where fewer nutrients are wasted, and more value is returned to farms and society.
The full list of references used in this article is available upon request.
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