Citation: Kioumarsi, H. (2026). Using Walnut Husks, Orange Peels, and Broad Bean Hulls in Sheep Diets. Research Communities by Springer Nature. https://go.nature.com/4qG7Tde
Abstract
The growing need for environmentally sustainable livestock production has fueled interest in the use of agricultural residues to produce alternative feed ingredients for ruminants. By-products such as Juglans regia husks, Citrus sinensis peels, and Vicia faba hulls are readily available from food processing and are rich in bioactive compounds and structural carbohydrates with potential nutritional value. This review aims to compile and assess existing research on their chemical composition, effects on ruminal fermentation, nutrient digestibility, animal performance, and potential effects on carcass quality in sheep. Existing research suggests that these by-products have the potential to affect rumen microbial populations, provide fermentable fiber fractions, and provide phenolic compounds with potential health benefits. Nonetheless, their overall efficacy is highly dependent on their inclusion rate, processing method, and balance.
Keywords: Walnut Husk, Orange Peel, and Broad Bean, Sheep, Nutrition
Introduction
Sheep, which belong to the ruminant category, have specialized digestive systems that allow them to break down fibrous materials using microbial fermentation in the rumen. This ability can be utilized to incorporate unconventional feeds that are high in fiber and biological compounds, such as fruit and legume by-products, into the diets of sheep, thus potentially lowering the dependence on conventional cereal and protein feeds.
My colleagues and I previously carried out a study investigating the in vitro antibacterial activity of extracts from Juglans regia, Citrus sinensis, Vicia faba, and Urtica urens, which was published in Bioactivities in 2023. I also considered the possibility of utilizing these agricultural by-products in sheep nutrition. However, these materials contain various bioactive and anti-nutritional components that may complicate their incorporation into sheep diets, and their practical use might not always be economically feasible. This short review was therefore developed as a preliminary exploration of the concept. Nevertheless, stronger conclusions would require substantial support from extensive experimental research in this field. Accordingly, it would be inappropriate to draw firm conclusions or make definitive recommendations solely on the basis of this review.
Walnut (Juglans regia) Husk
Walnut husk is an agro-forestry waste that is normally discarded after walnut harvest, but it is rich in phenolic compounds such as flavonoids, tannins, and antioxidants. It has been indicated in literature that green walnut husk has a high level of diversity in phenolic compounds and antioxidants.
Effects on Rumen Fermentation and Digestibility
In vitro experiments have explored the supplementation of diet with walnut green husk, and the results have revealed that the addition of husk resulted in an increase in the digestibility of organic matter, gas production, microbial protein production, and metabolizable energy. These findings suggest that walnut husk can serve as a fermentable material and can increase microbial production, but the presence of tannins and other phenolic compounds can affect fermentation and methane production. From my perspective, its use in sheep diets is not highly advisable.
A few recent studies have explored the effects of dietary supplementation with walnut shell biochar on growth performance, nutrient digestibility, and rumen fermentation characteristics in fattening male lambs. These findings suggest that walnut shell biochar can enhance fiber and organic matter digestibility. The result has shown that the digestibility of nutrients and volatile fatty acid concentration in lambs was improved without affecting growth rate. The enhancement in rumen fermentation characteristics suggests the potential use of processed walnut residues as a functional feed ingredient in sheep diets.
Bioactive Effects and Implications
Although the bioactive compounds like phenolics are known to possess antioxidant and antimicrobial properties, care should be taken in their direct use, as high levels of tannins can lead to protein binding and decreased nutrient availability. So pre-processing of husk materials may be necessary to ensure the best benefits without any adverse effects.
Orange (Citrus sinensis) Peel and Citrus By-Products
Citrus by-products such as peel, pulp, and dried pulp have been extensively researched for their potential in ruminant diets. The by-products contain highly fermentable soluble carbohydrates and digestible fibers, which can increase the production of volatile fatty acids (VFA) in the rumen.
Rumen Microbial Modulation
Studies assessed the processed orange peel waste in sheep’s diet for prolonged periods, showing changes in rumen microbial populations and a decrease in methanogenic bacteria, suggesting beneficial effects on the environment through decreased methane production. Microbial manipulation can affect ruminal fermentation efficiency and nutrient utilization.
Orange peel has essential oils and flavonoids, which have been reported to have antimicrobial properties and could be similar to feed supplements, although findings on their effect on fermentation variables and nutrient utilization are inconsistent. The fibrous component of orange peel can improve ruminal fermentation by serving as a substrate for fiber-degrading bacteria without increasing the risk of ruminal acidosis compared to high-starch diets.
Orange By-Products
Citrus pulp ensilage with forages can enhance the quality of feeds and stabilize fermentable materials in the form of feed matrices. However, low protein levels and digestibility variability should be considered, and sometimes supplementing with other protein sources may be necessary for optimal performance. Nargeszadeh et al. (2024) examined the impact of an ensiled mixture of orange pulp on the feed intake and growth rate of fattening male lambs. The results of the study showed that the ensiling of fresh orange pulp with dry ingredients of feed is an effective way of preserving orange pulp as animal feed, which could potentially reduce the cost of lamb production without affecting fattening performance.
Broad Bean (Vicia faba) Hulls and Residues
Broad bean hulls are the fibrous covers of the seeds obtained during bean processing. Although there is little information on research specifically conducted on hulls, their composition suggests a moderate level of crude fiber and a substantial amount of neutral detergent fiber (NDF), making them potentially useful as a source of roughage for ruminants.
Research conducted on dehulled faba bean seeds has indicated that their inclusion in ruminant diets can influence protein digestibility and nitrogen retention, suggesting that the hulls could have a similar effect.
Faba Bean By-Products
In reviews on faba bean by-products, it is noted that ensiled bean residues can be used as feed for sheep and can enhance the ammonia nitrogen content in ruminal fluid, although the quality of the feed is only moderate and may need supplementation with higher-quality forages or concentrates. The presence of tannins and other anti-nutritional factors in the hulls indicates the need for processing or blending with other feed ingredients.
Effects on Growth Performance
There is limited direct research that specifically investigates the impact of walnut husk, orange peel, and faba bean hulls on carcass characteristics in sheep. However, evidence from other studies related to by-products suggests that the replacement of conventional feeds with fibrous by-products, such as almond hulls, does not impair average daily gain and carcass characteristics but can enhance meat oxidative stability because of the antioxidant bioactive compounds.
The phenolic compounds in walnut husk and citrus peel could also have a similar effect on meat quality due to reduced lipid oxidation and improved antioxidant potential in muscle tissues.
Conclusion
By-products of agriculture like walnut husk, orange peel, and broad bean hulls offer great potential for sustainable sheep feeding. These by-products can be used as fermentable substrates, rumen microbial population modifiers, and potentially nutrient utilization enhancers when properly processed and incorporated. Although the nutrient content and anti-nutritional factors differ, combining these by-products with well-balanced diets can help lower feed costs and environmental disposal issues without compromising growth rate. More in vivo studies are required to clarify their individual effects on carcass quality and determine optimal inclusion rates in sheep diets.
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