Recently, the interest in industrial by-products produced at the local level in Mediterranean areas, resulting from fruit and vegetable processes, has increased because of their considerable amounts of bioactive compounds, including polyphenols. In this review, we analyze the most recent scientific results concerning the use of agro-industrial by-products, naturally rich in polyphenols (BPRP), in the diets of small dairy ruminants. Effects on milk production, milk and rumen liquor fatty acid profile, metabolic parameters, and methane production are reviewed. The feed intake and digestibility coefficients were generally depressed by BPRP, even though they were not always reflected in the milk yield. The main observed positive effects of BPRP were on quality of the milk’s FA profile, antioxidant activity in milk and blood, a reduction of rumen ammonia, and, consequently, a reduction of milk and blood urea. The expected beneficial effects of dietary polyphenols in small ruminants were not always observed because of their complex and variable matrices. However, owing to the large quantities of these products available at low prices, the use of BPRB in small ruminant nutrition offers a convenient solution to the valorization of residues arising from agricultural activities, reducing feed costs for farmers and conferring added value to dairy products at the local level, in a sustainable way.

Sheep and goat production is an important economic resource in many countries around the world. The industry produces wool (2 million tons/yr), meat (14 million tons/yr, 21% of cattle meat production), and milk (28 million tons/yr, 4.4% of cow milk production). The total world number of small ruminants is growing quickly. As shown in Figure 1, the world population of goats (1.05 billion in 2017) increased by 49% in the last 20 yr, whereas that of sheep (1.20 billion in 2017) and cattle (1.49 billion in 2017) increased more slowly (+15% and +14%, for sheep and cattle, respectively) (FAOSTAT, 2019. http://www.fao.org/faostat/en/#home). Sheep and goats are raised in a wide range of farming (from extensive to highly intensive) and feeding (from grazing diets to total mixed rations) systems, geographical areas, and by using diverse breeds, populations, and crosses, due to their high nutritional and environmental adaptability.

This bibliographic review presents and discusses the nutritional strategies able to increase the concentration of beneficial fatty acids (FA) in sheep and goat milk, and dairy products, with a particular focus on the polyunsaturated FA (PUFA), and highlights differences between the two species. In fact, by adopting appropriate feeding strategies, it is possible to markedly vary the concentration of fat in milk and improve its FA composition. These strategies are based mostly on the utilization of herbage rich in PUFA, or on the inclusion of vegetable, marine, or essential oils in the diet of lactating animals. Sheep respond more effectively than goats to the utilization of fresh herbage and to nutritional approaches that improve the milk concentration of c9,t11-conjugated linoleic acid (c9,t11-CLA) and α-linolenic acid. Dietary polyphenols can influence milk FA profile, reducing or inhibiting the activity and growth of some strains of rumen microbes involved in the biohydrogenation of PUFA. Although the effectiveness of plant secondary compounds in improving milk FA composition is still controversial, an overall positive effect has been observed on the concentration of PUFA and RA, without marked differences between sheep and goats. On the other hand, the positive effect of dietary polyphenols on the oxidative stability of milk fat appears to be more consistent.

The continuous increase in the cost of feeds and the need to improve the sustainability of animal production require the identification of alternative feeds, such as those derived from the agro-industrial sector, that can be effectively used for animal nutrition. Since these by-products (BP) are sources of bioactive substances, especially polyphenols, they may play an important role as a new resource for improving the nutritional value of animal-derived products, being effective in the modulation of the biohydrogenation process in the rumen, and, hence, in the composition of milk fatty acids (FA). The main objective of this work was to evaluate if the inclusion of BP in the diets of dairy ruminants, as a partial replacement of concentrates, could improve the nutritional quality of dairy products without having negative effects on animal production traits. To meet this goal, we summarized the effects of widespread agro-industrial by-products such as grape pomace or grape marc, pomegranate, olive cake, and tomato pomace on milk production, milk composition, and FA profile in dairy cows, sheep, and goats. The results evidenced that substitution of part of the ratio ingredients, mainly concentrates, in general, does not affect milk production and its main components, but at the highest tested doses, it can depress the yield within the range of 10–12%. However, the general positive effect on milk FA profile was evident by using almost all BP at different tested doses. The inclusion of these BP in the ration, from 5% up to 40% of dry matter (DM), did not depress milk yield, fat, or protein production, demonstrating positive features in terms of both economic and environmental sustainability and the reduction of human–animal competition for food. The general improvement of the nutritional quality of milk fat related to the inclusion of these BP in dairy ruminant diets is an important advantage for the commercial promotion of dairy products resulting from the recycling of agro-industrial by-products.

The warming impact of methane (CH₄) emissions calculated using the metrics proposed by the Intergovernmental Panel on Climate Change (IPCC), which measure its global warming potential in 100 years (GWP₁₀₀) expressed as carbon dioxide equivalents (CO₂e), accounts for the greatest impact in animal production chains. This work uses the new metrics, proposed to consider the difference between short living climate pollutants (SLCP), such as CH₄, and long living climate pollutants (LLCP), such as carbon dioxide (CO₂), which measure the warming equivalent (we) effect relative to that of CO₂ in a given time frame (GWP*) and expressed as CO₂we. The GWP* was applied to CH₄ emissions from all Italian livestock supply chains and compared with GWP₁₀₀ for annual and cumulative assessment from 2010 to 2020 of the impact of this gas on climate change. Using official data published by Istituto Superiore per la Protezione e la Ricerca Ambientale (ISPRA) from 1990 to 2020, almost all species, except for buffalo (+272.6% of emissions calculated with the new metrics), revealed lower CH₄ emissions with the greatest re-dimensioning for non-dairy cattle (-53786 kt of CO₂we of calculated with GWP* compared to +66437 kt of CO₂e estimated with the GWP₁₀₀ method). The total cumulative contribution of Italian livestock production to global warming over the past 10 years, including the nitrous oxide (N₂O) emissions, has been greatly negative (-48759 kt of CO₂we) compared to the data calculated using the GWP₁₀₀ method (+206091 kt of CO₂e). In conclusion, the application of GWP* metric to CH₄ emissions of all Italian livestock supply chains allowed to better identify the role of Italian livestock on climate change. Over the 2010–2020 time frame, the Italian animal supply chains reduced the warming impact related to its CH₄ emission, with the ruminants (expect buffaloes) being the major contributor to this positive effect.